plicative. That is, the lifetime risk of lung cancer posed by radon exposure did not simply add to the lifetime risk of lung cancer associated with smoking, but neither did the risks multiply. The risk related to radon and smoking appeared to be between the two extremes.

The exposure data on the Eldorado cohort were not considered carefully by the BEIR IV committee. A reported Eldorado mining exposure of 1 WLM gave a 50% excess lung-cancer mortality—clearly an erroneous value. It is known that the miners had prior exposure in other mines, but the additional exposure is not known (Chambers and others 1990). The exaggerated risk per WLM in this study for the 1-WLM exposure cohort is important in controlling the overall BEIR IV model. This exposure category included a large number of person-years. Therefore, when the four cohorts were combined to yield a ''best estimate'' of the relative-risk coefficient, the 1-WLM group carried substantial weight. If this data point were omitted, the risk coefficient in the model would be less than the 0.025/WLM used in the final BEIR IV model. That possible effect should be carefully considered in any future models that use the Eldorado cohort. Thus, considering that inaccuracies might be incorporated into the BEIR IV model, the calculated risk estimates for both smokers and nonsmokers at environmental exposures are likely to be overestimates.

The values of lifetime risk as calculated by the BEIR IV TSE model are also shown in table 5.5.

NCI Model

The National Cancer Institute coordinated an effort to pool the epidemiologic data from 11 underground-mining studies. The authors from the various countries pooled results, and these were reported by the National Institutes of Health (Lubin and others 1995; 1994). The report Radon and Lung Cancer Risk: A Joint Analysis of 11 Underground Miners Studies , is the most complete analysis of the health detriment to underground miners. For the pooled analysis, there were 2,701 lung cancer deaths among 68,000 miners who accumulated about 1.2 million person-years of exposure.

In all 11 cohorts, the excess relative risk (ERR) of lung cancer (the fractional increase in lung cancer) was linearly related to the cumulative exposure estimated in working level months (WLM). Thus, although other carcinogens might be present in the mine atmosphere, a clear exposure-response relationship was associated with 222Rn decay products. Smoking history was complete only in the Colorado mining cohort. Because of the lack of smoking information the combined risk for smoking and radon could only be inferred qualitatively.

The Colorado uranium-miner data are shown as a typical example of the 11 cohorts in figure 5.7. The ERR/WLM for all 11 studies is shown in figure 5.8. Parts a and b of figures 5.8 show the ERR for all the cohorts combined, and for all cohorts with exposure under 600 WLM.

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