, CENTERS FOR DISEASE CONTROL ‘ AND PREVENTION _ Technical Report Control of Nitrous Oxide in Dental Operatories US. DEPARTMENT OF HEALTH AND HUMAN SERVICES Public Health Service Centers for Disease Control and Prevention National Institute for Occupational Safety and Health u ’E‘; k ’50 H_/"_‘ M4 Auk/133;“ V66I leqmendes 923$? 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Two of the scavenging C systems were chosen for the field evaluations because of nationwide {9:sgig availability and differing nasal cone scavenging design. The other scavenging system was evaluated in the laboratory because of its unique design to capture N20 due to patient mouth breathing. All scavenging systems, as designed, were found to be inadequate in consistently controlling the gas to the NIOSH REL. The NIOSH Recommended Exposure Limit (REL) for N20 is 25 parts per million parts (ppm) of air or less during administration. The REL is based on avoidance of adverse reproductive and impaired psychomotor health effects. Infrared thermography, real—time sampling, and integrated personal and general area sampling were conducted to quantify exposures to dental personnel in the field studies. The fourth in—depth survey included a combination of laboratory and field work on two new local exhaust systems, intended primarily to control patient mouth emissions of N20. Laboratory testing on a head form in conjunction with the fourth field evaluation, established that mask leakage due to poor fit was the primary cause of N20 emissions. An improved mask fit and the addition of a slotted skirt around the outer mask shell individually resulted in greatly reduced leakage rates in the laboratory tests. Also, exhaust systems placed on the chin, on the chest, or in the mouth, proved effective in capturing mouth emissions simulated by a breathing machine and head form. Based on these in—depth field surveys and laboratory tests, it was determined that N20 concentrations may be consistently controlled to approximately 25 ppm or less by the following: (1) maintaining a leak free N20 delivery system from the cylinder to the scavenging mask, (2) adjusting the system exhaust ventilation to recommended flow rates of approximately 45 liters per minute, (3) installing an air measuring device (such as a flowmeter) to assure that the exhaust rate is set properly, (4) redesigning the scavenging mask for better fit on the patient, and (5) using an auxiliary exhaust ventilation placed near the patient’s mouth to capture excess N20 from patient mouth breathing. -» 1 iii CONTENTS INTRODUCTION RESEARCH OBJECTIVES THE NEED FOR CONTROLS OVERVIEW OF N20 Physical Properties Toxicological Properties Reproductive Effects Human Studies—— Animal Studies—— . Carcinogenicity and Mutagenicity Liver and Kidney Effects Central Nervous System Effects EXPOSURE LIMITS . DENTAL PRACTICES AND ANESTHESIA EXPOSURES Analgesic/Anesthetic Dental Practices . . Nitrous Oxide Exposure During The Dental Procedure CONTROL PRINCIPLES Engineering Controls Substitution—— Isolation—— . Respiratory Protection—— Scavenging Systems—— Nitrous Oxide Concentrations in Dental operatories with Scavenging Systems . Efficiency of Different Scavenging Systems Local (Auxiliary) Exhaust Ventilation—— General Ventilation-— Work Practices Administrative Controls . . . Equipment Inspection and Maintenance—— Monitoring—— STUDY DESIGN STUDY SITE SELECTION . Survey #1: Pediatric Dental Facility . Survey #2: Oral Surgical Clinic Survey #3: Dental Clinic for the Developmentally Disabled Survey #4: Pediatric Dental Facility . METHODS . SAMPLING METHODS . Personal and Area Sampling Real— Time N20 Sampling Video Recording and Documentation of Work Practices Infrared Thermography . . EVALUATION OF VENTILATION SYSTEMS General Ventilation . . SCAVENGING SYSTEM VENTILATION . . . Survey #1: Pediatric Dental Facility . iv OOKOKDKOODVCthfiLnLnJ-‘PWUONMNI—‘I—‘I—I |-'|-' 15 16 17 l9 19 19 19 19 19 2O 20 20 20 21 21 21 21 23 24 26 26 27 27 Survey #2: Oral Surgical Clinic Survey #3: Dental Clinic for the Developmentally Disabled Local (Auxiliary) Exhaust Systems Survey #4: Pediatric Dental Facility . . Laboratory Test Facilities and Instrumentation Laboratory Test Procedures and Observations LEAK TESTING SCAVENGING EQUIPMENT . DATA ANALYSIS . . . . . . . . . . . . . Survey #1: Pediatric Dental Facility . Survey #2: Oral Surgical Clinic Survey #3: Dental Clinic for the Developmentally Disabled . Limitations in Data Analysis RESULTS . . . . . . . . . . . . . . . SURVEY #1: PEDIATRIC DENTAL FACILITY . Air Sampling Personal—— General Area—— Real- Time—— . Infrared Thermography . Ventilation . General—— . Scavenging System—— . . . . Work Practices and Changes in N20 Exposure SURVEY #2: ORAL SURGICAL CLINIC Air Sampling Personal—— General Area—— Real— Time—— . . Infrared Thermography . Ventilation . General—— . Scavenging System—— Work Practices and Changes in NZO Exposure Surgical Teams and Type of Operation—— Personal Versus Real—Time Sampling Data—— Oral Surgeon and Surgical Assistant—— Oral Surgeon and Real— Time—— . Surgical Assistant and Real— Time-— Summary of Work Practices and Changes in N20 . SURVEY #3: DENTAL CLINIC FOR THE DEVELOPMENTALLY DISABLED Air Sampling Personal—— Real—Time—— Ventilation . General—— . . Scavenging System (Auxiliary) Exhaust—— Performance of Auxiliary System Type—— Work Practices and Changes in N20 Exposure SURVEY #4: PEDIATRIC DENTAL FACILITY Air Sampling . . Ventilation -— Laboratory Experimentation . V 27 27 27 29 29 33 4O 41 41 41 42 42 42 42 42 42 45 46 46 47 47 47 47 48 48 48 48 50 52 52 52 52 53 53 53 53 53 54 54 55 55 55 57 57 57 57 58 6O 60 6O 63 Scavenging System (Auxiliary) Exhaust—— DISCUSSION SCAVENGING SYSTEM CONTROLS . SURVEY #1: PEDIATRIC DENTAL FACILITY a . Performance of Fraser— Harlake Scavenging System . Control of Airborne Exposures Personal—— General Area—— Work Practices . Mask Leakage and Patient Activities Ventilation . SURVEY #2. ORAL SURGICAL CLINIC . . Performance of Porter— Brown Scavenging System . Control of Airborne Exposures Personal—— General Area—— Work Practices Mask Leakage and Patient Activities Ventilation . . Modification of Porter— Brown scavenging System to Improve Effectiveness SURVEY #3: DENTAL CLINIC FOR THE DEVELOPMENTALLY DISABLED Performance of Porter— Brown Scavenging System with Local Exhaust . Personal—— Real— Time—— Ventilation —— Auxiliary Exhaust SURVEY #4: PEDIATRIC DENTAL FACILITY AND LABORATORY FINDINGS EDUCATION AND TRAINING IN USING DENTAL SCAVENGING EQUIPMENT . MAINTENANCE OF ANESTHETIC DELIVERY AND SCAVENGING SYSTEM EQUIPMENT CONCLUSIONS . SCAVENGING SYSTEM CONTROLS MASK LEAKAGE AND PATIENT ACTIVITIES VENTILATION . General . . . Scavenging System with Auxiliary Exhaust ventilation EDUCATION AND TRAINING IN USING DENTAL SCAVENGING EQUIPMENT . ADMINISTRATION OF N20 DURING RECOMMENDATIONS . EXPOSURE SOURCES AND CONTROL METHODS . Anesthesia Delivery and Scavenging System Controls Exposure Sources—— Control Methods—— VENTILATION . . General Ventilation . . . Local Exhaust Ventilation . ADMINISTRATIVE CONTROLS AND N20 MONITORING EQUIPMENT MAINTENANCE . . . . . vi 63 66 67 67 68 68 68 68 69 69 69 69 69 70 7O 70 71 72 73 73 73 73 73 74 74 75 77 77 78 78 78 79 79 79 79 79 79 80 80 80 80 82 82 83 83 84 WORK PRACTICES REFERENCES APPENDIX A . ANESTHESIA EQUIPMENT AND EMISSION SOURCES Compressed Gas Containers Gas Piping Systems The Analgesia Machine The Breathing System . The Mapleson Breathing System . Reservoir Bag . Breathing Tubes The N20 Gas Delivery Mask REFERENCES Vii 84 85 95 95 95 95 98 101 102 102 102 103 108 Table Table Table Table Table Table Table Table Table Table Table \l 10. ll. TABLES NIOSH Hazard Evaluation & Technical Assistance (HETA) results for N20 in nonscavenged dental operatories . NIOSH Hazard Evaluation and Technical Assistance (HETA) results for N20 concentrations in dental operatories using nasal scavenging masks . . Mean and standard deviation of N20 concentrations in dental operatories for scavenging masks evaluated in Wisconsin . Summary of personal and real—time sampling data (ppm) for N20 during administration in a pediatric operatory Summary of general area data (ppm) for N20 during administration Summary of personal & real— time N20 sampling (TWA during administration of N20), percent of N20 administered, time of administration, and time of operation . Results for general area sampling (ppm N20) . . Real- time N20 concentrations by team, time, and type of operation . . . . . . . . . . . . . . . . . . . Breathing zone, immediate area, and real—time, N20 concentration, based on auxiliary ventilation system used . Local exhaust system measurements . Step—by— step approach for controlling N20 to the NIOSH REL in dental operatories. . . . . . viii 13 16 44 45 49 50 51 56 58 81 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 10. ll. 12. l3. l4. l6. l7. 18. 19. 20. 21. 22. 23. 24. FIGURES Schematic of an anesthetic nasal mask retrofitted with a plastic scavenging dome and exhaust port. . . Schematic of the principal components of a common scavenging nasal mask Schematic of the principal components of a scavenging system with nasal mask . . . N20 concentration and scavenging system exhaust rates —— Wisconsin Data. , An auxiliary exhaust system for capturing N20 from patient mouth breathing . . . . . . . . General configuration for personal, real—time, and infrared videography monitoring of NéO . Data acquisition system for evaluating N20 exposure in dental offices . Basic configuration of the infrared thermography system to detect N20 in dental operatories Locations of auxiliary exhaust systems for control of N20 mouth breathing . . Breathing simulator for qualitative mask leakage testing. Quantitative mask leakage test facility. . . . . Standard and skirted mask laboratory leakage rates as a function of scavenging flow rate. . . Relative concentration of breathing gas inside the mask as a function of scavenging flow rate as measured in the laboratory for the standard and skirted masks. Laboratory leakage rates of an unmodified mask with tight and loose fits, nose and mouth breathing, and with a supplementary chin exhaust located on the head form’s chin or neck. . . Laboratory leakage rates of a skirted mask with tight and loose fits, nose and mouth breathing, and with a supplementary chin exhaust located on the head form’s chin or neck. Laboratory leakage rates of the Medicvent mask with tight and loose fits, nose and mouth breathing, and with a supplementary chin exhaust located on the head form’s chin or neck. . . Changes in N20 concentration during dentistry . Summary of N20 exposure and immediate area concentration data when using three different auxiliary exhaust systems Real— time N20 concentration changes from auxiliary exhaust ventilation . . . Diagram of auxiliary exhaust. . . N20 concentration versus time in the dentist’ s breathing zone with various combinations of the auxiliary and mouth— prop exhausts in place. Mouth— —prop exhaust. . . Dentist’ s and assistant’ 5 breathing zone concentrations of N20 averaged over the procedure. ix 12 l3 14 17 18 22 25 26 28 3O 32 34 35 36 38 39 43 59 61 61 62 64 65 Figure Figure Figure Figure Figure 25. 26. 27. 28. 29. Schematic of a central oxygen and nitrous oxide supply system with reserve supply a a ( Schematic of anesthesia machine with the arrangement of the components grouped according to high, medium, and low pressure 02 and N20 gas delivery . An analgesia delivery system from the 02 and N20 Wflowmeters to the mask used for patients A common nasal mask used for nitrous oxide delivery to the patient . . Sources of N20 exposure resulting from potential leaks in the anesthetic delivery system 97 99 104 105 106 INTRODUCTION The Occupational Safety and Health Act of 1970 has given the National Institute for Occupational Safety and Health (NIOSH), Centers for Disease Control and Prevention (CDC), a number of responsibilities, including the identification of occupational safety and health hazards, evaluations of these hazards, and recommendations for standards to regulatory agencies for control of hazards. Located in the Department of Health and Human Services, NIOSH conducts occupational safety and health research to assist the standard setting function of the Occupational Safety and Health Administration (OSHA) in the Department of Labor. An important area of NIOSH research deals with methods for controlling occupational exposure to potential chemical and physical hazards. The Engineering Control Technology Branch (ECTB) of the Division of Physical Sciences and Engineering studies the engineering aspects of health hazard prevention and control in the workplace. Currently, ECTB has been conducting an assessment of nitrous oxide exposures to dental personnel working in dental operatories. Nitrous oxide (N20) mixed with oxygen has been used in dentistry as an analgesic and as a sedative for more than 100 years. Today more than 424,000 workers who practice dentistry (i.e., dentists, dental assistants, and dental hygienists) in the United States, are potentially exposed to N20.1'2 Research was initiated to provide specific information about consistently controlling waste N20 in dental operatories below the NIOSH Recommended Exposure Limit (REL) of 25 parts N20 per million parts (ppm) of air during dental surgery. There were three premises for this investigation. First, there were commercially available N20 scavenging systems to control waste N20 in dental operatories. Second, there was published information in the literature, indicating that such systems did not control waste N20 to below the NIOSH REL. Third, there was insufficient evidence to indicate why such systems did not consistently control waste N20 to below the NIOSH REL. RESEARCH OBJECTIVES The objectives of this research were the following: (1) To determine why commercially available N20 scavenging systems were not controlling concentrations of N20 to below the NIOSH REL; (2) To determine whether the performance of commercially available systems could be improved to consistently reduce exposures to below the NIOSH REL; and (3) If the improved performance of commercially available scavenging systems could not consistently reduce exposures to below the NIOSH REL, to determine what further developments were needed to reach this goal. THE NEED FOR CONTROLS The suspected long—term adverse health effects associated with exposure to low concentrations of N20 have been demonstrated in a number of epidemiologic studies. These adverse health effects include irritability, headache, nausea, congenital abnormalities, spontaneous abortion, infertility, lymphoid malignancies, cervical cancer, hepatic and renal disease, and neurological disease.3""5"6 Short—term adverse health effects of behavioral performance 1 decrements have also been reported when N20 was administered at 50 ppm in controlled laboratory settings.7 The behavioral performance decrements are not present at 25 ppm. Based on the epidemiologic studies and the short—term health effects of performance problems, NIOSH recommends that exposures be limited to a time—weighted average (TWA) concentration of 25 ppm during the period of administration.8 Since the NIOSH recommendation was established, research efforts have focused on the mechanisms of N20 exposure and reproductive outcome. In the early 19805, N20 concentrations at 1000 ppm were reported to inactivate the vitamin B12 component of the enzyme methionine synthetase. It was theorized that the interference of the enzyme activity can impair deoxyribonucleic acid (DNA) synthesis and thus may partially explain the role of N20 in reproductive toxicity.9'10 However, more recent research suggests that this is not well supported. What may be happening is a multifactorial etiology, which may involve changes in uterine blood flow.“112 OVERVIEW OF N20 In the late 19805, research demonstrated that rats exposed to 500 ppm N20 for eight hours per day for one or two months had reduced fertility.13 These researchers hypothesized that N20 increases the fertility blocking secretion of luteinizing hormone reducing hormone in the hypothalamus, thus, disrupting ovulation. Most recently, a retrospective epidemiologic study of female dental assistants exposed to N20 showed significantly reduced fertility compared to unexposed females. In addition, those females with five or more hours of exposure per week had a 59 percent decrease in the probability of conception compared to unexposed females.1“ In 1977, NIOSH published a technical report entitled "Control of Occupational Exposure to N20 in the Dental Operatory."15 In this report, methods were recommended to control waste N20 to 50 ppm during administration, based on the technical feasibility of existing controls. Since then, several reports have shown that not only is N20 not being consistently controlled to 50 ppm, but also not to the NIOSH REL of 25 ppm during administration (based on health effects), when anesthetic gas control scavenging systems are used 16,17,18,19,20,21 Physical Properties N20 is an odorless, stable, noncombustible, colorless, tasteless gas that is approximately 1.5 times heavier than air. It is manufactured commercially by thermal decomposition of ammonium nitrate and purification of its byproducts.22 Toxicological Properties N20 does not combine with hemoglobin but is dissolved in the blood as a gas.23 It is eliminated, virtually unchanged, from the body by way of the lungs; a slight amount may be excreted through the pores of the skin.2‘ N20 is a weak anesthetic with rapid onset and rapid emergence,25 most of it disappearing from the body in 17 to 35 minutes after being discontinued. N20 2 can produce several changes in cardiovascular function. It may depress the myocardial action while stimulating the heart by central activation of the brain nuclei.26 It decreases cardiac output, stroke volume, mean arterial pressure, stroke work, and minute volume.27 Decreases are also seen in blood pressure, pulse rate, and respiration.28 In 1979, Vean and King stated that N20 acted solely on the cerebral cortex, thus causing a mild depression and that N20 was not allergenic.29 Amess et al. (1978) pointed out that N20 may interfere with the function of vitamin En.3° The toxic effects of N20 have been attributed to its ability to inactivate the enzyme methionine synthetase by oxidizing the enzyme’s vitamin B12 cofactor. Supporting documentation by Sweeney et al. (1985) provided evidence that occupational exposure to N20 may cause depression of vitamin B12 activity, resulting in measurable changes in bone marrow secondary to impaired synthesis of DNA.“- Other researchers have suggested that N20 may not depress vitamin B12 activity, but that another mechanism may be causing the reproductive health effects.”*33 Reproductive Effects Human Studies—— Exposure to N20, along with other anesthetic agents including halogenated anesthetic compounds, has been identified by epidemiological studies to be a suspected reproductive health hazard. Vaisman published the first report in 1967 of adverse reproductive effects from working in operating theaters.34 Dr. Vaisman noted that 18 of 31 female anesthesiologists who had been pregnant experienced at least one miscarriage. A number of studies in Sweden, the United Kingdom, and the United States have shown adverse reproductive effects in females working in operating rooms.5"5'35 The most comprehensive epidemiological study of health dysfunction associated with work in the operating room was from research data obtained from 40,044 respondents.6 Females working in the operating room showed an increased incidence of spontaneous abortion and carcinoma. The incidence of birth defects in their offspring was elevated, as well as in the offspring of nonoccupationally exposed wives of exposed male anesthetists. Spontaneous miscarriage and birth defects also were reported in a survey of female anesthetists in the United Kingdom.3 The findings of several epidemiologic surveys were summarized by James T. Purdham of the Occupational and Environmental Health Unit, University of Toronto.36 Another study summarized the in vitro animal and retrospective studies from N20 exposure.37 A consistent result in these summaries showed that women exposed to waste anesthetic gases had a higher than expected incidence of spontaneous abortions. Congenital abnormalities in the offspring of exposed women were less strongly associated but were slightly higher than normal. A recent epidemiologic study of California female dental assistants found that women exposed to nonscavenged N20 were at significant risk of subfertility compared to unexposed women, and those with five or more hours of exposure per week had a 59 percent decrease in their probability of conception for any given menstrual cycle compared to unexposed female dental assistants.1“ This same study also reported that female dental assistants who worked with N20 had no evidence of reduced fertility when working with scavenged N20 systems compared to controls. 3 Animal Studies—— Supporting evidence of the toxic effect of anesthetic agents is shown in laboratory studies. The evidence includes the following: teratogenic effects in various animal species upon exposure to a wide group of inhalation agents at anesthetic concentrations, decreased survival rate in various species, structural changes in the central nervous system of rat fetuses following a single maternal exposure, decreased ability to solve maze problems in rats, and evidence of testicular damage after a minimum of two days exposure to 20 percent N20.38*39 Several animal studies have focused on anesthetic gases, principally N20 and halothane, as a cause of miscarriage or congenital abnormalities. When the animals were exposed to high concentrations of these anesthetics, spontaneous abortion (animal fetal resorption) and congenital abnormalities were observed. In one study by Viera et al. (1980), spontaneous abortion was observed in rats at 1000 ppm or more.“0 Similar concentrations of 1000 ppm have been found in operating rooms and in dental operatories not equipped with scavenging systems. In a recent study, female rats were exposed to high concentrations (30 percent equal to 300,000 ppm) of N20 eight hours/day for four days to allow completion of one ovulatory cycle. All exposed rats exhibited abnormal ovulatory cycles. Rats exposed to oxygen and compressed air maintained a normal four—day estrous cycle.13 More recent animal experiments suggest that the reproductive hazards may be related to decreased release of luteinizing hormone reducing hormone.41 Other studies show that exposure to concentrations of 50 percent or more of N20 for 24 hours during early pregnancy result in high incidence of fetal wastage and skeletal and visceral abnormalities.32”2 The mechanisms which result in the fetal wastage and skeletal and visceral abnormalities, as well as other teratogenic effects, are yet to be defined. As mentioned earlier, one popular opinion suggests that N20 reacts with the reduced form of vitamin B12, thereby inhibiting the action of methionine synthetase, and thus interfering with DNA synthesis.43 However, this opinion has been challenged recently; an alternate theory suggests that N20 stimulation of alpha—1 adrenergic receptors may account for some of the adverse reproductive effects.33 Fujinaga et al. (1991) suggest that two mechanisms can be postulated for linking adrenergic stimulation and adverse reproductive effects: reduced uterine blood flow, and/or overstimulation of G protein— dependent, membrane signal transduction pathways.33 Both mechanisms are reported to be linked to teratogenic and tumorigenic effects. While the animal studies are not directly transferable to human studies, the reproductive effects warrant prudent use and control of N20. Carcinogenicity and Mutagenicity Excess cancer was found in a small group of nurse anesthetists in Michigan by Corbett in 1973.