The total mean annual dose of radiation from all natural sources is about 2.4 mSv. This radiation comes from several sources. About 15% of this background radiation comes from natural radioactivity in our bodies – primarily potassium K-40. Large part comes from soil and buildings and remains in atmosphere as progeny of radon, therefore varies with geographical location. There are a few places, including one in Brazil, and one in India, where the background radiation is a factor of 3 to 40 times greater than average. Typically, 15% to 20% of the background radiation comes from cosmic rays from outer space.

The atmosphere acts as a shield to absorb some of this radiation as well as hazardous component of the ultraviolet spectrum. Therefore, at high elevations, this protection will be lost.

Background radiation has increased due to advancement of technology too, such as fallout from nuclear explosions. As most of these nuclear tests are carried out in the northern hemisphere, the people in those areas receive about 3–4 micro Gray extra annually when compared with the people in the southern hemisphere. There are other sources of radioactivity in industrialised countries, which are indicated below.

The annual whole body dose of radiation due to background and all other sources, which varies from place to place, is estimated as 2.4 mSv. This is at the surface of the earth and varies with altitude.

NATURAL BACKGROUND RADIATION

Everyone is exposed to low levels of radiation all the time. The main contributors to the natural component of background radiation are external radiation from both cosmic sources and terrestrial sources, and internal radiation from sources such as radon and potassium-40.