key: cord-0985498-1ftayqho authors: Zhang, Yinping; Mo, Jinhan; Li, Yuguo; Sundell, Jan; Wargocki, Pawel; Zhang, Jensen; Little, John C.; Corsi, Richard; Deng, Qihong; Leung, Michael H.K.; Fang, Lei; Chen, Wenhao; Li, Jinguang; Sun, Yuexia title: Can commonly-used fan-driven air cleaning technologies improve indoor air quality? A literature review date: 2011-08-31 journal: Atmospheric Environment DOI: 10.1016/j.atmosenv.2011.05.041 sha: e47402fcd7e6c22a95beeaa06f1f7e94cb7a4e42 doc_id: 985498 cord_uid: 1ftayqho Abstract Air cleaning techniques have been applied worldwide with the goal of improving indoor air quality. The effectiveness of applying these techniques varies widely, and pollutant removal efficiency is usually determined in controlled laboratory environments which may not be realized in practice. Some air cleaners are largely ineffective, and some produce harmful by-products. To summarize what is known regarding the effectiveness of fan-driven air cleaning technologies, a state-of-the-art review of the scientific literature was undertaken by a multidisciplinary panel of experts from Europe, North America, and Asia with expertise in air cleaning, aerosol science, medicine, chemistry and ventilation. The effects on health were not examined. Over 26,000 articles were identified in major literature databases; 400 were selected as being relevant based on their titles and abstracts by the first two authors, who further reduced the number of articles to 160 based on the full texts. These articles were reviewed by the panel using predefined inclusion criteria during their first meeting. Additions were also made by the panel. Of these, 133 articles were finally selected for detailed review. Each article was assessed independently by two members of the panel and then judged by the entire panel during a consensus meeting. During this process 59 articles were deemed conclusive and their results were used for final reporting at their second meeting. The conclusions are that: (1) None of the reviewed technologies was able to effectively remove all indoor pollutants and many were found to generate undesirable by-products during operation. (2) Particle filtration and sorption of gaseous pollutants were among the most effective air cleaning technologies, but there is insufficient information regarding long-term performance and proper maintenance. (3) The existing data make it difficult to extract information such as Clean Air Delivery Rate (CADR), which represents a common benchmark for comparing the performance of different air cleaning technologies. (4) To compare and select suitable indoor air cleaning devices, a labeling system accounting for characteristics such as CADR, energy consumption, volume, harmful by-products, and life span is necessary. For that purpose, a standard test room and condition should be built and studied. (5) Although there is evidence that some air cleaning technologies improve indoor air quality, further research is needed before any of them can be confidently recommended for use in indoor environments. Indoor air quality (IAQ) Air cleaner By-product High efficiency particulate air (HEPA) Sorption Ultraviolet germicidal irradiation (UVGI) Photocatalytic oxidation (PCO) Thermal catalytic oxidation (TCO) Plasma Ozone Ion generator Electrostatic precipitator Clean air delivery rate (CADR) a b s t r a c t Air cleaning techniques have been applied worldwide with the goal of improving indoor air quality. The effectiveness of applying these techniques varies widely, and pollutant removal efficiency is usually determined in controlled laboratory environments which may not be realized in practice. Some air cleaners are largely ineffective, and some produce harmful by-products. To summarize what is known regarding the effectiveness of fan-driven air cleaning technologies, a state-of-the-art review of the scientific literature was undertaken by a multidisciplinary panel of experts from Europe, North America, and Asia with expertise in air cleaning, aerosol science, medicine, chemistry and ventilation. The effects on health were not examined. Over 26,000 articles were identified in major literature databases; 400 were selected as being relevant based on their titles and abstracts by the first two authors, who further reduced the number of articles to 160 based on the full texts. These articles were reviewed by the panel using predefined inclusion criteria during their first meeting. Additions were also made by the panel. Of these, 133 articles were finally selected for detailed review. Each article was assessed independently by two members of the panel and then judged by the entire panel during a consensus meeting. During this process 59 articles