Before the air enters the lungs it must be warmed and moistened. Any impurities present, such as dust, must be screened out, otherwise we would soon be ill with pneumonia or some other serious infection.

So we have been provided with a most remarkable air-conditioning system within the nose. Here the dust is screened out and the air is moistened and warmed to the right temperature and humidity required by the lungs. If the weather is hot and dry, more fluid must be poured out within the nose and added to the air before it reaches the lungs. On cool, moist days the reverse will take place.

image courtesy: www.merck.com