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Abstract
Respiratory tract injury resulting from inhalation of mixtures of ozone (O3) and nitrogen dioxide (NO 2 ) and of O 3 and formaldehyde (HCHO) was studied in Sprague‐Dawley rats under exposure conditions of rest and exercise. Focal inflammatory injury induced in lung parenchyma by O 3 exposure was measured morphometrically and HCHO injury to the nasal respiratory epithelium was measured by cell turnover using tritium‐labeled thymidine. Mixtures of O 3 (0.35 or 0.6ppm) with NO 2 (respectively 0.6 or 2.5 ppm) doubled the level of lung injury produced by O 3 alone in resting exposures to the higher concentrations and in exercising exposures to the lower concentrations. Formaldehyde (10 ppm) mixed with O 3 (0.6 ppm) resulted in reduced lung injury compared to O 3 alone in resting exposures, but exercise exposure to the mixture did not show an antagonistic interaction. Nasal epithelial injury from HCHO exposure was enhanced when O 3 was present in a mixture. Mixtures of O 3 and NO 2 at high and low concentrations formed respectively 0.73 and 0.02 ppm nitric acid (HNO 3 ) vapor. Chemical interactions among the oxidants, HNO 3 , and other reaction products (N 2 O 5 and nitrate radical) and lung tissue may be the basis for the O 3 ‐NO 2 synergism. Increased dose and dose rate associated with exercise exposure may explain the presence of synergistic interaction at lower concentrations than observed in resting exposure. No oxidation products were detected in O 3 ‐HCHO mixtures, and the antagonistic interaction observed in lung tissue during resting exposure may result from irritant breathing pattern interactions.