Answer to Question #754 Submitted to "Ask the Experts"

The toxicity of uranium has been under study for over 50 years, including life-span studies in small animals. Depleted uranium and natural uranium both consist primarily of the uranium isotope ²³⁸U. They are only very weakly radioactive and are not hazardous radioactive toxicants, but uranium is a weak chemical poison that can seriously damage the kidneys at high blood concentrations. Virtually all of the observed or expected effects are from nephrotoxicity associated with deposition in the kidney tubules and glomeruli damage at high blood concentrations of uranium. The ionizing radiation doses from depleted and natural uranium are very small compared to potential toxic effects from uranium ions in the body (primarily damage to kidney tubules).

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 and glomeruli of kidney, where damage may occur at high concentrations. This kidney damage is dosage-dependent and somewhat reversible. The occupational exposure threshold limit air concentration value is 0.2 milligrams uranium per cubic-meter based on chemical toxicity. The main route of potentially hazardous exposure is inhalation since gastrointestinal uptake is relatively small. 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 micrometers and the radiation is very diffuse, so the bone marrow is not effectively irradiated by uranium in the bone. Ionizing 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 ²³⁸U in the bone because the marrow is not efficiently irradiated.

Solid pieces of uranium metal emit measurable amounts of beta and gamma radiation that originates from decay products and can be easily detected with a radiation detector. Uranium-238 decays slowly by alpha emission to form ²³⁴Th. A total of 99.8 percent of the ²³⁴Th decays (6.7 h half-life) by beta emission (average energy 0.05 MeV) to form the ^234mPa. The ^234mPa decays (1.17 minutes half-life) by beta emission (average energy 0.8 MeV) to form ²³⁴U (a weakly radioactive alpha emitter with a 244,500 year half life). Protactinium-234m and ²³⁴Th are in activity equilibrium with the ²³⁸U parent each at about 15 becquerels per milligram of uranium. The beta radiation surface dose rate is about 2 millisieverts per hour (200 millirem per hour). This is a short-range radiation that can penetrate the protective layer of the skin and has the potential to cause some skin damage if the exposure continues for extended periods of time (months or years). It would be unacceptable to wear jewelry or make tokens or coins from uranium metal because of the possible extended exposure of the skin or lens of the eye to beta radiation from pure metallic uranium.

References:

• Friberg et al. Handbook of the toxicology of metals. 1990.
• Hodge et al. Uranium, plutonium, transplutonium elements. New York: Springer-Verlog; 1973.
• Leach LL, Yuile CL, Hodge HC, Sylvester GE, Wilson HB. A five year inhalation study with natural uranium dioxide. Health Phys 25:239-258; 1973.
• Environmental Exposure Report, Depleted Uranium in the Gulf (II).

Professor Otto G. Raabe, PhD, CHP