30 exposure days) caused a reduction in erythrocyte numbers and hemoglobin levels (Roberts, 1949). Significant increases in myeloblasts and lymphoid cells of the bone marrow occurred after inhalation exposure to uranium peroxide or uranium trioxide (15.4 mg U/m3, 5 hours per day, 23 days, and 16 mg U/m3, 6 hours per day, 24 days, respectively) (Dygert et al., 1949). However, a series of intermediate-duration inhalation studies showed no adverse effects of uranium on the blood (ATSDR, 1999b). Similarly, the majority of animal studies show no adverse effects of orally administered uranium compounds on blood. For example, in New Zealand white rabbits exposed for 91 days to uranium (uranyl nitrate hexahydrate) in drinking water (0.96, 4.8, 24, 120, or 600 mg U/L), hematological and biochemical parameters did not change (Gilman et al., 1998a,b,c).

EPIDEMIOLOGIC STUDIES: DESCRIPTION OF THE STUDIES

General Considerations

This section contains descriptions of epidemiologic studies of the human health effects of exposure to uranium. The section begins with an overview of the studies of uranium miners. The studies in this cohort have limited relevance to the depleted uranium exposures of Gulf War veterans because, as described below, the primary disease-causing exposures for the miners were not to uranium, but to radon. The remainder of the section provides detailed descriptions of studies on workers occupationally exposed to uranium in uranium-processing plants. The results of these studies appear in the next section of the chapter.

Although depleted uranium is the form of uranium that was present in the Gulf War theater, there are only a few studies of its health effects. Therefore, the committee relied on studies of the health effects of natural and processed uranium. As noted earlier in the chapter, the chemical characteristics of an element are independent of its isotopic form. As a result, DU has the same chemical effects as naturally occurring or enriched uranium. Given the same dose by the same route, the health effects that are due to the chemical characteristics of uranium should be identical from natural, enriched, or depleted uranium exposures. Thus, studies of the chemical effects of natural and processed uranium will provide a good indication of what studies of DU would show.

The literature examining the health effects of exposure to ionizing radiation is extensive. Recent reports including those by the NRC (National Research Council) and ATSDR (NRC, 1988, 1990, 1999; ATSDR 1999a) summarize this work. High-dose human and animal studies have shown that radiation is carcinogenic and that the incidence of cancer increases with the dose of radiation (ATSDR, 1999a). The principal isotope in natural and depleted uranium, 238U, has a long half life (4.5 billion years) and primarily decays by alpha particle emission. Alpha particles have a very short range and little penetrating power. They are therefore a hazard to humans only in close proximity to human tissue.



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