Fig. 2

Images of a subset of masks subjected to performance testing and manufactured in China. a. A dome-type mask manufactured to N95 standards and listed on the NIOSH website for sale in the US that has all of the required markings. This mask performed as expected (Fig. 3, label # 24). b. A flat-fold mask that claims compliance with European FFP2 but contains an FDA logo, which is not allowable. This mask performed well in our tests across all particle sizes and has the performance expected of a legitimate product (Fig. 3, label #2). c. A flat-fold mask supplied in bulk with no markings other than the embossed KN95 label; this mask had negative filtration efficiency, and more particles were detected at the output of our test apparatus than at the input (Fig. 3, label #12). Additional mask photographs are available in  Additional file 2.