It's chronic, habitual mouth breathing that is so bad for our health .. When we breathe through our noses, we humidify air, we heat it, and we condition it so that by the time it reaches our lungs, it's treated—it's purified in some ways. When you breathe through your mouth... you get none of those benefits. You can think of the lungs as an external organ when you're breathing through the mouth—they're just exposed to allergens, and pollution, and whatever viruses, whatever else is in the air. Our nose is our first line of defense against all these things.