Uranium is a naturally occurring, mildly radioactive, chemical substance found in almost all soil and rocks that can be processed into a silvery-white metal. Natural uranium is a combination of three isotopes: 234U, 235U, and 238U. 238U is the most common isotope, making up over 99% of natural uranium. All three isotopes behave the same way chemically, yet have different radioactive properties. The half-lives of uranium isotopes (the amount of time needed for half of the isotope to give off its radiation and change into a different element) is very long. The least radioactive isotope is 238U with a half-life of 4.5 billion years.

Uranium is almost as hard as steel and much denser than lead. Enriched uranium occurs through an industrial process during which natural uranium separates into enriched uranium and depleted uranium. Depleted uranium is the byproduct of the enrichment process. There are military and civilian uses for both enriched and depleted uranium. The main use of uranium in the civilian sector is the use of enriched uranium to make fuel for nuclear power plants. Application of uranium in the military sector uses depleted uranium as an equalizer on helicopter rotors and airplane control surfaces, as a shield to protect against ionizing radiation, as a component of weaponries to help penetrate enemy armored vehicles, and as armor in some parts of military vehicles.


Uranium is found almost everywhere, therefore, small amounts of uranium are found in the human body. The primary route of exposure for the general population is ingestion of food and water. The general population can be exposed to uranium in the air; however, it is small in comparison to exposure from food and water. The highest levels of uranium in food have been found in beef liver and kidneys, cow’s milk and root vegetables. People living in close proximity to uranium mills or mines have higher levels of uranium in their soil, therefore possibly increasing levels of uranium in locally grown vegetables.

People working in certain occupations using uranium may be exposed to higher levels of uranium. People who work in uranium mines and mills, and nuclear weapons production, for example, are more likely to be exposed. Glass makers and potters who use uranium containing enamels may be exposed to higher levels as well.

According to the Agency for Toxic Substances and Disease Registry (ATSDR), the normal adult body burden is approximately 90 µg of which 66% is found in bone, 16% in the liver, 8% in the kidneys, and 10% in other tissues.

The range of geometric mean levels of uranium in the urine of the U.S. population is 0.006–0.009 µg U/g creatinine or 0.005–0.010 µg/L urine.

According to the ATSDR, the minimal risk levels (MRLs) for inhalation and oral routes of exposure are as follows:

The 4CSBC uses the National Health and Nutrition Examination Survey (NHANES) 95th percentile level as a screening threshold. Only five percent of the NHANES participants (representing the American population) have levels that exceed the 95th percentile level. Everyone else is below that level. Thus, that level is considered elevated. It may or may not be harmful. The 95th percentile screening threshold for uranium is 0.03 μg/L

Health Concerns

Uranium is not easily absorbed following inhalation, oral or dermal exposure. Absorption is dependent upon the solubility of the compound. Most inhaled and ingested uranium is not absorbed and leaves the body through the feces. Absorbed uranium, however, leaves the body in the urine. Depending on the solubility of the compound, inhaled uranium can stay in the lungs for a long time, ranging from weeks to years.

Toxicity of natural, enriched and depleted uranium can cause damage to the kidneys, reproductive system and the developing organism. Toxicity of enriched uranium also causes an increased risk of radiotoxicity. This has been shown to increase serum testosterone levels and decrease memory in animals.

Neither the National Toxicology Program, International Agency for Research on Cancer nor the Environmental Protection Agency has classified uranium as a possible carcinogen.

Sources of additional information

Agency for Toxic Substances and Disease Registry (ATSDR)

Centers for Disease Control and Prevention (CDC)

U.S. Environmental Protection Agency (EPA)

Last Update: May 5, 2016