Penetration of hepa filters by submicron aerosols

Abstract

The efficiency of High Efficiency Particulate Air (HEPA) filters is commonly measured with dioctyl phthalate (DOP) fog of 0.3 μm dia. Little is known about HEPA filter performance for smaller sizes because of the difficulty of producing submicron aerosols at high enough concentrations to give measurable penetrations. HEPA filter sheets and pleated filters were tested with ambient aerosol 0.025–0.050 μm number median diameter (n.m.d.) and gold and silver aerosol 0.005–0.026 μm n.m.d. at flow velocities from 4 to 70 cm/sec. At the standard test velocity of 3–4 cm/sec the penetration of the filter sheets for all particle sizes was much less than the standard penetration of 0.3 μm DOP. At 70 cm/sec a most penetrating size was observed at about 0.04 μm. The pleated filters are more retentive and could be tested with silver aerosol only since it could be generated at concentrations up to 10 7/cm 3 cc at the standard flow velocity for these filters of 3 cm/sec. Again the penetration was less than for DOP and particularly so for the smaller particle sizes. It is concluded that at the standard test velocity the efficiency of HEPA filters is much greater for the particle dia. range from 100 down to 1 nm than for 0.3 μm because of the increased effect of molecular diffusion.