“‘ However, Ferstandig evaluated Corbett’s work and found that the high cancer rate occurred only for one year, and when all the data were considered, there was no significant difference between the nurse anesthetists and the control group.“5 Tests for mutagenicity (a test for screening carcinogenic agents in bacterial systems) are negative for N20.46 Liver and Kidney Effects A national study sponsored by the American Society of Anesthetists found that liver disease occurred more frequently among males and females exposed to anesthetic agents; kidney disease was less strongly associated with anesthetic exposure.“7 Studies supporting these conclusions were performed in England.A8 Because the workers were exposed to a mixture of anesthetic agents including nitrous oxide, halothane and methoxyflurane, it was not known what impact N20 alone had on liver and kidney dysfunction. In animal experiments N20 was shown to be without effects to the liver and kidneys.36 Central Nervous System Effects Human studies testing cognitive and motor skills show that exposure to trace concentrations of anesthetic gas mixtures, NZO/halothane or NZO/enflurane, and N20 by itself results in decreased ability to perform complex tasks.””“7 However, experimental attempts to duplicate human performance decrements have not supported these earlier studies.5° While habitual use of N20 has been linked to damage of the peripheral nervous system, the literature does not define a safe limit of occupational exposure that will not impair performance. The epidemiologic and behavioral toxicity studies are not without controversy. The literature citing limitations of the research for long term, low concentration exposure to N20 is summarized by Yagiela (1991).“3 Yagiela concludes that there is evidence that a potential danger exists for adverse health effects to occur as a consequence of N20 exposure, and that there is a known mechanism by which N20 could induce deleterious health effects. In addition, some studies have not been able to duplicate the deficiencies in behavioral performance among test subjects reported in the literature.5’1'52'53 EXPOSURE LIMITS In May 1977, NIOSH published a criteria document entitled "Occupational Exposure to Waste Anesthetic Gases and Vapors."8 This document recommended a N20 concentration no greater than 25 ppm during administration. This document also recommended the use of engineering and work practice controls and discussed health effects and methods for monitoring anesthetic waste gases and vapors. The NIOSH REL for N20 in the criteria document was based on several studies showing adverse health effects at higher anesthetic concentrations, including the following: irritability, headache, nausea, congenital abnormalities, spontaneous abortion, involuntary infertility, lymphoid malignancies, cervical cancer, hepatic and renal disease, and neurological disease compared to controls.%“’7'16’ However, the recommendation for a 25 ppm maximum limit was based primarily on a NIOSH—sponsored study performed by Bruce and Bach“7 and published by NIOSH in April 1977.15 This study showed that human volunteers who were exposed to N20 at concentrations of 50 ppm had audiovisual decrements with delayed reaction times to audiovisual stimuli. No such decrements were observed at 25 ppm.8 The criteria document concluded with the following recommendation: "The adverse effects of prime concern involve decrements in performance, cognition, audiovisual ability, and in dexterity during exposures to nitrous oxide. Such effects have been observed 5 at exposure levels to NéO at 500 ppm. At levels as low as 50 ppm, audiovisual decrements were observed in exposed volunteers. This shows the potential for this substance to impair functional capacities of exposed workers. Similar decrements were not observed at 25 ppm nitrous oxide with 0.5 ppm halothane. Based on this information NIOSH recommends that where exposures are limited to N20 alone, the permissible level of exposure should be a TWA concentration of 25 ppm during the period of administration."8 In April 1977, a NIOSH technical report was published which developed and evaluated controls for waste anesthetic gases in dental operatories.15 Studies presented in this report and based on technical feasibility of existing controls demonstrated that in dental operatories it was possible to achieve a N20 concentration of 50 ppm during administration. In October of that same year, an Ad Hoc Committee of the American Dental Association published a report entitled "Trace Anesthetics as a Potential Health Hazard in Dentistry."54 The Committee recognized the potential that a health hazard could occur and urged that every effort be made to reduce the trace concentration of anesthetic/sedative agents in the dental environment to concentrations as low as possible using the existing technology. In 1989, the American Conference of Governmental Industrial Hygienists (ACGIH) recommended a N20 Threshold Limit Value (TLV)Q of 50 ppm for an 8—hour day.55’56 One problem with the 8—hour TWA is that it permits short—term exposures to high N20 concentrations when the anesthetic is used intermittently. For example, if N20 is administered for only one hour during the 8—hour day, then it may be interpreted by the dental community that an excursion of up to 400 ppm TWA is allowed under the ACGIH guidelines. However, in order to control for intermittent exposure, the ACGIH notes that exposure should not exceed three times the TWA (i.e., 150 ppm during administration of N20).55 OSHA does not currently have a standard for N20. However, it has drafted guidelines for waste anesthetic gases and vapors. While the guidelines on anesthetic gases and vapors do not specify a limit, it provides information to employers and employees on the potential health risks, ways to reduce concentrations through engineering or work practice controls, means of implementing medical or training programs, procedures for monitoring gases and vapors in dental operatory, and implementation of preventive maintenance. Presently there are few specific state regulations governing the handling and administration of N20 by dentists. The Boards of Registration in Dentistry of 24 states have rules that regulate the use of N20. Massachusetts, Tennessee, Utah, and Wisconsin have implemented more detailed regulatory language regarding the use of N20; Massachusetts and Wisconsin also have implemented laws to control N20 by using scavenging systems. DENTAL PRACTICES AND ANESTHESIA EXPOSURES Analgesic/Anesthetic Dental Practices Dental practices may vary according to the type of dental setting, dental operation, and patient needs. Certain basic practices performed by the 6 dentist when using N20 are similar. Before N20 is administered, the dental assistant may position the patient in a dental chair and perform various other functions needed before the dentist begins work. These may include organizing the dental tools, setting up the mask for N20 delivery to the patient, and arranging intravenous sedation (if needed). Following preparation procedures, the dentist positions himself next to the patient and begins the operation. The dentist or assistant places the mask over the patient’s nose, turns on the oxygen and N20, and waits a few minutes, possibly five to ten minutes, for the N20 to take effect. N20 can be administered to the patient from a range of 1 to 70 percent; the usual range is 30 to 50 percent N20. For safety reasons, certain anesthesia machines are designed so that no more than 70 percent N20 and no less than 30 percent 02 can be delivered to the patient. The amount of N20 administered is based on patient needs as determined by the dentist. Some dentists administer N20 at higher concentrations at the beginning of the operation, then decrease the amount as the operation progresses. Others administer the same amount of N20 throughout the operation. When the operation is completed, N20 is turned off. Oxygen may be continued for a few minutes, after which the mask is removed from the patient. Some dentists turn the N20 on only at the beginning of the operation, using N20 as a sedative during the administration of local anesthesia, and turn it off before operating procedures. Based on variations in dental practices and other factors in room air, N20 concentrations can vary considerably for each operation and also vary over the course of the operation.57 Nitrous Oxide Exposure During The Dental Procedure When N20 equipment leakage is prevented, gas concentrations will be highest around the breathing zone of the patient, especially the nosepiece where the anesthetic is administered. The anesthetic gas mixture is exhaled by the patient, either from the nose or from both the nose and mouth, and is diluted by mixing with room air. Mixing occurs from the movement of supplied air through ducts or wall—mounted air conditioners and from the movement of the dentist and dental assistant. N20 concentrations vary according to the amount of fresh air supplied to the dental room and the room configuration (i.e., open or closed architecture).57 Personal exposure of the dentist and dental assistant to the anesthetic will vary according to their proximity to the breathing zone of the patient and the general room concentration. Previous survey observations have shown that the dentist usually works from 6 to 12 inches above the patient’s breathing zone, while the dental assistant works from 12 to 24 inches of this zone.57 In an environment where there is little air movement, high concentrations of N20 may occur between the working level of the dental personnel and the patient. If the room is not well ventilated, gas concentrations may be very high at times and not return to baseline levels. Over time, background concentrations may increase, as other operations using N20 are performed. Personal exposures of N20 found in earlier surveys conducted by NIOSH researchers varied from 25 ppm to 3,500 ppm.21 Table 1 shows the results of NIOSH Hazard Evaluation and Technical Assistance (HETA) evaluations for nonscavenged occupational exposure to nitrous oxide in dental operatories. The NIOSH findings are consistent with other studies showing high concentrations of N20 in dental operatories. As Table 1 shows, there is a large range of N20 concentrations for nonscavenged delivery systems. Scheidt et al. showed that the concentration of waste gas in the ambient air during administration of NZO/O2 is dependent upon three primary factors: (1) the distance from the nosepiece escape valve; (2) the position in relation to the direct line of waste gas dissemination; and (3) the changes in concentration of analgesia.58 Table 1. NIOSH Hazard Evaluation & Technical Assistance (HETA) results for N20 in nonscavenged dental operatories. HETA # General Area Sampling Personal Sampling Report Number Range, (ppm) Range, (ppm) 78-959 10 — 170 150 — >1400 79—5—5646o 75 — 3000 90 — 3500 79—4361 170 — > 1000 180 - >1000 79—5962 54 — 500 258 — 2650 80—1663 100 - 210 25 — 300 81—200—9996“ 150 — >1000 200 — 700 81—342—100565 4 — >250 175 — >250 84—126—155566 20 — 350 ——- 84—204—160067 100 — 750 670 — 2270 84-412—161268 70 — 315 2400 85—408—166669 100 150 — >1000 86—157—167870 50 — 800 700 — >1000 Angle of position in relation to the direction of waste gas dissemination was shown to be the most dominant factor. These findings may explain the large variation reported in the literature with regard to concentrations from nonscavenged systems. CONTROL PRINCIPLES Occupational exposures can be controlled by the application of a number of well-known principles including engineering measures, work practices, personal protection, and monitoring. These principles may be applied at or near the hazard source, to the general workplace environment, or at the point of occupational exposure to individuals. Controls applied at the source of the hazard, including engineering measures (material substitution, process/equipment modification, local ventilation) and work practices, are generally the preferred and most effective means of control in terms of both occupational and environmental concerns. In dental operatories, exposure to N20 may be controlled by the following activities: (1) effective scavenging devices that remove excess anesthetic gas at the point of origin (e.g., from the proximity of the mask); (2) good work practices on the part of the dentist and dental assistant, including the proper use of controls; (3) proper maintenance of equipment to prevent leaks; and (4) regular monitoring of environmental exposure for leaks in the anesthesia equipment delivery systems and to assure the effectiveness of equipment and controls. Additional controls that may be applied include dilution, general ventilation, and good housekeeping. In general, a system comprised of the above control measures is required to provide worker protection under normal operating conditions. Workplace monitoring devices, personal exposure monitoring, and medical monitoring are important mechanisms for providing feedback concerning the effectiveness of the controls in use. The education and training of dental personnel to reduce and eliminate occupational health problems are also important elements for a complete, effective, and durable control system. The sections that follow briefly examine the existing guidelines and current controls that are used to reduce sources of N20 in the dental operatory. Appendix A provides additional information on controls that are used for N20 at various points in the analgesia delivery system. Engineering Controls Substitution—— The substitution of N20 with a nontoxic analgesic gas, which can perform to the specifications required by this profession, would eliminate the hazards to the dental personnel from exposure to N20. Currently no such analgesic gas is available. Although N20 is routinely used in dental practice, many dental schools are training their students to use local anesthetics in combination with injectable drugs to get the same results. However, many dentists continue to prefer N20 because of its relative safety.71 Isolation—— Isolating dental personnel from N20 emissions by a physical barrier or by increasing the distance between the dentist and the patient while N20 is in use is a potential control method to reduce exposure. However, physical separation with a clear plastic barrier placed between the patient and dentist may not be practical because of the awkwardness and distance constraints of such a barrier. If barriers such as clear plexiglass are used, then functional design elements, such as size, distance, and effects on lighting, need to be considered. Consultation with dental practitioners will help with design, as well as acceptability of such control devices in the dental profession. It has been reported that a "rubber dam," a plastic 6X6 inch sheet placed in the patient’s mouth during N20 administration, serves to 9 reduce waste N20 by trapping this gas in the patient’s mouth. However, as discussed later in this report, N20 emissions may not be reduced by the use of rubber dams. The use of infrared thermography indicated that N20 was not trapped in the patient’ 5 mouth but was redirected through the left and right sides of the mouth where the rubber dam was not fastened. 57 Respiratory Protection—— Workers should wear respiratory protection when N20 concentrations are not consistently below 25 ppm; however, practical considerations may prevent them from wearing such protection. Therefore, it is essential that employers use the engineering controls and work practices to reduce N20 concentrations below 25 ppm. When N20 concentrations are not consistently below 25 ppm, workers should take the following steps to protect themselves: Wear air—supplied respirators. Air—purifying respirators (that is, respirators that remove N20 from the air rather than supply air from a clean source) should not be used because respirator filters do not efficiently remove N20. As specified by the NIOSH Respirator Decision Logic, minimal protection for an air—supplied respirator is provided by a half—mask respirator operated in the demand or continuous—flow mode. [Note: the assigned protection factor (APF) for this class of respirator is 10. The APF indicates the amount of protection provided by a class of respirator. An APF of 10 means that the respirator should reduce the air concentration of N20 for the wearer by a factor of 10 (or to 10% of the concentration without respiratory protection).] More protective air— supplied respirators are described in the NIOSH Respirator Decision Logic.72 When respirators are used, the employer must establish a comprehensive respiratory protection program, as outlined in the NIOSH Guide to Industrial Respiratory Protection, and as required by the OSHA respiratory protection standard [29 CFR 1910.134]. Important elements of this standard are (1) an evaluation of the worker’s ability to perform the work while wearing a respirator, (2) regular training of personnel, (3) periodic environmental monitoring, (4) respirator fit testing, (5) maintenance, inspection, cleaning, and storage, and (6) selection of proper NIOSH— approved respirators. The respiratory protection program should be evaluated regularly by the employer. Scavenging Systems—— The NIOSH technical report entitled "Control of Occupational Exposure to N20 in the Dental Operatory,".published in 1977, presented information on the development of engineering controls to consistently control waste N20 to 50 ppm during administration. The main engineering control was the design and development of a nasal scavenging mask. This scavenging mask consists of an inner and a slightly larger outer nasal mask. The inner mask has two 10 3/8 inches hoses connected which supply anesthetic gas to the patient. A relief—valve is attached to the inner mask to release excess N20 into the outer mask. The outer mask has two smaller hoses connected to a vacuum system to capture excess gases from the patient and from the analgesia machine. A flow rate of approximately 45 liters per minute (1pm) is the optimal flow necessary to prevent significant N20 leakage into the room air. In addition to scavenging masks, other engineering controls were developed to control N20, such as a suction hook and evacuated plastic hood which fit over the patient’s head.15 Researchers found the hood and suction hook were compatible with a conventional nasal mask; however, it was inconvenient to the dentist and unacceptable to the unanesthetized patient. The three—stage industrial scrubber, which contained a water spray, a calcium sulfate (CaSOA) absorber, and a high efficiency filter did not significantly reduce N20 concentrations. The authors concluded that optimal control of N20 could not be achieved by any single control system, and that the primary systems should include a regular maintenance program of the anesthetic equipment to reduce leakage, ventilation of the waste N20 to a safe disposal site, and use of a scavenging nasal mask.15 The usual analgesia equipment used by the dentist includes a N20 and 02 delivery system, a gas mixing bag, and a nasal mask with a positive pressure relief valve.73 The analgesia machine is usually adjusted to deliver more of the analgesic gas mixture than the patient can use. x A scavenging system, simply defined, is a means to collect and remove excess gases to prevent them from being vented back into the operating room. Installation of an efficient scavenging system is the most important step in reducing trace gas concentrations. It has been demonstrated that ambient concentrations have been lowered by 90 percent through the use of an efficient system.7“7536 A scavenging system has five basic components: (1) a gas collection assembly, which captures excess anesthetic N20 at the site of emission, then delivers it to the transfer tubing; (2) transfer tubing, which conveys the N20 to the interface; (3) the interface, which provides pressure relief and may provide reservoir capacity; (4) gas disposal assembly tubing, which conducts the N20 from the interface to the disposal assembly; and (5) the gas disposal assembly. Some or all of these components may be combined into a single device. The first prototype vacuum connected scavenging system was developed and tested in the middle 19705. There are currently several commercially available scavenging systems; however, several studies have shown that these systems cannot consistently meet the NIOSH REL.16’17'18'19’20'77'78'7g The most common scavenging system design includes a scavenging circuit (Mapelson D), a nasal mask, and a vacuum system. Figure l is a simplified schematic of a common anesthetic nasal mask that is retrofitted with scavenging equipment (a plastic dome with an exhaust tube is attached to the nasal exhaust port to reduce ambient N20 concentrations). Another common nasal mask for scavenged systems has two concentric masks in which anesthetic gases are supplied through a pair of tubes to the center of the mask. A second set of tubes is 11 Figure 1 Schematic of an anesthetic nasal mask retrofitted with a plastic scavenging dome and exhaust port. SCAVENGING CONE EXHALED GASES SCAVENGED GASES O2 / N20 INHALATION attached to the outer space of the mask to provide exhaust.15 This shape allows for scavenging of excess gas supplied to the patient, as well as excess gas that may escape around the edges of the mask. Figure 2 is a simplified schematic of the principal components of this common scavenging nasal mask. Figure 3 is a schematic of the anesthetic gas delivery and scavenging system with a detail of the Porter—Brown Scavenging Mask. Nitrous Oxide Concentrations in Dental Operatories with Scavenging Systems Table 2 shows NIOSH HETA results for scavenged occupational exposure to N20 in dental Operatories. These data show that scavenged dental Operatories have lower N20 concentrations compared to nonscavenged dental Operatories. However, such systems do not consistently reduce N20 to the NIOSH REL. Similar results have been reported by other researchers.18'80 A recently published article showed high concentrations of N20 for both scavenged and nonscavenged systems when used during pediatric sedation.18 This study was performed on 20 uncooperative 2 to 4 year old children, randomly assigned to a performed on 20 uncooperative 2 to 4 year old children, randomly assigned to a scavenged versus nonscavenged dental operatory. The results showed the N20 concentrations exceeded the NIOSH REL by more than ten times, regardless of whether a scavenging system was employed or not (mean concentration 300 ppm for scavenged versus 375 ppm for nonscavenged dental operatory).18 12 Figure 2 Schematic of the principal components of a common scavenging nasal mask. AIR INLET TO VACUUM Table 2. NIOSH Hazard Evaluation and Technical Assistance (HETA) results for N20 concentrations in dental operatories