Removal of particles from air is discussed in terms of three classes of filtering media--air-sampling, respirator, and air-cleaning. Plots of efficiency versus filtering velocity are presented. The pressure drop characteristics of the filters are discussed in terms of initial pressure drops of the clean filters, variations in pressure drop of the individual filters, and comparison of loading rates.

In 1949 Barad presented two solutions to the general diffusion equation. Basic in both solutions is the assumption that in very stable atmospheres a point source may be replaced by a vertical area of uniform concentration at a short distance downwind. This vertical area is considered to exist at the distance at which the plume finally "levels-off" and assumes a flat ribbon-like appearance. In addition if the distance over which diffusion takes place is limited to one or two miles and if only the stable atmosphere with its narrow range of eddy sizes is considered, the average diffusion co-efficient (K) has a signficance which it does not have in the case of neutral equilibrium.