On the retention of air-borne particulates in the human lung.

Penetration and retention of various particulates was measured in the respiratory tract of two human volunteers by impingement. Data was related to particle size. There is a particle diameter giving minimum retention, i.e., higher retention with larger particles because of settling and with smaller particles because of Brownian diffusion. Increasing tidal volume or decreasing respiratory rate (for equal flow) increases retention. More smaller particles are retained with increased flow.