Question

why is exhaled air is free of dust

Answer 1

Tiny hairs called cilia protect the nasal passageways and other parts of the respiratory tract, filtering out dust and other particles that enter the nose through the breathed air. ... The pharynx is part of the digestive system as well as the respiratory system because it carries both food and air.

Answer 2

because when we inhale the air it gets filtered by the nose hairs and when it reaches our pharynx area mucus traps all the dust particles

and so is the case is in exhalation so the air we exhale is dust free