The influence of the particle size of homogeneous insecticidal aerosols on the mortality of mosquitoes in confined atmospheres

Abstract

Cooperative tests were made between the Bureau of Entomology and Plant Quarantine, and the Central Aerosol Laboratory, Columbia University, to compare the mortalities produced in confined atmospheres by aerosols of different sized particles. Apparatus for the generation of homogeneous aerosols, and instruments and methods of measuring the particle size and concentration, which had been developed by the Central Aerosol Laboratory in previous studies related to screening and other smokes, were utilized. Exposures of 10–30 minutes in aerosol concentrations of around 100 γ/l. were necessary to produce high mortalities when particles of 1 μ or less in diameter were used. Exposure of 0.5–2 minutes in aerosol concentrations of around 25 γ/l. produced high mortality when particles were 5–16 μ in diameter. When Ct (concentration of oil in γ containing 8% DDT multiplied by time of exposure in minutes) values which produced 50% mortality of females (calculated by graphical interpolation) were plotted against particle size, the toxicity was shown to be increased by 250 or more times by increasing the diameter of the particle from 0.33 to 11 μ. This curve corresponds to a curve in which the time necessary for an aerosol to settle out of 1 cc. is plotted against particle size, as calculated according to Stokes' Law and Cunningham's correction. It also shows that the toxicity is proportional to the rate of deposition which, in turn, is proportional to the square of the radius (or diameter) of the aerosol particle for the sizes discussed.