Mouth breathing in cold, dry environments can dry nasal passages and increase susceptibility to infections.

“And when I say robust, what I mean is that if your mouth breathing, for instance in a cold, dry environment, I'm thinking from the time I spent in Cambridge, Massachusetts in winter at the T station. If you're I'm just imagining myself because it was so darn cold back there breathing through your mouth or having gone for a run, and your mouth isn't covered, or even if you're deliberately nasal breathing, you are drying out your nasal passages and making them more susceptible to infections.”