Health effects of radiation exposure at uranium processing facilities

Abstract

There is substantial public and scientific interest in the health consequences of exposures to ionising radiation in nuclear installations, in particular at nuclear fuels enrichment and production facilities. In this issue of Journal of Radiological Protection, McGeoghegan and Binks report on the follow-up of a cohort of over 19 000 uranium fuel and uranium hexafluoride production workers employed at the Springfields uranium production facility between 1946 and 1995. Workers included in the study were followed, on average, for almost 25 years. Findings from this study present potentially important information about mortality and cancer morbidity in populations receiving low-dose protracted exposures to ionising radiation. At uranium fuel production facilities such as Springfields, inhalation of airborne uranium dust may represent an important potential source of radiation exposure. Workers in these facilities therefore have two main possible sources of radiological exposure to tissues of the whole body: external gamma-ray exposure—which results in fairly uniform distribution of dose—and internal depositions that deliver radiation doses primarily to the lung and lymphatic system. If the uranium dust is solubilised, exposure may also result in other tissues such as the liver, the kidney and the bone. Tumours occurring in these organs are therefore, a priori, of particular interest in epidemiological studies of workers at uranium production facilities. Assessment of past internal exposure of nuclear workers is often complicated by methodological difficulties of internal dosimetry, as well as by inadequate historical information with which to accurately quantify internal radiation doses. These exposure measurement problems pose significant problems for epidemiology: the inability to accurately classify workers by level of internal radiation exposure may lead to confounding of the analyses of radiation-cancer associations, since workers with significant dose from internal contamination are often persons with substantial external exposure. It is important to consider these concerns with regards to the findings reported by McGeoghegan and Binks, as analyses in this paper are restricted to external doses only. In this paper, standardised mortality and morbidity ratios (SMRs), which did not rely upon exposure classification, were reported. For the full cohort, deficits in mortality were observed in a number of broad categories of cause of death, including all causes, all cancers, and cardiovascular disease. This conforms to expectations about health-related selection and survivorship in relatively well-paid industrial cohorts [1-3]. SMRs for these broad categories of cause of death were lower for radiation workers than for non-radiation workers, suggesting stronger processes of health-related selection in radiation jobs than in non-radiation jobs. Analyses based upon internal comparisons of mortality and cancer registration rates by level of radiation dose were of primary interest in this paper. Such analyses allow investigation of radiation-disease associations that do not suffer the biases associated with the use of external comparison populations. McGeoghegan and Binks present tests for trends in the association between radiation exposure and cause-specific mortality and morbidity, after adjustment for age, sex, calendar year, industrial and worker status. Using a 10 year lag between exposure and disease, statistically significant associations were reported for mortality from Hodgkin's disease and cancer of the bladder. Associations were also seen with mortality due to all causes and all cancers; they were, however, statistically significant only when doses were lagged by 20 years. No statistically significant association between leukaemia (one of the cancer types most consistently associated with external radiation exposure) and radiation dose was observed. A particularity of this study is the analysis of cancer incidence, since few studies of nuclear workers have involved cancer morbidity follow-up. As indicated by the authors, cancer diagnoses are generally more reliably registered in cancer registries than on death certificates. In addition, such analyses allow the study of the association between radiation dose and the risk of non-fatal tumours. In this paper, significant associations were reported for morbidity from all cancers excluding leukaemia, cancer of the pleura, Hodgkin's disease, non-Hodgkin lymphoma and all lymphatic and haemopoetic cancers. A significant association with incidence of lung cancer was also observed, but only when doses were lagged by twenty years. This is one of only a few studies of nuclear industry workers that report an association between radiation dose and all cancer incidence or mortality [4-7]. In this paper, the association appears to be largely driven by the association with lung cancer incidence. Analyses by histological type of lung cancer were carried out, but the large number of unconfirmed tumours and of unspecified histologies precludes an interpretation of these results. As mentioned above, no attempt was made in this paper to classify workers by level of possible internal exposure; further, information on tobacco smoking is not available. Although the effects of internal exposures are addressed in the discussion, their impact on the findings of the study is unclear. It is therefore important to recognize that estimates of association between external radiation and cancer in this cohort may at least partially reflect confounding due to occupational and non-occupational exposures. Results of the planned analyses of internal exposures should be very valuable to clarify this issue. The paper contributes to a growing literature about disease in the nuclear industry and specifically in the nuclear fuels fabrication industry. Comparison of findings between uranium processing facilities is complicated by the fact that processes and historical periods of operation have differed between facilities, leading to differences in exposure conditions and follow-up between cohorts. Lung cancer has been the primary outcome of interest in these studies. Lung cancer mortality was found to be elevated, compared to national rates, among workers in some nuclear fabrication facilities [8], but not in others [9]. An association between external radiation dose and lung cancer mortality was observed in two cohorts in the USA (Fernald and Y-12) [10, 11]; an association with estimated dose from alpha radiation was also observed at Y-12. In contrast, a US multi-facility case control study of lung cancer among workers exposed to uranium dust at TEC, Y-12, Fernald and Mallincrodt found no such association; there was suggestion, however, of positive associations among workers hired at ages over 45 [9]. Future research with these cohorts may help to understand the roles of external and internal radiation exposures in these associations, as well as the roles of potential modifiers of risk. The findings among Springfield workers of positive associations under long-lag assumptions, and the findings in a small number of studies of uranium production workers [6,7,12] of apparent differences in radiation risk estimates with age at exposure, suggest that further attention should be focused on the investigation of temporal patterns of radiation risks. In this discussion, internal radiation exposure has primarily been discussed as a potential confounder of external radiation-cancer associations. While these concerns are important, further epidemiological research is also needed to understand the effects of internal radiation exposure, and to investigate potential interactions between external and internal radiation exposures. Unfortunately, estimation of internal doses using historical records is often a difficult and imprecise task; such efforts may, however, substantially enrich our understanding of the effects of occupational radiation exposures. 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