using nasal scavenging masks. HETA General Area Sampling Personal Sampling Report Number Range, (ppm) Range, (ppm) 78-6281 TRACE 38 — 171 78—129—54482 10 — >160 40 — 430 79—107—63283 10 — 60 10 — 300 80—102—76484 3 — 36 16 — >250 80—113—81385 25 400 — 500 80—249—83386 200 250 — >1000 81—111—147187 40 - >250 30 — >1000 82—070—114888 500 — 650 650 — >1000 84—126—155566 7 - 182 130 — 1300 84—412—161268 30 — 270 830 86-179—169989 0 — 500 200 — 1000 87—281—185490 65 — 140 30 — 220 l3 VI DETAIL / \ \ ___________ / \ l \ ‘\ /' nuaaen DISK \\ //’ CUTTER MASK \ INNER MASK GU | CK DI SCONNECT \\ TO CHANGE FROM ADULT / \\\ TO CHILD 'S MASK \\ \ 5/ 1S ELBOW \\ GOES THROUGH \ WTEH MASK ONLY \ \ 1/qx3/32 TUBING \To VACULM \ ANALGES IA MACHINE —’ \ \ \ \ \\ W VACUUM CONNECTOR \\ \ \ \ ANALGESIA i \ CONNECT ION FLOW METER VACUUy/TUB I NG VACUUM ON—OFF VALVE FLOW N “WW ADJUST VALVE ‘ OUTSIDE VENT OI: SCAVENG|NG MASK pop—cs: VALVE RETAINING 12le 7/ 15 ELBOW—GJES THPOLGH BOTH INNER AND omen MASK 3/ Exa/ 32 TUB | NE ANESTHET 1c 0 GAS INLET 'XSEIII '[ESEU L13 3:0 OIQEIIIEIIOS 'g alnfijg IDUIId a qnlm measfis BuifiueAeos e 30 snueuodmoo {ed Efficiency of Different Scavenging Systems A study to determine the efficiency of scavenging devices by a standardized experimental model was conducted by Hollonsten in 1982..16 Eight different masks were tested using well—controlled sedation techniques. Efforts were made to reduce N20 leakage by employing leak—proof equipment and carefully positioning the nose mask. The breathing zone N20 concentrations for the dentists varied from 4 to 385 ppm and the ambient air concentrations (i.e., in the dental operatory, but not in the dentist’s breathing zone) ranged from 0 to 55 ppm. These results are consistent with the NIOSH studies cited above, which show that operatories using scavenging systems have a range of 10 ppm to 1,300 ppm for personal sampling, and from O to 800 ppm for ambient air, or general air concentrations. Researchers comparing other scavenging systems have reported similar results.1“’17'18'91 In a study performed by Hollonsten, seven commercially available scavenging systems were evaluated: Blue”, Brown”, Porter” #1, Porter” #2, two conventional masks fabricated in Denmark, and the Fraser—Harlake". Sampling results for the Blue Mask showed a mean concentration of 4 ppm (2—65 ppm) for 12 experiments; the Brown mask showed a mean concentration of 5 ppm (2—178 ppm) for 13 experiments. The other masks ranged from a mean concentration of 17 to 53 ppm and met the Swedish standards of occupational exposure of 100 ppm. The Fraser—Harlake system did not perform as well as the others tested with a mean concentration of 385 ppm (110—1400 ppm).16 Donaldson et a1. published information in which the mechanisms, testing, and effectiveness of various scavenging systems were evaluated.20 Donaldson was able to show differences in mask performance under controlled conditions. Six scavenging masks were evaluated: Brown, Porter, Parkellm, Dupacom, and Fraser— Harlake. The Brown mask performed best with a mean concentration of 43.4 ppm for 35 trials in eight different dental offices; the Fraser—Harlake performed worst with a mean concentration of 62.7 ppm for 35 trials in eight dental operatories. Reasons for leakage ranged from talking (48%), movement (48%), poor mask fit (36%), restlessness (12.4%), technical problem (915%), mouth breathing (2.8%), mask movement (4%), and moustache (3%). Donaldson concluded that (1) the scavenging systems did not perform as well under actual conditions of dental surgery when compared to controlled experimental conditions and (2) that the scavenging systems appeared to perform best when used with nitrous oxide delivery systems designed for the mask. Mixing and matching different anesthesia delivery and scavenging components produced poorer results. The state of Wisconsin Department of Health and Social Services, Occupational Health Section, has performed over 300 dental surveys dating from 1978. Analysis of data from dental operatories where the scavenging system and vacuum exhaust rate information was collected showed that these operatories had significantly lower waste N20 concentrations than dental operatories without scavenging systems and masks (scavenging: [N=101 dental sites] mean N20 concentration 686/median 265; no scavenging: [N=64 dental sites] mean N20 concentration 2031/median 658). Table 3 shows the immediate work area N20 concentration results from different scavenging systems in dental operatories for the state of Wisconsin. 15 Table 3. Mean and standard deviation of N20 concentrations in dental operatories for scavenging masks evaluated in Wisconsin. Scavenging Masks Dental Sites Mean N20 Standard Surveyed Concentration Deviation Porter 33 636 650 Brown 20 325 357 Porter—BrownTM 1 223 ——— Blue 2 53 10 MDT—McKessenTM 3 257 67 Fraser—Harlake 16 1105 2203 Comfort Cushionm 3 112 50 Other 4 196 56 Unknown 17 1263 2411 Based on this information, the state of Wisconsin Board of Dentistry required that scavenging systems be installed in dental operatories when N20 is used and that the vacuum flow rate for the scavenging mask should be 45 1pm when administering a mixture of N20 and 02 to the patient. The Board of Dentistry also recommended that the masks be purchased or retrofitted with a flowmeter to verify vacuum flow rates.92 This data also showed that the scavenging system exhaust rate was related to the ambient concentrations of N20 in dental operatories, regardless of the type of scavenging system evaluated.93 Figure 4 shows the concentrations of N20 in various operatories as a function of scavenging system exhaust rates. Local (Auxiliary) Exhaust Ventilation—— In order to control for leakage around valves and fittings of the nasal mask, the use of local exhaust ventilation has been shown to be effective.94 In addition, the use of local exhaust ventilation in tandem with scavenging systems was effective in reducing occupational exposure to N20 by 75 percent during dental surgery, as reported by Carlsson et al.17 This type of ventilation effectively captures the waste anesthetic gases at their source, costs little to build, and works well in operatories with poor general ventilation. The major disadvantage is the proximity of the local exhaust opening to the patient (4 inches as recommended by Carlsson et al.). The proximity of this exhaust is distracting and may interfere with access to the patient’s mouth by the dentist. l6 Figure 4. N20 concentration and scavenging system exhaust rates —— Wisconsin Data.93 N20 Exposure vs Scavenging Flow 1_5 ‘With Ceiling 0 1 lo — D 1.3 —. U 1.2 — 1.1 ‘- 1 " D D D f‘ 0.9 " EA 0 3% 0.8 - D 1:: fl 3% U :3 .OJ - 0 EE 0J‘- B u a an 0.5 -- u D n 0.2. _ a o D 03 _ D D 0.2 - El D n E 0 OJ — a a o a i a a a I I I I 1 I I I l '12 0 20 40 60 80 100 Scavenging Flow Rate - Cubic Feet Per Hour (CF11) 95.3 cm - 45 1pm In another study, Middendorf and Jacobs fabricated a portable local exhaust ventilation system and tested its effectiveness in controlling nitrous oxide exposures.19 For one operatory, peak exposures were reduced from 600 ppm to less than 70 ppm. The authors concluded that a permanently installed local exhaust ventilation system could be designed that would be feasible for most operatories and should not interfere with dental procedures. Figure 5 illustrates an auxiliary exhaust scavenging system used to capture excess N20 from nasal mask leakage and patient mouth breathing. General Ventilation—— The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) recommends up to 25 room air changes/hour for surgical suites.95 However, there are no recommendations for room air exchange rates for dental operatories. It has been suggested that the guidelines provided by ASHRAE for surgical suites be adopted for dental operatories. l7 Figure 5. An auxiliary exhaust system for capturing N20 from patient mouth breathing. LOCAL EXHAUST DUCT N 20 / LEAKAGE Néj/O2 SUPPLY I /TO VACUUM PUMP 18 General ventilation is important when considering total control strategies for airborne contaminants. However, it can be inefficient when trying to control for N20. Additional concerns for general ventilation are reentrainment of contaminated air through ventilation systems and migration of N20 to other rooms. Work Practices Good work practices by the dentist and dental assistant are essential for controlling N20 exposure during administration. Good work practices by dentists to reduce N20 exposure have been reported in the literature. These include adjusting the scavenging system exhaust flow rate to 45 1pm; selection of the right size nasal mask for good fit; turning the N20 on only after the nasal mask has been secured on the patient; and flushing the anesthesia delivery unit and scavenging system with 02 following N20 delivery. To control N20 emissions from the patient, dental personnel should instruct their patients to avoid mouth breathing during dental surgery, avoid excessive talking while N20 is being administered, and minimize facial movement to maintain the nasal mask seal.20 Administrative Controls Equipment Inspection and Maintenance—— Good equipment and proper maintenance are important when controlling N20 in the dental operatory. Routine inspection and maintenance of dental equipment are essential in order to reduce N20 leaks and to have the best performance of dental scavenging equipment. Procedures for evaluating and maintaining dental equipment have been published.73 Monitoring—— Routine monitoring of N20 concentrations in the dental work environment is needed to ensure that the engineering controls work properly and the environment maintenance program continues to perform effectively. Monitoring can be performed through conventional time—weighted average air sampling and by real—time air sampling. STUDY DESIGN STUDY SITE SELECTION The project protocol included the following requirements for in—depth survey site selection: The facility (1) used at least one of the five scavenging systems selected to be evaluated; (2) had a minimum of four dentists; (3) had a minimum of four dental surgeries, with performance of operations in separate operating rooms; and (4) had the appearance of good work practices. Four sites were selected for the in—depth evaluation of the Fraser—Harlake and Porter—Brown N20 scavenging systems: two surveys at a pediatric dental facility, one at a oral surgical clinic, and one at a dental clinic for the 19 developmentally disabled. In addition, walk—through surveys were conducted to evaluate qualitatively the Blue Mask and Comfort Cushion at two other facilities: a family and cosmetic dental clinic and an Indian Reservation Dental Clinic. Use of the MDT McKessen system was observed at the oral surgical clinic. The Blue Mask, Comfort Cushion, and MDT McKessen were not quantitatively evaluated because the design, function, and use of these masks were not sufficiently different from the Fraser—Harlake and Porter—Brown systems to warrant in—depth evaluation. Survey #1: Pediatric Dental Facility At the time of this survey, 9 dentists, 8 dental assistants, and 8 support staff were employed. The facility performed dental work on an average of 41 patients per day with approximately 7 percent of the patients receiving N20 during dental surgery. This facility had ten dental chairs, all equipped with anesthetic gas delivery and scavenging systems. The dental work area had over 3,260 square feet of working space with three types of dental operatories: closed (one chair per one room), semi—open (two chairs separated with 6 foot high partitions), and open (three chairs with no partitions). This dental facility had used the Fraser—Harlake scavenging system for seven years prior to this survey. Survey #2: Oral Surgical Clinic This facility employed approximately 6 oral surgeons, 8 surgical assistants, and 12 support staff who performed dental work on an average of 15 patients per day; approximately 50 percent of the procedures used N20. This facility had ten dental chairs, all equipped with anesthetic gas delivery and scavenging systems. The surgical suites had floor to ceiling walls with single door entrances. This clinic had used the Porter—Brown scavenging system for more than ten years. Survey #3: Dental Clinic for the Developmentally Disabled All procedures were performed by dental hygienists in the dental clinic serving patients with disabilities. There was a common hall connecting two groups of four side—by—side chairs. A partition separated the two groups of chairs, but there were no partitions between chairs within each group. Limited use of N20 at chairs other than for the procedure under investigation was noted and effects were evaluated. Management personnel at this dental clinic permitted NIOSH researchers to install and evaluate a local exhaust system to determine its effectiveness in reducing N20 from scavenging mask leakage and from the patients’ mouth breathing during dental surgeries. Survey #4: Pediatric Dental Facility The fourth in—depth survey was conducted at the same pediatric dental facility as described in the first survey. Physical plant and personnel resources were very similar during the fourth survey as described during the first NIOSH survey. Scavenging systems were changed from the Fraser—Harlake to Porter— 20 Brown, and the ventilation improved, between the time of the first and fourth in—depth survey. METHODS While the basic method for evaluating N20 was similar in all four in—depth surveys, some differences based on knowledge gained from the prior in—depth surveys were applied to the subsequent ones. Because of this approach, better controls to reduce N20 exposure were used in each succeeding in—depth survey. "In—depth survey" will be referred to as "survey" from this point on. SAMPLING METHODS Personal and Area Sampling During the surveys, air samples were taken in the breathing zone of the dentists or oral surgeons and the dental assistants or dental hygienists. General area samples were taken at the room air supply and/or exhaust vents and areas outside the room. Personal and general area air exposures to N20 were collected in 30—liter Tedlar bags and analyzed at the dental facility using a calibrated infrared gas analyzer (Miran lA, Foxboro Instruments, Inc., Foxboro, Massachusetts). Battery—powered, universal flow sampling pumps (SKC 224—PCXR7, SKC Incorporated, Eighty Four, Pennsylvania) modified for bag filling were used to draw air through a section of tygon tubing into a bag. MSA Flo—Lite ProTM Pumps, (Mine Safety Appliances, Pittsburgh, PA), were used in the third study. The sampling pumps were calibrated at a flow rate of 1.5 1pm for both personal breathing zone samples and for general area samples. The sampling pumps were started when N20 was turned on and stopped when N20 was turned off. General area sampling was conducted at the entry to the operatory, in the main hallway of the facility, at the room air supply and the room exhaust fixtures, and at the appointment desk (separate from the operatories). Analysis of N20 samples was performed using a direct—reading, portable, variable—path length infrared spectrophotometer (Miran 1A) in accordance with NIOSH Method 6600.96 The general configuration for personal and real—time N20 sampling locations, dental or surgical suite layout, and visual and infrared videography setup for the case studies are shown in Figure 6. Real—Time N20 Sampling During each dental operation, N20 was measured and recorded continuously. The infrared gas analyzer (Miran 1A) was used to measure the anesthetic gas concentrations. This instrument is a variable filter, variable path length infrared analyzer with 20.25 meter cell. It has direct-reading scales with a gas cell mixing time constant of approximately 15 seconds under continuous operation. The lag time is caused by a combination of factors, including transport of the gas to the analyzing chamber, mixing of gas in the chamber, and instrument response. Because of the time lag, the output values at the peak and low exposures are truncated. However, the accuracy and ease of use of this instrument for real—time sampling greatly outweighs this limitation. 21 Figure 6. General configuration for personal, real—time, and infrared Videography monitoring of N20. Personal VISUAL CAMERAS 6S IR CAMERA Sampler Personal DENTAL OR ORAL SURGEON Real Time Sampler Dental Chair DENTAL ASSISTANT OR SURGICAL ASSISTANT Sampler I><] ‘— ANALGESIA CONTROL NITROUS OXIDE MEASURING INSTPUWNT MIRAN 'IA \ RUSTRAK DATA LOGGERS pomAsLE COMPUTER O 0 WA STE N 2:: CDNTAM I NANT DATA ANALYSIS DATA INTEGRATION AND RECOMMENDED/ NDDI F 1 ED SCAVENG | NG SYSTEM/ WORK PRACT | CES 'seoxggo Tequep u: elnsodxe OZN SUIQBUIBAB 10; measfis u013151nboe eqeq 'L eanIg Figure 8. Basic configuration of the infrared thermography system to detect N20 in dental operatories. STIPPER DANEL RGB MONITOR CONTROL ’///_ RADIANT PANEL GASEOUS a; :H‘ HUD AGA DISPLAY EMISSION DISCON SCANNER the mask; and, (4) the proximity of N20 emissions to the breathing zones of dental personnel performing the operations. EVALUATION OF VENTILATION SYSTEMS General Ventilation For the first two surveys, general ventilation measurements were taken for the dental operatories and surgical suites, and in locations where general area sampling was conducted, including dental laboratories, consultation room, equipment areas, dark rooms, sterilizer room, waiting room, offices, and hallways. The Kurz Model No. 480”, TSI Model No. 1650“, and Alnor Balometerm were used to measure air velocity and average flow rate, respectively. Smoke tubes were used to observe airflow patterns in each room, especially near the ceiling, in an attempt to determine if there was adequate mixing of air throughout the room. Measurements were taken to determine if the supply register louvers directed most of the air toward the ceiling or if there was good mixing of the air around the dental chair. Building blueprints were used for locating air duct locations and comparing flow rates with building design specifications. 26 For the third survey, the volumetric airflow rate of each supply and exhaust vent opening in the dental clinic building wing was measured. Fresh air supply rates could not be measured because the fresh air supply inlet for the building was not accessible. However, the air sampling results showed that the concentration of N20 was below detectable limits in the return air stream. SCAVENGING SYSTEM VENTILATION Survey #1: Pediatric Dental Facility The exhaust ventilation for the Fraser—Harlake Scavenging System was checked by using an airflow meter (Kurz). The exhaust rates of the scavenging system were measured using an in—line connection of the airflow meter between the scavenging mask and the exhaust port of the vacuum hose. The measurement was made after one of the dental operations in which the vacuum rate was adjusted by the dental assistants. The assistant adjusted the flow by listening to the noise level of airflow through the line; there was no flowmeter in the exhaust line to provide a visual indication of the flow rate. Survey #2: Oral Surgical Clinic For the Porter—Brown scavenging system, the exhaust vacuum rates were set to 45 lpm at the beginning of oral surgery by manually adjusting a valve connected to the scavenging vacuum line. A flow rate of 45 lpm was visually verified with a flowmeter (DwyerTM). At the conclusion of each operation N20 was turned off and the vacuum system valve was manually closed to stop the airflow through the vacuum line. Survey #3: Dental Clinic for the Developmentally Disabled The Porter—Brown scavenging system was used for all dental procedures. As in the second survey, the exhaust flow rate was set at approximately 45 lpm. The flow rate was measured using a by—pass flowmeter supplied with the scavenging system. The scavenging system was manually turned on and off with N20. Local (Auxiliary) Exhaust Systems The use of auxiliary exhaust systems were also evaluated at the Dental Clinic for the Developmentally Disabled. Of the 20 dental hygiene procedures (teeth cleaning) observed, six of the procedures were performed without the auxiliary exhaust ventilation system to provide a baseline assessment of exposure. Three auxiliary local exhaust configurations were used during the remaining 14 procedures. Positions for each exhaust system are schematically shown in Figure 9. The three local exhaust ventilation systems had in common a hood opening located near the patient’s mouth, a conveying duct, and a 10—inch centrifugal fan with a one horsepower motor located outside the building. The length of the conveying duct, suspended from the ceiling, was approximately 25 feet from the hood opening to the fan. 27 Figure 9. Locations of auxiliary exhaust systems for control of N20 mouth breathing. Real—time and area sampling probe ~l. System 1 employed an auxiliary exhaust system and a commercially—available hood and duct (Nederman Mini Extractor, Westland, MI) typically used in bench top industrial operations (e.g., soldering). This unit consisted of a 2.5—inch diameter nonflanged circular hood and 4 feet of 2.5—inch inner diameter duct equipped with three adjustable pivot points. Using the pivot adjustments, the hood opening was positioned over each patient’s chest, 6 to 8 inches from each patient’s mouth. The hood and pivot point system was connected to the exhaust fan using a 3—inch diameter flexible duct. The system 2 auxiliary exhaust system consisted of only the 3—inch diameter flexible duct used in system 1. The end of the duct served as a plain, nonflanged hood opening. The hood opening was placed directly above the nose and mouth area and was located 6 to 10 inches away from each patient’s mouth. Removal of the Mini Extractor resulted in an increased exhaust airflow rate. System 3 was similar to system 2 except that the 3-inch duct was replaced by a 6—inch duct. This system had the hood opening positioned above the chest in front of the nose and mouth area, about 12 inches away from each patient’s mouth. 