In summary, the studies of uranium miners consistently show a large increase in deaths from lung cancer. However, these studies are of limited relevance to veterans of the Gulf War because the miners were exposed to high concentrations of radon gas as a consequence of working underground in poorly ventilated confined spaces. Radon decays into short-lived alpha-particle-emitting isotopes that have been found to increase the risk of lung cancer (NRC, 1999). Although long-lived isotopes of uranium, plutonium, and thorium are also present in lung tissue and thoracic lymph nodes of uranium miners (Singh et al., 1983, 1986, 1987, 1989), it has not been possible to distinguish their contribution to disease from that of radon progeny.
The most important sources of evidence about the human health effects of exposure to uranium are studies of people who worked in plants whose purpose was to process and refine raw uranium ore into 235U-enriched uranium metal for use in weapons and nuclear reactors (Table 4.3). Their exposure history differs from that of uranium miners because they worked in an environment that had little radon gas, an exposure that confounds any attempt to link uranium exposure in mine workers to effects on the lung, an important potential site for disease caused by uranium. The principal exposure of uranium processing workers is to uranium oxides and derivative uranium compounds produced during the uranium refinement process. It is important to note, however, that uranium is not the sole potential hazard in the industrial settings described below. Studies vary in the nature of the work being conducted and the extent to which workers were exposed to enriched uranium, soluble and insoluble uranium compounds, other radioactive elements (e.g., radium, thorium), and other potentially hazardous industrial chemicals (e.g., sulfuric acid, fluorocarbons).
Occupational studies involve exposure that occurred in the course of day-to-day tasks, rather than as a single large exposure. The exposure was greatest in the early days of the uranium processing and machining industry; as the industry adopted improved safety measures, the degree of exposure decreased. The principal route of entry for uranium in occupational exposures was inhalation of dusts to which uranium compounds were attached. In this sense, the exposure was similar in character, albeit far more prolonged, to that of Gulf War veterans involved in cleanup actions after friendly fire incidents. The exposure in the case of uranium processing plant workers occurred over a period of months to many years, in contrast to the much shorter period of exposure in veterans involved in cleanup actions.
This section is a study-by-study description of the epidemiologic studies of uranium-processing workers and includes information on the exposure measures (where available) and the analytic methods used in each study. This section does not contain any results. Results appear in the following section, which is a disease-by-disease analysis of the strength of the evidence for associations between health outcomes and exposure to uranium.