From the comments section. "Pilots in the United States aren’t allowed to wear beards because they compromise the seal of oxygen masks on the face, primarily used for emergency decompression and for fire and smoke. Without a good seal, even an N95 won’t provide proper protection, which is why most include instructions for achieving a proper fit. In general, having two bands going around your head makes it far easier to get a good seal than with ear loops.
Respirators are nice; it’s amazing to handle chemicals without smelling them, but they are heavy and uncomfortable to wear for long." I have a beard, and I don't believe the masks I've worn would do anything to prevent exposure to a respiratory virus.