were deemed conclusive and their results were used for final reporting at their second meeting. The conclusions are that: (1) None of the reviewed technologies was able to effectively remove all indoor pollutants and many were found to generate undesirable by-products during operation. (2) Particle filtration and sorption of gaseous pollutants were among the most effective air cleaning technologies, but there is insufficient information regarding long-term performance and proper maintenance. (3) The existing data make it difficult to extract information such as Clean Air Delivery Rate (CADR), which represents a common benchmark for comparing the performance of different air cleaning technologies. (4) To compare and select suitable indoor air cleaning devices, a labeling system accounting for characteristics such as CADR, energy consumption, volume, harmful by-products, and life span is necessary. For that purpose, a standard test room and condition should be built and studied. (5) Although there is evidence that some air cleaning technologies improve indoor air quality, further research is needed before any of them can be confidently recommended for use in indoor environments. Ó 2011 Elsevier Ltd. All rights reserved. Because indoor air quality is an important determinant of human health, comfort and productivity, high quality indoor air is desirable. Air cleaning technologies are of increasing importance, especially when building ventilation rates are being reduced to conserve energy. Numerous air cleaning technologies have been developed and used, but there have been no systematic assessments of these technologies. This is particularly true with regard to (1) application at realistic indoor conditions, (2) long-term performance, and (3) production of unwanted by-products during operation. The lack of widespread acceptance of reliable protocols for estimating the effectiveness of air cleaning systems has made it difficult to develop a standardized labeling system for indoor air cleaners, including standard methods for estimating Clean Air Delivery Rate (CADR). Consequently, a literature review was undertaken to collect state-of-the-art information on air cleaning technologies focusing on both their effects at removing indoor air pollutants and the problems that may occur during their application. The scientific peer-reviewed literature on the effects of commonly-used gas-phase and particle phase air cleaners on indoor air pollutants in non-industrial indoor environments was reviewed by a multidisciplinary group of scientists with expertise in medicine, epidemiology, toxicology and engineering. The focus was only on air cleaning techniques for which indoor air flows through a device and is returned to the indoor environment ("fandriven" air cleaners). Technologies like "catalyst in paint" or other passive air purification materials, masks and other personal protective devices were not included. Botanic air cleaners that did not involve flow-through systems were also excluded. Air cleaning devices that are intended only for outdoor air intakes (e.g., filters in mechanical ventilation system) were not considered. Consequently the air cleaners reviewed included only: high efficiency particulate air (HEPA), adsorption, ultraviolet germicidal irradiation (UVGI), photocatalytic oxidation (PCO), thermal catalytic oxidation (TCO), plasma, botanic air cleaners, ion generators, and electrostatic precipitators. The selected air cleaning technologies were reviewed regarding their efficiency to reduce/remove indoor air pollutants including particles, microorganisms, inorganic and organic gases; radon was not included. The effects on health and/or occupant performance were not considered. For example, we did not consider articles which only reported the effects of an air cleaning device on health unless the effects on air pollutants were also reported. The selected articles were limited to those which reported the tests involving pollutant concentrations within an order of magnitude of concentrations reported by the US EPA, WHO, and others to be typical in non-industrial indoor environments. This approach may have excluded some information related to particle filtration because standard test protocols are completed at elevated particle concentrations and there is general consensus that removal efficiency is not affected by standard test concentrations. Only demonstrated changes of concentration of one or more pollutants due to the use of an air cleaning device were considered, where "demonstrated" means that the methodology was validated and other effects such as air leakage and natural decay were considered. Demonstrated changes in odor intensity or perceived indoor air quality were also considered as evidence in this review. The scientific literature was gathered by searching through the following databases: ISI Web of Science (1910epresent), Science-Direct (1823epresent), MEDLINE (1965epresent) and Engineering village 2 (1884epresent). Google Scholar was used as a supplementary search. As a source of search records, the following keywords were used: Conference papers were excluded. During the literature search, over 26,000 articles were identified. 