A new method of filter efficiency evaluation using the nanoparticle surface area monitor (NSAM) for a nanoparticle health relevant filter efficiency measure

Lung deposited surface area (LDSA) has been identified as an important metric for determining the toxicity of an aerosol. Surface area is especially important when the aerosol is composed of nanoparticles due to their high surface area to mass ratio. When a filter is used to remove nanoparticles from the air to reduce human exposure, it is important to rate the filter based on surface area, or LDSA, to obtain a more health relevant filter efficiency value. The Nanoparticle Surface Area Monitor (NSAM) was developed to measure LDSA in real-time. In this study, we used the NSAM and a Scanning Mobility Particle Sizer (SMPS) to sample upstream and downstream of a test filter, both gave very consistent LDSA measurements. The LDSA based filter efficiency values showed a discrepancy of only 0.3 to 1.8% between the two instruments, demonstrating the capability of the NSAM for evaluating filter performance based on LDSA. Both laboratory and field studies show that when dealing with an aerosol mainly composed of nanoparticles, the LDSA based filter efficiency is closer to the minimum filter efficiency than the number based efficiency. This LDSA filter efficiency measurement also represents a more health relevant filter evaluation.