All exposure to radioactivity, whether natural or artificial, is measured in doses which take into account the biological effect the radiation can have. This is the effective dose, which has one given value for the whole body. For small amounts of radiations, the relevant unit is the thousandth of a sievert or millisievert (mSv).
In a country like France or Belgium, the annual dose is calculated to be around 3.5 mSv per person in this first decade of the 21st Century. One hundred years ago that value would have been at 2.4 mSv. This rise is largely due to the development of radiation techniques in medicine; as the doses of natural exposure have undergone no variation.
The high dose of radiation one receives in a medical scan is applied over a very short period of time. Exposure to natural radioactivity, however, is permanent. Spread over a period of days, months, years or decades, the chronic dose we absorb is fortunately comparatively low. The long time periods over which the absorption takes place also allows the cells of our body to repair the damage they undergo as a result.
Natural radiations come from a variety of sources: the telluric radiations emitted by rocks (0.45 to 0.54 mSv), cosmic rays (0.30 to 0.36 mSv), from the body own radioactivity (0.25 to 0.30 mSv) and, most significantly, from radon gas (1.0 to 1.2 mSv) – a radioactive descendant of uranium which escapes from the rocks around us. Ranges are given rather than precise values because measurements of exposure levels vary depending on the body that carried them out or the period in which the tests were conducted.
Radon and its radioactive decay products are the principal contributors to the natural radioactivity we absorb. The radioactivity of the human body is mainly caused by the presence of two natural radioisotopes; potassium 40 and carbon 14 (which undergoes 8,000 disintegrations per second).
Apart from exposure to radon gas, which can, to some degree, be reduced, the value of natural radioactivity is invariant.
In France, natural exposure is responsible for an average dose of 2 mSv. Apart from our body own radioactivity, the ratios of the other sources can vary dramatically depending on where we live in the world – without having to go the extremes of astronauts who absorb greater doses of cosmic rays or villagers in Kerala, who have greater exposure to telluric rays.
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