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The onset of the COVID-19 pandemic in spring 2020 resulted in a spike in the demand for face masks and respirators. Due to their effectiveness at filtering aerosols that could potentially contain viruses, the N95-type filtering facepiece respirators (FFRs) are frequently used by healthcare workers and first responders. However, due to a shortage of domestic N95 FFRs in the US at the beginning of the pandemic, internationally produced respirators were imported and deployed under an Emergency Use Authorization by the Food and Drug Administration. Due to concerns raised at the time, there was an urgent need to verify their effectiveness and usability. In this study, we summarize our characterization of the nanoparticulate filtration performances of 136 such respirators, measured between April 1 and June 30, 2020. Our results indicate that about 42% of the respirators showed filtration efficiencies better than 90% (≤10% penetration), but only 17% performed better than 95% (≤5% penetration). On the other hand, about 35% showed filtration efficiencies below 80% (≥20% penetration). A representative subset of devices was analyzed for the origin of such variations in filtration performance. We found that filtration efficiency increased with the level of electrostatic charge on the FFRs and that the poor performance of the internationally sourced FFRs could be traced to a lack of electrostatic filtration mechanisms. Furthermore, electrostatics shifted the particle size at which aerosol penetration through the FFR was maximal from around 200 nm to less than 100 nm for the highest-performing FFRs, a size range that largely goes undetected in standardized tests.

Examination of Nanoparticle Filtration by Filtering Facepiece Respirators During the COVID-19 Pandemic