In mid-April the City and County of San Francisco ordered its residents to wear face coverings when waiting in line, shopping, riding public transportation, and engaging in other social activities. Although the evidence is inconclusive, masks may prevent the spread of viruses from the wearer; multiple layers of fine woven fabric covering the nose and mouth filter aerosol droplets.
However, most “consumer masks” are designed to protect the wearer from inhaling dust, not filter exhalated breath. These covers have simple flapper valves and an unfiltered exhaust port. The square plastic piece in the center of N95 “valved” respirator dust masks, prevalent in California as protective during wildfires, don’t have an exhaust filter. Wearing these masks – including reusable “shop” and “dust” masks used by woodworkers and home renovators – won’t contain the COVID-19 virus. They’re suitable for protecting the wearer from inhaling particles, but don’t filter exhaled germs. Wearing an unfiltered respirator mask isn’t “better than nothing;” it makes the wearer a potential germ spreader.
Masks with an exhaust port may be effective against virus transmission if the port is filtered. Sieves can be added by stuffing tissue or toweling inside the mask, in front of the port, or closing it off entirely with tape.