Ozone carcinogenesis revisited

The question was asked whether ozone would act as a lung carcinogen in mice . To test the hypothesis, female strain A/J mice were exposed for 6 h/day, 5 days/week to 0.12 ppm, 0.5 ppm, or 1.0 ppm of ozone; control animals were kept in filtered air. No ozone-related deaths were observed at any time du ring the experiment, After 5 months, one-third of the animals were killed. The remaining animals were split into two groups: exposure to ozone continu ed for one group, whereas the other group was transferred into filtered air . Four months later, these animals were killed. No significant increase in lung tumor multiplicity (average number of tumors per lung) or lung tumor i ncidence (percentage of tumor-bearing animals) was found in the animals exp osed to ozone when compared to animals kept in filtered air, regardless of ozone concentration. Morphometric analysis of lungs of animals exposed to t he highest ozone concentration (1.0 ppm) showed a small, statistically not significant increase in centriacinar lesions. It was concluded that ozone i s not a lung carcinogen in strain A/J mice at those exposure levels. Moreov er, this mouse strain appears to be particularly resistant towards chronic ozone toxicity.