A radionuclide is an atom with an unstable nucleus which, to become more stable, emits energy in the form of rays or high speed particles. This is called ionizing radiation because it can create “ions” by displacing electrons in the body e.g. in the DNA, disrupting its function. The three major types of ionizing radiation are: alpha particles, beta particles and gamma rays.
Approximately 80% of our exposure to radioactivity is natural and another 20% is from man-made sources, although more frequent use of diagnostic imaging involving radiation (x-rays, CT scans) is increasing exposure from this source. We are exposed to naturally occurring radiation for example from radon gas emanating from rocks and soil, and cosmic radiation from space. We also carry small amounts of potassium-40 in our bodies from the foods containing potassium. Depending on the type of rocks where you live, 55 to 70% of natural exposure comes from radon gas, while cosmic radiation (which is greater at higher altitude) represents about 11%, and potassium-40 about 5%. Radiation may exist in drinking water from nuclides dissolved in the water from natural sources in the earth or occasionally from releases from laboratories or nuclear power plants.
EPA regulates the following radionuclides in drinking water: (Adjusted) Gross Alpha Emitters, Beta Particle and Photon (gamma) Radioactivity, Radium 226 and Radium 228 (Combined) and Uranium.