Question

why respiration through mouth is harmful?

Answer 1

because mouth does not have hair cells (which are in nasal vestibule)
so breathing from mouth cannot prevent dust particles entering in mouth.

Answer 2

As respiration through nose results in dust particles sticking through the hair in our nostrils and mucus, thus, keeping our lungs safe...But while respiring through mouth, the dust is not trapped anywhere, thus the dust goes to our lungs or gets stuck in our larynx...Thus harming us...