Mouth breathing is a health hazard and unnatural. The nose filters air, warms it, and prepares it for the lungs. There is an instilled, nearly intractable belief that mouth breathing helps get more oxygen because you can take in more air, but it is the amount of C02 you can conserve and tolerate that regulates oxygen to the cells. An excellent recent book, "Breath" (noted below in comments) as well as "Oxygen Advantage" (P. Mckeown) and "Breathe to Heal" (Yakovleva, Novozhilov) are requird reading on this topic. The Buteyko Method teaches that breathing less is the first step to improving overall oxygenation--paradoxical but proved. As Dr. Buteyko put is, "Breathe through your mouth as often as you eat through your nose." Learning to breath correctly, beginning with 100% nasal breathing, will change your health and life!