Studies of rats at Harwell and CEA [Monchaux et al., Radiat. Res. 152 (Suppl.), S137-S140, 1999] are currently in progress to determine the factors affecting the risk of induction of lung tumors after exposure to radon and radon progeny. Knowledge of the effect of dose, dose rate and characteristics of the aerosol on lung tumor induction in rats may be used to improve estimates of risk from domestic exposure. At Harwell, three studies are in progress, studying the effect of dose, dose rate, and dose rate at low total exposures. Approximately 2000 adult male rats have been exposed. A small number of rats were taken to determine deposition in the respiratory tract and the early effects of exposure on cell proliferation and nuclear aberrations. The remaining animals have been held for their life span. To date 65% of the animals in the first study have been examined. Current results (for 421 rats) suggest that exposure to radon and radon progeny causes elevated incidences of both benign and malignant lung tumors. These findings are based on incomplete tumor incidences. Competing causes of death may affect the results, and full statistical analysis is required before firm conclusions can be drawn about the effect of dose and dose rate.

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