Our physiological design is to breathe through the nose, and i know we can breathe through the mouth. But for that reas, mouth breathing is a long term, quite dangerous,. If you're in an environment that's very dusty your opportunity to breathe has been magnified compared to the nose. So then there is this misconception that if i push myself through exercise, that i have to breathe through my mouth. The reason that you're doing it is because there's a part in your brain called the brain stem, right at the back of your neck. Its reading not oxygen, its read carbon dioxide. It's, in fact, i's reading hydrogen irons, but basically as carbon dioxide