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Uranium is a naturally occurring, chemically toxic, and radioactive element composed of three isotopes. Relative to other radionuclides, natural uranium is only slightly radioactive because of its low specific activity.288 When the uranium isotope used for nuclear reactors and weapons is extracted from natural uranium, DU is the byproduct.

DU is nearly twice as dense as lead-a property used to improve the performance of both armor and armor penetrating munitions. During the Gulf War, some U.S. tanks and U.S. aircraft fired DU munitions, which produced shrapnel and an aerosolized dust on impact with armor or on ignition in accidental munitions fires. DU retains natural uranium's toxicological properties and approximately half its radiological activity.267 Most of DU's radiation cannot penetrate skin, and DU poses little threat to human health while it is external to the body.288

Because it is slightly radioactive, natural uranium is considered to be a potential carcinogen-albeit with a small cancer risk relative to other radionuclides.288 Taken together, human and animal studies do not indicate conclusively that natural uranium causes cancer in humans. Epidemiologic studies of uranium miners experiencing extremely high, lifetime, occupational exposures to uranium show an increase in mortality due to lung cancer, but such cancers are thought to be caused by miners' concurrent exposures to radioactive radon gas and its decay products, tobacco smoke, silica and other dusts, or exhaust fumes from diesel engines.172,321 Animal studies conclude that exposure to uranium for long periods of time does not result in increased incidence of cancer, except in the case of one study. This study found prolonged (more than five years) inhalation of high levels of uranium dioxide led to lung neoplasms in dogs.130,131

The chemical toxicity of uranium as a heavy metal is well characterized. In fact, the kidney is the most sensitive organ affected by exposure to uranium and is the critical target organ for risk assessment.133,218,322,341 For this reason, uranium exposure is regulated based on its chemicaltoxicity and not its radiological properties.129,156 Even so, more than 50 years of occupational health data from uranium miners reveal little epidemiologic evidence of excess kidney disease among workers exposed for years or decades.322

The health risks of internalized uranium or DU particles depend on dose, exposure pathway, and solubility of the ingested particle. Ingestion of insoluble uranium compounds poses little health hazard because they pass rapidly through the body and are eliminated in the feces. However, animal studies have shown that ingestion of large doses of relatively soluble uranium compounds are associated with kidney toxicity.129,288 Inhaled uranium particles that are nonrespirable are cleared from the respiratory tract and either expelled from the body (cough) or swallowed and passed to the GI tract. Respirable and relatively soluble particles are cleared to blood and can affect kidney toxicity.14,129 Less soluble particles can remain in the lung longer and in theory could pose a radiological hazard.

The U.S. Army has conducted tests to characterize aerosols associated with DU munitions impacts with armor and with accidental DU munitions fires; it concluded a service member's risk exceeds civilian safety standards only when he or she is inside a vehicle when it is penetrated by DU munitions.39,96,97 The adequacy of the research supporting this conclusion has been questioned by some reviewers.229,267

No studies of long-term human health effects of uranium metal implanted in tissue exist. Nevertheless, toxic effects are likely to be similar to the kidney toxicity observed from inhaled or ingested uranium. To date, VA has reported no kidney toxicity among soldiers wounded by DU fragments in friendly fire episodes.112 VA currently monitors the health of approximately 30 veterans suspected of retaining embedded DU fragments, and the U.S. Army Medical Research and Materiel Command is funding animals studies to investigate the health hazards associated with short- and long-term exposure to DU metal fragments.296