Face masks are widely considered by infectious disease experts to be a key layer of protection against catching or spreading SARS-CoV-2, the virus that causes Covid-19. A new study confirms this consensus and provides helpful details for buying or making a mask. Well-fitting masks can easily trap 50% or more of infectious particles, according to a series of lab tests led by Linsey Marr, PhD, an aerosol expert at Virginia Tech. Masks made of two or three layers of cotton and other filtering materials can capture up to 90% of the particles. The effectiveness was similar for exhaling and inhaling, Marr and her colleagues found. The findings are detailed in a preprint paper that has not yet been peer-reviewed. But several scientists not involved in the work touted its validity and importance. “This detailed lab study shows how masks can be effective bi-directional filters when worn properly,” says Kimberly Prather, PhD, an aerosol expert and atmospheric chemist at the Scripps Institution of Oceanography. “Such careful lab studies provide much needed critical mechanistic insights into why masks are critical for reducing the spread of SARS-CoV-2,” says Prather, who was not involved in the study. “In addition, this study provides guidance on the best materials and number of layers that are most effective in filtering micron-sized aerosols that could contain infectious virus.”