400 articles were selected as relevant based on their titles and abstracts by the first two authors, who also further reduced the number of articles to 160 based on the full text. These articles were reviewed by the panel using predefined inclusion criteria during their first meeting and some articles were found to be outside the scope of this review. Additions were also made by the panel. In this process 133 articles were selected for thorough review. Each article was reviewed by two scientists, one assigned to be a prime reviewer and the other one assigned to be secondary reviewer. Each scientist reviewed 17 to 18 articles. The articles were assigned to the reviewers completely at random and not depending only on their expertise; no article was assigned to a scientist if he was one of the authors. When reviewing the article, information on different aspects of the study was collected including design, methods, data analysis, measurements of airflow rates and air pollutants, possible bias, single-pass removal efficiency, CADR, creation of by-products, results and main conclusions. Reviewed articles were then classified as: relevant and conclusive e providing sufficient information on air cleaning effect, data processing and reporting; relevant but non-informative e lacking essential information; relevant but inconclusive e with incomplete data processing or reporting. Classification of each article was first made independently by each reviewer. Then, during the plenary meetings, the whole group agreed on a final classification. The articles judged during plenary discussions of the whole group as conclusive were used to formulate the final consensus statement and conclusions. During the review process the following definitions and terms were used: Indoor air pollutants refer to contamination of the indoor environment by any chemical, physical or biological agent that is harmful to human health or uncomfortable to humans, and new airborne pollutants with unknown health effects. (ISIAQ, 2010; WHO, 2010) Secondary indoor air pollutants refer to the intermediates or by-products produced by air cleaning devices, and released to indoor air. Single-pass removal efficiency is defined as the percentage (or fraction) of the target pollutant that enters an air cleaner and is removed by the cleaner. Effectiveness is defined as the fractional reduction in indoor pollutant concentration that results from application of a control device relative to the identical conditions without the control device in place (Nazaroff, 2000) . Clean air delivery rate (CADR) is the equivalent clean airflow rate delivered by an air cleaner in which "clean" only refers to the absence of the target pollutants removed by the air cleaner. It is equal to the single-pass efficiency multiplied by the airflow rate through the device. Single-pass or flow-through test method is a method in which an air cleaner is placed between two tightly sealed chambers in such a way that one chamber is connected to the air cleaner intake and the other to the air cleaner outlet. The pressure drop between the chambers is adjusted to zero, so that the air cleaner operates as it would in a normal room. There is a constant airflow. Target pollutants (particles or gases) are constantly generated upstream and the concentration is measured in the upstream and downstream chambers. Decay test method involves an air cleaner that is positioned in a sealed chamber. The airborne pollutant(s) is (are) injected as a short-term release and mixed with the chamber air before activating the air cleaner. The concentration of the pollutant in the chamber air is measured over a specific time period. The CADR is calculated from the decay curve accounting for losses by deposition to chamber surfaces and air exchange rate, if any (Niu et al., 1998) . Non-industrial indoor environments include any indoor environment not related to industrial exposures. The number of published articles found using the abovementioned keywords in ISI Web of Science (1910epresent) vs. publication year is shown in Fig. 1 . Although studies and articles related to indoor air cleaning have increased rapidly since 1993, we have not been able to read and analyze carefully all these articles