Natural background radiation comes from two primary sources: cosmic radiation and terrestrial sources such as soil, rocks, water, air, and vegetation.
This is far greater than human-caused background radiation exposure. In 2000, this amounted to an average of about 0.005 mSv per year from historical nuclear weapons testing, nuclear power accidents and the nuclear industry operation combined. The background radiation dose is also greater than the average exposure from medical tests, which ranges from 0.04 to 1 mSv per year.
The level of natural background radiation varies depending on location, and in some areas the level is significantly higher than average. Such areas include Ramsar in Iran, Guarapari in Brazil, Kerala in India, parts of the Flinders Ranges in South Australia and Yangjiang in China. In Ramsar, a peak yearly dose of 260 mSv has been reported.
Radiation inside the human body
Some of the essential elements that make up the human body, mainly potassium and carbon, have radioactive isotopes that add significantly to our background radiation dose. An average human contains about 30 mg (10−3g) of potassium−40 (40K), and about 10 nanograms (10−8 g) of carbon−14 (14C).