Uranium and Depleted Uranium
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 (increasing U-235, the fissionable isotope concentration) in the nuclear power industry in which nearly all the radioactive isotope U-234 and about two thirds of the U-235 are removed. Thus, DU is almost entirely U-238 and is about 60% as radioactive as natural uranium. DU can also contain traces of other radioactive isotopes introduced during processing.
Chemically, physically and toxicologically, DU behaves in the same way as the metallic form of natural uranium. Fine particles of both metals ignite easily, producing oxides.
Uses of Depleted Uranium
Peaceful applications of DU have included counterweights in aeroplanes, shields against radiation in medical radiotherapy units and for transport of radioactive isotopes.
DU is used for heavy tank armour, anti-tank munitions, missiles and projectiles due to its high density and melting point, and availability. DU weapons are regarded as conventional weapons and are used freely by armed forces.
Concerns Raised by the Use of Depleted Uranium
DU is released from fired weapons in the form of small particles or dust that may be inhaled, ingested or remain in the environment.
DU weapons may affect the health of the populations living in the conflict areas in the Gulf and the Balkans. There is speculation that the "Gulf War Syndrome" is linked to DU exposure, but no causal relationship has yet been established.
DU was released into the environment by planes that crashed (e.g. Amsterdam, Netherlands in 1992; Stansted, United Kingdom in January 2000); this has raised the concern of governments and non-governmental organizations.
Depleted Uranium and Human Health
The effects of DU on human health are complex, due to its chemical form that enters the body, and may be caused by both chemical and radiological mechanisms.
Information on health and environmental effects of uranium is limited. However, since uranium and DU are essentially the same, except for the composition of their radioactive components, scientific studies on natural uranium are applicable to DU.
For the radiation effects of DU, the picture is further complicated since most data refer to the health effects of natural and enriched uranium.
Health effects depend on: the route and magnitude of exposure (ingestion, inhalation, contact or in wounds) and the characteristics of DU (particle size and solubility). The likelihood of detection of possible effects may depend on the setting (military, civil, occupational).
Types of Exposure
On average, approximately 90 µg of uranium exists in the human body from normal intakes of food, air and water; approximately 66% in the skeleton, 16% in liver, 8% in kidneys and 10% in other tissues.
External exposure occurs during proximity to DU metal (e.g., working in a munitions store or vehicle equipped with DU munitions or DU armour) or through contact with dust or shrapnel created following explosion or impact. Effects due to external exposure only (no ingestion, inhalation or dermal absorption) would be limited to radiological effects.
Internal exposure occurs by ingestion and inhalation. In the military environment, wounds represent an additional route of exposure if these are caused by the impact of DU projectiles or armour.
Absorption of Uranium into the Body
Most (>95%) uranium entering the body is not absorbed, but is eliminated via the faeces.
Of the uranium that is absorbed into the blood, approximately 67% will be filtered by the kidney and be excreted in the urine in 24 hours.
Uranium is distributed to the kidney, bones and liver. The time for half of the uranium to be excreted in the urine has been estimated to be in the range from 180 to 360 days.
Uranium causes kidney damage in experimental animals, and some studies indicate that long-term exposure may result in damage to kidney function in humans. The types of damage that have been observed are nodular changes to the surface of the kidney, lesions to the tubular epithelium and increased levels of glucose and protein in the urine.
DU decays mainly through emission of alpha particles that do not penetrate the external skin layers but may affect internal body cells (which are more susceptible to the ionizing effects of alpha radiation) when DU is ingested or inhaled. Exposure to alpha and beta radiation from inhaled insoluble DU particles may therefore lead to lung tissue damage and increase the probability of lung cancer. Similarly absorption into the blood and retention in other organs, notably the skeleton, is assumed to carry an additional risk of cancer in these organs, depending on the level of radiation exposure. However, at low radiation exposure levels, the risk of cancer is thought to be very low.
Up to now, no adverse health effects have been established in the limited epidemiological studies of internal exposure to radiation through ingestion and inhalation of DU particles or through skin lesions and wounds contaminated by DU or in studies of uranium workers exposed to natural or enriched uranium.
