These can kill us if they are not immediately allowed to spread out and get diluted by the atmospheric air. Hence the necessity of proper ventilation. The air comes into close contacts with blood in the lungs and purifies it. Many people do not know the are of breathing. This defect prevents an adequate purification of their blood.
Some people breath through their mouth instead of their nose.
This is a bad habit, Nature has so designed the nose that it acts as a sort of filter for the ingoing air and also warms it. In mouth breathers the atmospheric air reaches the lungs without the preliminary filtration or warming. It follows therefore that those who do not know how to breathe should take breathing exercises. They are as easy to learn as are useful. I do not wish to go into a discussion of the various asanas or postures.