Respirator particulate filter test methods were evaluated. Four brands of respirator particulate filters (dust and mist (DM), dust, fume, and mist (DFM), and high efficiency particulate (HEPA) filters) were tested for penetration using aerosols of di-n-octyl- phthalate (117840) (DOP), silica (7631869), dust, lead (7439921) fume, and sodium-chloride. Silica dust showed very little penetration indicating that it was a relatively insensitive indicator of the efficiency of respirator particulate filters. When lead fume was used, the DM filters showed a higher degree of penetration, 8.8 to 14.2 percent, than either the DFM or HEPA filters, 0.19 and 0.14 to 0.63 percent, respectively. Percentage penetrations for the sodium-chloride aerosol ranged from 0.03 percent for HEPA filters to 1.07 percent for DFM and 6.70 percent for DM filters. DOP penetration ranged from 0.001 percent for the HEPA filters to 88.1 percent for DM filters. Additional testing was done with the sodium-chloride aerosol to evaluate the effects of humidity and electric charge on short and long term (1 hour) penetration. Penetration of the sodium-chloride aerosol decreased with increasing humidity and increased when the aerosol charge was reduced by conditioning with a radioactive source. When exposed to the sodium-chloride aerosol for 1 hour, penetration at high humidity (90 percent) was sharply increased, compared with instantaneous penetration. Smaller increases were observed for lower humidities (10 and 50 percent relative humidity). The authors conclude that DOP and sodium-chloride aerosols can be used to measure the efficiency and degradability of respirator particulate filters reliably. The charge and relative humidity should be controlled when using sodium-chloride aerosols.