His team looked at 20 healthy people, average age 37, who ran on a treadmill to peak exhaustion while wearing an N95 mask, a cloth mask and no mask. None of the participants had any safety issues while working out in either type of mask, and monitors showed no abnormal heart rhythms or unsafe drops in oxygen. The main reason participants stopped running was due to mask discomfort, according to the study published online June 30 in JAMA Network Open.