28 Airflow measurements for each system were conducted with a pitot tube to determine the flow rate, and a swinging vane velometer (Alnor Instrument Company, Niles, IL) or hot—wire velometer (TSI, Inc, St. Paul, MN) measured the velocity at the face of the hood opening. Because noise levels from the auxiliary ventilation were considered a potential problem by dental personnel, sound pressure levels were measured for local exhaust ventilation systems 2, 3, and 0 (without the local exhaust ventilation systems). The sound pressure levels were measured near the dental hygienist’s ear with a GenRad Type I Precision Sound Level Meter (General Radio, Concord, MA) calibrated before and after each use. Survey #4: Pediatric Dental Facility The fourth survey was performed at the same facility as the first survey. Shortly after the completion of the first NIOSH survey, scavenging systems were changed from the Fraser—Harlake to the Porter—Brown scavenging system. Scavenging system exhaust vacuum rates were set to approximately 45 1pm by manually adjusting a valve connected to the scavenging vacuum line. N20 exposures were measured in a dental operatory using two Mirans continuously sampling the breathing zones of the dentist and the dental assistant. One end of plastic tubing with an inside diameter of % inch was fastened to the lapels of the dental personnel and run first to a diaphragm pump and then to a Miran. The flow rates of the sampled air were each about 17 1pm. Analog data produced by these Mirans were digitized and stored in data loggers (Rustrak Ranger I, Model RR 400, Rustrak Instruments, East Greenwich, Rhode Island), then downloaded to a portable personal computer for later analysis. Also, a video recording was made of most of the operations in which N20 exposure concentrations were measured. The video was synchronized with the digitized N20 concentration data for later use in correlating events in the dental operation with features of the concentration data. The speed of unconfined air was measured with a digital air velocity meter (Kurz Model 1440, Kurz Instruments, Incorporated, Monterey, California)s Ambient air velocities in the laboratory were adjusted to the same range as those found in the operatory based upon air velocity meter measurements. Laboratory Test Facilities and Instrumentation Some of the qualitative laboratory testing of the various scavenging mask configurations and supplementary controls was performed using a head form connected to a breathing machine and a smoke generator (Figure 10). The breathing machine was driven by a variable speed motor. The travel of the piston also was adjustable. For the data reported here, the breathing machine was set for 15 cycles/minute and the volume per inhalation (exhalation) was 580 cm3, which corresponds to a resting breathing rate.97 The smoke was delivered by tubing to the head form, either to its nose, or to its mouth, or to both the mouth and the nose. Qualitative evaluation of the performance of the equipment under test was based on visual observations of smoke capture. 29 0E SMDKE GENERATEIR BREATHING STMULATDR FDR NED EMTSSTDN CDNTRDL TESTING nfiig '01 91 H BAIQEQIIenb 10; IOJEIHMIS Buyqneel afiefieax asem Quantitative laboratory testing of the leakage of N20 administration or control equipment was accomplished using the breathing machine and head form just described along with additional apparatus, as shown in Figure 11. Infrared Analyzer 1 measured the concentration c of tracer gas, either N20 or sulfur hexafluoride (SFG), released into the hood where the head form and equipment to be tested was located. The flow rate, f, of tracer gas entering the hood was determined by the following equation, using the tracer gas concentration, c, and measurements of the flow rate, Q, of air entering the hood and traveling down the duct: CE volume tracer gas] X'Q [volume gas] = fl volume tracer] volume gas tlme time where: c was measured with Infrared Analyzer 1, Q was measured in the duct with a Pitot tube, and f was the flow rate of tracer gas in the duct, and thus, the flow rate of tracer gas not captured by the equipment under test. Infrared Analyzer 2 allowed measurement of the sum of the flow rates of tracer gas captured by the test equipment and by the hood, and provided assurance that tracer gas was not escaping from the test system. (The infrared analyzers used in both the laboratory and the field work were Miran, Models 1A or 1B2”, The Foxboro Company, East Bridgewater, Massachusetts.) Each Miran sampled gas in the exhaust duct using a diaphragm pump, operating at 17 1pm, located between the duct and the Miran. The gas was extracted from the exhaust duct through k—inch diameter stainless steel tubes inserted into the duct along its diameter. Gas entered the steel tubes through five 1n6—inch diameter holes drilled in the tube and spaced evenly across the duct diameter. The exhaust of Infrared Analyzer” 1 was routed back into the exhaust duct between the two Miran inlets. The exhaust of Infrared Analyzer 2 was routed back to the exhaust duct downstream of its inlet, preventing tracer gas contamination of the laboratory, which supplied fresh air for the hood. The breathing gas was supplied from a regulated compressed gas tank of air, containing 2 percent of either N20 or SF5. The breathing bag was a plastic bag of about l—liter capacity. The breathing gas was maintained at a flow rate of 10.27 lpm. The flow rates of the breathing gas and the vacuum pump were measured with calibrated rotameters. The flow rate to the exhaust blower was determined by two lO—point pitot tube traverses made at right angles in the 14-inch diameter exhaust duct. The flow characteristics of the breathing machine were determined using a Medistor Pulmonary Function Analyzer (Model M— 010, Cybermedic, Boulder, Colorado). Calibration of the Mirans was accomplished with the arrangement of Figure 11. By turning off the scavenging exhaust flow, a known concentration of tracer gas was generated at the sampling points of both Mirans, since the tracer gas and hood flow rates were measured. This measured concentration agreed well with the concentration determined using the internal library of the 1B2. In conjunction with the above quantitative leakage measurement system, an infrared (IR) imaging system (Thermovision 782, AGEMA Infrared Systems, 31 ZE FLDVMETER FDR BREATHING GAS L—J BREATHING \ BAG \\‘~ E E BREATHING VACUUM TRACER MACHINE FLDVMETER GAS VACUUM PUMP f + TD EXHAUST ‘ + BLUWER INFRARED INFRARED ANALYZER #1 ANALYZER #2 CDNCENTRATIDN TDTAL UF TRACER GAS DUE TU CDNCENTRATIDN UF MASK LEAKAGE TRACER GAS LABERATDRY TEST FACHJTY 'II exnfitg 'AQIIIOEJ 3593 939x991 332m 9A112113uenb Secaucus, New Jersey) was used to locate leaks in the systems under test. The source of IR energy was a square panel 18 inches on a side maintained at 120 °F. The apparatus under test was placed between the IR camera and the hot panel. Leaking tracer gas, such as N20 or SF6, strongly absorbed the IR radiation generated by the panel and was visible as a flowing plume or cloud on the IR system’s video display. Laboratory Test Procedures and Observations Control of mouth emissions was the original goal of this effort. The approach to this goal involved the following three steps: First, a series of local exhaust systems was constructed and tested qualitatively in the laboratory. Systems were tested using the apparatus shown in Figure 10 to determine which ones had the basic capability to capture mouth emissions. If this test was successful, the second step was to obtain an initial opinion of the system’s acceptability in the dental practice. The director of the dental operatory in which the field evaluation was accomplished provided this opinion. The third step was a field evaluation in the operatory of those systems which had acceptably met the criteria of the first two steps. With controls in place and operating, personal sampling of the dentist and dental assistant was carried out for N20 exposure concentrations. Because none of the controls passed the third step, tests were run to confirm the original assumption that the primary source of N20 exposure was mouth emissions. The apparatus of Figure 11 was developed to measure mask leakage. Also, an infrared imaging system was used to locate the leaks. Observations made with these two methods showed that the mask leaked when placed on the head form in what seemed to be a typical manner. Because the breathing bag was generally passive in the operatory, indicative of an ill—fitting mask, it was concluded that the mask leaked in most operations, as it had in the laboratory, and was the usual cause of overexposure to N20. Improved control of mask leakage was attempted in several ways and leakage was again measured using the apparatus of Figure 11. First, increased flow of the mask’s scavenger system was evaluated. When the scavenging flow was increased from 40 1pm to 62 1pm, the mask leakage decreased to 17 percent of its original value (Figure 12). The N20 concentration inside the mask was reduced to 73 percent of its original value as a result of this increase in scavenging flow (Figure 13). A second approach was to improve the mask fit. The data appears in Figure 14. Although a good—fitting mask (achieved by increasing the pressure of the mask’s inner shell against the head form) resulted in low leakage, it may not be possible to assure this quality of fit under conditions commonly occurring in the operatory. The third approach was the addition of a slotted skirt to the outer shell of the mask type used in the operatory (construction diagrammed in Figure 15). The leakage was decreased considerably as shown in Figure 16 and was not dependent for success on the mask’s quality of fit. A mask recently introduced on the market also was evaluated for comparative purposes, using the laboratory leak testing facilities (data shown in Figure 17). The three mask systems will be referred to as the following: 33 Figure 12. Standard and skirted mask laboratory leakage rates as a function of scavenging flow rate. O of] O O 03 N 5‘. O i o O [\ 2 '0 q) .EB (”‘5 2 E5 030C U _I(/') U; . o r’) a) x: a} g *5 O? or DC (1).— Ln 0; U‘ \V L02 _lg Q3/ ‘2 ex 5 0 8‘8 Xa '52 U) '0 t!) c 0% "3 o' $3 29 °‘ 0'0 m m > L3 0 r\ o g A1 r") 3 V) /,B/ D V O uuu/j‘sfioxog1xsow 34 wwmcnm Hw. memnw A ’/—E|u'ter' Shell Natl Duct Inner Shell _>A\-Sklr‘t Vacuum Duct Chin View Porter—Brown MQSR With Skirt Added '91 exnfiid 'xsem paalIfis 30 MEIBEIQ meCHm Hm. fimUOHmnOH% Hmmwmmm Hmnmm om m mWHHan Emmw swmr nwmrn mba Hoomm demu 50mm mda Boin? UHmmnrwsm. msa swnfi w mcwwwmsmdnmmw 05H: mmeCmn Hoomnma 0: firm Emma mowE.m orwd 0H Dmow. 9 mm t w mm m t .m «a “M m .6 Lu nu m ad H n B S m CH M _ m .+I.. a.+n.. H _ t a W . e ~ _ h d O n . 2 e1. .I j ‘ .mu 0 9 .. k .. H m g .4 N ham 8 . _ _ r H 60. _ w m .. . 0 * L O ..-um ....................... nv.r * o C m m 0.9 - h n v H H * 60. H H * 4. O . 9 .. H or; V $90 ..... O 46 Q m 0.9 I H 1 H ., y» u H 900 .U M... ADVVOQ . 9 Q n _ o I m M Q / 5 4 \Q «.0 A\o m .. CH :- $99WMV L ® 6: / .. 0 I 9 v m .. x9 % ///// . . Aflv Ax L _ _ _ ,_ _ _ 9 9+ \mxb 0\ ave 3 5 2 5 1 5 0 0“ .\W\ 00 0 2 0 1. O 0 em. 0 0 0 0 ® Tumcfim HV. HNUOHmnOHv. Hmmwmmm Hmnmm om firm ZmQHo/den Emmw and? nwmrn mum Hoomm mwnm. 50mm BE :65”: UHmmnrwnmu mdm sway m wcfivaBmDanv~ 03.5 mxrmCmn Hoomnma 05 $5 Toma. mowad or?» 0H Dmow. 5.14 ”'4Mouth Breathing -Loose Fit * * = No data taken °Leakage without °With chin exhaust on chin (no mask): control: 10.3 L/min L/min uh. F t h m. .4 ///¢ H. .. k 5 . M 8“ .. m 4. MW w 0 .. m d .r. e V M -. 0398 @tb Leakage, L/min o Unmodified mask —— The latest available Porter—Brown mask (Porter Instrument Company, Hatfield, Pennsylvania) as received from the manufacturer. 0 Skirted mask —— A Porter—Brown mask with a flexible, slotted skirt added to the outer shell. 0 Medicvent“ mask —— A recently—introduced mask (Model Anevac—D, Medicvent AB, Umea, Sweden) which includes a supplementary chin—mounted exhaust and has a much higher scavenging flow than the Porter—Brown masks. LEAK TESTING SCAVENGING EQUIPMENT N20 delivery equipment was visually inspected to ensure that all components were in place and that there were no obvious tears, cracks, abrasions, or worn spots. If there was an obvious problem with the general repair or maintenance of the equipment, components of the equipment was either repaired or replaced before leak testing was conducted. During the first survey, leak testing of the low—pressure components of the anesthesia scavenging machine was performed as follows: the breathing bag was removed from the anesthesia machine, overfilled with oxygen, and the end of the bag was corked. It was submerged in water, and the bag was inspected.and palpated to reveal leakage in the form of air bubbles. The hoses from the anesthesia machine were removed and a blood pressure gauge was adapted to fit the hoses. The hoses were sealed at one end and supplied with air to a pressure of 30 millimeters mercury (mm Hg). They were then submerged under water to reveal air bubble leakage. Leaks in high—pressure connection ports were checked by swabbing soap solution around the N20 and 02 valve connections and checking for soap bubbles. On the second survey, leak testing of low—pressure components of the anesthesia scavenging machine was determined by using the Miran 1A. After the end of the scavenging mask hose was blocked with the thumb, the gas delivery system was turned on. A tygon tube connected to the Miran 1A was used as a "sniffer" to detect N20 leaks, tracing the system from the wall connection to the mask connection. For high-pressure leaks, soap solution was applied with a swab applicator around the valve connections of N20 supply cylinders to check for leaks appearing in the form of soap bubbles. The supply cylinders of N20 and oxygen were located in a different section of the building, and the gases were supplied to the operatories through gas line connections. 0n the third survey, leak testing of the scavenging system, all high—pressure connections, and the tank and manifold connections was performed with a Miran 1B (Foxboro Instruments). The gas delivery and scavenging systems at each chair were turned on, and the scavenging mask was placed in a gas—tight bag, which was then sealed. The N20 and 02 valves were then opened to a flow rate of about 3 1pm. The probe for the Miran 1B was directed at various high— and low-pressure fittings along the gas delivery system to identify leaks. If a reproducible marked increase in N20 readings (10 ppm or more) was observed, a significant leak was considered to have been identified. For leaks identified in this manner, a soap solution was applied to locate the specific leak point. 40 The supply cylinders of N20 and oxygen were located in a closet in the building wing and were found to be free of leaks. DATA ANALYSIS Statistical analysis of the personal sampling results for the dentists, their assistants, dental operation, and type of dental operation was conducted for the first two surveys. For the third survey, statistical analysis was conducted for personal sampling results of the dental personnel and for samples taken in the immediate area of the patient’s mouth and local exhaust ventilation. Because N20 concentrations in other parts of the dental suite were low relative to the dental operatories, no statistical tests were performed to compare sampling from these areas with personal sampling results. Survey #1: Pediatric Dental Facility \ Paired Student’s t-tests were used to compare differences in N20 concentrations between the personal breathing zone results of the dentists and dental assistants. Paired Student’s t—tests were also performed for the real—time probe results and compared to the personal breathing zone results of the dentists, as well as the dental assistants.98 Analysis of variance (ANOVA) was used to compare N20 real—time sampling results for the three operatory configurations (i.e., open bay, semi—open, and closed room) that were evaluated.98 Mallows CP statistic (which measures the sum of squared biases plus the squared random errors in Y at all N data points) was used to evaluate the contribution of the work activity as a function of changes in N20 concentration for the dental operations mentioned above. Separate models were tested for each dental operation. The Statistical Analysis System (SAS) Procedure, General Linear Model SAS PROC GLM, was used for this analysis.99 Survey #2: Oral Surgical Clinic Data representing operations on nine patients by four dental teams were analyzed to determine how dental practices and other factors, such as probe distance and changes in N20 delivery concentration, affected N20 exposure concentrations. If dental practices can affect the amount of escaped N20, then one might expect to find similar effects resulting from different dental teams. The type of dental operation might also be a factor. Analysis of team and operation differences included the mean level (on a log scale) of N20 concentration as the dependent variable. Several limitations of using the logarithm mean level of N20 occurred: (1) positive concentrations of N20 were observed even before the source was turned on (apparently because residual N20 was trapped in the anesthesia delivery system from a previous operation); (2) in some of the data sets, positive N20 concentrations occurred before and at the same time the anesthetic gas was turned on; and (3) there was lag of one 41 or more time intervals before positive concentrations were observed after the source was turned on. Survey #3: Dental Clinic for the Developmentally Disabled For statistical analysis, the concentration of N20 in the dental hygienist’s breathing zone, dental assistant’s breathing zone, and area samples were considered the dependent variables. Local exhaust ventilation (with flow rates of O, 1, 2, and 3), patient compliance (compliant as opposed to struggling and in need of physical restraint), and flow rate of delivered N20 were the independent variables.99 Dental hygienist was not used as an independent variable because 18 of 20 procedures were conducted by the same dental hygienist. The SASTM general linear model (GLM) was used to analyze these relationships among dependent and independent variables. The geometric means and the upper 95 percent confidence limits of the dependent variables were evaluated against the NIOSH REL of 25 ppm to determine if the N20 concentrations in the dental operatory were statistically less than the NIOSH REL. Limitations in Data Analysis For each of the dental surgeries analyzed in the first and second studies, there were more than 150 possible models to compare. In all cases, the "best" of these differed from the next 20 to 30 models only slightly, based on the Mallows Cp statistic, the multiple correlation, the adjusted multiple correlation, and the site selection criteria. The selected model is at best only suggestive of important relationships. The selected model for N20 exposure sources exhibited some limitations: (1) In some cases, N20 from previous operations appeared to remain in the N20 delivery system, even after operations where oxygen flushing was performed. It is hypothesized that most of the N20 was in the reservoir bag, and/or leaking from the flowmeter. As a next operation began, this residual N20 was breathed in and exhaled by the patient, thus making it harder to analyze the data for work practice effects. (2) There was some confounding among the variables which might have blurred the effects and produced numerical problems for least squares statistical analysis. (3) The sequence of occurrence effects factors were ignored. For example, the presence or absence of mouth breathing might have affected the manner in which other factors affected N20 concentration. (4) Sample size differences and differences in patient behavior before and after the patient was fully sedated might have affected results. RESULTS SURVEY #1: PEDIATRIC DENTAL FACILITY Air Sampling Personal—— N20 concentrations within the breathing zones of the dentist and dental assistant ranged from 25 ppm to 950 ppm. The average real-time concentration 42 was 350 ppm. Seven lpm of gas were supplied to the patient’s nasal mask throughout the operation. N20 was supplied at 2.5 1pm while oxygen was supplied at 4.5 1pm. The mixture provided the patient with 40 percent N20 and 60 percent oxygen. During the 45—minute operation, the dentist "stepped—down" the N20 from 40 to 20 to 10 to 0 percent. The general area concentration of N20 subsequently decreased from over 200 ppm to 35 ppm 55 minutes after the operation began. By combining direct N20 readings with the videotape analysis, several work elements appeared to influence the concentration of N20 during the course of surgery. These elements included use of the scavenging unit, the regulation of N20 concentration administered by the dentist, the use of a rubber dam (i.e., a 6- x 6—inch rubber sheet inserted into the patient’s mouth to isolate the operative site from oral fluids), and the dental assistant’s use of the saliva aspirator. Also, the patient contributed to the exposure of the dentist and dental assistant through exhalation of N20 by talking, coughing, and yawning. A profile of the realmtime sampling results for N20 and dental work activities during the operation is shown in Figure 18. Figure 18. Changes in N20 concentration during dentistry. Real-Time NItl‘OUS Oxnde Concentrations Percent Nltrous Oxlds 01 gas admlnlstered to the pellent. o s 40 x 20 s 10 s o s 1000 I A - Alrjol uuu. C I - Aaplralor used. 0 800 ‘ 0 - Rubber can «no and ulna. n o - Lou ol muck coal. 0 I - Fllllna walled. e F - Muk romovoa. n . O - N.o hon dloeonnoolod. ; 600 - a F l 2 400 — e , g \ ' m 200— \I.\ 0 : i i i i O 10 20 30 4O 50 60 TI m e . nmln ut e s 43 The initial survey showed an average real—time N20 concentration of 352 ppm during dental surgery. It also showed that the use of this scavenging system did not guarantee a reduction to safe working concentrations of N20. A NIOSH HETA study of the same facility in 1979 showed N20 concentrations for personal exposure ranged from 90 to 3500 ppm without a scavenging system.60 While the scavenging system reduced N20 to lower concentrations, scavenging alone did not decrease it below the NIOSH REL. Work practices, including the regulation of N20 by the dentist during the course of the operation, location of the dentist’s breathing zone to the patient’s mouth, and use of a dental saliva aspirator and air jet appeared to influence the amount of N20 exposure dental personnel received while working. Information gathered on the Fraser—Harlake scavenging system during follow—up surveys at the pediatric dental facility showed that N20 exposures were generally lower in the open bay (Operatory 3—6) and semi—open bay (Operatory 3—9), compared to the closed room. Table 4 shows the N20 concentrations for the dentists and dental assistants by operation and dental operatory. The mean N20 concentration for dentists was 487 i 366 ppm; for dental assistants, Table 4. Summary of personal and real—time sampling data (ppm) for N20 during administration in a pediatric operatory. Personal Real—time Monitoring Dental Procedure ID Room Dentist Assis ’ t AVG STD MAX MIN Preliminary 3—8 * * 372 149 934 78 Survey Dental Op. 1 3—11 233 120 206 125 511 1 Dental Op. 2 3—11 >1000 432 770 476 >1000 0 Dental 0p. 3 3—11 904 142 502 503 >1000 0 Dental Op. 4 3—6A 133 88 295 161 638 2 Dental Op. 5 3—9A 347 44 282 150 623 2 Dental 0p. 6 3-9A 290 47 416 202 713 41 Dental Op. 7 3—6A 160 113 473 305 >1000 0 Dental Op. 8 3-6C ** ** (473) (605) (>1000) (4) Dental Op. 9 3—6C ** ** 469 484 >1000 48 NOTES: * = No personal sampling data was collected during the preliminary survey. ** = No personal sampling data was collected. () = Nitrous Oxide turned on prior to beginning of sampling period. 