Respiratory Deposition

:: Section 4

Deposition Mechanism

III. Diffusion/Brownian Motion

In the small airway where the distance is short and residence time is long, diffusion is an important mechanism for the deposition of small particles (< 0.5 μm). Brownian motion is the microscopic random motion of small particles due to the numerous random collisions by gas molecules. Macroscopically, we see the overall movement of particles from a high concentration region (in this case, the center of air stream) to a lower concentration region (in this case, the airway wall). Since it is caused by gas molecule collisions, the effectiveness of this mechanism increases as particle size decreases.