Depleted uranium projectiles

["Otto G. Raabe" <ograabe@ucdavis.edu>]

January 10, 2001
Davis, CA

There are several reports in the news about the implied toxicity of
depleted uranium used for projectiles and shielding material in modern
warfare. It has been suggested to be a potent carcinogen and leukemia inducer.

The toxicity of uranium has been under study for al least 50 years
including life span studies in small animals. Depleted uranium is only very
weakly radioactive, and virtually all of the observed or expected effects
are from nephrotoxicity associated with deposition in the kidney tubules
and glomeruli damage at high doses. The radiation doses from depleted
uranium (specific activity only 15 Bq/mg)(U-238 has a 4.5 billion year half
life)are very small compared to potential toxic effects from uranium ions
in the body (primarily damage to kidney tubules). The main route of
potentially hazardous exposure is inhalation since gastrointestinal uptake
is very small (<1/10,000). 

Consider, for example the deposition of a respirable particle of depleted
uranium dioxide in the human lung. If that particle is approximately
spherical and has a diameter of 1 micrometer (aerodynamic diameter about 3
micrometer), it will emit an average of only one alpha particle every 100
days. Meanwhile the cells of the lung are being irradiated in a milieu of
even more energetic alpha particles from natural radon and its decay
products that are present in all the air on the surface of the earth. The
total radiation dose to the lung from even relatively high exposures to
airborne depleted uranium particles is not remarkable. The TLV is 0.2
mg/cubic-meter based on chemical toxicity. 

After inhalation, uranium will be slowly mobilized and enter the systemic
circulation. The uranyl ion is the form of mobile uranium within the body.
It deposits at bone surfaces and remains in the bone matrix with a half
time of up to one year. It is slowly cleared to the blood and excreted via
the kidneys. While in the bone, alpha radiation is emitted, but with very
low intensity since depleted uranium is not very radioactive. The range of
alpha radiation in the bone is about 30 micrometer and the radiation is
very diffuse, so the bone marrow is not effectively irradiated by uranium
in the bone. Radiation induction of leukemia requires effective high
dose-rate irradiation of the bone marrow. There is no known or expected
leukemia risk associated with small amounts of U-238 in the bone because
the marrow is not efficiently irradiated. [The same is true for much more
highly radioactive radium-226 and plutonium-239.]

As to its "heavy Metal" toxicity, the closest analogy is lead. However,
metallic lead has considerably higher toxicity than metallic uranium.
Compounds of lead are much more hazardous than compounds of uranium since
uranium tends to form relatively insoluble compounds which are not readily
absorbed into the body. Also, lead within the body affects the nervous
system and several biochemical processes, while the uranyl ion does not
readily interfere with any major biochemical process except for depositing
in the tubules of kidney where damage occurs if excess deposition occurs.
Glomeruli damage has been reported at high doses as well. The kidney damage
is dosage dependent and somewhat reversible. Lead bullets are probably more
dangerous than uranium bullets.

References: "Handbook of the Toxicology of Metals", Friberg et al.(1990),
"Uranium, Plutonium, Transplutonium Elements", Hodge et al. (1973), "A five
year inhalation study with natural uranium dioxide", HEALTH PHYS 25,
230-258 (1973), "Depleted Uranium In The Gulf":
http://www.gulflink.osd.mil/du_ii

Otto
**********************************************
Prof. Otto G. Raabe, Ph.D., CHP
Institute of Toxicology & Environmental Health
(Street Address: Bldg. 3792, Old Davis Road) 
University of California, Davis, CA 95616
E-Mail: ograabe@ucdavis.edu
Phone: (530) 752-7754   FAX: (530) 758-6140
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