ANALYSIS OF LUNG-TUMOR RISKS IN RATS EXPOSED TO RADON

Using data on 3117 rats exposed by inhalation to radon, radon progeny and uranium ore dust, the hazard function (or age-specific risk) for l ung tumor incidence was modeled as a function of exposure, exposure ra te and other factors. The overall estimate of lifetime risk was 237 ca ses per 10(6) rats per WLM (237 per 10(6) WLM), reasonably comparable to estimates obtained from data for humans. The data below 1000 WLM (2 0-640 WLM) were consistent with linearity with positive excess risks a t all levels; however, evidence of statistically significant excess ri sk was limited to exposures of 80 WLM or greater. Evidence for an inve rse exposure-rate effect was limited primarily to cumulative exposures exceeding 1000 WLM (1280-10,240 WLM) and to comparison of results at 100 and 1000 WL. Even at these levels, the possibility that the effect might be explained by time since last exposure or by heterogeneity ac ross experiments could not be entirely excluded. The inverse exposure- rate effect was strongest for epidermoid and adenosquamous tumors, and the only indication of such an effect at exposures below 1000 WLM was modest evidence (P = 0.024) in analyses limited to these tumors. When all lung tumors, or all malignant lung tumors, were included, there w as no evidence of such an effect below 1000 WLM. These data support th e viewpoint that the inverse exposure-rate effect is primarily a high- dose phenomenon. (C) 1996 by Radiation Research Society