Depleted uranium (DU) is a dense metal that exists as a byproduct of the process used to create enriched uranium for nuclear reactors or weapons. This process removes most of the highly fissile uranium-235 isotope from natural uranium ore. The resulting material, DU, is predominantly composed of the uranium-238 isotope, making it less radioactive than the original ore. Concerns about DU causing long-term health problems, particularly cancer, arose after its extensive use in military munitions and armor. This assessment provides a scientific overview of the properties of depleted uranium, the ways humans can be exposed, and the current evidence regarding its connection to cancer risk.
The Dual Nature of Depleted Uranium
Depleted uranium poses two distinct risks to human health: chemical toxicity and radioactivity. Uranium, regardless of its isotope composition, is a heavy metal, similar to lead or mercury. Its chemical properties are identical to those of natural uranium, and this chemical toxicity is the primary acute hazard associated with DU exposure.
The main target organ for chemical toxicity is the kidney. Once absorbed, the uranium compound can cause damage to the renal tubules, a condition known as nephrotoxicity. This effect occurs because the uranyl ion (UO₂²⁺) is corrosive and impairs kidney function at high exposure levels. Chemical toxicity is a concern even at levels of exposure where the radiological risk is considered minimal.
DU is less radioactive than natural uranium, possessing 40% to 60% of its radioactivity. This is because the more radioactive isotopes, uranium-235 and uranium-234, are largely removed during the enrichment process. DU primarily emits alpha particles, along with some minor beta and gamma radiation. Alpha particles cannot penetrate the skin and pose little external threat. However, they become biologically hazardous if DU is internalized, as the radiation is then released directly into internal tissues.
Routes of Human Exposure and Internal Movement
Exposure to depleted uranium occurs through several specific pathways, with the most significant one being the inhalation of fine dust. When DU munitions strike a hard target, the intense heat and impact aerosolize the metal, creating microscopic, respirable particles of uranium oxide. These particles can be inhaled deep into the lungs by military personnel or civilians in the vicinity of the impact.
The second primary route is through wound contamination, where fragments or shrapnel of DU metal become embedded in soft tissue. These fragments can remain in the body for many years, slowly dissolving and providing a continuous, low-level source of systemic exposure. Ingestion of DU through contaminated water, food, or soil is a third, less efficient pathway for internal exposure.
Once depleted uranium enters the body, its biological fate depends on the compound’s solubility. Insoluble uranium oxide particles inhaled into the lungs can remain there for years, while soluble compounds are quickly absorbed into the bloodstream. Uranium in the blood is rapidly filtered by the kidneys, which is why the kidney is the most susceptible organ to chemical damage. Soluble uranium has a relatively short biological half-life in the kidney of about 15 days. Uranium that is absorbed and not immediately excreted is eventually deposited in the skeleton, where it can be retained for years or even decades.
Scientific Evidence Linking DU to Cancer
Determining if depleted uranium causes cancer involves examining large-scale population studies and theoretical mechanisms of carcinogenesis. Epidemiological studies focusing on veterans exposed during conflicts, such as the Gulf War and the Balkans, have been the primary tool for long-term health surveillance. These studies have not found a statistically significant increase in overall cancer incidence or mortality that can be definitively attributed to DU exposure.
Long-term monitoring of occupational workers in uranium processing facilities has also failed to establish a direct link between uranium exposure and increased cancer rates. The elevated lung cancer risk historically observed in uranium miners is primarily attributed to their co-exposure to radon and its decay products, which are significantly more radioactive than uranium itself.
The theoretical cancer risk from DU stems from two potential mechanisms: chemical genotoxicity and localized radiation damage. Laboratory studies have shown that DU particles can induce chromosomal breaks and damage DNA in human lung cells, suggesting a potential for carcinogenesis through chemical action. However, these in vitro findings have not been conclusively validated in human populations at environmentally relevant exposure levels.
For individuals with embedded DU fragments, the concern is localized radiation from alpha particles damaging surrounding tissue over time. Despite this theoretical risk, a long-term surveillance program tracking Gulf War veterans with retained DU fragments has not yet shown an increased incidence of leukemia, bone cancer, or lung cancer. Major health and regulatory bodies have acknowledged the need for continued monitoring but have not classified depleted uranium as a definitive human carcinogen. The International Agency for Research on Cancer (IARC) classifies mixtures of uranium isotopes as having limited evidence in humans for carcinogenicity, and neither the U.S. Environmental Protection Agency (EPA) nor IARC has classified depleted uranium itself with respect to carcinogenicity.