Radiation that is emitted by naturally occurring radioactive materials, such as uranium, thorium, and radon in the earth. (see also: naturally occurring radioactive materials)
U.S. Environmental Protection Agency, Radiation Glossary, Office of Radiation and Indoor Air Radiation Protection Division, Washington, DC, USA 2013. http://www.epa.gov/radiation/glossary/index.html#a , (Dzstr/ Rad Emr - Amer) {823}