Stomach cancer in the United States has been qualitatively linked to smoking habits in some studies though no analytic study has yet been mounted to attempt to derive risk estimates. Data in the United States (NIH 1996) generally show an increasing mortality risk ratio for stomach cancer with increasing smoking rates, but the data are inconclusive. The two studies of other cancers provide reasonable support for the conclusion that the dosimetric significance of inhaled 222Rn and decay products for the induction of cancer in other organs is absent or minimal.

Evaluation Of Risk Per Unit Exposure From Inhaled 222RN in Air

The lifetime risk of lung cancer associated with indoor radon concentration of 150 Bq m-3, calculated from the various models, is summarized in table 5.5. The most recent estimate of risk of lung cancer in the United States due to inhalation of radon decay products is from the model published in the BEIR VI report.

The BEIR VI calculated estimates of lung cancers per year in the United States from the two models are 15,000 and 21,000. These result from an average exposure of the population to 46 Bq m-3; the exposure distribution was documented from the national residential radon survey by the Environmental Protection Agency (Marcinowski and others 1994).

The lifetime risk can be calculated per unit Bq m-3 directly from the relative risk tables given in BEIR IV, NCI and BEIR VI for smoking and nonsmoking males and females. The fractional values used by the BEIR VI committee for ever-smoking males and females are 0.58 and 0.42, respectively. The values for the lifetime base risk of lung cancer in ever-smoking males, never-smoking males, ever-smoking females, and never-smoking females are 0.116, 0.0091, 0.068, and 0.0059, respectively (J. Lubin, personal communication).

The lifetime inhalation risks per unit of exposure to 222Rn in air and in air from water use are shown in table 5.7. The risk is estimated for ever-smokers and never-smokers and men and for women with the average of the two BEIR VI preferred risk models. The lifetime risks are derived as a product of the BEIR VI preferred relative risk estimates and the baseline lung cancer risks given above. The lifetime risk per unit of exposure to 222Rn in air derived from water use is the risk in air multiplied by the average water transfer coefficient.

All the risk computations described above are based on up to 11 cohorts of underground miners. The atmospheric characteristics in the various mines have a wide array of values with regard to unattached fraction, decay product equilibrium, etc. The absorbed dose delivered to the target cells in bronchial epithelium differs somewhat among mines and among different homes. The various factors such as higher unattached fraction in homes versus mines and lower breathing rates in homes versus mines compensate, in such a way that the application of the



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