Sometimes when wearing a mask I found myself slipping into mouth breathing. I wondered if this was more dangerous than nasal breathing. I found some articles that suggest breathing through the nose is prudent. Nitric Oxide is produced in the nose and may provide some protection against viruses. Of course it isn't going to fully protect you but when given a choice between nasal breathing and mouth breathing, the former is clearly better. I imagine other people wearing masks might also slip into mouth breathing to get more air. But if everyone consciously avoided that I wonder how much a difference this would make to infection rates.
Some references here:
Nitric Oxide (not to be confused with Nitrous oxide) is also being tested in treatments as well as a preventative for healthcare workers.