The data presented show that the yearly background dose equals on the average about 2 mSv (200 mrem). Two thirds of the dose is attributable to internal radiation from natural radionuclides—primarily the decay products of radon and thoron, entering the human body with inhaled air. The external irradiation is due approximately equally to cosmic radiation and natural γ emitters such as40K and radionuclides of the thorium and uranium series.