LUNG-CANCER INCIDENCE AFTER EXPOSURE OF RATS TO LOW-DOSES OF RADON - INFLUENCE OF DOSE-RATE

To study the effect on lung cancer incidence of a long exposure to low levels of radon, 500 male 3-months-old Sprague-Dawley rats, were expo sed to a cumulative dose of 25 WLM of radon and its daughters, 6 hours a day, 5 days a week, during 18 months. Exposure conditions were cont rolled in order to maintain a defined PAEC: 42 x 10(-6) J.m(3) (2 WL), in the range of domestic and environmental exposures. Animals were ke pt until they died or given euthanasia when moribund. Mean survival ti mes were similar in both irradiated and control groups: 828 days (SD = 169) and 830 days (SD = 137), as well as lung cancer incidence, 0.60% at 25 WLM and 0.63% for controls. The incidence of lung lesions was c ompared statistically with controls and those previously obtained at c umulative exposures of 25 and 50 WLM delivered over a 4-6 month period , inducing a significant increase of lung cancer, 2.2% and 3.8% respec tively. Such a comparison showed a decreased lung cancer incidence rel ated to a decrease in the dose rate for low levels pf radon exposure.