What is the reason why mouth breathing is forbidden during exercise in cold season?

In the cold season, whether during exercise or at ordinary times, we should develop the habit of breathing through the nose. Because there is a lot of hair in the nostrils, it can filter the air and protect the trachea and lungs from dust and germs. Exercise in winter, the air temperature is low, the cold air has been preheated when it passes through the nasal cavity, and then it will not produce strong stimulation when it enters the lungs. Breathing through the mouth makes cold air directly enter the lungs, which produces strong stimulation and causes adverse consequences. Of course, when the activity is intense, breathing through the nose alone can't meet the body's demand for oxygen. At this time, you can use the method of mouth and nose breathing, but don't open your mouth too wide to prevent inhaling too much cold air.