44 the mean N20 exposure was 150 i 144 ppm. Paired Student’s T—tests comparing N20 personal sample results of dentists with dental assistants showed a significant difference (X=6, p<.03). This difference may have resulted from a closer working proximity for the dentist than the dental assistant to the patient’s mouth. If there is N20 leakage from the mask, and/or from patient mouth breathing, differences in exposure between the dentists and dental assistants may be significantly higher. General Area—— General area sampling (Table 5) concentrations above the NIOSH REL were observed at the entry to the operatory in five of the six dental operatory runs and in three of the six main hallway runs. N20 was not detected at the appointment desk on any of the sampling runs. Table 5. Summary of general area data (ppm) for N20 during administration. INIT ROOM ROOM ROOM HALL APPT CONC SUPPLY EXHAUST DOOR DESK Preliminary Survey 25 # # # # # Dental Op. 1 4 0 31 ## ## 0 Dental Op. 2 2 0 114 38 29 0 Dental Op. 3 0 O 33 38 10 0 Dental Op. 4 4 30 114 95 0 0 Dental Op. 5 2 36 49 45 64 0 Dental Op. 6 45 50 73 48 50 1 Dental 0p. 7 4 4 8 l4 6 0 Dental Op. 8 5 ### ### ### ### ### Dental 0p. 9 42 ### ### ### ### ### NOTE: # No general area sampling data was collected during the preliminary survey. ## = Not all general sampling data was collected for dental operation number 1. ### = These runs were conducted to assess the feasibility of infrared thermography. The closed, semi—open, and open bay operatories were evaluated for N20 concentrations in the dental operatory room air exhaust and the hallway. Dental operatory room-supplied air did not show recirculation in the closed 45 bay but did show N20 recirculation from the wall air handling unit. The open bay, also showed recirculation of N20 from the wall units, but at a much lower concentration. These lower concentrations for the open bay may have been a function of several variables: (1) the larger area, allowing for more dilution of the N20; (2) the amount of N20 administered over time; (3) the dental procedure underway; and (4) the proximity of the air—conditioning wall units to open bay chairs (approximately 10 feet further away than the semi—open bay). The air conditioning wall units provided N20 recirculation because these units could not be adjusted to entrain outside air alone. The wall unit dampers were usually closed to outside air during winter to conserve energy. Real—Time—— Real—time sampling results for the dental operations ranged from an average NéO concentration of 206 ppm in Operating Room 3—11 (Dental Operation #1) to 770 ppm in the same operating room (Dental Operation #2). N20 concentrations exceeded 1000 ppm in five of the ten operations monitored. The real—time sampling and personal sampling results followed the same N20 concentration patterns. Analysis of variance showed the means of the three sampled areas (i.e., closed, semi-open, and open bay dental operatories) were not significantly different (p<.70). This may be attributed, in part, to the low sample size and high variance in N20 concentrations within the different operatories. There was no significant difference (p <.68) between the means of the average real—time sampling results (442 ppm) and the average dentists’ personal breathing zone results (487 ppm). However, the difference between the average real—time sampling results and the average breathing zone N20 concentrations among dental assistants (150 ppm) was significant (p <.Ol4). The real—time probe was positioned to maximize capture of N20 emission from the patients. Because the dentist worked in close proximity to the patient’s mouth, it may be assumed that the real—time sampling results were more representative of the dentist’s exposure than that of the dental assistant’s. Infrared Thermography An infrared scanning camera was used during Dental Operations #8 and #9 to determine scavenging mask leakage during administration of N20. This technique was very useful in determining N20 leakage around the patient’s mask. The scanner helped determine that the Fraser—Harlake mask did not fit the patient’s face properly, allowing N20 leakage between the mask and face seal. Furthermore, it was observed that sudden increases in N20 exposures observed from the real-time data could be traced to the patient’s expired breath; when the patient inhaled, the N20 concentrations decreased. This indicated that patient breathing was an important factor in exposure to the dentist and dental assistant. 46 Ventilation General-— The data from the first survey showed that there were differences in the measured flow rates for specified ventilation systems in the building. These differences were accounted for in part by acceptable variations from design flows and by changes in the ventilation systems from the original blueprint specifications. The periphery of the building was fitted with wall, recirculation heating/air conditioning units manufactured by the Singer Company that met American Refrigeration Institute (ARI) Standard 310—70 specifications for function EAlS. These specifications called for a cooling blower capacity of 290 cubic feet per minute (cfm) with 70 cfm (24 percent) maximum fresh air. The heating mode specification called for 280/60 cfm, which is a 21 percent maximum fresh air. The airflow from two Singer units was measured during the April 1988 survey by traverse velocity measurements, using the T81 hot—wire anemometer. The calculated flow rates were 287 cfm and 320 cfm, which is reasonably close to the specification of 290 cfm. Scavenging System-- The effectiveness of the capture capacity of the anesthetic gas from the scavenging nasal mask is evaluated by inserting a flowmeter in the exhaust ports of the scavenging mask tubing, following a dental operation. The exhaust valve for the scavenging mask is adjusted at the beginning of an operation by the dental assistant and not changed throughout the surgery. For Dental Operation #7, the flowmeter showed the exhaust to be approximately 7 to 12 1pm. The scavenging system ventilation was not evaluated until this operation. It had been assumed that the scavenging system flow rates were automatically set at 45 1pm, the effective scavenging amount when 4 to 7 1pm of N20 and 02 are mixed and delivered to the patient.15 Work Practices and Changes in N20 Exposure For the first seven runs, dental surgical activities were observed to determine if they influenced changes in N20 concentrations. When the dentist performed certain tasks, significant N20 concentration changes occurred: (1) turning the N20 gas on; (2) adjusting the concentration during the operation; and (3) turning the N20 gas off. Up to 98 percent of the changes in N20 exposure could be accounted for, based on the concentration in the gas delivered to the patient. Other dental work (i.e., the use of the rubber dam, the aspirator, and the air and water syringes) appeared to have little, if any, influence on changes in N20 concentration to which the dentist and dental assistant were exposed. This pattern was also evident in the initial preliminary survey. Figure 18 shows that certain work activities were observed to change N20 concentrations. However, the changes were small and transient compared to the overall N20 concentration, as shown by the area beneath the graph curve. This was more apparent when the dentist changed the amount of N20 administered to the 47 patient from 40 to 20 to 10 percent, demonstrating that the primary source of exposure for this scavenging system was from N20 delivery concentration and the low scavenging system exhaust rate. SURVEY #2: ORAL SURGICAL CLINIC Personal and general area air sampling was conducted for N20 exposure in nine dental operations using four surgical teams. All operations were performed using the Porter—Brown scavenging system. The duration of oral surgery for these operations ranged from 32 to 100 minutes. The percentage of time N20 was on during surgery ranged from 18 to 88 percent. The concentration of N20 administered to the patients ranged from 20 to 50 percent. It was constant throughout five of the procedures, but was varied for the others. The N20 supplied to the patient ranged from 2 to 3 lpm, while oxygen was supplied at 3 to 4 lpm. The total anesthetic mixture airflow administered to the patient for all operations was between 5 to 6 lpm. The oral surgery included (1) six operations for the removal of one wisdom tooth; (2) one for two wisdom teeth; (3) one tooth implant; and (4) one removal of mandibular canine. There were seven female patients and two male patients ranging in age from 20 to 75 years. Air Sampling Personal—— Table 6 shows the N20 TWA concentrations during administration for personal and real—time samples for the nine operations. These concentrations ranged from less than the detection limit (<1 ppm) to 277 ppm for the oral surgeons and from less than the detection limit to 77 ppm for the surgical assistants. The overall average N20 concentration for the oral surgeons in the operations measured was 101 (i 117) ppm, and for the surgical assistants 27 (i 31) ppm. Concentrations for the oral surgeons averaged from less than 1 ppm for team #3 to 257 (i 29) ppm for team #1. For the surgical assistant, concentrations varied from less than 1 ppm for team #3 to 77 ppm for team #4. The greatest difference in N20 concentrations between the oral surgeons and surgical assistants on the same team was found in team #1; the difference in concentration was approximately an order of magnitude higher (236 versus 20 ppm). N20 concentrations shown by personal sampling did not appear to be related to the type of operation performed. The average N20 concentrations for the oral surgeons was 131 (1129) ppm for Room D1218 and 25 (:28) ppm for Room D1222; for the surgical assistants it was 24 (:31) ppm and 34 (:43) ppm, respectively. General Area—— Results of the general area sampling data are shown in Table 7. There was no detectable initial N20 concentrations in eight of nine surgeries monitored by NIOSH personnel. The ninth surgery showed 13 ppm prior to the N20 being turned on. It is suspected that there was some residual N20 from the previous dental surgery where this anesthetic had been administered approximately 48 Table 6. Summary of personal & real—time N20 sampling (TWA during administration of N20), percent of N20 administered, time of administration, and time of operation. LENGTH OF OPERATIONS OPER # ROOM # MASK PATIENT/NOTES SURGICAL N10 TWA N20 TWA N10 N20 RANGE TIME (MIN) TIME (MIN) TIME (MIN) TIME(MIN) TEAM SURGEON ASSIST. REAL-TIME N10 ADMIN N20 DETECTED ’N20>25 ppm SAMPLING 1 01218 Female, 70 yrs. #1 236 20 110 (20-40) 18.50 28.50 18.75 32.50 P-Brown Mandibular Canine 2 D1218 Male, 24 yrs. #1 277 16 135 (20-40) 31.00 39.25 31.25 39.25 P-Brown Removed 4 Wisdom 3 D1218 Female, 24 yrs. #2 1 5 170 (33-50) 36.75 52.25 49.25 52.25 P-Brown Removed 4 Wisdom 4 D1218 Female, 38 yrs. #3 * * 11 (33) 15.25 55.00 43.75 62.50 P-Brown Removed 4 Wisdom 5 D1218 Male, 26 yrs. #3 * * 118 (33) 21.00 32.25 24.75 44.00 P-Brown Removed 2 Wisdom 6 D1218 Female, 22 yrs. #4 138 77 173 (50) 27.72 33.00 28.5 45.75 P-Brown Removed 4 Wisdom 7 D1218 Female, 75 yrs. #3 <1 <1 17 (33) 17.75 93.00 19 100.25 P-Brown Tooth Implant 8 D1222 Female, 20 yrs. #2 45 64 30 (33-50) 45.20 46.20 40.4 51.50 P-Brown Removed 4 Wisdom 9 D1222 Female, 24 yrs. #2 6 3 37 (50) 27.50 38.50 33.5 38.50 P-Brown Removed 4 Wisdom * = Sampling time < 2 minutes for N20 49 Table 7. Results for general area sampling (ppm N20). OPERATION INIT ROOM ROOM ROOM APPT No. CONC SUPPLY EXHAUST DOOR HALL DESK Oper. #1 <1 <1 4 <1 <1 <1 Oper. #2 <1 <1 3 <1 <1 <1 Oper. #3 <1 <1 <1 <1 <1 <1 Oper. #4 <1 <1 <1 <1 <1 <1 Oper. #5 <1 <1 1 <1 <1 <1 Oper. #6 <1 <1 27 3 <1 <1 Oper. #7 <1 <1 <1 <1 <1 <1 Oper. #8 <1 <1 8 2 <1 <1 Oper. #9 13 <1 22 8 <1 <1 30 minutes earlier. No N20 was detected in the surgical suite supply air, indicating that the N20 was not being entrained into the building’s supply ventilation or that the recirculation of this air diluted concentrations below detectable limits. The N20 concentrations in the surgical suite exhaust air was low (range was <1 to 27 ppm) for all NIOSH—monitored surgeries. The N20 concentrations were low at the door of each suite, ranging from <1 to 8 ppm, indicating that the rooms were not under positive pressure. This was confirmed by using smoke from smoke tubes to observe the direction of airflow. No detectable concentrations of N20 were found in the hallway or at the appointment desk indicating that the N20 was not migrating from the operating suites or being ventilated into the hallway from other sources. Real—Time—— The average N20 concentrations during administration for the nine operations ranged from 11 ppm to 173 ppm (Table 8). The overall mean N20 concentration was 89 (:66 ppm). When the values were averaged over the duration of the surgical operation (i.e., from when the operation started to when it ended), the values were slightly less: 6 to 137 ppm (Table 8). Peak N20 concentrations of over 1000 ppm were detected in two of nine operations. N20 concentrations decreased after the gas was turned off and averaged 2 to 61 ppm from the time the gas was turned off until surgery was completed. Table 8 shows the real—time N20 concentrations by team, time, and type of operation. 50 Table 8. Real—time N20 concentrations by team, time, and type of operation*. Oral Surgical Clinic - Summary of Surgery Time & N20 Averages in ppm for Each Operation 0PER_# Oper.#1 Oper.#2 Oper.#3 Oper.#4 0per.#5 Oper.#6 Oper.#7 Oper.#8 Oper.#9 TEAM # 1 1 2 3 3 4 3 3 2 0PER_TYP MCANE WISD4 WISD4 WISD4 WISD4 WISD4 IMPLA WISD4 WISDZ TIME_0P(min) 32 39 52 62 44 46 100 51 38 N20_ON 19 31 37 15 21 28 18 46 28 PERCENT TIME_ON 57 79 70 25 48 61 18 89 72 N20_OFF 8 7 12 42 18 9 73 3 6 AVGALL(ppm) 67 108.3 137 48 60 113 6 27 35 MAX 361 794.3 1672 321 759 915 125 239 271 S.D. 83 110.0 211 61 106 164 16 35 43 AVGDET(ppm) 75 110.1 137 54 84 155 6 30 39 MAX 361 794.3 1671 321 759 915 125 239 271 S.D. 84 109.2 211 54 117 174 17 35 45 AVGON(ppm) 110 134.7 170 11 118 173 17 30 37 MAX 361 794.3 1672 81 759 915 125 239 271 S.D. 83 108.4 237 20 129 184 28 37 47 AVGOFF(ppm) 12 5.5 61 68 9 33 2 9 13 MAX 134 24.5 348 321 85 114 20 65 83 S.D. 32 6.8 82 65 16 34 4 15 17 ' Key: Oper_#: Operation Number, Team #: Team Number; Oper_Typ: Operation Type; MCANE: Remove Mandibular Canine; WISD4: Remove 4 Wisdom Teeth; IMPLA: Implantation; WISDZ: Remove 2 Wisdom Teeth; TIME_0P: Time of Operation in Minutes; N20_0N: Time N20 Was On; percent TIME_ON: Percent Time N20 is On; N,O_OFF: Time N20 is Off During Operation: AVGALL: Average N10 Concentration During Operation; AVGDET: Average N20 Concentration for the Total Time N20 is Detected; AVGON: Average N10 Concentration from Time the Gas was Turned on to Time it was Turned Off; AVGOFF: Average N20 Concentration After the Gas was Turned Off; MAX: Maximum N20 Concentration During Operation; S.D.: Standard Deviation. 51 Infrared Thermography As in the first survey, infrared thermography was used to observe N20 emissions from mask leakage and patient mouth breathing. Ventilation_ General—— The general air supply system consisted of several units. Blowers manufactured by Barry Blower, Model No. 220, 90 BBC (DWDI), were used to supply air to several rooms, including the two sampled by NIOSH researchers. Of the total specified capacity of 8645 cfm, 690 cfm was dedicated to rooms 1218 and 1222. The air was exhausted from these and other rooms by two exhaust systems manufactured by Barry Blower, Model 7600 AF (DWDI). The specified capacity of each of these two systems was 56,200 cfm; 500 cfm was dedicated to room D1218 and 190 cfm to room D1222. At the time of the NIOSH survey, the ventilation rates were within 24 to 60 percent in room D1218 (operations 1—7), and 13 to 18 percent of the original ventilation specifications in room D1222 (operations 8-9). Both rooms were under negative pressure; the total volume of air exhausted was 60 to 180 cfm greater than the supply volume. The remaining make—up air entered through the open door to each room. The dimensions of Room D1218 were 17.2 by 17.2 by 9.9 feet high, a total volume of 2,300 cubic feet. The ventilation to the room was through one supply duct located in the ceiling and an open door and exhausted through five ceiling registers. The total air exhausted from this room provided an average of ten air changes per hour. The dimensions of Room D1222 were 11.3 by 10 by 8.5 feet high, a total volume of 960 cubic feet. The ventilation to the room was supplied through one ceiling duct and an open door and exhausted through three ceiling registers. The total air exhausted from this room provided an average of 14 air changes per hour. Smoke tube observations indicated that the supply air to each room remained near the ceiling and did not readily mix with the air already present in the rest of the room. Additional observations showed that the supply register louvers directed most of the air toward the ceiling and did not mix well with the air around the oral surgical chair. Scavenging System—— A flowmeter was connected into the scavenging exhaust system and flow rates were adjusted to approximately 45 1pm after the gas was turned on for both oral surgeries monitored. The exhaust rate for the scavenging unit was generally adjusted at the beginning of the operation by a member of the surgical team and not changed throughout the surgery. In one operation, the scavenging system exhaust was not turned on until a few minutes after the gas 52 was turned on (Operation #2). This potential problem could have been eliminated by an interlock between the gas delivery and exhaust system. Work Practices and Changes in N20 Exposure Surgical Teams and Type of Operation—— Four surgical teams and four types of operations were performed. The work practice analysis results based on team, time, and type of operation are shown in Table 8. Statistical analysis showed that of the differences (four teams and four operations) examined, only one was significant (team #3, Operation #5 at 118 ppm versus team #3, Operation #7 at 17 ppm; p<.05). Personal Versus Real-Time Sampling Data—— Generally, average real—time sampling N20 concentrations (89 ppm) were more closely correlated with the oral surgeon’s personal sampling results (101 ppm) than with the surgical assistant’s results (27 ppm). However, the range of exposure for the personal sampling results was greater (<1 to 277 ppm) compared to the real—time results (11 to 173 ppm). There was no significant difference between the real—time and personal sampling results. Oral Surgeon and Surgical Assistant—— Statistical comparisons between the oral surgeon and the surgical assistant for seven paired samples for Operations #1, #2, #3, #6, #7, #8, and #9 showed the mean for the oral surgeon was 100 (i118), and 26 (131) ppm for the surgical assistant. The median was 45 for the oral surgeon and 16 for the surgical assistant. The paired two—tailed T-test did not show significant differences between the two means (p=.142). While the exposure of the oral surgeon tended to be higher in N20 concentration compared to the surgical assistant, there was a large variation between operations for the surgeon (<1 to 277 ppm); the variation for the surgical assistant was much lower, ranging from <1 to 77 ppm. Oral Surgeon and Real—Time—— The oral surgeon and real—time N20 concentrations for the seven paired samples were 89 (i114) and 94 (162) ppm for real—time, respectively. The medians were 26 ppm for the oral surgeon and 98 ppm for the realftime measurements. The paired two-tailed T—test with 7 degrees of freedom did not show significant differences between the two means (p=.870). The oral surgeon showed more variability between operations with a range of <1 to 277 ppm, compared to the real—time range from 11 to 173 ppm. The difference in results may be related to the closer proximity of the oral surgeon’s breathing zone, to the patient’s breathing zone compared to the real—time sampling probe placement. One oral surgeon showed higher N20 values (Operations #1 and #2), compared to the other surgeons. This surgeon’s concentrations were an order of magnitude higher when compared to his surgical assistant (256 i29 versus 18 i3 ppm). As in the first study, the results consistently showed that real—time N20 results more closely reflected those of the oral surgeon than the assistant. 53 Surgical Assistant and Real—Time—— The surgical assistant and real—time N20 concentrations for seven paired samples were 26 (:31) and 96 (£110), respectively. The median for the surgical assistant was 31 ppm and 67 ppm for the real—time measurements. The paired two—tailed T—test showed significant differences (p=.034) between the mean concentrations for the surgical assistant and the real—time sampling results. The surgical assistant showed less variability between operations with a range from <1 to 77 ppm, compared to the real-time range of 11 to 173 ppm. Summary of Work Practices and Changes in N20 Analysis of variance was used to examine team differences overall for N20 concentration when the N20 was on, off, and for both periods combined during surgery. The results suggest that team differences, if any, were too small to be detected; statistical significance was p< 0.35. None of the pairwise differences among teams was large enough for statistical significance, even when each was treated as a planned comparison. Analysis of differences among types of dental surgery was performed by analysis of variance, ignoring possible differences associated with dental teams. Differences associated with type of dental surgery were too small to be detected with these data; significance level was p< 0.5. The mean N20 concentrations, as a function of oral surgery type, ranged from 17 ppm (: 74 ppm), for tooth implantation (operation #7), to 110 ppm (1 74 ppm), for removal of the mandibular canine tooth (operation #1). None of the pairwise differences among surgery types were large enough for statistical significance, even when each was treated as a planned comparison. Analysis was performed to relate the N20 concentration as a function of N20 percentage administered and for changes in the N20 sampling probe distance during oral surgery. This analysis indicated that when N20 percentage administered increased, the waste N20 concentrations tended to increase. The slope of the regression for waste N20 was based on the concentration of N20 administered. Thus, when N20 was administered at 33 percent, the waste N20 concentration was estimated to be 81 ppm. This N20 concentration was based on the slope estimate, the intercept estimate, and a real—time sampling probe distance of 11 inches. Under these conditions, when the concentration of N20 was administered at 40 percent, the waste N20 concentration was estimated to be 95 ppm; at 50 percent administration, the waste concentration was estimated to be 109 ppm. As the probe distance from the patient’s mouth was increased, there was generally a decrease in waste N20 concentration. The slope of the regression of waste NéO concentrations, based on the distance from the patient’s mouth and the probe used to measure waste N20, was estimated to be —7 ppm (:18). Therefore, the concentration of waste N20 was estimated to decrease by approximately 7 ppm for every inch the sampling probe was moved away. 54 Statistical analysis was also performed to determine if there were significant differences in N20 concentrations between the two surgical suites. Analysis was performed when the gas was on, off, and overall. While the observed N20 concentrations were lower in Room D1222 (i.e., 34 i 41 ppm for the gas on), versus Room D1218 (i.e., 105 i 21 ppm for the gas on), the differences were not statistically significant. SURVEY #3: DENTAL CLINIC FOR THE DEVELOPMENTALLY DISABLED N20 concentrations were monitored for 20 operations at this dental clinic. All operations were performed using the Porter-Brown Scavenging System. Six of the operations were monitored for N20 using only the scavenging system; the remaining operations were monitored using three different configurations of auxiliary exhaust ventilation in conjunction with the scavenging system. Air Sampling Personal—— The first seven dental procedures somewhat confounded the effectiveness of the scavenging and auxiliary exhaust ventilation controls due to procedural errors or leaks in the N20 delivery system. During the first procedure, the scavenging system was inadvertently not turned on until the last three minutes of the procedure. This resulted in the highest observed exposure (1,800 ppm for the dental hygienist). This procedure was not included in the data analysis. In addition, all the N20 delivery systems within the clinic were found to have leaks. A leak free N20 delivery system was not obtained and used until after the seventh procedure. While the amount of N20 leaking from each delivery system was not quantified, it was determined by the on—site researchers that the leaks found during procedures 2 through 6 were not significant enough to exclude data from analysis. Table 9 is a summary of the N20 concentration measurements for the 20 operations monitored. Excluding the first operation, N20 concentrations for dental hygienists ranged from 4 to 205 ppm, with an average concentration of 55 i 57 ppm. For the dental assistants, the range was 1 to 163 ppm, with an average concentration of 40 i 55 ppm. When the scavenging system was used without auxiliary ventilation, the dental hygienists’ N20 concentrations ranged from 33 to 205 ppm, with an average 80 i 70 ppm. For the dental assistant the range was 16 to 147 ppm, with an average concentration of 55 i 62 ppm. When the small hood was used (2.5" hood, 160 cfm flow rate, and a capture distance of 6 to 8") the dental hygienists’ N20 concentration ranged from 48 to 146 ppm, (average of 112 i 55 ppm). For the dental assistants, the range was 46 to 121, with an average concentration of 110 i 59 ppm. The second system (3.0" hood, 250 cfm flow rate, and 6 to 10" capture distance), showed that dental hygienist’s N20 concentration ranged from 2 to 48 ppm, with an average concentration of 22 i 19 ppm. The dental assistant’s range was 2 to 65 ppm, with an average concentration of 15 i 24 ppm. 55 Table 9 . Breathing zone, immediate area, and real—time, N20 concentration, based on auxiliary ventilation system used. N20 Concentration (ppm) Obser- Hygienist Assistant Real- N4) flow vation Duration breathing breathing time Vent rate Other No. (min) zone zone Area integrated system (1pm) notesA 1 24 1809 89 440 727 01 3 a,b 2 24 146 46 123 255 1 3 b 3 26 55 ——- 117 129 0 2 b 4 25 142 163 106 125 1 4 b,c 5 29 48 121 47 52 1 2 b 6 30 205 147 357 380 0 3 b 7 24 42 65 102 111 2 5 b 8 25 13 9 9 14 2 3 c 9 16 3 5 4 5 2 3 10 36 44 ——— 8 9 2 3 11 22 61 16 119 146 0 4—3 c,d 12 26 47 24 142 129 0 2 13 26 7 8 7 14 2 4-5 c,d 14 12 33 31 190 148 0 4 15 23 18 —-— 6 ——— 2 3 16 25 48 3 9 ——— 2 4 17 27 2 2 ——— -—— 2 3 18 24 14 1 3 --— 3 3 19 24 118 2 6 ——— 3 3 20 23 4 2 3 ——— 3 3 ——— Not measured. 