as this was beyond our collective ability. Of the 133 articles thoroughly reviewed and discussed by the panel, 59 articles were judged relevant and conclusive. We found that the number of articles with real environment data (field test) depends on the specific air cleaning technologies. For example, catalytic oxidation, ozone, and plasma are mostly tested in laboratory settings. For the traditional technologies such as filtration, sorption and UVGI, many studies were in completed in real environments. Table 1 summarizes the distribution of reviewed articles in either laboratory or real environments. The performance of air cleaners is best measured and compared (between cleaning devices) by a Clean Air Delivery Rate (CADR) which is defined as the product of the single-pass removal efficiency and volumetric airflow rate through the cleaning device. However, many of the articles that were reviewed in this study did not include an explicit determination of CADR, nor were single-pass removal efficiency or volumetric flow rates provided to allow for an implicit calculation of CADR. Figs. 2 and 3 summarize the reported CADR and single-pass removal efficiency in the 59 reviewed articles (Tables 2e6). Most catalytic oxidation air cleaning studies focus on photocatalytic oxidation (Table 1) . TiO 2 is the most commonly-used material in PCO research. In some studies ozone was applied to enhance the performance of the catalysts (Ellis and Tometz, 1972; Kwong et al., 2008b) . PCO is a general air cleaning technology, which can degrade almost all contaminants (such as aldehyde, aromatics, alkanes, olefins, halogenated hydrocarbons, odor compounds etc.). The competitive adsorption effect for contaminants and water vapor has a significant effect on the oxidation rate (Obee and Brown, 1995) . Hybrid catalysts (combined TiO 2 with adsorption materials such as activated carbon and zeolite) are used to enhance the PCO degradation of VOCs . In most studies only a single compound was tested, although often with good results. However, indoor air contains numerous contaminants, so tests of only one or a few compounds may be misleading. Furthermore, the generation of by-products is a serious problem for catalytic oxidation processes. Indeed, PCO can generate by-products (formaldehyde, acetaldehyde etc.) that are more harmful than the target pollutant (Hodgson et al., 2007; Muggli et al., 1998; Mo et al., 2009) . Table 1shows that there is lack of field studies for PCO air cleaning in the 59 reviewed papers. Most of the studies were performed in small-scale laboratory settings, which resulted in their low CADR values (Fig. 2) . PCO has high efficiency in the single-pass tests, but its efficiency is significantly reduced in the chamber tests (Fig. 3 ). This indicates that PCO technology is not ready for practical application. Fifteen articles on air filters (some with activated carbon) underwent a detailed review process; recall that filters used on outdoor air intakes were not included in the review. Some of the articles focused on particle removal efficiency, but the particles studied were quite different, ranging from large microbes to very small particles. They all report a positive effect with regard to particle removal (higher removal efficiencies for larger particles), but sometimes not as high as the manufacturers' data indicate. VOC removal was investigated in four studies, with results ranging from zero removal (no activated carbon) (Batterman et al., 2005) to some removal (with activated carbon) (e.g., Bekö et al., 2008) . Removal of ozone by reactions with filters has also been observed by Bekö et al. (2006) and Zhao et al. (2007) . However, ozone reaction products released from filters have been reported (Bekö et al., 2006 (Bekö et al., , 2008 (Bekö et al., , 2009 Hyttinen et al., 2007; Schleibinger and Rüden, 1999) . In summary, mechanical filters can efficiently remove particles, but are not as effective for organic and inorganic chemical pollutants. The main problem with mechanical filters is that they act as a pollution source if they are not properly used. A solution seems to be a combination of particulate filter and activated carbon, as shown by Bekö et al. (2009) , and Metts and Batterman (2006) . Ozone is an oxidant that can react with some indoor pollutants. The combined use of ozone and various micro-or meso-porous adsorbents can take advantage of the oxidizing capability of ozone and reduce the residual ozone due to enhanced catalytic reaction in the porous structure (Kwong et al., 2008a) . Considering that ozone itself is quite harmful and that reactions with compounds such as terpenes can produce potentially harmful secondary organic aerosol (SOA) in the