Chemical and Radiological Toxicity of Depleted Uranium

Military Medicine, Mar 2004 by Sztajnkrycer, Matthew D, Otten, Edward J

A by-product of the uranium enrichment process, depleted uranium (DU) contains approximately 40% of the radioactivity of natural uranium yet retains all of its chemical properties. After its use in the 1991 Gulf War, public concern increased regarding its potential radiotoxicant properties. Whereas in vitro and rodent data have suggested the potential for uranium-induced carcinogenesis, human cohort studies assessing the health effects of natural and DU have failed to validate these findings. Heavy-metal nephrotoxicity has not been noted in either animal studies or Gulf War veteran cohort studies despite markedly elevated urinary uranium excretion. No significant residual environmental contamination has been found in geographical areas exposed to DU. As such, although continued surveillance of exposed cohorts and environments (particularly water sources) are recommended, current data would support the position that DU poses neither a radiological nor chemical threat.

Introduction

Fear of radiation is prevalent in modern society. 1 Anxiety and distress are the most pronounced primary health effects after most radiation accidents.1-5 Depleted uranium (DU) has been linked in the lay-press to cancer, the Gulf War syndrome, and the Balkans syndrome. It continues to foment confusion and controversy. The U.S. Armed Forces Radiobiology Research Institute explicitly declared that "DU is neither a radiological nor chemical threat. It is not a weapon of mass destruction."6 However, the United Nations Subcommission on Prevention of Discrimination and Protection of Minorities passed a resolution declaring "weaponry containing depleted uranium" a weapon of mass destruction and called for the end of its use.7

Multiple reports have emphatically stated a lack of cancer risk from exposure to DU.8-11 Yet, a recent report by the Royal Society suggested the possibility of a twofold increase in lung cancer in individuals exposed to DU munitions.12,13 The purpose of this study was to evaluate the current knowledge and understanding of DU and its potential for both chemical and radiological toxicity.

What Is DU?

Uranium, the heaviest naturally occurring element, is a ubiquitous soil component found at an average concentration of 3 �g/g soil. A typical plot of soil 1 square mile in area and 1 foot deep contains 4 tons of uranium.8,14 Naturally occurring uranium is composed of three isotopes, ^sup 234^U, ^sup 235^U, and ^sup 238^U, in the following proportions, respectively: 0.005%, 0.711%, and 99.283%.15 During the process of nuclear fuel and weapons production, naturally occurring uranium is processed to increase the percentage of available ^sup 235^U. This "enriched uranium" contains more than 0.711% ^sup 235^U by mass. The by-product of the enrichment process is DU, which by definition contains less than 0.711% ^sup 235^U by mass.15 DU typically contains 70% less ^sup 235^U and 80% less ^sup 234^U than naturally occurring uranium.14,15 DU manufactured from spent uranium fuel rods additionally may contain trace levels of plutonium, neptunium, americium, technetium, and ^sup 236^U. These impurities are found in parts per billion concentrations, and increase radioactivity by less than 1%.16

Sources of DU Exposure

DU has been used in both civilian and military technology with particular emphasis placed upon its military use. DU has several properties that make it useful as a military kinetic penetrator munition. Uranium is the heaviest naturally occurring element, approximately 1.7 times denser than lead. In addition, DU is pyrophoric and has self-sharpening properties.15,17 These properties make DU an ideal munition with which to defeat protective armor. In contrast, other armor-piercing incendiary munitions such as tungsten tend to mushroom and become blunt upon armor penentration.18

During the 1991 Gulf War, DU rounds were extensively deployed by the United States. In addition to 288 DU-containing Tomahawk cruise missiles, between 14,000 and 940,000 armor-piercing incendiary rounds were fired.19 More recently, U.S. military aircraft fired 10,000 30-mm DU rounds (approximately 3.3 tons of DU) at 12 sites in Bosnia-Herzegovina between 1994 and 1995 and another 31,000 DU rounds (10.2 tons) at 85 sites in Kosovo in 1999.18,19

Although less well publicized, DU is used commercially as ballast in yachts and wide-body commercial jet liners, including the DC-10 and 747.20 The Boeing 747 involved in the 1992 Amsterdam accident reportedly contained 282 kg of DU counterweights.21 DU has also been used as shielding in radiation therapy.

Chemical Characteristics of DU

DU is unique in that it possesses both potential chemical and radiological toxicant properties. Based upon reports by both the Agency for Toxic Substances and Disease Registry and the World Health Organization, the major health concern from DU is toxicological rather than radiological.8,11

Chemically, uranium causes toxicity as a heavy metal with characteristics similar to the alkaline earth metal ions.14 The principal target organ is the kidney.8,11,22 Toxicity is dependent upon the solubility of the uranium compound.19,23 Soluble compounds, such as halides and urinates, demonstrate absorption rates from the gastrointestinal and respiratory tracts 10- to 20-fold greater than insoluble uranium oxides (U^sub 3^O^sub 8^, UO^sub 2^, UO^sub 3^), and demonstrate increased nephrotoxicity.23