One of the more perplexing situations in which humans are exposed to low levels of ionizing radiation involves the ubiquitous element known as radon gas. The carcinogenic effects of this gas are most evident in underground miners, especially those in uranium mines, where substan- tial levels of radon gas occur. Figure 12.3 shows the relative risk of lung cancer for cohorts of miners for all exposures and for exposures restricted to under 600 WLM (working-level months). For all exposures, any deviation from linearity was not statistically significant (Lubin et al., 1995). The dashed line in the right-hand figure indicates a relative risk of 1.0. For residen- tial radon exposure, living 30 years in a home with concentrations of radon gas at 1 and 4 pCi/L
Figure 12.3 Relative risks (RR) of lung cancer for categories of working-level months (WLM) for all exposures to radon by underground miners and for exposures restricted to individuals of less than 600
WLM. For all exposures, deviation from linearity was not statistically significant. The dashed line indicates a relative risk of 1. For comparison, residential radon exposure, living 30 years in a home at 1 and 4 pCi/L is equivalent to about 6 and 24 WLM, respectively (from Lubin et al., 1995, with permission of the authors and publisher.)
is equivalent to about 6 and 24 WLM respectively. Thus, while the dose-response curve indicates that the very low levels of exposure over extended periods in homes carry a significant risk, it is almost impossible to determine unequivocally the actual carcinogenic risk of this environmental agent to the human at these extremely low doses. However, a study in Sweden (Pershagen et al.,
1994) concluded that residential exposure to radon gas is an important cause of lung cancer in the general population, and a recent metanalysis of eight epidemiological studies on the long- term effects of exposure to indoor radon concentrations also concluded that this environmental agent was a factor in cancer induction in the human (Lubin and Boice, 1997). Furthermore, an earlier investigation (AMA, 1987) concluded that variations of radon levels in private dwellings in the United States are 100-fold or greater, with many homes exceeding the 4 pCi/L seen in the figure (AMA, 1987). Radon gas is clastogenic and mutagenic in experimental systems (Jostes,
1996). However, the total impact on the general population of exposure to this ubiquitous radio- active gas may never be completely known.