nium-214, deliver to target cells in the respiratory epithelium the energy that is considered to cause radon-associated lung-cancer (NRC 1991).

Evidence on radon and is now available from about 20 epidemiologic studies of underground miners, including 11 studies that provided quantitative information on the exposure-response relationship between radon and lung-cancer risk (Lubin and others 1995). Those studies and several epidemiologic findings before them, continue to support the implementation of regulatory programs to reduce exposures of underground miners to radon and to provide compensation for occupational lung-cancer (Samet 1992).

Although the progeny of radon are now a well-recognized cause of lung-cancer, radon itself has again become a topic of controversy and public-health concern because it has been found to be a ubiquitous indoor air pollutant to which all persons are exposed (Cole 1993 and Proctor 1995 review the controversy). Radon was found to be present in indoor air as early as the 1950s, but the potential health implications received little attention until the late 1970s (Proctor 1995). In Scandinavia, housing surveys in the 1970s documented the presence of relatively high radon concentrations in homes built with materials containing medium-rich alum shale. Sparse data from the United States provided a similar indication of contamination of indoor air with radon. In 1984, a man triggered the radiation detector on entering the nuclear power plant where he worked. His home was found to have radon concentrations well beyond those permitted in underground mines. Other homes with high concentrations were identified later, and an enlarging database on indoor radon concentrations showed that the problem was widespread (Nero 1986).

The evidence on radon and lung-cancer is now extensive. Initially, research was driven by the need to characterize the risks faced by underground miners so that exposure limits that would keep risks to an acceptable level could be set. The work emphasized epidemiologic studies of the uranium and other underground miners exposed to radon, but animal studies were also conducted to address the modifying effects of such factors as the presence of ore dust and diesel exhaust, cigarette-smoking, and dose rate. Models of the respiratory tract were developed to characterize the relationship between exposure to radon progeny and dose of alpha energy delivered to target cells in the respiratory epithelium. In the last decade, research has reflected the need to improve the understanding of the risks posed to the general population by indoor radon. Epidemiologic studies have been conducted to assess the general population's risk of lung-cancer associated with indoor radon and complementary animal and laboratory studies have been carried out to address uncertainties in assessment of the risks associated with indoor radon. As a result, we have gained a rich body of evidence on radon and lung-cancer that addresses all facets of the problem within the framework of exposure, dose, and response (Figure 1-2). The new techniques of cellular and molecular biology also have brought new insights into how alpha particles injure the genetic material of cells and cause cancer (NRC 1994).

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