The authors examined the components and modifiers of the healthy worker effect using mortality data from three occupational cohorts: the employees of Atomic Energy of Canada Limited followed between 1950 and 1981, a 10% sample of the Canadian labor force followed between 1965 and 1979, and workers at the Eldorado Resources Limited Beaverlodge uranium mine followed between 1950 and 1980. Two important components of the healthy worker effect have been identified in these cohorts, namely, initial selection of and continuing employment of healthy individuals. There is less evidence for a contribution from the existence of differential risk factors among employed individuals as compared with the general population. The healthy worker effect is, however, substantially modified by time since employment, sex, age, specific cause of death, and specific occupation. Because of this variation, it is inappropriate to account for the healthy worker effect by a single parameter, and all of the above factors must be taken into account in any appropriate analysis. When the only available comparison group for an occupational cohort is the general population, the healthy worker effect is unlikely to have any substantial influence on the process of assessing causality for any observed association or attributing cause in an individual case. This would be particularly true for cancer, and even more so for lung cancer, a disease often associated with industrial compensation cases.
A cohort study of 8,487 workers employed between 1948 and 1980 at a uranium mine in Saskatchewan, Canada, has been conducted. A total of 65 lung cancer deaths was observed (34.24 expected, P less than 10(-5)). There was a highly significant linear relationship between dose and increased risk of lung cancer giving estimates for the relative and attributable risk coefficients of 3.28% per working level month (WLM) and 20.8 per WLM per 10(6) person-years. Age at first exposure had a significant modifying effect on risk. The interaction of exposure with age at observation fits a relative risk model well. The similarity of these results to a recent study of Swedish iron miners with similar levels of relatively low exposure suggests that exposure to radon daughter products may be a major contributory factor to lung cancer occurring among nonsmokers in the general population. The results also reinforce concerns as to the appropriateness of present occupational exposure standards.
A cohort study of 2,103 workers employed between 1942 and 1960 at a uranium mine in the Northwest Territories, Canada, was conducted. A total of 57 lung cancer deaths was observed (expected = 24.73, P less than .0001). There was a highly significant linear relationship between exposure and increased risk of lung cancer, giving estimates for the relative and attributable risk coefficients of 0.27 per working level month (WLM) and 3.10 per WLM per 10(6) person-years. These risk coefficients were substantially less than those estimated from the experience of miners in the Beaverlodge mine, which have previously been reported. Any biases in the present estimates are likely to have been upward, and therefore they probably represent an upper limit. The major difference between the two mine cohorts is in the exposure rate, since the Port Radium miners were exposed to much greater concentrations of radon daughters than the Beaverlodge miners. It is postulated that risk of lung cancer from radon daughter exposure may be modified by exposure rate, for which hypothesis there is some support from other epidemiologic data.
An epidemiological follow-up study of 16,000 uranium mine and refinery employees has made use of computerized techniques for searching a national death file. The accuracy of this computerized matching has been compared with that of corresponding manual searches based on one-eighth of the worker file. The national death file--Canadian Mortality Data Base--at Statistics Canada includes coded causes of death for all deaths back to 1950. The machine search was carried out using a generalized record linkage system based upon a probabilistic approach. The machine was more successful than the manual searchers and was also less likely to yield false linkages with death records not related to the study population. In both approaches accuracy was strongly dependent on the amount of personal identifying information available on the records being linked.
Exposures to radon decay products have been re-estimated for 65 men who died of lung cancer between 1950 and 1980, and 126 matched controls selected from the Beaverlodge cohort of 8,487 workers at the Beaverlodge mine in Northern Saskatchewan. The revised exposure estimates were based on a more thorough review of individual employment records for the study subjects than originally conducted, together with historic data from area-specific measurements of exposure rates in the mine. The revised cumulative exposures are approximately 60% higher than the original exposure estimates, which were based on mine-wide averages for those exposures incurred prior to 1967, and which used geometric rather than arithmetic means for area exposure rates. Despite the increase in estimated exposures using the revised estimates, the excess relative risk per 100 working level months has increased from 2.70 to 3.25, most likely due to a substantial reduction in random exposure measurement error. The new data show similar modifying effects of risk by time since exposure and age at risk as other studies of underground miners, but provide no evidence of an inverse exposure-rate effect, in contrast to a strong effect seen in the analyses based on the original exposure estimates.
The use of dead controls in a case-control study, the primary purpose of which is to control for confounding, leads to consistent relative risk estimates within stratum of the confounder, even if the causes of death of controls are associated with that confounder. Dead controls may be more comparable to dead cases in terms of data quality, and therefore, under these circumstances, the use of dead controls may be advantageous.