We examined interactions in rats between NO2 gas and respirable aerosols of sulfuric acid (H2SO4) or sodium chloride (NaCl). Rats were exposed for 1, 3, or 7 days to 5 ppm of NO2 gas, alone or in combination with 1 mg/m3 of H2SO4 or NaCl aerosols. The apparent rate of collagen synthesis by lung minces was measured after 7 days of exposure, and the protein content of whole lung lavage fluid was measured after 1 or 3 days of exposure. Responses from rats exposed to 5 ppm of NO2 alone were significantly different from controls by these assays. A synergistic interaction was demonstrated between 5 ppm of NO2 and 1 mg/m3 of either H2SO4 or NaCl aerosol as evaluated by measurement of the rate of lung collagen synthesis. A synergistic interaction was also demonstrated by the criterion of increase protein content of lung lavage fluid in rats exposed to 5 ppm of NO2 and 1 mg/m3 of H2SO4 aerosol after 1 day of exposure and between 5 ppm of NO2 and 1 mg/m3 of NaCl aerosol after 3 days of exposure. These observations with 5 ppm of NO2 alone and in combination with 1 mg/m3 of NaCl aerosol support the hypothesis that formation of nitrosyl chloride may contribute to a synergistic interaction between NO2 gas and NaCl aerosol. These results suggest that, in general, combinations of oxidant gases with respirable acidic aerosols or with acidogenic gases will demonstrate interactive effects on rat lungs. Such a hypothesis is testable and makes specific predictions about effects of inhalation of pollutant mixtures.
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