The deposition of particles within the airways and lungs causes dust diseases, such as asbestosis, mesothelioma and black lung disease. Deposition depends upon one of four actions: interception, impaction, sedimentation, and diffusion.
When a particle travels close enough to an airway surface that it touches the surface of the airway, it is intercepted or deposited there. This is typically what happens when an asbestos fiber or similar fiber enters the airways. The size of these fibers causes them to be deposited in the bronchial tree because they are too large to pass through the width of the bronchial tree branches.
During inhalation, some particles impact or stick to a surface in the respiratory system where there is a bend in the air passages. The likelihood of impaction depends on the air velocity and the particle mass. Velocities are greater in the upper air passages. During exertion, such as heavy physical labor, breathing rates and, hence, air velocities increase throughout the system, and air enters the lungs more deeply during exertion.
Some particles will settle within the bronchi and bronchioles due to gravitational forces and air resistance. Particles settle in "dead-space" areas of the respiratory system where air is nearly stagnant (not moving). Sedimentation is most commonly seen during shallow breathing when particles of a larger aerodynamic diameter enter the airways.
Diffusion is the most important mechanism for deposition in the small airways and alveoli. Smaller particles tend to move in a random motion and deposit on the lung walls mostly by chance. This movement is also known as the “Brownian Motion”. The effect increases as the size of the particles decreases.
Source: Canadian Centre for Occupational Health & Safety (CCOHS)
