8 January 2001

Background Material on Depleted Uranium (DU) prepared by Dr. Michael H. Repacholi of the World Health Organization, Geneva What is DU? Uranium is a naturally occurring element used, among other applications, in the generation of nuclear power. Naturally occurring uranium has three principal radioactive isotopes, namely U-238, U-235 and U-234. Depleted uranium (DU) is a by-product of the process of uranium enrichment in the nuclear power industry in which nearly all the radioactive isotopes U-234 and about two thirds of the U-235 are removed. Thus, DU is almost entirely U-238 and is about 40% as radioactive as natural uranium. Chemically, physically and toxicologically, the metallic form of DU behaves in the same way as the metallic form of natural uranium. Fine particles of the metal ignite easily, producing oxides. It is used for heavy tank armour, anti-tank munitions, missiles and projectiles due to its high density and availability. DU weapons are regarded as conventional weapons and as such can be freely used by the armed forces. Depending upon the type of impact a fraction of the DU may be released in the form of small, relatively insoluble particles of uranium oxide as well as relatively large pieces of metallic uranium. Peaceful applications of DU include counterweights in aeroplanes and boats, shields against radiation in medical radiotherapy units and for transport of radio isotopes. Concerns raised by the use of DU DU is released from fired weapons in the form of small particles that may be inhaled, ingested or remain in the environment. The use of DU weapons in the Balkans, particularly in Kosovo, has raised concerns about the health risks of the populations living in the affected areas. There is speculation that the "Gulf War Syndrome" is linked to exposure to DU, but no causal relationship has been established. DU as ballast in planes that crashed (e.g Amsterdam in 1992.; Stansted, UK in January 2000) has raised the concern of governments and NGOs. The report of the Dutch commission of enquiry concluded that some of the DU unaccounted for had been dispersed as small particles which could have been inhaled by rescue workers and local populations. What are the effects of DU on human health? They are complex, due to the chemical, radiological and physical characteristics of DU. Sound scientific information on various health and environmental effects is limited since most data refers to the health effects of uranium. However, since uranium and DU are essentially the same except for the composition of their radioactive components, scientific studies on metallic uranium will be applicable to DU. Health effects depend on the type of exposure (internal or external), route of exposure (ingestion, inhalation, wounds), characteristics of DU (soluble or insoluble particles), and setting (military, civil, occupational.). External exposure occurs during the physical handling of DU metal (e.g., working in a munitions store or vehicle equipped with DU munitions or DU armour) or through contact with dusts created following explosion or impact. Effects due to external exposure only (no ingestion or inhalation) would be limited to ionizing radiation. Internal exposure occurs by ingestion, inhalation and also through wounds. Effects are caused by both chemical and radiological toxicity. Inhalation and ingestion of large quantities of soluble compounds of DU may produce acute effects on the kidneys. Even for soluble forms, only a few percent of ingested uranium is absorbed into blood. [ICRP Publication 69]. Of this, about 10% is deposited in bones, kidneys and other tissues, and this retention of uranium could potentially cause long-term effects, including cancer. However, at low doses the risks of cancer are minimal. Long-term exposure has been shown to result in damage to kidney function. The ingestion of insoluble particles of DU poses a much lower chemical health risk, as the fraction absorbed is lower still and nearly all the material is eliminated through the GI tract. Absorption by inhalation depends on the size and solubility of the particles. Generally, inhaled uranium oxide particles larger than a few micrometers in diameter will mainly deposit in the upper respiratory tract (nose, mouth and larger airways), from which they quickly pass into the digestive system. For smaller particles, a larger fraction will deposit in the lungs, where they main remain for months or years, unless they dissolve. Very small amounts may be retained in the lymphatic system for longer. For soluble compounds deposited in the lungs the fraction absorbed into blood is generally higher than when ingested, and can be greater than 50%. Insoluble particles pose a higher radiation risk owing to the longer stay in the lungs. In the military environment, wounds represent an additional route of exposure if caused by the impact of DU projectiles or armour. Military personnel are exposed to shrapnel as well as dusts that can be inhaled, ingested or deposited on wounds. Radiological toxicity and exposures DU decays mainly through emission of alpha particles, which cannot penetrate the external skin layers but may affect internal body cells (more susceptible to the ionizing effects of alpha radiation) when DU is ingested or inhaled. However, from the studies undertaken on uranium workers, no negative health effects have been established after internal exposure to radiation through ingestion and inhalation of DU particles or through skin lesions and wounds contaminated by DU. There