Quantification of local deposition patterns of inhaled radon decay products in human bronchial airway bifurcations.

Aerosol deposition studies with tracheobronchial casts and models have demonstrated that inhaled particles are preferentially deposited within transitional bifurcation zones, exhibiting hot spots in the vicinity of carinal ridges. The goal of the present study is to quantify the inhomogeneity of theoretically predicted deposition patterns by local deposition enhancement factors. First, inspiratory particle deposition patterns of unattached (1 nm), ultrafine (10 nm and 20 nm), and attached (100 nm and 200 nm) radon progeny within three-dimensional models of segmental bronchial airway bifurcations were simulated by a numerical fluid dynamics and particle trajectory model. Second, local deposition enhancement factors were computed by scanning along the surface of the bifurcation models with prespecified surface area elements. Maximum values and frequency distributions of local deposition enhancement factors of inhaled radon progeny were derived for different sizes of the scanning element in a "narrow" and a "physiologically realistic" bifurcation model and for two different flow rates (10 L min(-1) and 60 L min(-1) in the trachea). Computed enhancement factors indicate that cells located at carinal ridges may receive localized doses which are 20-40 times (1 nm) and 50-115 times higher (10 nm-200 nm), respectively, than the corresponding average doses. This may have important implications for the microdosimetry of inhaled radon progeny and the resulting lung cancer risk.