1. system 0 (i.e., no auxiliary exhaust system) A a, Scavenger was off during procedure; b, leak detected in N20 delivery system; c, patient was not compliant; d, N20 adjusted during procedure. For system 3 (6.0" hood, 630 cfm flow rate, and 12" capture distance), the N20 concentration for the dental hygienists ranged from 4 to 118 ppm, with an 56 average of 45 i 63 ppm. For the dental assistants, the range was 1 to 2 ppm, with an average of 2 i 0.6 ppm. Real—Time—— When the real—time N20 concentrations were mathematically integrated over the duration that N20 was in use, they were not significantly different from the TWA background samples collected in the same location. Excluding the first operation, overall N20 concentrations for the immediate area sampling ranged from 5 to 380 ppm, with an average concentration of 117 i 108 ppm. [When the results were analyzed for the scavenging system with auxiliary ventilation, the N20 concentrations ranged from 129 to 380 ppm, with an average of 186 i 109 ppm. When the auxiliary exhaust systems were used in conjunction with the scavenging systems, the first system reduced N20 concentrations, which ranged from 52 to 255 ppm, with an average of 144 i 103 ppm. The second system reduced N20 concentrations and ranged from 5 to 111 ppm, with an average concentration of 31 i 45 ppm. No real—time data were collected for the third auxiliary exhaust system. The overall area results were higher than the personal sampling results because the sampling probe was located more in—line with the patient’s mouth. The average concentration for the 18 procedures was 75 (i 93) ppm. The average concentration without auxiliary exhaust was 185 (i 101). When auxiliary exhaust ventilation systems were used, the immediate area results were 92 (i 40) ppm for the first system, 21 (i 36) ppm for the second system, and 4 (i 2) ppm for the third system. Ventilation General-- Seven registers supplied a total of 1000 cfm of air to the dental clinic, which had a volume of approximately 12,800 cubic feet. Variations in NiO concentration have been shown, with higher concentrations closer to the N20 delivery system. Even in operating theaters with high general dilution ventilation capacity, the effect of dilution ventilation on occupational exposure was limited.16 Scavenging System (Auxiliary) Exhaust—- Table 10 lists hood diameters, sound pressure levels, airflow rates, hood capture distance range from each patient’s mouth, and calculated capture velocities for each system. The local exhaust capture velocity was calculated from the following equation:1°C 57 Q Equation (2) (10X2+A) where: V = The capture velocity in feet per minute. Q = The airflow rate in cubic feet per minute. X = The distance in feet, from the hood to the point of capture. A = The hood opening area in square feet. Table 10. Local exhaust system measurements. Capture Syste Vent Hood Noise Flow Capture Velocity m No. Diameter Level Rate Distance V (fpm) (in) (dBA) Q (cfm) Range X (in.) 0 No Hood 56 NA NA NA 1 2.5 —— 160 6—8 64—36 2 3.0 67 250 6—10 96—35 3 6.0 81 630 12 62 NA Not applicable —- Not measured Although nitrous oxide flow rate and patient compliance may have an effect on N20 exposure, they had no significant effect on breathing zone or immediate work area N20 concentrations for the data reported in this study. Nitrous oxide was below the detectable limit for all of the general area samples collected just outside the operatory. Performance of Auxiliary System Type—— Because the variances about the mean N20 concentrations were not homogeneous and increased with the mean, the N20 concentration values were log transformed before the comparisons among systems were made. Mean values were calculated using all the data except procedure one. Means and upper and lower 95 percent confidence limits are presented in Figure 19. With system 0, observed exposure results were within the range observed by other researchers who used similar controls.91 System 1 results were not significantly different from system 0 results. The analysis showed with 95 percent confidence that the dental hygienist’s and dental assistant’s N20 exposures and the immediate area N20 concentrations were greater than the NIOSH REL for these systems. 58 Figure 19. Summary of N20 exposure and immediate area concentration data when using three different auxiliary exhaust systems. Nitrous Oxide (ppm) 1000 5 , 1°C); : .L A} ’ 10 g %’ J 1 g A Dental Hygienist E X Dental Assistant : U Immediate Area 0.1 I I I I I I I l I 1 No Vent 2.5-inch Vent 3-inch Vent 6-inch Vent Type of Ventilation The dental assistant’s exposures and the concentration of N20 in the immediate area were less than the NIOSH REL when using systems 2 or 3. The upper 95 percent confidence limit of the exposure (9 ppm) when using system 3 was less than the NIOSH REL. For system 2, the upper 95 percent confidence limit (27 ppm) was just above the NIOSH REL. Compared to the exposures without a local exhaust system, the assistant’s exposures were significantly reduced when using either system 2 (p=0.05) or 3 (p=0.001). System 2 was capable of controlling the exposures of the dental hygienist to concentrations below 60 ppm, based on the upper 95 percent confidence limit. System 3 was capable of controlling the exposures of the dental hygienist to below 135 ppm. For the dental hygienist, the upper 95 percent confidence limits of the exposures when using exhaust systems 2 (70 ppm) and 3 (170 ppm) were greater than the NIOSH REL. Compared to the exposure without a local exhaust system, the hygienist’s exposures were reduced when using systems 2 (p=0.07) or 3 (p=0.l60), but not significantly (a=0.05). It is important to note that one dental procedure (procedure 19, 118 ppm) contributed substantially to the 59 hygienist’s mean exposure when using system 3 and was well above the other values, 4 and 14 ppm. When the analysis was done using only data collected after the N20 leaks in the delivery system were corrected, the following changes were obtained: (1) the dental assistant’s upper 95 percent confidence limit of the exposures was less than the NIOSH REL for both system 2 (9.2 ppm) and 3 (3.3 ppm); and (2) compared to the exposures without local exhaust, the assistant’s exposures were significantly reduced for both system 2 (p=0.003l) and 3 (p=0.002). Work Practices and Changes in N20 Exposure The real—time data allowed qualitative correlation between several work practices and changes in N20 concentration: (1) a marked reduction in N20 was observed when the scavenging system was turned on for the last three minutes of the first procedure; (2) the initial peak concentration of over 400 ppm during the first three minutes of procedure 7 quickly subsided when the exhaust system 2 was moved into its proper position, from 24 inches away to six to ten inches from each patient’s mouth (for all other observations, each exhaust system was properly located prior to N20 delivery); and (3) the N20 concentration rapidly changed when the N20 was turned on or off for most of the procedures. In Figure 20, the real—time N20 concentration for observation 7 is plotted as a function of time. The real—time N20 concentration data were averaged at 30—second intervals and plotted. SURVEY #4: PEDIATRIC DENTAL FACILITY Air Sampling The cooperative operatory was located in the pediatric ward of a teaching hospital. The patients ranged in age from 8 to 17 years. The unmodified masks were used to administer N20. N20 concentration measurements made in the operatory were carried out in October and November of 1992, and were intended to demonstrate the effectiveness and acceptability of two new controls designed primarily to capture emissions from the patient’s mouth. The first, an auxiliary exhaust (Figure 21), rested on the patient’s chest. It was connected to a blower with flexible and rigid sections of vacuum cleaner hose and pipe. The blower was exhausted outside the building in which the operatory was located. The exhaust flow was 100 cfm. The air velocity in the operatory measured at the dentist’s breathing zone with an air velocity meter ranged from 25 to 40 fpm. It is apparent from Figure 22, which shows the concentration of N20 in the dentist’s breathing zone versus time for one operation, that the auxiliary exhaust was not effective in controlling the dentist’s exposure while the high—speed drill was on. In the time interval from approximately 1400 to 3000 seconds, with the drill off, it appears that the auxiliary exhaust did have a significant effect on the dentist’s exposure. The auxiliary exhaust normally was placed on the patient’s chest so that the inlet to the tubing was 3 to 6 inches from the patient’s chin. In this location, it occasionally interfered with the dentist’s technique and was, therefore, lowered for sufficient access to the patient, but also was judged by several dentists not to be a significant interference problem. 60 Figure 20. Real—time N20 concentration changes from auxiliary exhaust ventilation for Survey #3. 500 ‘— Posltlon exhaust system .h C O I OH, nltrous oxlds M O O Nltrous Oxlde Cone. (ppm) '2‘ a q 2 0- 1 l 1 1 1 | ' -5 0 5 10 15 20 25 30 35 40 45 Time (minute) Figure 21. Diagram of laboratory designed auxiliary exhaust used in Survey #4. PVC Flex Hose (15' ID, 2.0' EID) g . O Latex Hose 0.79113, 1.00431] (NoteIIMet tube ends cut 45 Deg to verflculoxBJexhuust tube ends gued mto PVC Hex Hose) onmer Exhoust 61 MHWCHm NM. zwo nonomsnmmnwos < OH onHm Hmbo Hubomm mH HH mmbm MM<2 .AMmHmzoaomMOMHommM QMMHmDmmHQ mmDv Mahmomxm nz< HHm MwoMm2H ~2mm and Qumoxm mmMDwomxm A mHHz mommov HHoz MH< MH< OH umoqo mm< mazm> Hwb¢mwmo OH mmmDH MMOZw umDv mHzm> wqmmbm MH< OH Hmoqo mm Hoz QADOMm HMH mm Hmb AHH>HHO< meow: Mom Uo Hoz mH UMNmHm “H0>Mmmmm M>MNmmo .QNDH>OMm ozHM =mHAm: mHHz Mm imamwm quozm> mmmomm mszmuHmo .o~z zo UZHZMDH mMommm hibm EDDO<> zo ZMDH nz< mmom OH Mm¢2 Homzzoo a* mmHm .ZMA n: H< oma<flmmo mH WHmq HmHoz H¢mH mmHOMm mmNHw wDon<> zH mzoo DADOMm Mm<2 .Mmdz 92¢ zuamwm wzHuzm>q<> .wHHMmA<> M2HHommmn HU H* mmam AOMHZOU WMDQMUOMM .nuauoumnuno Hmuflov EH gum mmon wsu ou fimz wcaaaouudoo Ham fiumonmmm Amanlhnlnuum .HH uHamH All suction pumps aspirating NZO—contaminated air from the patient’s mask or mouth should be vented outside the building and away from fresh air inlets. The scavenging system should always be on when N20 is used: its use should be continued during recovery from analgesia in order to capture N20 retained by the patient. An automatic interlock system is recommended to assure that the N20 cannot be turned on unless the scavenging system is activated. A flowmeter should be connected to the scavenging system vacuum line and positioned so that it is visible at all times to dental personnel. A bypass flowmeter may have an advantage over an in—line flowmeter because the former avoids moisture problems from the dental operations in the vacuum line and interference with the flowmeter ball and airflow scale. Scavenging system manufacturers should supply a flowmeter kit with their scavenging system so that such systems can be monitored for recommended flow rates to make the system as effective as possible. Scavenging masks should be available in a variety of sizes to fit easily over the patient’s nose. The mask should be pliable, so that when the mask is secured around the patient’s nose leaks are minimized. To secure the mask gently to the patient, the mask should be fastened to the patient’s nose prior to surgery by using a slip clamp, or comparable device, connected to the analgesia hoses which gather near the back of the patient’s head and dental chair headrest. Most commercially available scavenging masks are equipped with slip clamps. A visual observation should confirm a reasonable fit. VENTILATION General Ventilation 1. Supply register louvers located in the ceiling should be designed so as to direct the fresh supply air toward the floor and toward the dental chair to provide mixing, dilution, and removal of the contaminated air from the operating room. Exhaust register louvers should not be located near the supply air vents because this will short circuit the airflow, thereby preventing proper mixing and flushing of the contaminants from the room. If the N20 concentration is above 25 ppm for dental personnel, then airflow should be increased in the room to allow for more air mixing and further dilution of the anesthetic gas. The recirculation of room air is not recommended; it should be exhausted outdoors and away from windows, doors, and air intake vents. Sweep fans have been shown to be effective in mixing room air and dilution of anesthetic gas.15 However, sweep fans should be installed with caution since placement of the fan may increase exposure by generating eddies in the breathing zone of the dentist or entrainment of 82 the anesthetic from the patient’s mask and mouth to the dentist’s breathing zone. If sweep fans are used, the location of the fan should be so that the room air is blown past the dentist toward the patient. An air velocity of approximately 50 to 75 cfm at a distance of 3 to 4 feet from the patient’s head is recommended.15 4. Users of N20 should consult the Department of Health and Human Services’ publication entitled "Guidelines for the Construction and Equipment of Hospital and Medical Facilities" (Publication No. [HRS—M—HF] 84—1, 1984) for more detailed information regarding ventilation guidelines.102 Local Exhaust Ventilation Local exhaust ventilation has been shown to be effective in reducing N20. However, there are practical limitations in using it in the dental operatory which must be considered. These include proximity to the patient, interference with dental practices, noise, and installation and maintenance costs. It is most important that the dentist does not work between the patient and the local exhaust duct, since this will cause the contaminated air to be drawn through the dentist’s breathing zone. ADMINISTRATIVE CONTROLS AND N20 MONITORING 1. Annual reviews of N20 use should be conducted, as well as reviews of the waste gas reduction methods employed at the facility. The annual review should include environmental air monitoring, leak testing of equipment, and personal and environmental monitoring. Air monitoring may be performed either by gas bag sampling, real—time sampling, and/or by diffusive sampling (passive monitors).103 2. When real—time sampling is conducted, the sampling train should be attached to the lapel of dental personnel to obtain personal exposure data. The sampling port of the sampling train should be connected on the dominant work side of dental personnel (i.e., right side if the dentist is right—handed; left side if left—handed). 3. If diffusive samplers (also known as passive dosimeters) are used, it is important to use the method of sampling exposure recommended by NIOSH: Uncap the dosimeter when N20 is turned on, and recap it when the N20 is turned off. Keep a time log for the administration of N20. Indicate to the company performing the analysis that the results are to be reported for the time the dosimeter is open and not for an 8—hour day. Present instructions for dosimeters indicate that exposure times will be for 8— hours unless otherwise indicated. 4. Dentists should request information from dental equipment suppliers on the proper use of the equipment and its effectiveness in reducing N20 prior to purchase. Suppliers need to be informed when and where their products leak. 83 5. Improving instructions and providing educational updates by the manufacturers to sellers and users may help reduce overall N20 exposure by more efficient and prudent use of such systems. EQUIPMENT MAINTENANCE This research suggests that leaks are a potential cause of excessive N20 exposure in dentistry. The analgesia equipment contains rubber and plastic components which may be degraded by the N20 and 02, as well as by repeated sterilization for infection control. 1. All rubber hoses, connections, tubing, and breathing bags should be frequently checked to assure that this equipment is in good working order. Rigorous leak testing can be performed according to the manufacturer’s recommendations or by the procedures outlined in the "Methods" section of this report. If new anesthesia gas delivery and scavenging systems leak because of design and/or quality control problems, manufacturers should be contacted, along with appropriate representatives of the American Dental Association and the Food and Drug Administration, to assure that such problems are fixed immediately. For gas cylinders, Teflon® tape should be used on all pipe—threaded connections through which N20 flows. Tape should not be used on compression fittings. High— to low—pressure connections should also be checked regularly. O—rings may become worn and should be replaced periodically. The nitrous oxide/oxygen gas mixing system should be evaluated for leaks when first installed and periodically thereafter. This should be performed daily with pressure gauge readings coupled with periodic use of an infrared gas analyzer. WORK PRACTICES 1. The dental personnel should inspect the anesthesia machines and all connections before starting anesthetic gas administration. Breathing bags should be attached to the anesthesia machine, and hoses and clamps should be in place before turning on the anesthetic gas. Avoid over— or underinflating breathing (reservoir) bag while patient is breathing. The scavenging mask should be connected properly to the gas delivery hose and the vacuum system. Indexing connection ports with different diameter hoses to reduce the possibility of incorrect connection of the gas delivery and scavenging hoses is recommended. N20 should not be turned on until the following is in place: (a) the vacuum system scavenging unit is operating at the recommended flow rate of 45 1pm; and (b) the scavenging nasal cone is secured over the patient’s nose prior to surgery. 84 To reduce leaks around the nasal cone during gas delivery, the slip clamp that is attached to the scavenging nasal inhaler hoses should always be used to seat the mask securely on the patient’s nose. Oxygen should be administered to the patient through the analgesia equipment for at least 5 minutes following dental surgery, before the gas delivery system is disconnected from the patient and before the scavenging system vacuum is turned off. Patients should be encouraged to minimize talking and mouth breathing during dental surgery. However, some talking may be necessary in assessing the level of analgesia. Dental personnel should avoid getting in the direct breathing path of the patient when mouth breathing is apparent. REFERENCES USDOL [1990]. Bureau of Labor Statistics, Outlook, 2000, Bulletin 2352, April. ~ Wallace WR [1989]. Keynote address, selected proceedings of the workshop on anesthesia education. J Dent Educ 53:5—6. Knill—Jones RP, Rodrigues LV, Moir DD, Spence AA [1975]. Anesthetic practice and pregnancy: controlled survey of women anesthetists in the United Kingdom. Lancet 227939—8079. Jastak JT, Greenfield W [1977]. Trace contamination of anesthetic gases —— a brief review. JADA 95:758-762. American Society of Anesthesiologists [1974]. Occupational disease among operating room personnel: a national study. Report of an ad hoc committee on the effect of trace anesthetics on the health of operating room personnel. Anesthesiology 41:321. Cohen EN, Brown BW, Bruce DL, Casocorbi HF, Corbett TH, Jones TW, Witcher C [1975]. A survey of anesthetic health hazards among dentists: report of an american society of anesthesiologists ad hoc committee on the effects of trace anesthetics on the health of operating room personnel. JADA 9021291. Bruce DL, Bach MJ [1975]. Laboratory report: psychological studies of human performance as affected by traces of enflurane and nitrous oxide. Anesthesiology 42:194. NIOSH [1977]. Criteria for a recommended standard: occupational exposure to waste anesthetic gases and vapors. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 77—140. 85 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. Koblin DD [1982]. Inactivation of methionine synthetase by nitrous oxide in mice. Anesthesiology 54:318. Sharer NM [1983]. Effects of chronic exposure to nitrous oxide on methionine synthetase activity. Brit J Anesth 55:693. Mazze RI, Fujinaga M, Baden JM [1988]. Halothane prevents nitrous oxide teratogenicity in sprague—dawley rats: folinic acid does not. Teratology 38:121-127. Fujinaga M, Baden JM, Shepard TH, Mazze RI [1990]. Nitrous oxide alters body laterality in rats. Teratology 41:131—135. Kugel G, Letelier C, Atallallah H, Zive M [1989]. Chronic low levels of nitrous oxide exposure and infertility [Abstract #1057]. J Dent Res 68:313. Rowland AS, Baird DA, Weinberg CR, Shore DL, Shy CM, Pierce J, Wilcox AJ [1992]. Reduced fertility among women employed as dental assistants exposed to high levels of nitrous oxide. New England J Med 327(14):993— 997. NIOSH [1977]. Control of occupational exposure to N20 in the dental operatory. