Determining the dose delivered to specific sites in the lungs is a critical first step in modeling the potential toxic effects of airborne pollutants. An important recent development in estimating site specific dosimetry has been combining sophisticated analytical models to determine the delivery of inhaled pollutants to the lungs with relatively simplistic anatomic models. These methods are particularly important in extrapolating experimental results in laboratory animals to predict effects in humans. The goal in such modeling efforts is to characterize the average response of the lungs. However, in evaluating lung pathology following low level, chronic exposures to reactive gases, one quickly becomes aware that the lesions are far from uniform. The presence of such patchy lesions resulting from chronic exposures of laboratory animals or lifetime exposures of humans to various environmental pollutants is a frequent finding. The observation of a diseased area of the lung immediately adjacent to a normal healthy region is so common that it almost stifles the question of why it occurs. (Copyright (c) 1989 by Academic Press, Inc.)