Background radiation: Radiation that comes from environmental sources including the earth's crust, the atmosphere, cosmic rays, and radioisotopes. Natural sources of radiation account for the largest amount of radiation exposure received by most people each year with medical and occupational sources accounting for only a fraction of that exposure. It is currently believed that radon, a gas produced by radium decay within rock, constitutes the major source of background radiation throughout many parts of the US. The buildup of radon in inadequately ventilated homes may pose a long-term health hazard. The deleterious effects of background radiation, estimated as causing 1-6% of spontaneous genetic mutation, rise with dose.