Edouard Bastarache on fri 24 aug 01
Executive summary=20
This scientific review on depleted uranium is part of the World Health =
Organization's (WHO's) ongoing process of assessment of possible health =
effects of exposure to chemical, physical and biological agents. =
Concerns about possible health consequences to populations residing in =
conflict areas where depleted uranium munitions were used have raised =
many important environmental health questions that are addressed in this =
monograph.
Purpose and scope
The main purpose of the monograph is to examine health risks that could =
arise from exposure to depleted uranium. The monograph is intended to be =
a desk reference providing useful information and recommendations to WHO =
Member States so that they may deal appropriately with the issue of =
depleted uranium and human health.
Information is given on sources of depleted uranium exposure, the likely =
routes of acute and chronic intake, the potential health risks from both =
the radiological and chemical toxicity standpoints and future research =
needs. Several ways of uptake of compounds with widely different =
solubility characteristics are also considered.
Information about uranium is used extensively because it behaves in the =
body the same way as depleted uranium.
Uranium and depleted uranium
Uranium is a naturally occurring, ubiquitous, heavy metal found in =
various chemical forms in all soils, rocks, seas and oceans. It is also =
present in drinking water and food. On average, approximately 90 =B5g =
(micrograms) of uranium exist in the human body from normal intakes of =
water, food and air; approximately 66% is found in the skeleton, 16% in =
the liver, 8% in the kidneys and 10% in other tissues.
Natural uranium consists of a mixture of three radioactive isotopes =
which are identified by the mass numbers 238U(99.27% by mass), =
235U(0.72%) and 234U(0.0054%).
Uranium is used primarily in nuclear power plants; most reactors require =
uranium in which the 235U content is enriched from 0.72% to about 3%. =
The uranium remaining after removal of the enriched fraction is referred =
to as depleted uranium. Depleted uranium typically contains about 99.8% =
238U, 0.2% 235U and 0.0006% 234U by mass.
For the same mass, depleted uranium has about 60% of the radioactivity =
of uranium.
Depleted uranium may also result from the reprocessing of spent nuclear =
reactor fuel. Under these conditions another uranium isotope, 236U may =
be present together with very small amounts of the transuranic elements =
plutonium, americium and neptunium and the fission product =
technetium-99. The increase in the radiation dose from the trace amounts =
of these additional elements is less than 1%. This is insignificant with =
respect to both chemical and radiological toxicity.
Uses of depleted uranium
Depleted uranium has a number of peaceful applications: counterweights =
or ballast in aircraft, radiation shields in medical equipment used for =
radiation therapy and containers for the transport of radioactive =
materials.
Due to its high density, which is about twice that of lead, and other =
physical properties, depleted uranium is used in munitions designed to =
penetrate armour plate. It also reinforces military vehicles, such as =
tanks.
Exposure and exposure pathways
Individuals can be exposed to depleted uranium in the same way they are =
routinely exposed to natural uranium, i.e. by inhalation, ingestion and =
dermal contact (including injury by embedded fragments).
Inhalation is the most likely route of intake during or following the =
use of depleted uranium munitions in conflict or when depleted uranium =
in the environment is re-suspended in the atmosphere by wind or other =
forms of disturbance. Accidental inhalation may also occur as a =
consequence of a fire in a depleted uranium storage facility, an =
aircraft crash or the decontamination of vehicles from within or near =
conflict areas.
Ingestion could occur in large sections of the population if their =
drinking water or food became contaminated with depleted uranium. In =
addition, the ingestion of soil by children is also considered a =
potentially important pathway.
Dermal contact is considered a relatively unimportant type of exposure =
since little of the depleted uranium will pass across the skin into the =
blood. However, depleted uranium could enter the systemic circulation =
through open wounds or from embedded depleted uranium fragments.
Body retention
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 excreted in the =
urine in 24 hours.
Typically between 0.2 and 2% of the uranium in food and water is =
absorbed by the gastrointestinal tract. Soluble uranium compounds are =
more readily absorbed than those which are insoluble.
Health effects
Potentially depleted uranium has both chemical and radiological toxicity =
with the two important target organs being the kidneys and the lungs. =
Health consequences are determined by the physical and chemical nature =
of the depleted uranium to which an individual is exposed, and to the =
level and duration of exposure.
Long-term studies of workers exposed to uranium have reported some =
impairment of kidney function depending on the level of exposure. =
However, there is also some evidence that this impairment may be =
transient and that kidney function returns to normal once the source of =
excessive uranium exposure has been removed.
Insoluble inhaled uranium particles, 1-10 =B5m in size, tend to be =
retained in the lung and may lead to irradiation damage of the lung and =
even lung cancer if a high enough radiation dose results over a =
prolonged period.
Direct contact of depleted uranium metal with the skin, even for several =
weeks, is unlikely to produce radiation-induced erythema (superficial =
inflammation of the skin) or other short term effects. Follow-up studies =
of veterans with embedded fragments in the tissue have shown detectable =
levels of depleted uranium in the urine, but without apparent health =
consequences. The radiation dose to military personnel within an =
armoured vehicle is very unlikely to exceed the average annual external =
dose from natural background radiation from all sources.
Guidance on chemical toxicity and radiological dose
The monograph gives for the different types of exposure the tolerable =
intake, an estimate of the intake of a substance that can occur over a =
lifetime without appreciable health risk. These tolerable intakes are =
applicable to long term exposure. Single and short term exposures to =
higher levels may be tolerated without adverse effects but quantitative =
information is not available to assess how much the long term tolerable =
intake values may be temporarily exceeded without risk.