ultra fine and fine size ranges (Waring et al., 2008) , as well as reactive organic compounds, caution should be taken when using ozone-emitting air cleaning techniques (e.g., UVGI, plasma, electrostatic precipitator, and ion generators) in indoor environments, no matter whether they intentionally or unintentionally produce ozone. There are several ways to generate plasma for air purification: corona discharge with alternating current, direct current and dielectric barrier discharge (DBD). Plasma air cleaners have been reported to remove particles at high efficiency, e.g., within the range of 76e99% (Park et al., 2008; Van Durme et al., 2007; Van Durme et al., 2009 ). The technology is not efficient at removing gas-phase pollutants (Park et al., 2008) . When combined with catalytic technology, plasma air cleaners have been observed to more effectively remove VOCs, such as toluene (Van Durme et al., 2007) . If plasma is combined with UV-catalytic technology, the improved removal efficiencies for formaldehyde, benzene, toluene and xylene is promising (Park et al., 2008) . The performance of Table 2 Catalytic oxidation air cleaning technology. Catalytic oxidation refers to a set of chemical treatment procedures designed to remove organic and inorganic materials in gas by catalysts. The common types for indoor air cleaning are photocatalytic oxidation (PCO), thermal catalytic oxidation (TCO) and ozone-catalytic oxidation. Results by the authors Research type/test procedure Target pollutants/ concentration Airflow rate, Air velocity, or residence time CADR (m 3 h À1 )/efficiency (%) By-product tested or not and results Ao and Lee (2003) UV-TiO 2 /AC was more effective in BTEX removal and less affected by the increasing humidity than AC alone. AC acted as a local pollutant concentrator by adsorbing pollutants from the air stream. Obee and Brown (1995) Competitive adsorption between water and trace (sub-ppmv) contaminants has a significant effect on the oxidation rate. Yes. Acetaldehyde, formaldehyde, benzaldehyde and formic acid are the main by-products. By-products (e.g., formaldehyde, acetaldehyde, formic and acetic acid, etc.) were generated during the PCO decomposition of various pollutants. Combining TiO 2 with adsorption material (activated carbon etc.) may lower the generation of the by-products. The effect of multiple indoor pollutants on UVPCO performance needs further investigation and should not be neglected. Most of the researches on UVPCO are laboratory studies. Table 3 Filtration air cleaning technology. Filtration is a mechanical or physical operation which is used for the removal of particles by physical separation from air by interposing a medium through which only the air can pass. plasma-catalyst technology for VOC removal can be inhibited by humidity (Van Durme et al., 2009) . In general, plasma technologies can enhance the performance of filters for particle removal and catalysts for gas-phase pollutants. However, the production of secondary pollutants such as NO x and O 3 is a major drawback of plasma technology (Van Durme et al., 2007) . Eight articles involved investigations of sorption air cleaning techniques. Sorption is good for gas pollutant removal. For adsorption and chemisorption, the following factors are involved: sorption mechanism (e.g., strong chemisorption vs. weaker hydrogen bonding), specific sorbent surface area, porosity, specific equilibrium adsorption quantity, diffusion coefficient of target pollution in adsorbent and half-life (Parmar and Grosjean, 1991) . Desiccant wheels may be promising in controlling indoor air humidity and removing indoor VOCs simultaneously . There are some problems when applying such techniques in practice: (1) The sorbed VOCs and O 3 may generate reaction byproducts, such as particles. Adsorbents such as activated carbon can also be effective at removing reactant species (such as O 3 ) by surface reactions; (2) Humidity and/or other indoor pollutants generally have a negative effect on target indoor pollutant removal due to competitive sorption; (3) The removal effect for multiple indoor compounds remains unknown; (4) For a target pollutant, the criteria for selecting the best sorption material to optimize removal over a given time period are generally not available; and (5) The lifetime for sub-ppbv level indoor air pollutants removal is unknown. The use of ultraviolet (UV) wavelengths of light in the germicidal range (200e365 nm) for air or surface disinfection is referred to as UVGI. Though the germicidal ability of UV (200e320 nm) has been known for more than 100 years (Kowalski, 2009) , conclusive field data are still lacking to demonstrate the effectiveness of UVGI. None of the 10 studies