Question

explain why it is healthy to breathe by nose and not by mouth

Answer 1

Because the nostrils are smaller than the mouth, air exhaled through the nose creates a back flow of air (and oxygen) into the lungs. And because we exhale more slowly through the nose than we do though the mouth, the lungs have more time to extract oxygen from the air we've already taken in.

Hope it will help you

Answer 2

Brcause in our nose, there are hair like structures which do not allow to breathe dust in our body. And if we breathe with mouth, it directly breathe oxygen with dust.