Uranium is a naturally occurring heavy metal that is both radioactive and
ubiquitous. Small amounts are present in rocks, soil, and materials made from
them. It is also present in air, water, and food, and thus people come into
contact with or consume tiny amounts of it daily. Uranium is best known in its
enriched form, which is used for nuclear power plants and nuclear weapons.
Uranium is enriched by increasing the proportion of the more radioactive
isotopes. The by-product of this process, which depletes uranium of its most
radioactive isotopes, is called depleted uranium (DU). It is about 40 percent
less radioactive than natural uranium and has a variety of commercial and
military applications. The latter include improved armor and antiarmor rounds
with increased penetrating power, both of which take advantage of the metal's
density and metallurgical properties.
Very little literature directly addresses the health effects of DU. However, a
wide body of literature deals with the health effects of natural and enriched
uranium, and a review of that literature is relevant. The toxicological
effects of natural uranium are identical to those of DU; while the radiological
effects of DU are always less pronounced because DU is less radioactive than
natural uranium. In general, heavy-metal toxicity is regarded as posing a more
serious health risk than its radiation. This is because DU produces a low
level of radiation per unit mass. The most abundant isotope in natural
uranium, 238U, has a very long half-life (4.5 billion years), which
means that it decays slowly and thus produces fewer disintegrations per mass
than an isotope that decays rapidly, such as 234U (half-life of
245,000 years). As mentioned, DU is less radioactive than natural uranium and,
indeed, is classified in the lowest hazard class of all radioactive materials.
In addition to being very dense (almost twice as dense as lead), DU, like any
uranium, is pyrophoric, that is, in fine particles it can ignite easily. When
a DU penetrator strikes armor or burns, it produces uranium dusts or aerosol
particles, which can be inhaled. Once internalized, a fraction of the
particles dissolve and enter the bloodstream, where most uranium is excreted
from the body through the kidneys. These inhaled fine particulates or dusts of
oxidized uranium metal, uranium oxides, are presently thought in the published
literature to be the primary compounds of interest.
Several different U.S. government agencies regulate and make recommendations
about exposure to uranium. This report provides the present radiological
standards for both occupational and population exposure. Both standards are
important because military personnel are not classified as radiation workers
unless their jobs specifically qualify as such. This report also provides the
chemical toxicity guidelines and recommended kidney concentration standard of 3
µg/g of kidney.
The health effects of DU depend on several factors. The first is whether the
exposure is internal or external. Internal effects can be caused by either
chemical or radiological toxicity. Health effects related to external exposure
are limited to ionizing radiation emitted by uranium and its immediate decay
The main routes of natural uranium into the human body are by inhalation and
ingestion. A very small percentage of the inhaled uranium is retained by the
lymph nodes for a long time, and another small fraction is solubilized and goes
to the blood, where most is subsequently excreted. A fraction of the blood
content is deposited in the kidneys, liver, other organs, and the skeleton. In
the military environment, additional routes of exposure exist. When a DU
weapon impacts a target, the fragments can penetrate the body or any dusts
produced can be inhaled, ingested, or be deposited on wounds.
Chemical Effects of Inhaled Uranium
Extensive information is available about the occupational exposure of workers
in the uranium industry. No increase in overall deaths has been observed as a
result of exposure to uranium in several epidemiological studies of workers
exposed to uranium. One animal study designed to establish exposure end points
induced mortality in rats when they were exposed to extraordinary
concentrations of uranium oxides much higher than in occupational settings.
Evidence about hepatic effects from exposure to uranium oxides is more
equivocal. Hematological effects have been observed in one study of miners
exposed to uranium for up to 20 years. However, most animal studies found no
hematological effects. Immune system effects have not been associated with
inhaled uranium oxide exposure in uranium industry workers, and no evidence
exists showing an association between uranium inhalation exposure and adverse
effects on the nervous system.
The kidney is the target organ for uranium, and, as such, would be expected to
experience the most dramatic health effects if sufficient uranium were present.