The general public's ingestion of soluble uranium compounds should not =
exceed the tolerable intake of 0.5 =B5g per kg of body weight per day. =
Insoluble uranium compounds are markedly less toxic to the kidneys, and =
a tolerable intake of 5 =B5g per kg of body weight per day is =
applicable.
Inhalation of soluble or insoluble depleted uranium compounds by the =
public should not exceed 1 =B5g/m3 in the respirable fraction. This =
limit is derived from renal toxicity for soluble uranium compounds, and =
from radiation exposure for insoluble uranium compounds.
Excessive worker exposure to depleted uranium via ingestion is unlikely =
in workplaces where occupational health measures are in place.
Occupational exposure to soluble and insoluble uranium compounds, as an =
8-hour time weighted average should not exceed 0.05 mg/m3. This limit is =
also based both on chemical effects and radiation exposure.
Radiation dose limits
Radiation dose limits are prescribed for exposures above natural =
background levels.
For occupational exposure, the effective dose should not exceed 20 =
millisieverts (mSv) per year averaged over five consecutive years, or an =
effective dose of 50 mSv in any single year. The equivalent dose to the =
extremities (hands and feet) or the skin should not exceed 500 mSv in a =
year.
For exposure of the general public the effective dose should not exceed =
1 mSv in a year; in special circumstances, the effective dose can be =
limited to 5 mSv in a single year provided that the average dose over =
five consecutive years does not exceed 1 mSv per year. The equivalent =
dose to the skin should not exceed 50 mSv in a year.
Assessment of intake and treatment
For the general population it is unlikely that the exposure to depleted =
uranium will significantly exceed the normal background uranium levels. =
When there is a good reason to believe that an exceptional exposure has =
taken place, the best way to verify this is to measure uranium in the =
urine.
The intake of depleted uranium can be determined from the amounts =
excreted daily in urine. depleted uranium levels are determined using =
sensitive mass spectrometric techniques; in such circumstances it should =
be possible to assess doses at the mSv level.
Faecal monitoring can give useful information on intake if samples are =
collected soon after exposure.
External radiation monitoring of the chest is of limited application =
because it requires the use of specialist facilities, and measurements =
need to be made soon after exposure for the purpose of dose assessment. =
Even under optimal conditions the minimum doses that can be assessed are =
in the tens of mSv.
There is no suitable treatment for highly exposed individuals that can =
be used to appreciably reduce the systemic content of depleted uranium =
when the time between exposure and treatment exceeds a few hours. =
Patients should be treated based on the symptoms observed.
Conclusions: Environment
Only military use of depleted uranium is likely to have any significant =
impact on environmental levels. Measurements of depleted uranium at =
sites where depleted uranium munitions were used indicate only localized =
(within a few tens of metres of the impact site) contamination at the =
ground surface. However, in some instances the levels of contamination =
in food and ground water could rise after some years and should be =
monitored and appropriate measures taken where there is a reasonable =
possibility of significant quantities of depleted uranium entering the =
food chain. The WHO guidelines for drinking-water quality, 2 =B5g of =
uranium per litre, would apply to depleted uranium.
Where possible clean-up operations in conflict impact zones should be =
undertaken where there are substantial numbers of radioactive particles =
remaining and depleted uranium contamination levels are deemed =
unacceptable by qualified experts. Areas with very high concentrations =
of depleted uranium may need to be cordoned off until they are cleaned =
up
Since depleted uranium is a mildly radioactive metal, restrictions are =
needed on the disposal of depleted uranium. There is the possibility =
that depleted uranium scrap metal could be added to other scrap metals =
for use in refabricated products. Disposal should conform to appropriate =
recommendations for use of radioactive materials.
Conclusions: Exposed populations
Limitation on human intake of soluble depleted uranium compounds should =
be based on a tolerable intake value of 0.5 =B5g per kg of body weight =
per day, and that the intake of insoluble depleted uranium compounds =
should be based on both chemical effects and the radiation dose limits =
prescribed in the International Basic Safety Standards (BSS) on =
radiation protection. Exposure to depleted uranium should be controlled =
to the levels recommended for protection against radiological and =
chemical toxicity outlined in the monograph for both soluble and =
insoluble depleted uranium compounds.
General screening or monitoring for possible depleted uranium-related =
health effects in populations living in conflict areas where depleted =
uranium has been used is not necessary. Individuals who believe they =
have been exposed to excessive amounts of depleted uranium should =
consult their medical practitioner for examination, appropriate =
treatment of any symptoms and follow-up.
Young children could receive greater depleted uranium exposure when =
playing within a conflict zone because of hand-to-mouth activity that =
could result in high depleted uranium ingestion from contaminated soil. =
This type of exposure needs to be monitored and necessary preventative =
measures taken.
Conclusions: Research
Gaps in knowledge exist and further research is recommended in key areas =
that would allow better health risk assessments to be made. In =
particular, studies are needed to clarify our understanding of the =
extent, reversibility and possible existence of thresholds for kidney =
damage in people exposed to depleted uranium. Important information =
could come from studies of populations exposed to naturally elevated =
concentrations of uranium in drinking water.
Later,
Edouard Bastarache
Irreductible Quebecois
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://www.absolutearts.com/portfolios/e/edouardb/
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