included in this review studied the formation of possible secondary pollutants by UV, e.g., as initiated by ozone chemistry. The characteristics of commonly-used indoor air cleaning technologies were compared in Table 7 . Some "new" air cleaning technologies, such as PCO, plasma, and ozone-related, can handle more than one type of indoor air pollutant. For example, PCO can decompose almost all indoor organic compounds, and can also sterilize indoor microbes. This more "general" potential has made them a hot research topic (Fig. 1) . However, they all produce harmful by-products and there is a lack of data on practical applications. Fig. 2 shows PCO and plasma have low CADR values, even if they achieve high single-pass efficiencies in laboratory studies (Fig. 3) . For the "typical" air cleaning technologies, such as sorption, filtration and UVGI, they generally can remove one type of indoor air pollutant. Many studies show that filtration and UVGI have high single-pass efficiencies in real environment applications (Fig. 3) . In addition, many of the articles reviewed in this study did not include an explicit determination of CADR, nor were single-pass removal efficiency or volumetric flow rates provided to allow for an implicit calculation of CADR. Though the potentially harmful byproducts created by the air cleaners are important, only a few articles refer to secondary products associated with the air cleaning process. Energy consumption by air cleaners is often overlooked Offermann et al. (1992) No. Formaldehyde and acetone were the main by-products from dust loaded filters. Emission of VOC from the filter material should be very low. However, there were unknown oxidation products due to reactions between O Table 4 Ozone-oxidation and plasma air cleaning technology. Ozone has a very high oxidation potential and can react with some indoor pollutants. The common types of ozone generator for indoor uses are the corona discharge method and ultraviolet light method. Plasma is a gas produced by electrical charge, in which a certain proportion of its composition (atoms, molecules, or ions) is ionized. The atoms, molecules, or ions with unpaired electrons cause them to be highly chemically reactive. Yes. Ozone. The production of harmful pollutants from ozone is a major problem. Ozone generated by-products include a wide range of carbonyls, dicarbonyls, carboxylic acids, secondary organic aerosols, peroxides and more. Negative ion generators produced excessive concentration of O 3 and NO x . For VOC removal, other harmful by-products were generated in addition to CO, CO 2 , NO x and O 3 . Sorption is effective for VOC removal. The sorbed VOCs and O 3 may generate some low level reaction by-products, including particles. However, some adsorbents, e.g., activated carbon, can also be effective at removing reactant species such as ozone by surface reactions. Humidity generally has a negative effect on VOC removal due to the sorption competition. (599 mW s cm À2 UV-C). No. Xu et al. (2005) Performance of the UVGI system degraded significantly when RH was increased from 50% to 75e90%. The inactivation rate increased linearly with effective UV fluence rate up to 5 mW cm ; an increase in the fluence rate above this level did not yield a proportional increase in inactivation rate. Field test, commercial product/ decay. Can generate harmful by-products such as formaldehyde, acetaldehyde, and acetone. Catalyst may be poisoned, resulting in decreased performance. Combined with other air cleaning technologies, such as adsorption and thermal catalytic oxidation, to reduce by-products and enhance performance. Gas pollutants: organic, inorganic Airborne microbe. Can simultaneously remove gas pollutants, airborne microbe and even particles. High single-pass removal efficiency. May produce O3, NO x and other harmful by-products. High voltage and high energy consumption. Combined with particle filter to increase filter performance and reduce pressure drop. Combined with catalysts to reduce or remove ozone. Ozone-related Gas pollutants: organic, inorganic Airborne microbe. Can reduce some targeted odors; Can enhance some catalytic oxidation reactions for VOC removal. Ozone itself is a harmful pollutant and may react with other indoor pollutants to produce harmful pollutants such as carbonyls, dicarbonyls, carboxylic acids, and secondary organic aerosols. Combined with catalysts to reduce or remove ozone. Gas pollutants: organic, inorganic. No harmful by-products. Good performance for gas-phase pollutants. Must be regenerated after long-term operation. May produce some airborne pollutants. Reactions with ozone may generate gaseous secondary pollutants. Dynamic continual or intermittent