Q. How effective are antimicrobial “courtesy masks” at preventing the spread of contagious airborne illnesses?

A. The best evidence suggests that, when sick, wearing a mask can help to protect others from getting sick. And when well, wearing a mask around those who are sick will probably decrease your own chances of becoming infected. But the masks are far from foolproof.

Courtesy masks, or what we doctors refer to as surgical masks, were introduced into the operating room in the late 1800s. They quickly became popular among a public eager to protect itself against the influenza pandemic of 1918.

A century later, the advent of modern molecular techniques confirmed that surgical masks can indeed provide good protection against flu. In a 2013 study, researchers counted the number of virus particles in the air around patients with flu. They found that surgical masks decreased the exhalation of large viral droplets 25-fold. The masks were, however, less effective against the fine viral droplets that can remain suspended in the air longer and are therefore more infectious, cutting them by 2.8 times.