Background Radiation
Naturally occurring background radiation is the main source of exposure for most people. Levels typically range from about 150 - 350 mrem per year but can be more than 5 rem/yr. The highest known level of background radiation affecting a substantial population is in Kerala and Madras States in India where some 140,000 people receive doses which average over 1.5 rem/year from gamma radiation, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures up to about 4 rem/yr to many people.
Several places are known in Iran, India, and Europe where natural background radiation gives an annual dose of more than 5 rem and up to 26 rem (at Ramsar in Iran). Lifetime doses from natural radiation range up to a couple thousand rem. However, there is no evidence of increased cancers or other health problems arising from these high natural levels.