Data obtained from animal exposures may, if appropriately extrapolated, be used to quantitatively assess the health risks of humans chronically exposed to environmental airborne pollutants. The ability to make correct extrapolations is critical in making full use of the results of acute and chronic exposures in laboratory animals. Extrapolation of experimental results between different animal species requires several types of data, one of which is detailed data on the geometry of the lungs for each species in order to determine the quantity of pollutant that reacts (or is deposited) in the various anatomic regions (trachea, bronchi, bronchioles, terminal bronchioles, proximal alveolar and distal alveolar regions). The existing data on the geometry of the lungs is almost exclusively concerned with the geometry of the major airways. As least in part the focus of previous work on the major airways reflects the fact that they are the major determinant of airway resistance. Following the alveolar ducts as they branch out from the bronchiole-alveolar duct junction using serial lung sections is difficult. Advances in computer processing power have supplied the prodigious memory and appropriate algorithms to make the process of 3D reconstruction of the lung technically feasible to implement.