Knowledge of the total and regional lung retention of particles inhaled continuously by man over long periods can be useful in understanding the potential role of inhaled particles in the pathogenesis of lung diseases. Owing to practical and ethical considerations, however, little or no experimental information exists. A mathematical model of particle retention simulating environmental and occupational exposures has therefore been developed that takes into account particle deposition, tracheobronchial clearance, and two phases of alveolar clearance in the Weibel A anatomical lung model. The derived equations of retention kinetics predict retention of particles as a function of exposure time. For a continuous exposure (simulating environmental conditions) to 4 microns particles, the model predicts that retained particles approach an equilibrium between deposited and cleared particles with the 95% level being reached in 293 days. For an intermittent exposure (simulating occupational conditions) equilibrium is approached in five years. The whole lung burden of particles is predicted to be 9% of the total mass that entered the lung after a one-year environmental exposure and 1.5% after a 25-year occupational exposure. The equilibrium surface concentration and integrated dose of particles per airway generation predict enhanced risk to the pathogenic effects of inhaled particles in the large airways and respiratory bronchioles.
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