Inhaling through the nostrils is preferred over the mouth because

Explanation: Breathing through the nose is the normal thing to do, and it has two major advantages over mouth breathing:

1. filtration of particulate matter by the vibrissae hairs, and

2. better humidification of inspired gas.

Breathing through the nose allows for better conditioning of the inhaled air, because the nostrils are much smaller than the mouth, the air that is exhaled through the nose creates a back flow of air (and oxygen) into the lungs. Also, because we exhale more slowly through the nose than we do though the mouth, the lungs have more time to extract the oxygen from the air that we have already taken in.