Deposition Efficiencies of Radon Progeny in Human Airway Bifurcations

The dominant physical mechanism affecting deposition of submicron particles, such as ambient radon progeny, in human airways in Brownian motion (molecular diffusion). The mean displacement of a particle passing through a cylindrical airway under laminar flow conditions depends on the diffusion coefficient, which is inversely proportional to the thermodynamic (geometric) particle diameter, and the residence time, which is inversely proportional to the flow rate. Consequently, deposition by Brownian motion increases wiht decreasing particle size and decreasing flow rate. In bronchial airway bifurcations, however, a laminar inspiratory flow profile in the parent airway becomes distorted in the branching zone and in downstream daughter airways, creating significant secondary flow patterns ( 1,2). Since these secondary flows are generally directed toward the surfaces of the bifurcation, they can actively transport particles to the airway walls (convective diffusion), thereby increasing the probability of deposition by Brownian motion. Since the magnitude of secondary flows increases with rising flow rate, submicron particle deposition will also be enhanced at higher flow rates, which is at variance with the inverse dependence of deposition by Brownian motion on flow rate.