"You Probably Don't Need to Wear a Mask While You Run ....a very valid concern," says Dr. Sarah Doernberg, an associate Professor at University of California San Francisco who specializes in infectious diseases. "Covering your nose and mouth while you're exerting yourself may lead to other medical problems—and the fact is that your mask is going to get wet. As soon as the mask gets wet, it's not going to be effective anymore."