cohort), and wi indicates the exposure rate in period i. It can be shown that if the exposure rate were constant over the entire exposure period, then - 1 would correspond to γ in the power model described above that was applied in the NCI combined analyses. For both the Czech and Colorado data, - 1 was estimated to be about -0.5. The comparable values estimated for those cohorts in the NCI analyses were similar: -0.66 for the Czech cohort and -0.78 for the Colorado cohort.
Two of the miner cohorts (China and Ontario) had quantitative data on arsenic exposure, and Ontario, Colorado, New Mexico, and France had data indicating whether miners had previous mining experience. Analyses were conducted to investigate the effect of those variables on lung-cancer risks after adjustment for radon VVLM. Risks were found to increase with increasing arsenic exposure and to be larger for subjects with previous mining experience than for subjects without such experience. ERR/exposure for radon exposure was estimated both with and without adjustment for arsenic exposure or previous mining experience. For the China cohort, that reduced ERR/exposure from 0.61% to 0.16% but did not have a large effect on estimates from the other cohorts. There was no significant variation in ERR/exposure across categories of arsenic exposure or previous mining experience. It is noted that in all NCI analyses discussed thus far, the baseline risk was adjusted for arsenic and other exposures in cohorts for which data were available. It is possible, of course, that inadequate data or lack of data on such exposures could have biased results for any of the cohorts.
The effect of exposure to silica was investigated by Samet and others (1994) By examining whether the presence of silicosis, a fibrotic lung disease caused by silica, was associated with lung-cancer in a case-control study of New Mexico underground uranium miners. No evidence of such an association was found, but data were too sparse to rule out the possibility that silica exposure could substantially bias lung-cancer risk estimates for miners. Radford and St. Clair Renard (1984) investigated the role of silicosis in a case-control study and found no evidence of association with lung-cancer risk.
Overall, the NCI analyses provide a comprehensive summary of nearly all the relevant data on underground miners exposed to radon and radon progeny. The application of the same methods to all cohorts (parallel analyses) facilitate comparing results across cohorts, and combining data across cohorts (joint analyses) provides greater power for investigating various issues than would be available from any single cohort. The statistical methods are appropriate and in