CAS ID#: 7440-61-1

Affected Organ Systems: Developmental (effects while organs are developing), Renal (Urinary System or Kidneys), Respiratory (From the Nose to the Lungs)

Cancer Classification:  Please contact NTP, IARC, or EPA with questions on cancer and cancer classification.

Chemical Classification: Inorganic substances, Radionuclides (radioactive materials)


Uranium is a common naturally occurring and radioactive substance. It is a normal part of rocks, soil, air, and water, and it occurs in nature in the form of minerals – but never as a metal. Uranium metal is silver-colored with a gray surface and is nearly as strong as steel. Natural uranium is a mixture of three types or isotopes called U-234/234U, U-235/235U and U-238/238U. All three are the same chemical, but they have different radioactive properties.

Typical concentrations in soil are a few parts per million ppm. Some rocks contain high enough mineral concentrations of uranium to be mined. The rocks are taken to a chemical plant where the uranium is taken out and made into uranium chemicals or metal. The remaining sand is called mill tailings. Tailings are rich in the chemicals and radioactive materials that were not removed, such as radium and thorium.

One of the radioactive properties of uranium is half-life, or the time it takes for half of the isotope to give off its radiation and change into another substance. The half-lives are very long around 200,000 years for 234U, 700 million years for 235U, and 5 billion years for 238U. This is why uranium still exists in nature and has not all decayed away.

The isotope 235U is useful as a fuel in powerplants and weapons. To make fuel, natural uranium is separated into two portions. The fuel portion has more 235U than normal and is called enriched uranium. The leftover portion with less 235U than normal is called depleted uranium, or DU. Natural, depleted, and enriched uranium are chemically identical. Du is the least radioactive and enriched uranium the most.

Community Members
Community Members

ToxFAQs - Fact sheet that answers the most frequently asked questions about a contaminant and its health effects.

Public Health Statement - Summary about a hazardous substance taken from Chapter One of its respective ATSDR Toxicological Profile.

National Report on Human Exposure to Environmental Chemicals - Provides an ongoing assessment of the exposure of the U.S. population to environmental chemicals using biomonitoring.

Toxicological and Health Professionals
Toxicological and Health Professionals

Toxicological Profile - Succinctly characterizes the toxicologic and adverse health effects information for a hazardous substance.

ToxGuidepdf icon[126 KB] - Quick reference guide providing information such as chemical and physical properties, sources of exposure, routes of exposure, minimal risk levels, children's health, and health effects for a substance.

Priority List of Hazardous Substances - Prioritization of substances based on a combination of their frequency, toxicity, and potential for human exposure at National Priorities List (NPL) sites.

Minimal Risk Levels (MRL) - The MRL is an estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse, non-cancer health effects over a specified duration of exposure. The information in this MRL serves as a screening tool to help public health professionals decide where to look more closely to evaluate possible risk of adverse health effects from human exposure.

Interaction Profiles - Succinctly characterizes the toxicologic and adverse health effects information for mixtures of hazardous substances.

Medical Education and Training
Medical Education and Training

Case Study in Environmental Medicine (CSEM) - Self-instructional publication designed to increase primary care provider's knowledge of a hazardous substance in the environment and to aid in the evaluation of potentially exposed patients.

Page last reviewed: February 10, 2021