Could DU Cause Leukaemia Among Military Personnel?
Leukaemia occurs in adult populations in the world at an incidence rate of about 50 cases per million per year for the age group range of 20-45 years. The exact rate of leukaemia will vary depending on the country. While exposure to DU could theoretically lead to a risk of cancer, this seems unlikely to have occurred among military personnel in the Balkans for the following reasons:
- Generally several years (at least two to five years) are needed between exposure to ionizing radiation and clinical detection of radiation-induced leukaemia.
- While ionizing radiation exposure is known to cause leukaemia, the risk is proportional to the level of radiation exposure. As is illustrated from previous experience, about half the leukaemia cases among survivors of the atomic bomb on Hiroshima and Nagasaki are attributable to gamma-ray and neutron exposure from the bombing. In contrast, about 10% of leukaemia deaths in a large multinational study of nuclear industry workers are thought to be attributable to external gamma-ray exposure. Furthermore, while a large increase in childhood thyroid cancer has been demonstrated 15 years after the Chernobyl nuclear reactor accident, as yet, no increase in leukaemia has been detected in populations environmentally exposed in the most contaminated territories.
- No radiation-related increases in leukaemia have been established in uranium miners or workers milling uranium metal for nuclear reactor fuel elements.
- In war zones, the inhalation and ingestion of DU contaminated dust, even under extreme conditions, and shortly after the impact of projectiles, as determined by the amount of dust that can be inhaled, has been calculated to result in a radiation exposure of less than about 10 millisieverts (mSv). This represents about half the annual dose limit for radiation workers. Such an exposure is thought to result only in a small proportional increase in the risk of leukaemia, of the order of 2% over the natural incidence.
While from the science it appears unlikely that an increased leukaemia risk related to DU exposure would be detectable among military personnel in the Balkans, the World Health Organization (WHO) does not have enough information on the exposure situations in the Gulf or Balkans to make firm conclusions. A detailed study is 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 to coarse particles, and whether or not the excess of leukaemia reported among military personnel is above the normal incidence. It is important that when studies of military personnel possibly exposed to DU are carried out, information on all possible risk factors (including other environmental exposures, etc.) for leukaemia are collected so that any possible cause of leukaemia is not missed.
DU in the Environment
In arid regions, most DU remains on the surface as dust. It is dispersed into soil more easily in 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.
Young children rather than adults could 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.
WHO has guidelines on uranium that would also apply to DU. Currently these are:
- The guideline for drinking-water quality (2 µg/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 µg/kg of body weight/day WHO (1998).
- Limits of ionizing radiation exposure of 1 mSv/year for the general public and 20 mSv/year averaged over five years for radiation workers (Basic Safety Standards,1996).
An extensive literature review has been undertaken to determine the generic health consequences of exposure to uranium and DU. A forthcoming WHO monograph summarizes the results of the review.
WHO's 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.
WHO continues to provide advice to the United Nations Balkan Task Force (United Nations Environment Programme-UNEP) and on the possible environmental health consequences of the Gulf War.
WHO, through its International Agency for Research on Cancer (IARC), continues to study the effects of low-level exposure to ionizing radiation in order to improve the scientific bases of radiation protection. In particular, a study designed to assess whether there has been an increased rate of cancer amongst military personnel who served in the Gulf War or the Balkans, as well as amongst exposed populations (and, if appropriate, to evaluate the possible role of DU in the increase) is being planned.
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. Such data need to be linked to the extensive knowledge base of the environmental and physiological behaviour of uranium compounds.
- Improving our understanding through valid scientific studies of the reproductive, mutagenic and carcinogenic properties of uranium and, by inference, depleted uranium.
There are many gaps in knowledge about DU that need further research. A coordinated effort is needed to obtain valid information through high quality research so that better health risk assessments can be made and more precise recommendations can be provided on the need to clean up after conflicts.
Given the remaining uncertainties about the effects of DU, it seems reasonable to undertake clean-up operations in impact zones where there are substantial numbers of radioactive particles remaining. If there are very high concentrations of DU, then areas may need to be cordoned off until the particles are removed. This is especially the case where children are likely to be present.
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