nium per liter) (Damon et al., 1984), the half-time for Type F uranium compounds was between 1 and 5 days. The body retains Type S compounds for much longer. The half-time of uranium deposited in the lungs of dogs, rats, and monkeys exposed up to 5 years to uranium dioxide dust (5.1 mg U/m3) was approximately 15 months (Leach et al., 1973).

Reports from human subjects occupationally exposed to insoluble uranium compounds suggest a two-phase clearance process, consisting of a short phase with a biological half-time between 11 and 100 days and a slow phase of clearance with a biological half-time between 120 and 1,500 days (Hursh and Spoor, 1973). The biological half-time of uranium dioxide in the lungs of occupationally exposed workers was estimated to be 109 days in one study (Schieferdecker et al., 1985).

The aerosol by-products of exploded DU munitions are primarily the uranium oxides—uranium trioxide, triuranium octaoxide, and uranium dioxide (OSAGWI, 1998). Uranium trioxide behaves more like a soluble uranyl salt than the insoluble oxides (U3O8 and UO2) and is rapidly removed from the lung (half-time, 4.7 days). More than 20 percent of the exposure burden of UO3 passes into the systemic circulation, and approximately 20 percent of the excreted uranium appears in the urine (Morrow et al., 1972). Conversion of UO3 to uranyl hydroxide hydrate followed by cation exchange with structural hydroxyl groups is a possible mechanism for the high solubility of UO3 in biological fluids (Stuart et al., 1979). Uranium dioxide and triuranium octaoxide have slow dissolution rates (Type S dissolution), and the mechanical processes (mucociliary transport) and particle size determine their pulmonary clearance rates.

Oral exposure. The low rate of gastrointestinal absorption of uranium in humans results in approximately 95 percent of ingested uranium being eliminated in feces without being absorbed; the remainder is excreted in urine (Wrenn et al., 1985; Spencer et al., 1990). The average gastrointestinal uptake of uranium in adult humans is estimated at 1.0–1.5 percent (Leggett and Harrison, 1995). Although differences in uranium uptake with age have not been reported, more definitive information is needed for children (Leggett and Harrison, 1995).

Animal studies indicate that uranium absorption through the gastrointestinal tract depends strongly on its chemical form when ingested and the length of time between the last meal and the ingestion of uranium. Both rats and rabbits absorb about 0.06 percent of ingested uranium in the gastrointestinal tract (Tracy et al., 1992). The distribution and retention of uranium in the skeleton and kidneys of rats are comparable to parameters reported for humans. Studies with rats indicate that the majority of ingested uranium (99 percent) is eliminated in the feces without being cycled through the bile. Most of the uranium absorbed through the gastrointestinal tract is excreted within a few days in urine, with a half-time of 2–6 days (Durbin and Wrenn, 1975).

Depleted uranium fragments. Pellmar and colleagues (1999a) studied the organ distribution of uranium dissolved from DU fragments. The study examined rats that had DU and/or tantalum pellets surgically implanted within the gastrocnemius muscle. Tantalum, an inert metal that is widely used in prosthetic devices,

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