Physical exertion, such as sport for instance, raises the oxygen requirement: During exercise 10 times as much air is inhaled compared to when resting.
To achieve an optimal functional capacity it is important to breathe through the nose for as long as possible. The nose is, however, very narrow in the region of the nasal vestibule, so that the raised oxygen requirement can only be breathed in through the nose by considerable effort exerted by the respiratory muscular system.
Therefore most athletes shift under exertion from nasal to mouth breathing. But at the same time important functions of the nose such as cleansing, warming and moistening the air, are thus also suspended. Then as well as disturbances of the respiratory system, health risks are imminent!