Pulmonary epithelial cells are continuously subjected to a variety of oxidant stresses. Of substantial concern are the potential health impacts induced by the environmental oxidants ozone (O3) and nitrogen dioxide (NO2), which likely contribute to the pathogenesis of lung disease. The lung surfaces are covered by an aqueous layer (epithelial lining fluid; ELF) which inhaled gases first encounter. The ELF is a complex mixture that contains significant concentrations of small molecular weight antioxidants, principally ascorbic acid (AH2), glutathione (GSH), and uric acid (UA), that directly react with O3 and NO2 during their absorption. The standard paradigm proposes that ELF antioxidants provide a protective screen against the injurious effects of inhaled oxidants. However, because (i) injury unequivocally occurs during exposure despite the high antioxidant concentrations, and (ii) recent data suggests that antioxidants may themselves participate in the cascade(s) leading to membrane damage, the applicant proposes that the dynamics of ELF antioxidants critically mediate the balance between oxidant quenching and production of secondary bioactive species. Thus, if ELF antioxidants cannot be maintained in sufficient concentration, pro-oxidant conditions may develop within the site-specific regions where airway injury occurs. It is hypothesized that the efflux and turnover of ELF antioxidants are rate-limiting determinants governing the epithelial toxicity of inhaled environmental oxidants. This hypothesis will be addressed by an interrelated progression of experimental aims utilizing the application of labeled antioxidants and tightly controlled investigational models to elucidate the kinetics of AH2, GSH, and UA appearance (efflux) and turnover within the ELF. Accomplishment of the proposed aims will: 1) provide novel information regarding ELF antioxidant kinetics and define rate limitations during inhaled O3 and NO2 exposures, 2) quantify the associations among exposure-induced epithelial injury, oxidant dosimetry, and antioxidant kinetics, and 3) establish new mathematical paradigms that, based on oxidant and antioxidant ELF flux rates, will reveal new information regarding mechanisms governing the pathogenesis of oxidant lung injury, potential basis of differential susceptibility, and the importance of lung surface phenomena in predicting human health effects.