dusts will penetrate the lungs. The rest will deposit in the upper respiratory tract and eventually will be swallowed and go through the gastrointestinal tract.

Oral exposure. The absorption of uranium across the gastrointestinal tract generally increases with increasing solubility of the compound. Absorption is greatest for the relatively soluble uranium compounds. Notably, even with the soluble compounds, only a small fraction of uranium is absorbed across the gastrointestinal epithelium. Humans ingesting uranyl nitrate hexahydrate or uranyl nitrate absorb only 0.5–5 percent of the ingested dose (Hursh et al., 1969; Karpas et al., 1998). Adult hamsters that received actinide preparations of uranyl nitrate and uranium dioxide through an intragastric tube absorbed 0.77 and 0.11 percent of the total doses, respectively (Harrison and Stather, 1981). After administering UO2 to rats by the same route, other investigators could not detect any uranium in liver, kidney, muscle, bone, brain, blood, and urine (Lang and Raunemaa, 1991).

Dermal absorption. Dermal absorption of uranium compounds in humans has not been characterized (ATSDR, 1999b). In animals, the soluble uranium compounds uranyl nitrate hexahydrate (0.5–7 g/kg body weight) and ammonium uranyl tricarbonate (7 g/kg body weight) penetrated the skin of experimental rats within 15 minutes of application (de Rey et al., 1983). Forty-eight hours after exposure, uranium was no longer present on the skin, and the rats had experienced severe toxic signs ranging from weight loss to death, indicating absorption of uranium into the systemic circulation. No penetration of uranium through the skin occurred after applying the more insoluble compound uranium dioxide (de Rey et al., 1983). Other uranium compounds, such as uranium tetrafluoride, uranium tetrachloride, and uranium trioxide, are absorbed through the skin of mice, rats, and guinea pigs (Orcutt, 1949). Although the absorption rate was relatively low (0.1 percent of uranium applied to the skin), the amount of absorbed uranium was sufficiently high to cause toxicity (Orcutt, 1949). These animal studies show that percutaneous absorption is an effective route for soluble uranium compounds to enter the systemic circulation. However, the application of these findings to human dermal exposure is unclear because the concentrations of uranium that were applied to the skin were extremely high.

Transport and Biotransformation

Once absorbed, uranium forms soluble complexes with bicarbonate, citrate, or proteins in the plasma (Dounce and Flagg, 1949; Stevens et al., 1980; Cooper et al., 1982). Approximately 47 percent of blood uranium forms a complex with bicarbonate in plasma, 32 percent of uranium binds to plasma proteins, and 20 percent binds to erythrocytes (Chevari and Likhner, 1968).

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