SD ee 
Settling Rate and Dispersion 
Aerosol particles tend to settle vertically at a rate related to 
their sizes The time required for oil particles to settle 10 feet is 
given below. 
Diameter Diameter 
in microns Time in microns Time 
1 - - - - 263 hours 20 - - - - 5 minutes 
5 - - - - 72 minutes 30 ----2 
10 ----19 50 - - - - 45 seconds 
15 --=--15 100 - - - = ll 
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. Some- 
times 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 
size released at the ceiling level in an unheated 
room 8 feet high 
Percent of particles of indicated 
Feet from mass median diameter 
15 microns 25 microns 45 microns 
0) 28 43 66 8h 
10 25 40 29 15 
20 20 13} 4 Trace 
30 10 4 Trace 
LO 7 Trace 
50 5) 
75 A 
