A multifactorial analysis of radon exposure and lung cancer in the Middle Urals, Russia

n 1987 the International Agency for Research of Cancer (IARC) evaluated radon and its decay products as human carcinogens (1). Large epidemiologic studies conducted in Western Europe in the Õ90s showed that this radioactive gas accounted for 50 to 90% of a radiological dose (2). Very high residential concentrations of radon were measured in Sweden and Finland (3-5). They were 5,000 times higher than its airborne concentrations. In Great Britain and the U.S.A. some concentrations of residential radon were 500 times higher than airborne ones (6). Natural background radiation is rather high in the Middle Urals, Russia. There are over 1,000 local clusters of natural radioactive uranium, thorium and uranium/thorium mineralization and about 350 reservoirs with high contents of natural radionuclides approaching maximum allowable concentrations of the World Health Organization. Results of radiation studies conducted by the Institute of Industrial Ecology of the Ural Branch of the Russian Academy of Sciences in the Middle Urals confirmed increased residential levels of radon. In some Ural towns radon concentrations were >100 Bq/m3 in 30% of investigated residences and in 11% of residences the level of radon exceeded the criterion of safe residence. The results of this study also showed that the major part of the territory of the Middle Urals, including the biggest cities of Yekaterinburg, Nizhni Tagil, Pervouralsk, Kamensk-Uralsky and some others, were located within areas with high radon exposure (7). Based on data of the Ural Geological Committee, the dose of gradiation in Yekaterinburg, the capital of the Middle Urals, ranges from 8-10 mr/hr, Nizhni Tagil 6-9 mr/hr, Pervouralsk 5-7 mr/hr (8). A high contribution of thoron in the dose of radiation formed by natural radionuclides is a distinctive feature of the Middle Urals. High levels of radionuclides of thorium decay series in ore and building materials account for thoron accumulation in residences. On the whole, the dose of natural and industrial background radi-