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-
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.
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
I ------ 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
Percent of particles of indicated
Feet from mass median diameter
5 microns 15 microns 25 microns 45 microns
0 28 43 66 84
10 25 40 29 15
20 20 13 4 Trace
30 10 4 Trace
40 7 Trace
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
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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 greenhouses with aerosols should
keep this factor in mind when selecting the particle size.
Some structures may be too open for the successful 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.
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.
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.