CDC does advise health care workers working with SARS patients to wear a special mask called an N-95 respirator. But even these masks offer limited protection from coronaviruses. The name of the mask says it all. The “95” means the mask, if properly fitted—and that “fit factor” presents a big if—can filter out particles down to 0.3 microns 95 percent of the time. (A human hair is roughly 100 microns in diameter.) Human coronaviruses measure between 0.1 and 0.2 microns, which is one to two times below the cutoff.
[But even these masks offer limited protection from coronaviruses.]