Sedimentation brings about deposition in the smaller bronchi,
the bronchioles, and the alveolar spaces, where the airways are small
and the velocity of airflow is low. As a particle moves through
air, buoyancy and the resistance of air act on the particle in an
upward direction while gravitational force acts on the particle in a
downward direction. Eventually, the gravitational force equilibrates
with the sum of the buoyancy and the air resistance, and the particle
continues to settle with a constant velocity known as the terminal
settling velocity. Sedimentation is not a significant route of particle
deposition when the aerodynamic diameter is below 0.5 μm