A study of uranium mill workers occupationally exposed to "yellowcake," a
soluble uranium compound, reported findings of reduced proximal renal tubular
reabsorption of amino acids and of low molecular weight proteins. Another
study of workers exposed to insoluble uranium dust reported no renal injury.
Animal studies show that the solubility of the inhaled uranium compound
dramatically affects renal outcomes. Although soluble uranium compounds have
caused renal damage, insoluble uranium oxides appear to be far less toxic to
kidneys. When exposure is less than American Conference of Governmental
Industrial Hygienists (ACGIH) recommended amounts, the scientific literature
does not indicate negative health effects.
Radiological Effects of Inhaled Uranium
Negative effects from the exposure to the ionizing radiation from depleted or
natural uranium have not been observed in humans. Some epidemiological studies
show evidence of lung cancer in miners, but this is associated with the
exposure to airborne short-lived decay products of radon and cigarette smoking.
Some animal studies have examined pulmonary damage from exposure to uranium
oxides. Exposures over three years to 5.1 mg UO2/m3 in
air did not result in lung damage but did cause minimal fibrosis, suggesting
radiation injury, in the lymph nodes of dogs and monkeys and the lungs of
Cancer rates in almost 19,000 highly exposed uranium industry workers who
worked at Oak Ridge between 1943 and 1947 have been examined, and no excess
cancers were observed through 1974. Other epidemiological studies of lung
cancer in uranium mill and metal processing plant workers (environments without
radon) have either found no excess cancers or attributed them to known
carcinogens other than uranium, such as radon.
Human studies have examined bone cancers, and no associations between them and
internal or external radiation exposure from natural uranium were reported. A
variety of cancers developed when rats were exposed to enriched uranium;
however, because DU is orders of magnitude less radioactive than enriched
uranium, these data have little relevance to the possible health effects of DU.
High exposures may also be nephrotoxic; however, there is no evidence that
either natural or depleted uranium can induce this effect. Gastrointestinal
effects produced from inhalation of high levels of radioactive material have
been reported in both human and animal studies. There is no conclusive
evidence of reproductive effects.
In sum, cancer is the only radiation-associated disease that has been shown to
be related to inhalation of radioactive particulates in humans, but there is no
evidence documented in the literature of cancer or any other adverse health
effect related to the radiation received from exposure even to natural uranium,
which is more radioactive than DU.
Chemical Effects of Ingested Uranium
Chemical toxicity of ingested uranium is determined largely by the water
solubility of the compound and, therefore, ease of uptake from the
gastrointestinal (GI) tract. Compared with industrial compounds, uranium
oxides are generally thought to be less soluble or insoluble and, therefore, of
very low toxicity.
No studies report human deaths or other health effects from oral exposures to
uranium oxides. Mortality, usually from renal failure, can be induced in
animals at very high oral intake levels. No human studies were found in the
peer-reviewed published literature that showed respiratory, cardiovascular,
hematological, musculoskeletal, hepatic, endocrine, dermal, ocular, body
weight, or other system effects in humans exposed to uranium compounds. No
epidemiological studies were located that examined neurological effects
following uranium ingestion.
While the possible toxic effects of uranium on other organ systems have not
been rigorously excluded, extensive work points to the kidney as the major
target organ. A recent study described analyses of renal function following a
lifetime of drinking water with uranium levels in the range 2-781 µg/day. The
exposed group excreted more glucose in urine than did the control group, but
the only valid measure of glomerular dysfunction tested (proteinuria) remained
Respiratory effects have not been observed in animals following ingestion of
uranium oxides. Hepatic effects have been observed in animals dosed with very
high levels of insoluble uranium. No harmful effects on body weight were seen
in intermediate-duration oral studies of dogs given up to 10 g
UO2/kg/day for a year.
Thus, only limited evidence suggests that even chronic exposure to natural
uranium in food or water, except presumably at extraordinary concentrations, is
associated with morbidity in man or animals. This conclusion makes it unlikely
that DU would have any such effects.
Radiological Effects of Ingested Uranium
No human or animal studies associate adverse health effects with ingested DU.
No evidence has been found to associate human exposure to ingested uranium
compounds and carcinogenesis. Likewise, no oral animal studies report evidence
of cancer induction.
