Experimental study of the combined effect of cigarette smoke and an active burden of radon-222

Previous studies on the carcinogenic effect of radon-222 derivatives have yielded accurate relationships, for each radon dose, between the dose value and the frequencies and latency times of lung cancers. In the present work, one hundred rats were subjected, over one-and-a-half months, to a total dose of 3600 WLM, chosen because it corresponds to a 30% occurrence of cancers. Fifty of these animals then inhaled cigarette smoke during 50 ten-minute sessions per week. The total time for these inhalations was 350 hours spread over about six months. In the 'radon' group (50 rats), 17 animals were found to have lung cancer. In the 'radon + tobacco' group (50 rats), 32 cancers were observed; moreover, the tumours in this group were much more extensive, multifocal and invasive. Animals subjected to cigarette smoke alone have never shown lung cancer. The effect of tobacco as a co-factor in carcinogenesis has thus been varified experimentally, although inhaled cigarette smoke alone is not carcinogenic in rats. (author)