prehensive exposure assessment that includes quantitative risk assessments for selected health end points.

This chapter examines the published scientific literature on potential health effects of uranium and depleted uranium. The chapter begins with an overview of the toxicology and animal studies and then examines the scientific literature on human health effects, most of which comes from epidemiologic studies of workers exposed to uranium and from human case reports. In summarizing the scientific research on the toxicology of uranium, the committee frequently references the Agency for Toxic Substances and Disease Registry’s (ATSDR’s) Toxicological Profile for Uranium (ATSDR, 1999b). ATSDR’s extensive report is a review and assessment of the peer-reviewed literature on the toxicological end points. The ATSDR report was reviewed by a nongovernmental panel and by scientists from federal agencies.

As discussed above, uranium is both a heavy metal and a low-specific-activity radioactive element. Studies on the toxicity of uranium have examined both its chemical and its radiological effects. The primary routes of exposure to uranium for humans are through ingestion or inhalation; the effects of dermal exposure and embedded fragments have also been studied.

The amount of uranium that the body absorbs depends largely on the route of exposure and the solubility of the uranium compounds to which the individual is exposed. Insoluble uranium compounds may remain within the pulmonary tissues, especially the pulmonary lymph nodes, for a long time and thus constitute a localized radiological hazard. As a general rule, uranium is less readily absorbed from the intestinal tract than from the respiratory tract, resulting in lower doses per unit intake. Chemical toxicity, characterized predominantly by renal dysfunction as a consequence of exposure to soluble uranium, and lung injury potentially caused by the ionizing radiation from uranium decay isotopes are the best-characterized consequences of exposure to uranium compounds. However, the chemical and radiological properties of uranium could act cooperatively to cause tissue damage, and therefore, it cannot be assumed that excess cancers would be due solely to the radiological effects of uranium or that organ damage is exclusively due to its heavy-metal properties.

Inhalation exposure. The site of deposition of uranium particles in the respiratory tract is the result of a combination of physical forces that govern particle behavior in an air stream, as well as the anatomy of the respiratory tract (Gordon and Amdur, 1991). The site of deposition affects the degree of ura-