is a theoretical possibility that exposure to alpha and beta radiation from inhaled insoluble DU particles might lead to lung tissue damage and increase the probability of lung cancer. Similarly absorption into blood and retention in other organs, notably the skeleton, is assumed to carry an additional risk of cancer, depending on the level of radiation dose. Leukaemia Media reports suggest that soldiers in the Kosovo conflict may be at a higher risk of leukaemia from radiation exposure to DU. (Leukaemia occurs in adult populations at an incidence rate of about 50-100 cases per million per year for the age group range of 20-45 years). From current evidence, this is unlikely for several reasons: While ionizing radiation exposure is known to cause leukaemia, significant radiation exposures are necessary. Some 15 years after the Chernobyl nuclear reactor accident, the main cancer resulting was a very steep increase in childhood thyroid cancer. No increase in leukaemia has been detected in the exposed population. No radiation-related increases in leukaemia have been established in uranium miners or workers milling uranium metal to make nuclear fuel elements. Generally several years (normally at least 2-5 years) are needed between exposure to ionizing radiation and clinical detection of leukaemia. Inhalation and ingestion of DU contaminated dust, even under reasonably extreme conditions, and shortly after bombing, as determined by the amount of dust that can be inhaled, it has been calculated to result in a radiation exposure of less than about 10 mSv. This represents about half the annual dose limit for radiation workers. If this amount of soluble DU was inhaled, then severe kidney malfunction would occur below this radiation dose. While from the science it does not appear that leukaemia would result from exposure to DU, WHO does not have any information on the exposure situation regarding military personnel in Kosovo to make definitive conclusions. Detailed surveys are needed to determine the numbers of soldiers exposed, the amount of DU used, how much exists on the surface, how much is buried in the ground, what is the composition of fine and course particle etc., before better conclusions can be made. Breathing ultra-fine particles could lead to a theoretical risk of cancer. DU in the environment In arid regions, most DU remains on the surface as dust. It is dispersed in soil more easily, particularly in the areas of higher rainfall. Cultivation of contaminated soil and use of contaminated water and food may pose health risks, but these are expected to be limited. Chemical toxicity would be expected to be the main health concern rather than the radiation exposure. Children rather than adults may be considered to be more at risk of DU exposure when returning to normal activities within a war zone through contaminated food and water, since typical hand-to-mouth activity of inquisitive play could lead to high DU ingestion from contaminated soil. Standards WHO has been involved in the setting of guidelines on uranium that would apply to DU. Currently these are: Guideline for drinking-water quality (2 ng/litre), a value considered to be protective for sub-clinical renal effects reported in epidemiological studies (WHO, 1998) Tolerable Daily Intake (TDI) for oral exposure to uranium of 0.6 ug/kg of body weight/day WHO (1998). Limits of ionizing radiation exposure of 1 mSv/year for the general public and 20 raSv/year averaged over 5 years for radiation workers (Basic Safety Standards, 1996). Activities in WHO Extensive literature review undertaken to determine the generic health consequences of exposure to Uranium and DU and preparation of a monograph summarising the results of the review (to be peer-reviewed and published before end of February 2001). Preparation of a WHO Fact Sheet. The terms of reference of the scientific review included a requirement to identify gaps in knowledge requiring further research in order to make better health risk assessments of human exposure to DU. A high-level scientific expert group will be convened by WHO to review these requirements for research and make proposals for in-depth research on this issue. Provision of advice to the UN Balkan Task Force (UNEP) Provision of advice on possible environmental health consequences of the gulf war. Research Interim research needs identified so far by the WHO review include: Clarifying our understanding of the degree of kidney damage associated with changes in renal function (and its reversibility) in human populations subject to different levels of uranium exposure Investigating the chemical and physical form, physiological behaviour, leaching and subsequent environmental cycling of specific forms of uranium from various industrial and military sources (e.g. depleted uranium alloys, UFe, phosphate byproducts). To link such data to the existing extensive knowledge base of the environmental and physiological behaviour of uranium compounds produced by the nuclear industry. Improving our understanding of the reproductive, mutagenic and carcinogenic properties of uranium and, by inference depleted uranium. Prepared by: Dr Michael H Repacholi Coordinator, Occupational and Environmental Health Protection of the Human Environment (PHE) World Health Organization, CH-1211 Geneva 27, Switzerland Tel: +41 22 791 3427, Fax: +41 22 791 4123, E-mail: repacholim@who.int In consultation with Dr Elisabeth, International Agency for Research on Cancer, Lyon, France and many reviewers for WHO. 08 January 2001