Tracheal epithelial cells isolated from rats exposed to 0, 0.12, 0.5, or 1.0 parts per million (ppm) ozone for six hours per day, five days per week, for one, two, or four weeks were examined in culture for increases in the frequency of preneoplastic variants. Although cells from these rats did not exhibit such increases, exposed tracheas did show dose-dependent morphological changes. Rat tracheal epithelial cells exposed once, for 40 minutes, to approximately 0.7 ppm or 10 ppm ozone also showed no increase in the frequency of preneoplastic transformation, compared with control cells. In contrast, epithelial cells exposed in culture to 0.7 ppm ozone twice weekly for almost five weeks exhibited an increased frequency of preneoplastic variants compared with control cultures. Rat tracheal epithelial cells were also exposed to highly- or weakly-transforming doses of the chemical carcinogen N-methyl-N'-nitro-N-nitrosoguanidine and to 0.7 ppm ozone. When a single ozone exposure preceded the high level of carcinogen, the transforming potency of the carcinogen was reduced. When a single low dose of carcinogen preceded multiple ozone exposures, there was no statistically significant interaction between the effects of ozone and the carcinogen on transformation.

Cover title. "April 1992." "Includes the commentary of the Institute's Health Review Committee." Includes bibliographical references. Microfiche.