Fig. 3

Filtration efficiency of N95-type masks using ambient particles and aerosolized KCl particles as testing agents. Mask filtration efficiency at 0.3 μm particle size is shown. Black lines denote standard deviations of filtration efficiency measurements; full data are provided in Additional file 4. Masks are grouped based on the testing standard they comply with (FFP2, KN95, or N95) but some masks incorrectly claim compliance with multiple standards. “NIOSH N95” refers to masks appearing on the list of “NIOSH-Approved N95 Particulate Filtering Facepiece Respirators” and regulated according to U.S. standards; six of these models are manufactured in the US and the Rizhao and San Huei masks are manufactured in China; all of these masks were available on the US market prior to the COVID-19 pandemic. “Appendix A” refers to masks that are listed in the FDA EUA “Non-NIOSH Approved Disposable Filtering Facepiece Respirators Manufactured in China,” first issued on April 2020 and subsequently updated