In a video posted to Twitter, Dr. Maitiu O. Tuathail puts on six masks -- one on top of the other -- while hooked up to a machine that checks oxygen levels. With each mask he adds, his oxygen level never changes; the machine shows he's between 98 and 99 percent the entire time.
Tuathail's point is clear: While protective face masks significantly reduce the spread of the coronavirus, they don't stop oxygen from getting in and out.