An official website of the United States government.

Due to a lapse in appropriations, EPA websites will not be regularly updated. In the event of an environmental emergency imminently threatening the safety of human life or where necessary to protect certain property, the EPA website will be updated with appropriate information. Please note that all information on the EPA website may not be up to date, and transactions and inquiries submitted to the EPA website may not be processed or responded to until appropriations are enacted.

We've made some changes to If the information you are looking for is not here, you may be able to find it on the EPA Web Archive or the January 19, 2017 Web Snapshot.

Radiation Protection


Radioactive forms of elements are called radionuclideHelpradionuclideRadioactive forms of elements are called radionuclides. Radium-226, Cesium-137, and Strontium-90 are examples of radionuclides.s. Some occur naturally in the environment, while others are man-made, either deliberately or as byproducts of nuclear reactions. Learn the Radiation Basics. 

Every radionuclide emits radiation at its own specific rate, which is measured in terms of half-life. Radioactive half-life is the time required for half of the radioactive atoms present to decay. Radioactive decay is when a radioisotope transforms into another radioisotope; this process emits radiation in some form. Some radionuclides have half-lives of mere seconds, but others have half-lives of millions of years. Learn more about radioactive decay.

Below you will find links to basic information about twelve radionuclides encountered in medical, commercial, and military activities. These pages contain information about the properties of the element, how it moves through the environment, where you may find it (the radiation source) and potential health effects.