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Earlier this month, in a measure to slow the spread of the coronavirus in the United States, the Centers for Disease Control and Prevention that cloth face coverings be worn in public places where “other social distancing measures are difficult to maintain.” Although certain DIY masks are clearly more effective than others, the consensus seems to be that any kind of facial covering is . Such measures can help us protect ourselves, but are even more crucial in preventing unwitting, asymptomatic carriers from infecting others. Since the vast majority of us will have access to an old T-shirt, that wearing a mask to the grocery store is now a form of civic duty.

But while it seems sensible to cover up when we don’t have , what about when we are moving around outside? Should runners in densely populated areas also be wearing masks?

At first glance, the something-is-better-than-nothing rationale would appear to apply here as well. Since most runners (especially runners in cities) are still going to encounter other people, why not err on the side of caution? On the other hand, the CDC the use of masks for anyone who has trouble breathing. So is it a good idea for us to cover our airways with homemade filters during bouts of physical exertion?��Does wearing a mask while exercising merely substitute one health risk with another, by making it harder to breathe?

“I think that’s a very valid concern,” says , 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.” Although she was tentative on this point, Doernberg also says that running around with a wet rag on your face could potentially even exacerbate the problem of contagiousness. Dr. Louis-Philippe Boulet, a professor of cardiology and pulmonology at Laval University in Quebec, that “drawing in breaths through damp cloth tends to feel more strenuous,” and wet masks also “lose antimicrobial efficiency.”��

Doernberg added that, while there is still a lot we don’t know about whether COVID-19 can be transmitted via fleeting casual contact like passing someone in the street, the majority of infections seem to occur from more sustained moments of close contact—i.e. from spending several minutes in close proximity to someone who is infected. Nonetheless, she emphasizes that runners should make every effort to exercise in a physically distanced way. The ideal situation, she says, is running someplace where you don’t even need to think about wearing a mask, because you’re on your own.��

, who is an expert in airborne disease transmission at Virginia Tech University,��says that while she says that runners should maintain as much distance from others as they can, there is not enough evidence to mandate that they wear masks as well.��

“As an athlete, I think it would be difficult to run with a mask, and if you’re out in an uncrowded area and steer a wide path around others you might encounter, I don’t think there’s any need,” Marr says. “To my knowledge, there aren’t any studies on transmission between people who are walking, running, or cycling; researchers always look at people who are stationary, because understanding that situation is hard enough.” (Case in point: last week, of the potentially contagious droplets��that runners leave in their slipstream�� for its apparent lack of academic rigor after being widely shared online.)��

Indeed, one of the many challenges that public health officials currently face is figuring out how to set behavioral protocol for millions of people when so much about the current contagion remains unknown. As Harvard epidemiologist Bill Hanage , “We’re trying to build the plane while we’re flying it.”

So what should one do if the only running option available involves navigating semi-crowded spaces? If you’re one of those lucky people who can comfortably exercise with a mask without it devolving into damp, unbearable agony, then that could be the way to go. But you’d probably be better off just getting more creative with your route.��

As Doernberg puts it: “The overall message that I’m trying to send is that the most important thing is being physically distanced and cleaning your hands. The face covering is for situations where that is not possible, but you should not enter risky situations just because you have a face covering—it’s not a substitute.”��