The pulmonary epithelium is a cellular, avascular layer of tissue that is the first point of contact between the lung and inhaled pollutants. Previous research has indicated that altered epithelial permeability may be an early marker of subsequent lung damage. Dr. Crocker and colleagues at the University of California, Irvine sought to study the study the sites of epithelial injury in rat airways following inhalation of formaldehyde, ozone, and nitrogen dioxide. The rats were tested during rest and during exercise, given differing concentrations of the three pollutants, and under conditions of repeated exposure which might lead to progressive injury of adaptation. The purpose of these studies was to compare the permeability changes in three areas of the respiratory tract: the nose, trachea, and bronchoaveolar area.