Airway Surface Irregularities Promote Particle Diffusion in the Human Lung

Current NCRP and ICRP particle deposition models employed in risk assessment analyses treat the airways of the human lung as smooth-walled tubes. However, the upper airways of the tracheobronchial (TB) tree are lined with cartilaginous rings. Recent supercomputer simulations of in vivo conditions (cited herein), where cartilaginous ring morphologies were based upon fibre-optic bronchoscope examinations, have clearly demonstrated their profound effects upon fluid dynamics. Since inhaled radionuclides of health effects concern such as radon progeny are entrained and transported by airstreams, their trajectories and deposition probabilities will be influenced by the very character (i.e. laminar or turbulent) of air motion and its local velocity profiles. A physiologically based analytical model of fluid dynamics is presented, focusing upon applications to particle diffusion within the TB tree. The new model is the first to describe particle motion while simultaneously simulating effects of wall irregularities, entrance conditions and tube curvatures. The results show that particle dose to the large bronchi may be underestimated by up to 35% if cartilaginous rings are not considered. The work has fundamental implications for modelling concepts and health effects applications. The findings indicate that caution must be exercised when extrapolating heat and/or mass transfer correlations derived from standard thermodynamic-hydrodynamic engineering problems to biological issues, and that human subject activity levels have pronounced effects on particle diffusion in the lung. For instance, the submicron particle deposition efficiency differences between a sedentary condition (i.e. office worker) and light activity (i.e. manual labourer) approach 40%. This study may explain the enhanced deposition by particle diffusion detected in replica case experiments and have salient implications for the clinically observed preferential distributions of bronchogenic carcinomas associated with inhaled radionuclides.