Despite widespread use of face coverings to protect against the transmission of COVID-19, there have been few evaluations of the efficiency of face coverings or mask enhancements at filtering airborne particles. Using an approach based on the U.S. Occupational Safety and Health Administration Fit Test, researchers evaluated the fitted filtration efficiency (FFE) of various consumer-grade and improvised face masks, in addition to several modifications of medical procedure masks.

The tests were conducted in an exposure chamber that reflects typical indoor conditions. The consumer-grade masks and medical procedure mask modifications were tested as personal protective equipment against an aerosol of small (0.05 micrometers) sodium chloride particles. The masks were fitted on a man with no beard. FFE measurements were recorded during a series of repeated movements of the torso, head, and facial muscles. The researchers tested several types of consumer-grade masks, including bandanas; gaiters; and single-, double-, and triple-layer masks with ear loops or ties. Five medical procedure mask modifications were also tested. These modifications included fastening ear loops behind the head with a claw-type hair clip and tying the ear loops and tucking in the side pleats.

The mean FFE of consumer-grade masks ranged from 79% to 26.5%; the washed, two-layer woven nylon mask had the highest FFE, and the three-layer woven cotton mask had the lowest. Unmodified medical procedure masks with elastic ear loops had a mean FFE of 38.5%, according to the researchers, which was a lower score than that of medical-surgical masks with tie strings (71.5%). Modifications that improved the fit of the mask to the wearer's face increased the mean FFE by a range of 60.3% to 80.2%. The authors point out that although all the modifications tested improved FFE, not all were comfortable or practical for extended use. It's important, they write, to choose a modification that isn't a deterrent to wearing the mask for prolonged periods.

All masks were tested on a single individual, and differences in facial features may affect mask fit and FFE. Also, the size of the sodium chloride particles used in the study may not reflect the most penetrating size for all the mask materials tested.