Natural Background Radiation
Natural background radiation comes primarily from radon, but also includes significant components from cosmic radiation and other natural radionuclides, including those within the body. The worldwide average natural background dose for a human being is about 2.4 millisievert (mSv) per year. This is four times more than human-caused background radiation exposure, which in the year 2008 amounted to a global average of about 0.6 mSv per year.
In Europe, average natural background exposure by country ranges from under 2 mSv annually in the United Kingdom to more than 7 mSv annually in Finland. Some areas have greater dosage than the country-wide averages. In the world in general, exceptionally high natural background locales include Ramsar in Iran, Guarapari in Brazil, Kerala in India, the northern Flinders Ranges in Australia and Yangjiang in China.
The highest level of purely natural radiation recorded on the Earth's surface is 90µGy/h (0.8 Gy/yr) on a Brazilian black beach composed of monazite. The highest background radiation in an inhabited area is found in Ramsar, primarily due to the use of local naturally radioactive limestone as a building material. Some 2000 of the most exposed residents receive an average radiation dose of 10 mGy per year, (1 rad/yr) ten times more than the ICRP recommended limit for exposure to the public from artificial sources. Record levels were found in a house where the effective radiation dose due to external radiation was 135 mSv/yr, (13.5 rem/yr) and the committed dose from radon was 640 mSv/yr. (64.0 rem/yr) This unique case is over 200 times higher than the world average background radiation.
Epidemiological studies are underway to identify health effects associated with the high radiation levels in Ramsar. Radiation hormesis has not been observed, and data from Ramsar does not provide justification to relax existing regulatory dose limits.
Background radiation doses in the immediate vicinities of particles of high atomic number materials, within the human body, have a small enhancement due to the photoelectric effect.
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