Effect of Nitric Acid Vapor on the Response to Inhaled Ozone

Abstract

Studies concerned with the adverse health effects of acidic atmospheres have focused primarily on sulfuric acid aerosols and other acid sulfates. Little information is available on the health effects of nitric acid (HNO₃), an important atmospheric pollutant in southern California. The purpose of this study was to investigate the effects of exposure to nitric acid vapor, alone and in combination with ozone (O₃), on the rat lung. Groups of rats were exposed once for 4 h to purified air, 0.6 ppm O₃, 7.0 mg/m³ HNO₃ or 0.6 ppm O₃ plus 1.0 mg/m³ HNO₃. Other rats were exposed for 4 days, 4 h/day to purified air, 0.15 ppm O₃ 0.25 mg/m³ HNO₃, or 0.15 ppm O₃ plus 0.25 mg/m³ HNO₃ The animals were lavaged 18 h after exposure, and cells and fluid were assayed for changes in lavage cell population and lavage fluid protein content, pulmonary macrophage respiratory burst activity and leuko-triene production, and elastase inhibitory capacity of lavage fluid. A separate series of rats was lavaged 2 h after exposure to high concentration atmospheres for measurement of lavage fluid pH and PCO₂ and macrophage intracellular pH. Exposure to 0.25 mg/m³ HNO₃ alone decreased spontaneous and PMA-stimulated respiratory burst activity in freshly isolated macrophages, while the high concentration mixed atmosphere was also found to cause a significant decrease in respiratory burst activity in freshly isolated macrophages, while the high concentration mixed atmosphere was also found to cause a significant decrease in respiratory burst activity of macrophages maintained overnight in culture. HNO, at both concentrations resulted in a modest but significant increase in elastase inhibitory capacity of lung lavage fluid. Exposure to 0.6 ppm O₃ resulted in an increased number of neutrophils in the lavage cell population as well as in increased lavage fluid protein content and elastase inhibitory capacity These changes were also observed following exposure to 1.0 mg/m³ HNO₃ plus 0.6 ppm O₃ although not to the same extent as following O₃ alone. Linear regression analysis indicated that an antagonistic interaction occurs between HNO₃ and O₃ for changes in lavage cell population, lavage fluid protein content, and elastase inhibitory capacity.