background radiation [AS, baec + OE, grund, ground] , naturally occurring radiation emitted by soil, groundwater, building materials, radioactive substances in the body (especially potassium 40), and cosmic rays from outer space. Each year the average person is exposed to 44 millirad (mrad) of external terrestrial radiation, 18 mrad of naturally occurring internal radiation, and 44 mrad of cosmic radiation. Background radiation levels may vary in different locales.
