The increase in pressure drop across glass HEPA filters has been measured as a function of particle mass loading using polystyrene latex particles (PSL). PSL particles in several different sizes were generated as challenge aerosols. For each particle size distribution, the specific resistance (K2) was calculated by measuring the mass of PSL particles loaded per unit area of filter and the pressure drop across the filters at a given filtration velocity. In all cases, the specific resistance of the filter cake increased as the aerodynamic mean particle diameter decreased at the same mass loading. This correlation equation was modified by using the lognormal conversion method suggested by Raabe [1971] for a polydisperse particle size distribution; then the modified equation was expressed as a function of geometric mean particle diameter and standard deviation which could be obtained by the measuring instruments (PDS 3603; TSI Inc.). The advantage of this approach over other methods is the use of a more convenient prediction of pressure drop, if we know the geometric mean particle diameter and standard deviation, which could be easily measured. The values of porosities, obtained from the pressure drop responses of loading in the filters using the Rundnick and First equation, were compared with other researches.