Influence of exposure rate on radon-induced lung cancer in rats.

A new series of experiments was carried out to investigate specifically the influence of exposure rate on lung cancer induction in rats at relatively low cumulative exposures of about 100 WLM, and at potential alpha energy concentrations (PAECs) ranging from 13 to 150 WL. The results indicate that at relatively low cumulative exposures comparable to lifetime exposures in high-radon houses or current underground mining exposures, the risk of lung cancer in rats decreases with decreasing PAECs, i.e. exposure rates. They confirm the results of previous experiments conducted at lower cumulative exposure, showing that for a similar cumulative exposure of 25 WLM, the risk of lung cancer decreases with decreasing exposure rates. These data suggest that the induction of lung cancer results from a complex interplay between cumulative exposure and exposure rate, with an optimal combination of these two parameters, i.e. a combination of cumulative dose and dose rate that results in a maximum risk of lung tumour induction. They support the hypothesis that, at low doses, the risk of lung cancer is governed by the rate at which the dose is delivered, and not by the total cumulative dose alone. These data are also consistent with that of underground uranium miners showing an inverse dose-rate effect at high cumulative exposures, but a diminution of this effect at cumulative exposures lower than 50 WLM.