Look at the deposition pattern of inhaled particles.
Particles of 1 µm diameter have very little mass, and hence these small particles can easily follow a turbulent flow pattern without impacting on the airway wall. However, in small airways and alveoli the linear speed approaches zero and the small distances towards the lining wall favour deposition due to sedimentation (gravitational forces) and diffusion. The proportion of 1 µm particles settling in the airway is small, and the greatest proportion is exhaled.