Chemical and Radiological Effects of Embedded Uranium
What is known about the health effects of embedded uranium results primarily
from the clinical follow-up of the wounded veterans of the Gulf War.
Thirty-three individuals are being followed by Department of Veterans Affairs
(VA) researchers, and about half of these have been identified as having
embedded DU fragments. To date, although these individuals have an array of
health problems related to traumatic injuries, no manifestations of kidney
disease attributable to the chemical toxicity of DU have been found. Neither
do they appear to have any manifestations attributable to radiation effects.
These patients continue to be followed. AFRRI is also conducting important
studies of the effect of embedded DU pellets on rats.
Radiation rather than chemical toxicity poses the external health hazard.
DU exposes the skin to alpha, beta, and gamma radiation. In the case of
short-term radiation from particulates deposited on skin, more than 95 percent
of the radiation present is in the form of alpha radiation, which has a very
short range and will not penetrate the dead outer layer of the skin and thus
poses no documented health risk. Beta and gamma radiation from 238U
decay products can irradiate cells in the deeper skin layers. Sufficient mass
of DU to create radiation sufficient to be of concern can occur with intact
munitions and armor. However, DU munitions are shielded to limit emitted
radiation, and thus people working with intact munitions or armor usually face
little risk. The measured exposure to gamma and beta radiation from bare
penetrator or armor is well below recommended occupational levels (CHPPM,
Studies of workers occupationally exposed to uranium show no skin cancers
resulting from this exposure. No animal studies have found skin cancers with
this cause. As a point of perspective, to reach the occupational radiation
dose limit for beta and gamma radiation, a soldier would have to hold an
unshielded DU penetrator for more than 250 hours.
The clinical discussion section outlines possible health effects from a
clinical perspective. After discussing common diagnostic procedures, including
a 24-hour urine analysis, the section details symptoms and the relation, if
any, of diseases of various body systems to exposure to DU. Areas discussed
include lung cancer, kidney disease, liver disease, osteosarcoma, and
reproductive and developmental conditions.
From the scientific literature, the review reaches the following insights and
Finally, the report encourages continued research because the use of DU is
likely to expand in the future.
- Although any increase in radiation to the human body can be calculated
to be harmful from extrapolation from higher levels, there are no peer-reviewed
published reports of detectable increases of cancer or other negative health
effects from radiation exposure to inhaled or ingested natural uranium at
levels far exceeding those likely in the Gulf. This is mainly because the body
is very effective at eliminating ingested and inhaled natural uranium and
because the low radioactivity per unit mass of natural and depleted uranium
means that the mass of uranium needed for significant internal exposure is
virtually impossible to obtain.
- External radiation takes the primary form of alpha radiation, but
amounts of beta and gamma radiation also exist. Alpha radiation is not capable
of penetrating cloth or skin and would therefore have no negative health
effect. Beta and gamma radiation, which can have negative health effects, have
been measured at levels below those expected to be of concern.
- Large variations in exposure to natural uranium in the normal
environment have not been associated with negative health effects.
- Radiation-related effects from embedded fragments will depend on the
size of the fragment and its proximity to vital organs.
- Exposure to uranium and other heavy metals in large doses can cause
changes in renal function and at very high levels result in renal failure.
- In spite of these findings, no increased morbidity or frequency of
end-stage renal disease has been observed in relatively large occupational
populations chronically exposed to natural uranium at concentrations above
normal ambient ones.
- The cohort of individuals, about half of whom have embedded fragments,
who are being followed at the Baltimore VA Medical Center as part of the DU
Follow-Up Program, represents a group of Gulf War veterans who received the
highest levels of exposure to DU during the Gulf War. Although many of these
veterans have health problems related to their injuries in the Gulf War and
those with embedded fragments have elevated urine uranium levels, researchers
to date report neither adverse renal effects attributable to chemical toxicity
of DU nor any adverse health effects they relate to DU radiation (McDiarmid,
1998b). They do, however, note several biochemical perturbations in
neuroendocrine parameters related to urinary uranium concentrations and in some
subtle neuropsychological test findings; the clinical significance of these is