Why simple face masks are unexpectedly efficient in reducing viral aerosol transmissions

During the current pandemic and in the past, shortages of high quality respirators have forced people to protect themselves with homemade face masks that filter poorly in comparison to N95 respirators 1-4 and are often designed in ways that makes them susceptible to leaks 5,6. Nevertheless, there is compelling epidemiological 7,8 and laboratory evidence 9-12 that face masks can be effective in impeding the spread of respiratory viruses such as influenza and SARS-CoV-2. Here we show that this apparent inconsistency can be resolved with a simple face mask model that combines our filtration efficiency measurements of various mask materials with existing data on exhaled aerosol characteristics. By reanalyzing these data we are able to reconcile the vastly different aerosol size distributions reported 13-19 and derive representative volume distributions for speech and breath aerosol. Multiplying filtration efficiency by those aerosol volumes, which are proportional to emitted viral load, shows that electrostatically charged materials perform the best but that even most uncharged fabrics remove > 85 % of breath and > 99 % of speech aerosol volume for exhaled particles < 10 {micro}m in diameter. A leak model we develop shows the best uncharged fabric masks are made of highly air-permeable and often thin materials reducing viral load by up to 45 % and 50 % for breath and speech, respectively. Less permeable materials provide reduced protection because unfiltered air is forced through the leak. This can even render some charged materials inferior to uncharged household materials. Our model also shows that thin fabric masks provide protection for the wearer from aerosols expelled by another person reducing inhaled viral load by up to 20 % and 50 % and if leaks are avoided up to 35 % and 90 % for breath and speech, respectively.