The retention of polycyclic aromatic hydrocarbons in the bronchial airways and in the alveolar region--a theoretical comparison.

Several experiments indicate that physical transport phenomena such as molecular diffusion and partitioning between aqueous and lipid phases have a profound influence on the pulmonary retention of polycyclic aromatic hydrocarbons (PAHs). Because the average distance of diffusion between the air interface and the capillary blood is only about 0.5 microm in the alveoli, whereas in the bronchi it probably exceeds 50 microm, there should be a fundamental difference between the bronchial airways and the alveolar region in the retention of PAHs. A theoretical model was developed to simulate the retention of lipophilic substances in the two regions of the lung. Results show that a substance like benzo[a]pyrene, a PAH, may be retained for hours in the bronchi, compared to less than 1 min in the alveoli. This predicted dramatic difference in retention could explain the characteristic, biphasic pattern in the pulmonary clearance of PAHs observed in many animal experiments, but more importantly, it could also explain the fact that human lung cancers occur predominantly in the bronchi, although only a small fraction of inhaled particles carrying PAHs are deposited there.