The three main mechanisms of deposition are inertial impaction, sedimentation and Brownian diffusion, the particle diameter determining the relative importance of these mechanisms.
Inertial impaction is the most important mechanism of deposition for particles greater than 5 pm in diameter25. If particles are large or are travelling at high velocities they may be unable to follow a change in direction in the airstream, for example in the upper airways at bifurcations, and hence they will impact on the airway walls. Impacted deposition is also enhanced where airways are partially obstructed at high flow rates, and by a turbulent airflow in the trachea and major bronchi. Sedimentation occurs when particles settle under gravity. The rate of settling is proportional to the square of the particle diameter (Stokes' law) and so becomes less important for small particles. Brownian diffusion is an important mechanism of deposition only for particles less than about 0.5 pm in diameter. The particles are displaced by random bombardment of gas molecules and collide with airway walls.
As a consequence of these diameter-sensitive processes, the deposition of particulates in the lung is highly dependent on their size (Figure 10.10). Droplets larger than 10 pm
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