Modification by ozone of lung tumor development in mice

Mice, either strain A/J or Swiss Webster, were exposed for 18 weeks either to filtered air or to 0.4 or 0.8 ppm ozone for 8 hours daily. Subgroups in each test group received a single ip injection of 1,000 mg urethan/kg or 0.9% sodium chloride vehicle 1 day prior to initiation of the exposure regimen. Tumor incidence in Swiss Webster mice was 0-3% in groups not receiving urethan and was 61-74% in groups receiving urethan. In A/J mice, the corresponding values were 9-38% and 100%, respectively. Exposure to ozone caused a decrease in the number of tumors per lung in urethan-treated mice of both strains, in a dose-dependent manner. There seemed to be a specific decrease in tumors derived from alveolar type II cells in the A/J mice given urethan plus ozone. Most interesting, perhaps, was a significant increase in the number of tumors per lung in A/J mice exposed to 0.8 ppm ozone without urethan, confirming a previous report by others. The corresponding ozone effect on lung tumor development was not observed in Swiss Webster mice.