generation systems need to be developed. Particles Good at removing particles in the range from 0.1 mm to 4 mm. Used particle filters are sources of sensory pollutants. No evidence of VOC removal by filter alone, except when filter is combined with other materials, such as activated carbon. Combined with electrostatic field. and will be increasingly important as building energy consumption is reduced. We believe that researchers who study air cleaning devices should determine and report CADR, by-products, and energy requirements (perhaps CADR/energy consumption), and that peer reviewers of related submissions should require this information. The following consensus statements were made based on the 59 articles identified as conclusive and relevant: (1) Filtration is an efficient technology for removing particles, although used particle filters can be a source of sensory pollution. (2) Sorption is an efficient technology for removing some gaseous pollutants, including VOCs, formaldehyde, O 3 , NO 2 , SO 2 , and H 2 S, provided that the sorption system is properly designed and operated. It may produce pollutants, especially if ozone reacts with contaminants that deposit on or are sorbed by the media. More information is needed on the long-term performance of air cleaners using sorption principles. (3) UVGI is a proven technology for inactivation of some airborne microorganisms such as bacteria, fungi and viruses, but ozone may be produced during operation. (4) PCOs can reduce concentrations of some gaseous pollutants (e.g., BTEX, ethanol, formaldehyde), but may generate harmful by-products. By-products need to be identified and controlled and catalyst poisoning must be understood when this technique is applied for prolonged periods. (5) Laboratory tests show that plasma air cleaning can reduce concentrations of some gaseous pollutants such as BTEX, ethanol, and formaldehyde. It can also produce harmful byproducts such as ozone and oxidation intermediates, and its energy consumption tends to be high. (6) Ozone is not recommended for indoor air cleaning because of the harmful by-products. (7) The performance criteria of commonly-used fan-driven air cleaners, such as efficiency and CADR, strongly depend on the type of indoor air pollutants and the test conditions. Benchmarks and standard condition and procedures for evaluating air cleaner performance are necessary, and labeling of air cleaners will be valuable in the future. This review has shown that the question "Can commonly-used, fan-driven air cleaning technologies improve indoor air quality?" does not yet have an answer. Once researchers in the indoor air cleaning field have solved the problems identified in our review, the answer will be clear. Enhancement effect of TiO 2 immobilized on activated carbon filter for the photodegradation of pollutants at typical indoor air level Photodegradation of volatile organic compounds (VOCs) and NO for indoor air purification using TiO 2 : promotion versus inhibition effect of NO Inhibition effect of SO 2 on NO x and VOCs during the photodegradation of synchronous indoor air pollutants at parts per billion (ppb) level by TiO 2 Long duration tests of room air filters in cigarette smokers' homes Sensory pollution from bag filters, carbon filters and combinations Sensory pollution from bagtype fiberglass ventilation filters: conventional filter compared with filters containing various amounts of activated carbon Initial studies of oxidation processes on filter surfaces and their impact on perceived air quality The use of a UV lamp for control of odour decomposition of kitchen and vegetable waste Ozone generation by indoor, electrostatic air cleaners UV-PCO device for indoor VOCs removal: investigation on multiple compounds effect Performance of air cleaners for removing multiple volatile organic compounds in indoor air Efficiency of a portable indoor air cleaner in removing pollens and fungal spores Comparison of Experimental and Theoretical efficiencies of residential air filters Room-temperature catalytic decomposition of ozone Desiccant wheels as gas-phase absorption (GPA) air cleaners: evaluation by PTR-MS and sensory assessment Removal of Aldehydes and acidic pollutants from indoor air Use of ultraviolet irradiation in a room air Conditioner for removal of bacteria Performance of ultraviolet photocatalytic oxidation for indoor air cleaning applications Photodegradation of gaseous volatile organic compounds (VOCs) using TiO 2 photoirradiated by an ozoneproducing UV lamp: decomposition characteristics, identification of byproducts and water-soluble organic intermediates Effects of ceiling-mounted HEPA-UV air filters on airborne bacteria concentrations in an indoor therapy