Approximately 2,300 nuclides have been identified and most of them are radioactive. Almost all elements that are heavier than bismuth, which has 83 protons, are unstable or radioactive.

While there are hundreds of radionuclides (radioactive contaminants), many of them are rarely encountered. The most commonly found radionuclides are those used for medical, military, or commercial purposes.

Most radioactive contaminants are at levels that are too low to be considered a public health concern. At higher levels, long-term exposure to radionuclides in drinking water may cause cancer. In addition, exposure to uranium in drinking water may cause kidney damage.

To protect public health the EPA has established drinking water standards for several types of radioactive contaminants.

For the EPA quick reference guide to the Radionuclides Rule see:
http://www.epa.gov/safewater/radionuclides/pdfs/qrg_radionuclides.pdf