UBZ * RY STATE PLANT BOARD March 1952 E-835 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine DIRECTIONS FOR INDUSTRIAL USE OF AEROSOLS By A. H. Yeomans Division of Stored Product Insect Investigations Aerosols are used in many industrial structures to control insects by direct contact or by applying a light deposit of insecticide on the top of exposed horizontal surfaces. An aerosol is composed of a number of fine liquid particles suspended in the air. It is considered that in an insecticidal aerosol all the particles should be smaller than 50 microns in diameter and 80 percent by weight of them should be less than 30 microns. Vapors or smokes composed of particles of smaller diameter than 0.1 micron are not classed as insecticidal aerosols because they exhibit slightly different character- istics. Aerosols may be produced by liquefied-gas formulations released through capillary or expansion chamber nozzles, by steam or air atomization of liquid, by spinning disks and rotors, by forcing liquid under high pressure through atomizing nozzles, by heat vaporization, or by a combination of these methods. A wide range of particle sizes can be produced, and the size of the particles has a great influence on the effectiveness of the aerosol. The particle size determines the time the aerosol remains suspended in the air and therefore the amount of dispersion by air currents throughout the enclosure. The particle size is a critical factor influencing the amount that collects on an insect as it flies through the aerosol. If the particles are too small, they are deflected from the flying insect as smoke is from a moving automobile. If they are too large, they settle rapidly and their dispersion is poor; therefore their chance of contacting the insect is also poor. When an insect does collide with an oversized droplet, the excess insecticide is wasted. -2 - Settling Rate and Dispersion Aerosol particles tend to settle vertically at a rate related to their size. The time required for oil particles to settle 10 feet is given below. Diameter in microns Time Diameter in microns Time 1 26 1/2 hours 5 72 minutes 10 19 15 15 20 5 minutes 30 2 50 45 seconds 100 11 Water droplets settle slightly faster. The lateral dispersion of aerosols is accomplished by air currents after the small impetus from the atomizer is expended. Aerosol particles will not be conveyed into dead-end cracks or into materials through which air does not circulate. As would be expected, small particles disperse laterally to greater distances than do larger ones as they are suspended in air for a longer time. In unheated buildings air currents are at a minimum, but heating sets up convection currents that are a great aid to dispersion. In some cases it is necessary to aid dispersion with large-volume air blowers. Table 1 shows the dispersion of aerosols of various particle sizes in an unheated room with a ceiling height of 8 feet. Table 1. --Lateral dispersion of aerosols of various particle sizes released at the ceiling level in an unheated room 8 feet high Pe rcent of particles of indicated Feet from mass median diameter source 5 microns 15 microns 25 microns 45 microns 28 43 66 84 10 25 40 29 15 20 20 13 4 Trace 30 10 4 Trace 40 7 Trace 50 5 75 4 100 Trace i -3- Deposit About 95 percent of an aerosol settles on the top of horizontal surfaces, the remaining 5 percent deposits on the walls and ceiling. The amount of deposit on a horizontal surface depends on the concen- tration of the aerosol directly above that surface. Therefore, if the aerosol is evenly dispersed throughout a room, the deposit will be proportional to the distance of the surface from the ceiling. The deposit on walls and ceilings is mainly on small protrusions, such as fibers, on rough surfaces which catch the particles as they go by on air currents or by settling. Insects resting on the walls and ceilings will also be struck by an occasional particle as it passes by. Surfaces colder than the air temperature slightly attract very small particles. Selecting Particle Size No single particle-size range is suitable for all conditions where an aerosol may be used. The particle-size range should be selected after the factors involved in a proposed treatment have been evaluated. The most important factor is time. If the aerosol is to be applied to a structure or room that can be closed for several hours, small particles will give the best dispersion and penetration into small crevices. An aerosol with particles of about 5 microns mass median diameter was applied in a large warehouse. A thermal generator was operated outside of the building, introducing the aerosol through an open door. The aerosol dispersed along the ceiling, and by the time the proper amount had been applied it was well distributed throughout the warehouse by convection currents. The warehouse was then closed overnight and the aerosol allowed to settle. Uniform distribution resulted. When the treatment must be limited to a short time, larger particles are necessary. A 10- to 15-minute exposure is sufficient with an aerosol having particles with a mass median diameter of 15 to 20 microns, as these particles will settle 10 feet within 5 to 15 minutes. Where flying insects are to be controlled, this type of treatment is quite satis- factory. However, since lateral dispersion is restricted when particle sizes of this range are used, in large rooms or structures, the aerosol must be released from several points in order to give uniform dispersion. Many types of application will fall between these two extremes. The proper relation between time of exposure and particle size can be cal- culated from the tabulation on page 2. The allowable settling time should be based on the smallest particles. The approximate distances for uniform dispersion can be determined from table 1. Better dispersion can be obtained when heating sets up convection currents or when fans are used. - 4- UNIVERSITY OF FLORIDA 3 1262 09239 6331 After considering the foregoing factors, the operator can select the most desirable particle size to meet his need, and then the type of generator that will produce the desired particle-size range and volume of aerosol. If the available equipment will not produce the desired particle sizes, then the time will have to be adjusted to the particle-size range that can be produced. It has been demonstrated that particles above certain sizes, when com- posed of some solvents and insecticides used in aerosol formulations, will injure plant foliage. Persons treating greenhouse's with aerosols should keep this factor in mind when selecting the particle size. Some structures may be too open for the successfull use of aerosols because of too much loss through the wall openings. For instance, in large open warehouses full of tobacco hogsheads, where an aerosol could not be contained in the structures, it was found that a mist spray with particles of about 50 microns mass median diameter, blown over the top of the hogsheads by a mist blower, gave a uniform deposit of insecticide and good insect control. Formulations We have used the following formulations indoors: 1 pound of technical DDT dissolved in 7 1/2 pints of Sovacide 544C (Socony Vacuum) to make 1 gallon. 1 pound of technical DDT dissolved in 2 quarts of carbon tetra- chloride and then 3 1/2 pints of Texaco 300 oil added to make 1 gallon. (This material is noxious to breathe but relatively safe from explosion.) 2 quarts of a 10-percent pyrethrum in deodorized kerosene with 1 quart of piperonyl butoxide and 1 quart of Texaco 300 oil added to make 1 gallon. Owing to the explosion hazard when oil solutions are used indoors, not more than 1 gallon of these solutions should be used per 100,000 cubic feet, and they should not be released in the vicinity of an open flame. All work indoors should be done while wearing a proper respirator. The pyrethrum formula is recommended for use around exposed foodstuffs. Some formulations contain a proportion of relatively nonvolatile oil to maintain the desired particle size while it is suspended in the air. Dosage The dosage will vary according to the insect and the insecticide used. Because of better dispersion, rooms with high ceilings require a smaller dosage than those with low ceilings. In closed warehouses there are indications that 1 pound of DDT in 1 gallon of solution per 6,500 square feet of floor space applied about every 2 weeks in summer will provide excellent protection of commodities against insect infestation.