Oxygen demand from exercise is only one reason for mouth breathing.
Humans, as well as most mammals, have the ability to breathe through their nose as well as through their mouths. Breathing through the nose is the accepted norm. Breathing through the mouth, however, can be normal depending on why it's happening. There are several reasons people breath through their mouth.
More Air
When you exercise strenuously, your body needs extra oxygen. To increase the air flow into and out of your lungs, it's natural to supplement the flow through your nose by opening your mouth and breathing through both nose and mouth. This should be a temporary condition and once the oxygen demand returns to normal, you should be able to supply your lungs with ample amounts of air through your nose.
Sinus Conditions
Head colds and other sinus conditions can cause swelling of sinus membranes as well as excess mucous in your sinuses. The result can cause the airways routing through your nose and sinus passages to become partially or completely occluded. Breathing through the mouth is the only way to inhale enough air. Breathing should return to normal after the infection or allergy has passed.
Odor Avoidance
Most of the odor receptors that allow you to smell are located in your nose so smelling is a normal part of breathing. In situations where intense or extremely malodorous smells are present, consciously switching from nose-breathing to mouth-breathing can help you avoid the unpleasant odors.
Habit
Some people mouth breath out of habit rather than any physical need. This is not an indication of any mental incapacity. Those who are aware of their propensity to mouth breath are able to stifle the habit most of the time. These individuals may be prone to lapse into mouth breathing when their attention wanes or they are otherwise focused and concentrating on a specific task at hand.
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