Masks also “become quickly wet” and wadded as we huff into them heavily while exercising and the moisture in our breath collects there, says Dr. Louis-Philippe Boulet, a professor of cardiology and pulmonology at Laval University in Quebec City, who has studied asthma in athletes. Drawing in breaths through damp cloth tends to feel more strenuous than when it is dry. Worse, he says, wet masks “lose antimicrobial efficiency.”

And then there is the oozing. “Exercising in a face mask will create a warm and humid microclimate around your face” as the mask traps your exhaled breaths, says Dr. Grant Lipman, a clinical professor of emergency medicine at Stanford University who studies extreme athletes and wilderness medicine. In effect, the mask turns the bottom half of your face into a “mini-sauna,” he says, leading to a buildup of sweat under the mask and a related rise in nasal secretions.

The result can feel “unpleasant,” he says, if, like so many of us, “you find the sensation of mucus pouring down your face to be unpleasant.” When he and his colleagues studied the effects of wearing a facial covering at night to make breathing more difficult and feign being at altitude, almost half of the participants reported that they could barely sleep because of the “copious nasal secretions” produced under their masks, he says.

Taken as a whole, research and experience show that “running with a mask is clearly different compared to running without a mask,” Dr. Eijsvogels says.

Will my glasses fog up?

Probably, says Morten Hostrup, an associate professor of physiology at the University of Copenhagen. “It depends on the size of the mask, the intensity of the breathing, and the size of the glasses,” he says.

Facial coverings that are loose around the nose, allowing warm, wet air to flow upward, will probably cause the most fogging, especially if your glasses sport large lenses and frames that rest snugly against your cheeks. You might be able to reduce any misting by washing the lenses with soapy water before slipping on your mask, according to an advisory for bespectacled surgeons that was published in 2011 in the Annals of the Royal College of Surgeons.

So, what kind of face covering is best for outdoor exercise?

That choice ultimately requires a difficult balancing of concerns about infection control and discomfort, the experts say.