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 77—171. Hollonsten AL [1982]. Nitrous oxide scavenging in dental surgery. I. A comparison of the efficiency of different scavenging devices. Sweden Dent J 62203—213. Carlsson P, Ljungqvist B, Hallen B [1983]. The effect of local scavenging on occupational exposure to nitrous oxide. Acta Anaesthesiol Scand 27:470—475. Henry RJ, Jerrell RG [1990]. Ambient nitrous oxide levels during pediatric sedation. Pediatr Dent 12(2):87—91. Middendorf PJ, Jacobs DE, Smith KA, Mastro DM [1986]. Occupational exposure to nitrous oxide in dental operatories. Anesth Prog 33:91—97. Donaldson D, Grabi J [1989]. The efficiency of nitrous oxide scavenging devices in dental offices. J Canadian Dent Assoc 55(7):541—543. McGlothlin JD, Jensen PA, Todd WF, Fischbach TJ [1988]. Study protocol: control of anesthetic gases in dental operatories at Children’s Hospital Medical Center, Dental Facility, Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. PB—90—155946, ECTB Report No. 166—03. 86 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. Wynne JM [1985]. Physics, chemistry, and manufacture of nitrous oxide — — nitrous oxide, N20. Edmond I, Eger II, Elsevier, eds. Goodman LS, Gilman A [1965]. The pharmacological basis of therapeutics. New York, NY: MacMillian, p. 1785. Adriani J [1962]. The chemistry and physics of anesthesia. 2nd ed. Springfield, IL: Thomas, p. 849. Dripps RD, Exkenhoff JE, Van Dorn LD [1961]. Introduction to anesthesia. Philadelphia, PA: Saunders, p. 413. Fukunaga AF, Epstein RM [1973]. Sympathetic excitation during nitrous oxide—halothane anesthesia in the cat. Anesthesiology 39:23—26. Everett GB, Allen GD [1971]. Simultaneous evaluation of cardiorespiratory and analgesic effects of nitrous oxide —— oxygen inhalation analgesia. ADAJ 83:129—33. Trieger N [1971]. Nitrous oxide —— a study of physiological and psychomotor effects. ADAJ 83:142—50. Vean A, King K [1979]. Measuring N20 levels in the dental operatory. J Dent Child 46(6):454-59. Amess JA, Burman JF, Rees GM, NanCekieville DG, Mollin DL [1978]. Megaloblastic hemopoieses in patients receiving nitrous oxide. Lancet II, 339—342. Sweeney B, Bingham RM, Amos RJ, Petty AC, Cole PV [1985]. Toxicity of bone marrow in dentists exposed to nitrous oxide. Br Med J 291:567—569. Fujinaga M, Baden MJ, Yhap E0, Mazze RI [1987]. Reproductive and teratogenic effects of nitrous oxide, isoflurane and their combination in sprague-dawley rats. Anesthesiology 67:960—964. Fujinaga M, Baden JM, Suto A, Myatt JJ, Mazze RI [1991]. Preventive effects of phenoxybenzamine on nitrous oxide—induced reproductive toxicity in sprague—dawley rats. Teratology 43:151—157. Vaisman AI [1967]. Work in surgical theaters and its influence on the health of an anesthesiologist. Eksp Khir Anesteziol 3:44—49. Ericson A, Kallen B [1979]. Survey of infants born in 1973 or 1975 to Swedish women working in operating rooms during their pregnancy. Anesth Analg 58:302. Purdham JT [1986]. Anesthetic gases and vapours (P86—21E). Canadian Centre for Occupational Health and Safety. Hamilton, Ontario: 22 pages/33 references. 87 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. Kestenberg SH, Young ER [1988]. Potential problems associated with occupational exposure to nitrous oxide. J Canadian Dent Assoc 54(4):277—286. Knill-Jones PC, Owen HW, Linde HW [1981]. Morphologic changes in mouse spermatozoa after exposure to inhalation anesthetics during early spermatogenesis. Anesthesiology 54:53. Kripke BJ, Kelman AD, Shah NK, Balogh K, Handler AH [1976]. Testicular reaction to prolonged exposure to nitrous oxide. Anesthesiology 44:104. Viera E, Kleaton—Jones P, Austin JC, Moyes DG, Shaw R [1980]. Effects of low concentrations of nitrous oxide on rat fetuses. Anesth Analg 59:175—177. Kugel G, Letelier C, Zive MA, King JC [1990]. Nitrous oxide and infertility. Anesth Prog 37:176—180. Lane GA, Nahrwold ML, Tait AR, Taylor—Busch M, Cohen PJ, Beaudoin AR [1980]. Nitrous oxide is fetotoxic, xenon is not. Science 210:899-901. Yagiela JA [1991]. Health hazards and nitrous oxide: a time for reappraisal. Anesth Prog 38:1—11. Corbett TH, Cornell RG, Liedling K, Endres JL [1973]. Incidence of cancer among Michigan nurse anesthetists. Anesthesiology 38:275—278. Ferstandig LL [1978]. Trace concentrations of anesthetic gases: a critical review of their disease potential. Anesth Analg 57:328—345. Baden JM, Simmon VF [1980]. Mutagenic effects of inhalation anesthetics. Mut Res 75:169—189. Cohen EN, Brown BW, Wu ML, Whiteher CE, Brodsky JB, Gift H, Greenfeld W, Jones TW, Driscoll EJ [1980]. Occupational disease in dentistry and chronic exposure to trace anesthetic gases. JADA 101:21—31. Spence AA, Knill—Jones RP [1978]. Is there a health hazard in anesthetic practice? Br J Anesth 50:713—719. Bruce DL, Bach MJ, Arbit J [1974]. Trace anesthetic effects on perceptual cognitive and motor skills. Anesthesiology 40:453. Smith G, Shirley AW [1978]. A review of the effects of trace concentrations of anesthetics on performance. Br J Anesth 50:701—711. Gambill AF, McCallum R, Henrichs T [1979]. Psychomotor performance following exposure to trace concentrations of anesthetics. Anesth Anal 58:475—482. 88 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. Smith G, Shirley AW [1977]. Failure to demonstrate effect of trace concentrations of nitrous oxide and halothane on psychomotor performance. Br J Anesth 49:65—70. Venables H, Cherry N, Waldron H, Buck L, Edling C, Wilson H [1983]. Effects of trace levels of nitrous oxide on psychomotor performance. Scand J Work Environ Health 9:391—396. Jones TW, Greenfield W [1977]. Position paper of the ADA ad hoc committee on trace anesthetics as a potential health hazard in dentistry. JADA 95:751—756. ACGIH [1989]. 1989—1990 Documentation of the threshold limit values and biological exposure indices. Cincinnati, OH: American Conference of Governmental Industrial Hygienists. (10024—97—2), pp. 32. ACGIH [1986]. Draft report from the ACGIH Committee on nitrous oxide. Cincinnati, OH: American Conference of Governmental Industrial Hygienists. McGlothlin JD, Jensen PA, Todd WF, Fischbach TJ, Fairfield CL [1989]. In—depth survey report: control of anesthetic gases in dental operatories at Children’s Hospital Medical Center Dental Facility. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Report No. ECTB 166—llb. Scheidt MJ, Stanford HG, Ayer WA [1977]. Measurement of waste gas contamination during nitrous oxide sedation in a non—ventilated dental operatory. Anesth Prog 24:38—42. Kronoveter KJ [1979]. Hazard evaluation and technical assistance report: Louisiana State University, School of Dentistry, New Orleans, Louisiana. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 78—000—009, NTIS No. PB—82—216—292. Johnson PL [1979]. Hazard evaluation and technical assistance report: Children’s Hospital Dental Department, Cincinnati, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health‘ Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 79—5-564, NTIS No. PB—81—143059. Pryor P [1979]. Hazard evaluation and technical assistance report: Randall Egbert, Jr., DDS and Michael Getz, DDS, Milford, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 79-43, NTIS No. PB—81—168—528. 89 62. 63. 64. 65. 66. 67. 68. 69. Patnode R [1980]. Hazard evaluation and technical assistance report: Hillsboro, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 79—59, NTIS No. PB—81—112—120. Love JR [1980]. Hazard evaluation and technical assistance report: U.S. Public Health Service, Dental Clinic, Cincinnati, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 80—16, NTIS No. PB-80—194—l94. Gunter BJ [1981]. Hazard evaluation and technical assistance report: Conifer Dental Group, Conifer, Colorado. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 81—200—999, NTIS No. PB—82—187048. Salisbury SA [1981]. Hazard evaluation and technical assistance report: Emory DF University, School of Dentistry, Atlanta, Georgia. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 81—342—1005, NTIS No. PB—8l—342105. Gorman RW [1985]. Hazard evaluation and technical assistance report: Drs. Scheer and Gardner, Cincinnati, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 84—126—1555, NTIS No. PB—86—105590. Crandall MS [1985]. Hazard evaluation and technical assistance report: Dental Health Associations, Paoli, Pennsylvania. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 84—204—1600, NTIS No. PB-86—145356. Crandall MS [1985]. Hazard evaluation and technical assistance report: Dr. Youdelman and Teig, Brentwood, New York. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 84—412—1612, NTIS No. PB—86—145372. Gunter BJ [1986]. Hazard evaluation and technical assistance report: Dr. Hiatt, Metcalfe, and Schaad, Denver, Colorado. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 85-408-1666, NTIS No. PB-86—221637. 9O 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. Gunter BJ [1986]. Hazard evaluation and technical assistance report: Stag Dental Clinic, Boulder, Colorado. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 86—157—1678, NTIS No. PB—86—223021. Davis MJ [1988]. Conscious sedation practices in pediatric dentistry: a survey of members of the American board of pediatric dentistry College of Diplomates. Pediat Dent 10(4):328—329. NIOSH [1987b]. NIOSH respirator decision logic. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 87—108. Dorsch JA, Dorsch SE [1984]. Understanding anesthesia equipment: construction, care and complications. 2nd ed. Baltimore, MD: Williams & Williams. Whitcher CE, Cohen EN, Treudell JR [1971]. Chronic exposure to anesthetic gases in the operating room. Anesthesiology 35:348—353. Krapez JR, Saloojee Y, Hinds CJ, Hackett H, Cole PV [1980]. Blood concentrations of nitrous oxide in theater personnel. Br J Anesth 52:1143—1148. Nikki P, Pfaffi P, Ahlman K, Ralli R [1973]. Chronic exposure to anaesthetic gases in the operating theater and recovery room. Surv Anesth 17:464—465. Davis PD, Parbrook GD [1979]. The Brown nasal mask, a new scavenging mask for dental anesthesia. Br Dent J 146:246. Allen GD, Goebel W, Scaramella J, Randall F, Smith RT [1978]. Apparatus to reduce trace nitrous oxide contamination in the dental operatory. Anesth Prog 26(6):181—185 Shamaskin RG, Campbell RL [1983]. Nitrous oxide scavenging in the brain circuit for conscious sedation. Anesth Prog 30:147. ' Kay I [1978]. Detection and control of waste gases in the dental operatory. Analgesia 1(1):13—20. Taft RM [1979]. Hazard evaluation and technical assistance report: Louisiana State University, School of Dentistry, New Orleans, Louisiana. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 78—62, NTIS No. PB—82—182-627. 91 82. 83. 84. 85. 86. 87. 88. 89. 90. Burroughs GE [1978]. Hazard evaluation and technical assistance report: Alvin Jacobs, DDS, Fort Lee, New Jersey. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 78—129—544, NTIS No. PB-81—143026. Pryor P [1979]. Hazard evaluation and technical assistance report: L. Kotlow, DDS., Albany, New York. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 79—107—632, NTIS No. PB—80—194038. Schick W [1980]. Hazard evaluation and technical assistance report: Randall Egbert, Jr., DDS, Milford, Ohio. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 80—102~764, NTIS No. PB—82—151010. Gunter BJ [1981]. Hazard evaluation and technical assistance report: U.S. Public Health Service, IHS Dental Clinic, St. Ignatius, Montana. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 80—113—813, NTIS No. PB—82—189796. Gunter BJ [1981]. Hazard evaluation and technical assistance report: Robert W. Olson, DDS, Conifer, Colorado. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 80—249—833, NTIS No. PB—82—l72107. Behrens V [1984]. Hazard evaluation and technical assistance report: Stephen Gold, DDS, Port Jefferson Station, New York. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 84—111-1471, NTIS No. PB—85—184257. Gunter BJ [1982]. Hazard evaluation and technical assistance report: West Gate Dental Clinic, Cheyenne, Wyoming. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 82-070—1148, NTIS No. PB-83—198408. Gunter BJ [1986]. Hazard evaluation and technical assistance report: Dr. Levin, DDS, Denver, Colorado. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 86—179—1699, NTIS No. PB-87-105003. Gunter BJ [1987]. Hazard evaluation and technical assistance report: Litteken Dental Clinic, Ardmore, Oklahoma. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers 92 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. for Disease Control, National Institute for Occupational Safety and Health, NIOSH Report No. HHE 87—281—1854, NTIS No. PB—88—203849. Jacobs DE, Middendorf PJ [1986]. Control of nitrous oxide exposures in dental operatories using local exhaust ventilation: a pilot study. Anesth Prog 33(5):235—242. Wisconsin Regulatory Digest [1988]. A Publication of the Dentistry Examining Board, Vol. 1, No. 2. Gruetzmacher GR, Lifson LT, Moen TE [1989]. Analysis of nitrous oxide and mercury exposure levels at over 300 dental offices in Wisconsin. Presentation at the American Industrial Hygiene Conference, Cervantes Convention Center, St. Louis, MO. Berner O [1972]. A volume and pressure controlling spill valve equipped for the removal of excess anesthetic gases. Acta Anesth Scand 16:252—258. ASHRAE [1981]. Handbook of fundamentals. New York, NY: American Society of Heating, Refrigerating, and Air—conditioning Engineers. NIOSH [1985]. Nitrous oxide: Method 6600, 6600—1, 6600—3 (supplement issued 2/15/84). In: Eller PM, ed. NIOSH manual of analytical methods. 3rd rev. ed. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 84—100. Altman, PL, Dittmer DS. eds. [1971] Respiration and Circulation. Bethesda, Maryland: Federation of American Societies for Experimental Biology, pp. 42. Armitage P [1974]. Statistical methods in medical research. Third printing. New York, NY: John Wiley and Sons. Kirk HJ, Sall J [1981]. SAS Views: Regression and ANOVA, 1981 Ed. Cary, NC: SAS Institute Inc. Dallavalle JM [1952]. Exhaust hoods. New York, NY: Industrial Press, Inc. Nilsson K, Stenqvist O, Lindberg B, Kjeletofy B [1980]. Close scavenging experimental and preliminary clinical studies of a method of reducing anesthetic gas contamination. Acta Anesth Scand 24:475-481. Bureau of Health Maintenance Organizations and Resources Development [1984]. Guidelines for the construction of equipment of hospital and medical facilities. DHHS Publication No. HRS/M/HF 84—1, NTIS Assession # HRP—8905974/2/XAB. 93 103. Bishop EC, Hossain MA [1984]. Field comparison between two nitrous oxide (N20) passive monitors and conventional sampling methods. AIHAJ 45 (12):812—816. 94 APPENDIX A ANESTHESIA EQUIPMENT AND EMISSION SOURCES The purpose of this section is to familiarize the reader with N20 use in dentistry and to briefly describe the sources of N20 exposure, from gas filled cylinders to the dentist’s end use of this gas. Exposure to N20 in dental operatories may occur from a variety of sources. These emission sources include the following: compressed gas cylinders (stationary and portable); the gas piping system for stationary gas cylinders; the anesthesia machine, the breathing system, reservoir bag, breathing tubes, and the N20 gas delivery mask. Compressed Gas Containers Gases used in dental operatories may be supplied from a bank of gas cylinders located in a central area and connected to a building piping system, or from smaller, portable gas cylinders located in or near the dental operatory.1 The capacity of a gas cylinder is designated alphabetically, ranging from size A (smallest) to size H (largest). The cylinder sizes commonly employed in a dental practice are the size E for portable units, and the size H (oxygen) and size G (nitrous oxide) for central systems.2 The size E cylinder of oxygen contains about 66 liters, while the size E nitrous oxide cylinder contains 1,590 liters. The H oxygen cylinder contains over 5,300 liters, while the G nitrous oxide cylinders contain 13,000 liters. Nitrous oxide cylinders are color coded light blue; the oxygen cylinder is color coded green. The American Dental Association (ADA) has provided recommendations for safe handling and use of these cylinders. These recommendations include the following instructions: (1) Open cylinder valves very slowly in a counterclockwise direction. Close all cylinder valves tightly when not in use; (2) Use no grease, oil, or lubricant of any kind or type to lubricate cylinder valves, gauges, regulators, or other fittings that may contact gases; (3) Store full cylinders in the vertical position; (4) Store cylinders in an area where the temperature does not fluctuate, particularly avoiding heat; and (5) Handle cylinders with care. Especially avoid dropping.3 Gas Piping Systems Large dental clinics typically use a piping system to deliver nonflammable gases, such as nitrous oxide and oxygen. The piping system has the following: (1) a central supply system with control equipment; (2) a piping network that delivers the gases to locations where they may be required; and (3) station outlets at each point of use. Central supply has facilities for storage of gases, controls to deliver the gases to the piping system at the desired pressure, and alarms and safety devices. A common type of gaseous oxygen and nitrous oxide supply system includes two banks of cylinders. When one bank of cylinders is depleted, a pressure sensor switches on the second bank of cylinders. The depleted bank is then replaced with full cylinders to continue the cycle. The cylinders are connected to a manifold (header) that converts them into one continuous supply. Check valves are placed in the lead between each cylinder and 95 manifold header. Switch—over from the empty to full cylinder is done by a pressure—sensitive switch known as a manifold changeover device.4 Each bank of cylinders has a pressure regulator that reduces the pressure and maintains the pressure on the downstream side within prescribed limits despite the pressure upstream. If nitrous oxide cylinders are located in a cold place, the regulator may freeze. High use of gas also can cause a regulator to freeze.5 Shutoff and check valves are important safety features of a central supply system. A manually operated shutoff valve is recommended upstream of each pressure regulator and a shutoff valve or check valve downstream.73 Figure 25 is a schematic of a central oxygen and nitrous oxide supply system with reserve supply. Boxes 1 and 2 are the manifold change over devices. Pipeline distribution systems consist of a main line, risers, and branch (lateral) lines. The main line connects the central source to either risers (vertical pipes), branches (lateral pipes), or both. The main supply line must be equipped with a shutoff (stop) valve located near the entry of the gas source into the building or room. The purpose of the shutoff is for an emergency, routine maintenance, or modifications of the piping system. The piping system terminates at the station outlet where the user connects and disconnects to equipment either directly or by a flexible hose. At the ‘ station outlet, there is a faceplate that is permanently labeled with the name and/or symbol of the gas it conveys. The identifying color also may be present. Each station outlet must contain a valve that opens to allow gas flow when the male probe is inserted and closes automatically when the connection is broken. The station outlet must incorporate a shutoff valve to permit repair or maintenance of other components without effect. Seals or 0— rings are used between the secondary and primary valve assemblies to provide a gas—tight fit. Degradation of these seals or O—rings due to age results in leaky outlet stations, requiring seal or O—ring replacement. Each station outlet must incorporate the fixed female component of a noninterchangeable connection, either a Diameter Index Safety System Connection or a quick coupler. The Diameter Index Safety System (DISS) was developed to provide threaded noninterchangeable connections for medical gas lines at pressures of 200 pounds per square inch gauge (psig) or less.6 DISS connections consist of a body, nipple, and nut combination. The safety system is based upon two concentric and specific bores in the body and two cOncentric and specific shoulders on the nipple. To have noninterchangability between different connections, the two diameters on each part vary in opposite directions, so that as one diameter increases, the other decreases. Quick couplers have become popular because gases are frequently needed without delay. The quick coupler should allow the desired apparatus to be connected from the station outlet by a one—step motion using one hand. Each quick coupler has male and female components that are noninterchangeable between gases. The male member is called a plug, striker, probe, or jack. The female component is called a socket. Insertion into an incorrect outlet is prevented by indexing —— the use of different shapes for mating portions, different spacing for mating portions, or a combination of these.6 96 Figure 25. reserve supply. Schematic of a central oxygen and nitrous oxide supply system with Bank #1 Cylinder Lead Manifold (Header) KEY # Manually controlled shutoff valve <:> Cylinder -*— Check Va Ive +£> Relief Valve _]1_ Pressure Regulator Cylinder Lead Bank #1 Manifold (Header) N O 2 Bank #2 h:> ‘1‘— A A— 4" WV 1 ‘F _____;»__—— To ____ ___ Pipeline System —12- 7 2 H> h Bank #2 O2 Hoses used to connect anesthesia machines and other apparatus to the fixed station outlet should have permanently attached, noninterchangeable connectors. The outlet connector (that connects the hose to the anesthesia machine) should be the D155, while the connectors at the end of the station outlet should be quick couplers.6 This configuration will encourage making connections and disconnections at the station outlet rather than at the back of the machine. The Analgesia Machine A 1979 American National Standards Institute (ANSI) standard for anesthesia machines establishes the basic performance and safety requirements for components of analgesia machines.7 All American anesthesia machine manufacturers have agreed that machines sold after 1984 will comply with the standard. Figure 26 is a schematic of the analgesia machine with the arrangement of the components grouped according to high, medium, and low pressure 02 and N20 gas delivery. The high—pressure system consists of machine parts that receive gas at cylinder pressure. These include the following: (1) the hanger yoke that connects a cylinder to the machine; (2) the yoke block, used to connect cylinders larger than size E or pipeline hoses to the machine through the yoke; and (3) the cylinder pressure gauge, showing the gas pressure that converts a high, variable gas pressure into a lower, more constant pressure suitable for use in the machine.73 The hanger yoke assembly orients and supports the cylinder, provides a gas— tight seal, and ensures a unidirectional flow of gases into the machine. This assembly has the following parts: (1) the body, which is the principal framework and supporting structure; (2) the retaining screw, that is used to tighten the cylinder into the yoke and helps establish a seal; (3) the nipple, through which gas enters the machine; (4) the index pins that prevent attachment of an incorrect cylinder to the yoke; (5) the washer, which also helps to form a seal between the cylinder and the yoke; (6) a filter to remove dirt from the cylinder contents; and (7) the check valve assembly, which ensures a unidirectional flow of gases through the yoke. A cylinder pressure gauge is required for each gas supplied in cylinders. The gauges are usually of the Bourdon tube type.8 The machine standard requires that the full scale pressure indication be at least 33 percent greater than the maximum cylinder pressure. All cylinder gauges on a machine must have an equal span angle (between 180 and 280 degrees) from the lowest to the highest pressure indication, with the lowest indication between 6 and 9 on a clock face. Oxygen pressure failure devices are required on all ANSI—approved machines. Delivery of a hypoxic gas mixture due to a gradual or abrupt failure of the oxygen supply during an anesthetic procedure can be a serious problem. The ANSI machine standard requires that an anesthesia machine be equipped with an oxygen pressure device, such that a reduction in oxygen flow due to a drop in the oxygen supply pressure of 50 percent below normal will result in the cessation of flow of all other gases, including compressed air, or will 98 Figure 26. Schematic of anesthesia machine with the arrangement of the components grouped according to high, medium, and low pressure 02 and N20 gas delivery. HIGH PRESSURE SYSTEM iNTERMEDIATE PRESSURE SYSTEM LOW E”QESSUFIE SYSYEM ”2P BACK ‘ ‘ w W W W i ‘ > I i . Oz .