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Deposit 
About 95 percent of an aerosol settles on the top of horizontal sur- 
faces, and the remaining 5 percent on walls and ceilings. The amount of 
deposit on a horizontal surface depends on the concentration of the aerosol 
directly above the 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 ceiling is mainly on 
small protrusions, such as fibers, and on rough surfaces which catch the 
particles as they go by on air currents or in settling. Insects resting 
on the walls and ceiling 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 are 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. For example, an aerosol with 
particles of about 5 microns mass median diameter was applied in a large 
warehouse. A thermal generator was operated outside the building and the 
aerosol was introduced 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 treatment must be limited to a short time, larger particles 
are necessary. A 15-minute exposure is sufficient with an aerosol having 
particles with a mass median diameter of 15 microns, since most of the 
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 particles 
of this size are used, in large rooms or structures the aerosol must be 
released from several points to give uniform dispersion. 
Many types of application require particles between 5 and 15 microns 
in diameter. The proper relation between time of exposure and particle 
size can be calculated from the tabulation on page 2. The allowable 
settling time should be based on the smallest particles. The approxi- 
mate distances for uniform dispersion can be determined from table l. 
Better dispersion can be obtained when heating sets up convection 
currents or when fans are used. 
