Radiation that is present in very low levels on earth and in the atmosphere. This radiation is emitted from many sources, including granite and other stones, cosmic rays, and certain elements. The average dose a person receives from background radiation is approximately 3 mSv per year.
Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine testing: a catalog. Radiology. 2008;248:254-263.