__.__._. ___...Wm-V. FLUSH [ I E _V i - PRESSURE “ "' ‘ SAFETY DEwce oz passsm: t “""'—"" ALARM { « 1 EN‘! tLAYOn POWER 1 OWLET i o, i ‘ __ ___.3,__JLAA_EA__M_, ___ _ _ ___ cowon ms 0‘17st 1 ‘1 i f i o ._ @ \_/ D KEY @ CYLINDER [>- FLOW CONTROL VALVE FLOW/METER X FILTER QQ VADOFIIZEFJ _.> CHECK VALVE --.-----» NITROUS OXIDE N20 ® PRESSURE GAUGE , "" "M" OXYGEN 02 REGULATOR ___________ ‘- I}- I < p D O D P‘ PEL I NE INLET 99 automatically produce a proportional drop in the flow of the other gases, Oxygen failure safety valves are located in the intermediate pressure lines upstream of the flow control valves of all gases, except oxygen. Oxygen pressure acts as a control for all other gas systems. Therefore, when oxygen pressure drops, the other valves close and halt the delivery of all other gases. Audible and/or visible warning of the loss of oxygen pressure is another approach to this pressure loss. There are two types of alarms: high— pressure and low—pressure.9 High pressure alarms are sensitive to depletion of oxygen in cylinders attached to the machine. Low—pressure alarms are sensitive to a reduction in oxygen pressure in the intermediate pressure oxygen system. Since most anesthesia machines are operated from pipeline supplies, low—pressure alarms are more common. Alarms are classified by what powers the alarm, of which there are three: (1) oxygen whistles are designed to direct a stream of oxygen through a whistle when the oxygen pressure falls below a certain value;10 (2) nitrous oxide whistles divert a stream of nitrous oxide through a whistle when the oxygen pressure falls;11 and (3) electronic alarms incorporate a pressure—sensitive switch that initiates an audible or visible signal when the oxygen pressure falls below a preset value. The flowmeter assembly controls, measures, and shows the rate of gas flow passing through it. Most current anesthesia machines currently in use have individual flow control valves for each gas (i.e., nitrous oxide and oxygen). The flowmeter subassembly has a tube through which the gas flows, an indicator, a stop at the tube’s top, and a scale that shows the flow. The indicator, or flowmeter, has a rotating bobbin, or rotor, usually made of aluminum, with a diameter larger than that of the body. Diagonal grooves, or flutes, are cut into the rim. When gas passes between the rim of the bobbin and the wall of the tube, it impinges on the flutes, causing the bobbin to rotate. Sequence of flowmeters is of great importance from a human factor’s point—of—view. The right—hand location of oxygen flowmeter is the standard in the United States and Canada, but is in disagreement with the world standard of oxygen on the left side. Confusion about flowmeter sequence could be a cause of hypoxia.12 One important hazard associated with flowmeters is the possibility that the operator may set the flows so that a hypoxic mixture will be delivered. To prevent this, various devices have been developed. Touch—coded oxygen flow control knobs are one safeguard for an anesthesia machine standard that requires the oxygen flow control knobs to have a characteristic fluted profile and be as large or larger than all other flow control knobs. The tactile and visual identity should reduce the hazard of confusion between the oxygen flow control knob and the knob of another gas and reduce unintentional turning off or adjustments to lower settings. Another control on several anesthesia machines regulates the minimum flow of oxygen required before other gases can flow. This minimum flow is preset at the factory and an alarm will sound if the oxygen flow goes below a certain minimum —— even if no other gases are being administered. A third control regulates minimum oxygen flow in proportion to total gas flow, ensuring that a hypoxic mixture will not be delivered. Oxygen—nitrous oxide proportioning devices offer an alternative to conventional flowmeter tubes.13 These devices combine nitrous oxide and 100 oxygen flowmeter assemblies so that the percentage of oxygen and total fresh gas flow is dialed directly. The relative concentration of nitrous oxide and oxygen is varied by adjusting the concentration dial from 30 to 100 percent oxygen. Adjustment of the second dial, the flow control dial, causes the flows for both nitrous oxide and oxygen to increase or decrease, but they remain in the proportion set on the concentration dial. Another type of gas mixer is the monitored dial mixer that allows nitrous oxide and oxygen to be mixed in any proportion from 30 to 100 percent oxygen at total gas flow rates between 1 and 20 liters/min.1‘ There can be several problems with flowmeters, such as inaccuracy,15 improper assembly or calibration,16 dirt,17 back pressure, improper alignment,18 static electricity,19 and float damage.20 The flow control valve should be closed when the cylinder valves are opened or the pipeline hoses are connected to the machine. This will prevent the sudden rise of the indicator to the top of the flowmeter tube, which might damage the indicator or allow it to remain unnoticed at the top of the flowmeter. Flowmeter tubes may get dirty and require occasional cleaning. Cleaning of flowmeters is usually part of the manufacturer’s servicing program and should not be carried out by the user unless instructed by the service manual. The oxygen flush valve receives oxygen from the pipeline inlet or cylinder regulator and directs a high unmetered flow to the common gas outlet. With this flush valve, the anesthesiologist can flood the breathing system with a high flow of oxygen. The ANSI standard requires that the flow be between 35 to 75 liters/minute. Anesthesia machines require regular maintenance to perform reliably. Often a machine is not inspected until there is a problem. Servicing can reduce the frequency of failures/malfunctions by replacing damaged or worn parts. Servicing of the anesthesia machine should be done by an authorized service representative and at regular intervals. Even with a routine servicing program, the user still has the responsibility for checking the machine before each use. Records should be kept of each major piece of equipment, including problems that occur, service performed, when it was performed, and by whom. The ADA has adopted an Acceptance Program for inhalation sedation units that allow the dentist to evaluate those units being considered for purchase. In recent years, the primary emphasis has been the addition of safety features to the units that are aimed at making it difficult to administer less than 20 percent oxygen to a dental patient. The Council on Dental Materials, Instruments and Equipment for evaluation has a list of guidelines that may help the dentist in purchasing a unit acceptable to the ADA. The Breathing System The breathing system allows the dentist to take an anesthetic mixture from the anesthesia machine and present this mixture to the patient. The anesthetics are conveyed to and from the patient without the disruption of the normal exchange of oxygen and carbon dioxide. 101 Rebreathing includes any gas that has been exhaled from which carbon dioxide may or may not have been removed. There is a tendency to associate rebreathing with carbon dioxide accumulation. However, it is possible to have partial or total rebreathing without an increase in carbon dioxide concentration. Breathing systems should prevent the accumulation of carbon dioxide, but the prevention of some rebreathing is not necessarily desirable. The amount of rebreathing depends on three factors: the fresh gas flow, the mechanical dead space, and the design of the breathing system. The amount of rebreathing varies inversely with the total fresh gas flow. The mechanical dead space is the dead space in a breathing system occupied by gases that are rebreathed without any change in composition. This dead space may be minimized by separating inspiratory and expiratory gas streams close the patient.21 The design of the breathing system may be arranged so that there is more or less rebreathing. The Mapleson Breathing System Mapleson breathing systems are a group of breathing systems characterized by the absence of directional valves to direct gases to or from the patient. Since there is no clear separation of inspired and expired gases, the composition of the inspired mixture is highly dependent on the fresh gas flows used. The most common system used in dentistry is the Mapleson D system. The Mapleson D system has a fresh gas inlet at the patient end, a length of corrugated tubing connecting the fresh gas inlet to the relief valve, a relief valve of the high-pressure type, and a reservoir bag next to the relief valve.22 Reservoir Bag The reservoir bag is also known as the respiratory bag, breathing bag, or, somewhat erroneously, the rebreathing bag.23 Most bags are made of rubber; some are plastic. The reservoir bag has four basic functions: (1) it allows accumulation of gas during exhalation so that a reservoir of gas is available for the patient’s next inspiration, thus, allowing greater economy of anesthetic gases and preventing air dilution; (2) it provides a means for the dentist to help or control respirations; (3) it can serve as a visual and tactile sensor as a monitor of a patient’s spontaneous respirations; and (4) it acts to protect the patient from excessive pressure in the breathing system.2“ Breathing Tubes Large bore, nonrigid breathing tubes, typically composed of corrugated rubber or plastic, are found in most breathing systems. The plastic tubes are often clear to permit visual observation of the interior; they are lightweight to provide less drag on the scavenging mask; and they are corrugated to prevent kinking and obstruction. The breathing tubes have two functions: to act as a reservoir in certain systems and to provide a flexible, low—resistance, lightweight connection from one part of the system to another.25 102 The N20 Gas Delivery Mask Nitrous oxide may be delivered to the patient in one of three ways: nasal masks, nasal cannulae, and full face masks. Nasal cannulae and full face masks are not routine sedation procedures in dentistry and are more typically used during emergency procedures, The nasal mask is used most often when delivering N20 and O2 to the patient during dental surgery. The mask is made of a flexible rubber compound, usually a silastic rubber, which adapts to the contours of the patient’s face. Nasal masks are commonly manufactured with two valves. The expiratory or relief valve permits gas to flow out of the system only. Pressure builds up in the system when the patient exhales, and the relief valve will open allowing the gas to escape into the atmosphere. The second valve is the inspiratory valve. Nasal masks are often available in three sizes; small size for children, medium for small adults, and large for large adults. Nasal masks are designed to be disconnected from the breathing hoses to allow cleaning and disinfection. Figure 27 shows the analgesia delivery system from the O2 and N20 flowmeter to the mask used for patients. Figure 28 shows a common nasal mask used for nitrous oxide delivery to the patient. Before administering anesthesia, it is necessary to check all equipment to make sure it is functional, calibrated, and leak proof. Figure 29 shows the potential leak sources in the anesthesia delivery system. The anesthesia machine must be turned on before gases can flow. Before gas supplies are inspected, all flow control valves should be closed by turning them clockwise. In hospital and large dental operatories, the hose should be securely connected to the machine. Pressure gauges should be checked to make sure that proper pressures are available. If the machine is equipped with an oxygen pressure failure alarm, it should be checked for proper function according to the manufacturer’s instructions. The flowmeter should be examined with no gas flow to make certain that the indicator is in the 0 position. Each flow control valve should be opened and closed slowly while the indicator is observed as it rises and falls within the tube. The flowmeter ball should rotate freely. Testing the machine for leaks is normally performed separately from the test for leaks in the breathing system.26 Testing for leaks by pressurizing the breathing system frequently will not detect leaks in the high pressure components of the machine. Most machines are equipped with unidirectional check valves, either near the common outlet or in a vaporizer. A pressure gauge (a standard sphygmomanometer) is attached to the common gas outlet or the fresh gas hose. The flow control valve of a flowmeter on the machine is opened slowly until the pressure on the gauge reaches 30 cm H20 (22 millimeters of Mercury). The flow is lowered until a static equilibrium between the gas flow and the leak has been established; usually at a pressure of 30 cm H20. Test for leakage at the yoke is recommended. After cylinder pressures have been checked and the valves closed, the cylinder pressure gauges are observed for 2 to 5 minutes. A drop in pressure of more than 50 psig shows significant leakage. 103 mwmfimm NV. >5 mdmwmmmwm amHH nearby reentrainment from reentrain— recirculated air and ment from adjacent dental operations from outside air I “20 1 vacuum pump KEY: LEAK SOURCES /> 'wensfis AJeAtIap otneqnseue sq: u; sneeI {etauanod m01; SutaInSQJ exnsodxe OZN go seolnos '6Z eInSIa The oxygen failure safety valve should be tested at the beginning of the day and/or before each case. This test can be performed using either the pipelines or cylinders as the gas source. Some recommend that it be performed with both sources. A cylinder of each gas on the machine is turned on, leaving the pipeline hoses disconnected. Flows of 2 liters/min. are established on the flowmeters for each gas. The oxygen cylinder is then turned off. As the pressure of oxygen falls, the oxygen flowmeter indicator will fall. At a certain oxygen pressure, the indicators for each anesthetic gas should suddenly fall to 0. To test pipeline gases, all cylinder valves should be closed and the flow control valves opened until the cylinder pressure gauges register 0 pressure. The pipeline hoses are connected and flows established on both anesthetic gas and oxygen flowmeters. The oxygen hose is disconnected. The indicators of the anesthetic gases should fall to 0 with the oxygen indicator. When the hoses and bag are firmly attached, leaks are minimized. To check for leaks, the relief valve is closed and the patient port of the y—piece occluded. The reservoir bag is filled using the oxygen flush until a pressure of 30-40 cm H20 water is shown on the pressure gauge. With no additional gas flow the pressure should not drop more than 5 cm H20 in 30 seconds. This corresponds to a leak of less than 50 cc/min. The Mapleson system pressure gauge should be checked to make certain that it reads 0. System integrity is tested by occluding the patient port, closing the relief valve, and beginning the oxygen flush on the anesthesia machine to distend the bag. The bag should maintain the distension and not deflate. When the relief valve is then opened, the bag should deflate. An assortment of masks in various sizes should be readily available in order to fit the patient and reduce potential leaks. It may be necessary to try a few mask sizes on the patient’s face before finding a suitable one. Suction equipment should be checked by placing a flowmeter with a range of 0 to 100 liters per minute in line. The suction valve should be opened fully to find the range of suction flow and then, the valve should be turned back to 45 liters per minute. New equipment should be checked with the aid of a user’s manual. It will contain assembly and installation instructions, a list of requirements, and daily checking procedures. The manual should be kept in the central department files and reviewed periodically. A copy of the daily checking procedures should be kept in the operatory with the equipment. Assembly, installation, and operation instructions should be carefully followed. Preventive maintenance should be contracted with the respective equipment manufacturers and their trained service representative on 3— to 12—month intervals depending on frequency of use. Preventive maintenance includes inspection, testing, cleaning, lubrication, and adjustment of various components. Worn or damaged parts are to be fixed or replaced. Such maintenance can result in detection of deterioration before an overt malfunction occurs. 107 Recordkeeping for anesthesia equipment is often neglected. Recordkeeping is important because it provides a number of checks and balances: (1) proof that an effort has been made to keep the equipment in proper working order; (2) a means of communication with the service representative; (3) a complete, up-to— date record for each piece of equipment; (4) a written record that maintenance by a service representative was performed and shows what was done; (5) a check on the service rendered by the representative; and (6) a reminder to the user of when the equipment needs to be serviced or a component replaced. Figure 30 shows the sources of N20 exposure resulting from potential leaks in the anesthetic delivery system. American Dental Association Guidelines for Scavenging Equipment Because of the variety and quality of N20 scavenging systems and the concern for patient safety, the ADA has developed guidelines for scavenging equipment.27 These guidelines, recommend that scavenging system equipment have the following characteristics: (1) be capable of providing NZO—O2 flow rates that comply with or improve upon minimum concentrations indicated in current NIOSH and OSHA documents; (2) be adaptable to existing sedation, anesthesia, and exhaust systems; (3) be constructed so as not to interfere significantly with the normal breathing system and delivery of selected gas concentrations; (4) be effective regardless of the heating and air conditioning system in use; (5) be constructed to permit safe and efficient disposal of the gases; (6) be effective when more than one device is being used simultaneously; and (7) be constructed such that patient rebreathing will be insignificant. of two commercially available scavenging systems provided to the ADA for testing to date, only the Porter—Brown system has been fully tested and approved.27 It should be noted that testing of scavenging systems by the ADA does not include meeting the NIOSH REL. REFERENCES 1. Compressed Gas Association, Inc. [1974]. Safe handling of compressed gases in containers. Pamphlet P—l, New York, NY. 2. American Dental Association [1983]. Dentists’ desk reference: materials, instruments and equipment. 2nd ed., pp. 407—409. 3. Anonymous [1980]. Modification of medical gas systems. Health Devices 9:181—185. 4. Howell RSC [1980]. Piped medical gas and vacuum systems. Anaesthesia 35:676-698. 5. Fair JL [1978]. Canadian standards for piped gases. Anesthesiology 48:155. 6. Compressed Gas Association, Inc. [1978]. Diameter—Index safety system, CGA V-5. New York, NY: Compressed Gas Association, Inc. 108 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. ANSI [1979]. Minimum performance and safety requirement for components and systems of continuous flow anesthesia machines for human use. New York, NY: American National Standards Institute, ANSI Z—79.8. Wright, JG [1971]. Wright’s veterinary anesthesia and analgesia. 7th ed., London, Bailliere, Tindall and Cassell. Schreiber P [1971]. Anaesthesia equipment, performance, classification, and safety. New York, NY: Springer-Verlag. Adler L, Burn N [1967]. A warning device for failure of the oxygen supply. Anaesthesia 22:56—159. Cartwright FF [1963]. Warning of an empty oxygen cylinder. Lancet 2:407. Eger EI, Hylton RR, Irwin RH, Guadagni N [1963]. Anesthetic flowmeter sequence —— a cause for hypoxia. Anesthesiology 24:396—397. Lundsgaard JS, Einer-Jensen H, Juhl B [1977]. High precision mixing of anesthetic gases based on a new principle. Acta Anaesthesiol Scand 21:308—313. Heath JR, Anderson MM, Nunn JF [1973]. Performance of the quantiflex monitored dial mixer. Br J Anesth 45:216—221. Sadove MS, Thomason RD, Thomason CL, Ries M [1976]. An evaluation of flowmeters. J Am Assoc Nurse Anesth 44:162—165. Kelley JM, Gabel RA [1970]. The improperly calibrated flowmeter —— another hazard. Anesthesiology 33:467—468. Russell FR [1961]. Deposits in the cyclopropane flowmeter. Br J Anesth 33:323. Ward CS [1968]. The prevention of accidents associated with anaesthetic apparatus. Br J Anesth 40:692—701. Clutton—Brock J [1972]. Static electricity and rotameters. Br J Anest 44:86—90. Hodge EA [1979]. Accuracy of anaesthetic gas flowmeters. Br J Anesfl 51:907. Sykes MK [1959]. Nonrebreathing valves. Br J Anesth 31:450—455. Bain JA, Sporel WE [1972]. A streamlined anaesthetic system. Canw Anesth Scand J 19:426—435. Wyant GM [1970]. Rebreathing bag. Br Med J 1:112. 109 24. 25. 26. 27. Parmley JB, Tahir AH, Dascomb HE, Adriani J [1972]. Disposable versus reusable rebreathing circuits: advantages, disadvantages, hazards, and bacteriologic studies. Anesth Analg 51:888—894. Cottrell JE, Bernhard W, Turndorf H [1976]. Hazards of disposable rebreathing circuits. Anesth Analg 55:743—744. Comm G, Rendell—Baker L [1982]. Back pressure check valves a hazard. Anesthesiology 56:327—328. ADA Council [1986]. Guidelines for the acceptance of nitrous oxide sedation machines and scavenging equipment. American Dental Association. 110 u. c. BERKELEY LIBRAEIE """"" Delivering on the Nation’s promise: Safety and health at work For all people Through prevention