Fig. 1

Apparatus assembled from common components and used to test FFRs in this study. Details of the fabrication and use of this device for testing the filtration efficiency of N95-type masks using ambient particles and KCl aerosol particles can be found in the Methods and at http://cleanmask.org/setup. No legitimate FFR should demonstrate less than 95% filtration efficiency using this test, but testing performed with this apparatus is not sufficient to confirm adherence to U.S., European, Chinese or other regulatory standards. Such testing involves greater control over a wider range of test conditions and a formal approach to quality assurance and calibration.