Brownian diffusion, inhaled particles

Particle deposition in the respiratory system is related to distinct physical mechanisms operating on inhaled particles. The most important of these mechanisms are gravitational sedimentation, impaction by inertial forces, and brownian diffusion (see Fig. 3). Electrostatic forces and interception, the latter being significant only for fibers, are generally less important. In this section the physical mechanisms are briefly introduced. Several excellent overviews are available in the literature (12-14). 

The inhalation airflow comes to a rest in the alveolar region. In still air, the collision of gas molecules with each other results in Brownian motion. The same happens with sufficiently small particles (which can be seen when the dust particles in a nonventilated room are hit by a sunbeam). For very small or ultrafine particles (when the particle size is similar to the mean free path length of the air molecules), the motion is not determined by the flow alone but also by the random walk called diffusion. The diffusion process is always associated with a net mass transport of particles from a region of high particle concentration to regions of lower concentration in accordance with the laws of statistical... 

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