pool building UV air cleaners and upper-room air ultraviolet germicidal irradiation for controlling airborne bacteria and fungal spores Removal of VOCs from indoor environment by ozonation over different porous materials Catalytic ozonation of toluene using zeolite and MCM-41 materials Airborne bacteria control under chamber and test-home conditions Removal of fine and ultra fine particles from indoor air environments by the unipolar ion emission. Atmospheric Environment 38 Indoor air particulate filtration onto activated carbon fiber media Characterization of UVC light sensitivity of vaccinia virus Effect of ultraviolet germicidal lights installed in office ventilation systems on workers' health and wellbeing: double-blind multiple crossover trial Heterogeneous reactions of ozone and D-limonene on activated carbon Effect of VOC loading on the ozone removal efficiency of activated carbon filters Effectiveness of inroom air filtration and dilution ventilation for tuberculosis infection control Determination and risk assessment of by-products resulting from photocatalytic oxidation of toluene Mechanism of the photocatalytic oxidation of ethanol on TiO 2 TiO 2 photocatalysis for indoor air applications e effects of humidity and trace contaminant levels on the oxidation rates of formaldehyde, toluene, and 1,3-butadiene Control of respirable particles in indoor air with portable air cleaners Performance of air cleaners in a residential forced air system Lab-scale test of a ventilation system including a dielectric barrier discharger and UV-photocatalyst filters for simultaneous removal of gaseous and particulate contaminants Sorbent removal of air pollutants from museum display cases Indoor air disinfection using a polyester supported TiO 2 photo-reactor Air filters from HVAC systems as possible source of volatile organic compounds (VOC) e laboratory and field assays Study on the indoor volatile organic compound treatment and performance assessment with TiO 2 / MCM-41 and TiO 2 /quartz photoreactor under ultraviolet irradiation Inactivation of virus-containing aerosols by ultraviolet germicidal irradiation Photocatalytic degradation of volatile organics on TiO 2 embedded glass spherules Postplasma catalytic technology for the removal of toluene from indoor air: effect of humidity Effect of ultraviolet germicidal irradiation on viral aerosols Ultrafine particle removal and generation by portable air cleaners PTR-MS assessment of photocatalytic and sorption-based purification of recirculated cabin air during simulated 7-h flights with high passenger density Efficacy of ultraviolet germicidal irradiation of upper-room air in inactivating airborne bacterial spores and mycobacteria in full-scale studies Impact of environmental factors on efficacy of upper-room air ultraviolet germicidal irradiation for inactivating airborne mycobacteria Degradation of indoor gaseous formaldehyde by hybrid VUV and TiO 2 /UV processes. Separation and Purification Technology Adsorption and regeneration on activated carbon fiber cloth for volatile organic compounds at indoor concentration levels Effectiveness of photocatalytic filter for removing volatile organic compounds in the heating, ventilation, and air conditioning system The correlation between photocatalytic oxidation performance and chemical/physical properties of indoor volatile organic compounds A comparative study on decomposition of gaseous toluene by O3/UV, TiO 2 /UV and O 3 /TiO 2 /UV A model for analyzing the performance of photocatalytic air cleaner in removing volatile organic compounds Characteristics of photocatalytic oxidation of toluene, benzene, and their mixture Theoretical study of simultaneous water and VOCs adsorption and desorption in a silica gel rotor Ozone removal by HVAC filters Odors and volatile organic compounds released from ventilation filters Vocabulary of the Indoor Air Sciences by International Society of Indoor Air Quality and Climate (ISIAQ) Ultraviolet Germicidal Irradiation Handbook: UVGI for Air and Surface Disinfection Effectiveness of air cleaning technologies Using large environmental chamber technique for gaseous contaminant removal equipment test World Health Organization: The Definition of Air Pollution This literature review was financially supported by a research project of the National Natural Science Foundation of China (Project Number: 50725620, 51006057). Thanks to Prof. J. Zhang, Syracuse University, for providing the space and on-site support for the 2nd expert meeting, Dr. Jeffery Siegel for the additional paper-selection, and thanks to Ph. D candidates J. Pei of Syracuse University and Z. Liu of Virginia Tech for their assistance.Primary references (reviewed articles by the panel)