A randomized clinical trial in 2008, for instance, found that families who used paper face masks after their children had visited the ER with a flu-like illness were about 80 percent less likely to have other members become sick than families who didn’t use masks, provided they used them as often and correctly as they should. Other studies of college dorms and households have found a similar protective effect, though people in these studies were also asked to practice good hand washing hygiene.

But you shouldn’t overestimate the effectiveness of a surgical mask, especially if you’re not sick and are just trying to avoid other people’s airborne germs.

“They are not designed to protect the wearer from others. Instead, they are designed to protect others from the wearer’s own expectorated droplets,” Robert Amler, a former chief medical officer at the CDC’s Agency for Toxic Substances and Disease Registry and currently a dean at New York Medical College, told Gizmodo via email.