Question

why should we breathe through our nose not through our mouth

Answer 1

We should breathe through our nose not through our mouth Because the nostrils are smaller thanthe mouth, air exhaled through the nose creates a back flow of air (and oxygen) into the lungs. And becausewe 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.

Answer 2

We know that Air contains many types of $\textbf{dust and impurities}$, since nose have hair like structure inside it and mucus.

This thier filters the air and allow the moist and filtered Air to enter in lungs, which reduces chance of dancing lungs.

Such hair like structure is not available in mouth, so it can't filter such type of air.