Look at the deposition pattern of inhaled particles of 5 µm diameter.
Particles whose diameter is > 5 µm deposit preferentially in large airways as they are heavy and impact on the mucosa during directional changes in airflow. A small portion of particles > 5 µm diameter pass into the large central airways, but impact upon the mucosa of the airway wall at bifurcations of large intrathoracic airways due to their high speeds. Distal from generation 4, the total cross-sectional area of intrathoracic airways increases rapidly, the particle velocity rapidly decreases, and impaction is no longer contributes importantly to particle deposition. Now sedimentation due to gravitational forces, as well as the progressively smaller distances to the airway mucosa (diffusion) contribute increasingly towards particle clearance from inhaled and exhaled gas.