1989; Borak 1988). Most domestic studies show either a slight positive or slight negative correlation between measured radon in the home and lung-cancer mortality.

Many studies are ecologic exercises that relate lung-cancer mortality in a region with indoor radon concentration (Cohen 1992; 1990). In some cases the radon is not measured, but rather is estimated as high or low, depending on the type of house. The ecologic studies are ambiguous because no attempt is made to determine actual exposure to individuals in the area of study and no correction can be made for smoking, the strongest confounder for lung cancer (Stidley and Samet 1994; Cohen 1989). The ecologic study of Cohen (1995) is the most comprehensive. It encompasses about 300,000 radon measurements in 1,601 counties in the U.S. The trend of county lung cancer mortality with increasing home radon concentration is strikingly negative, even when attempts are made to adjust for smoking prevalence, and 54 socioeconomic factors. The measured average county radon concentrations do not exceed 300 Bq m-3, thus the typical low home exposure region is studied. This finding contradicts the existing risk estimates at low exposure, and a sound reason for the significant negative trend should be sought.

To date, there are 8 published case-control studies that compare the relative risk of lung cancer between high-and low-exposed groups. An attempt is made to measure the 222Rn exposure in the home. The largest case-control study to date was performed in Sweden (Pershagen and others 1994). There were 1,360 cases and 2,847 controls, and exposure was assessed with 3-mo measurements during the heating season, retrospectively for homes lived in for more than 2 y after 1947 up to 3 y before diagnosis of cancer. The lung-cancer excess was not statistically significant even for smokers or nonsmokers with over 400 Bq m-3 in the home for over 32 y.

A meta-analysis was performed with results of the eight published domestic studies. The lung-cancer excess is not statistically significant, but the trend with increasing concentration in the homes is significant (Lubin and Boice 1997). The graph of the eight studies from Lubin and Boice (1997) is shown in figure 5.9.

All that can be said about domestic risk is that it is low and difficult, if not impossible, to detect given the high background lung-cancer mortality in the populations studied. Although a pooling of data from the largest current and past case-control studies from all countries will be performed at NCI by the year 2000, it is unlikely to provide quantitative domestic risk estimates. Because of the poor precision of the individual studies. Lung cancer from environmental exposure might eventually be documented, but it is most likely that numerical risk estimates for lung cancer from 222Rn and decay-product exposure will rely on projection models from the underground-miner experience.

The difficulty in pooling the domestic studies is described by Neuberger and others (1996.).

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