The Earth crust contains a number of radioisotopes which are the principal natural sources of radioactivity: thorium 232, uranium 235 and uranium 238. With half-lives stretching out into the billions of years, none of them are even close to disappearing and for that reason are still key elements in our environment.
Along with their respective radioactive lineages, these three radioisotopes cause what is known as 'telluric' radiation, the natural radioactivity of rocks. These telluric radiations are a key external low source of gamma radiation, both inside and outside of buildings. The presence of these radioisotopes in the building materials may cause the levels of radiation indoors to be greater than those found outside. Whereas buildings made from wood do not have this problem, the downside is that wood acts as a poor shield from the gamma radiation coming from the soil.
In a country such as France, the average annual exposure from telluric radiation can be estimated at around 0.5 mSv. The estimation assumes humans spend in average 80% of their time indoors and 20% outdoors.
In the granite-rich areas of Brittany, where the rocks contain higher doses of radioactive elements, the exposure can be four times as high. Even then, however, the numbers are insignificant when compared to the doses absorbed on the black sand beaches of Brazil or Southern India, where the abundant mineral monazite consists of almost 10% radioactive thorium.
The world's highest background radiation rate is found in the city of Ramsar in Iran, with the astonishing maximum near a volcanic hot spring of 250 millisieverts a year. An iranian publication in 2002, indicates that Cytogenetic studies show no significant differences between people in the high background areas compared to people in normal background areas.
The study of the consequences of this natural radioactivity on certain communities in Southern India has not shown increase in the mortality rate, although certain chromosomal mutations have been observed. In any case, the analyses that have been carried out are not precise enough to be conclusive (*).
To the natural radioactivity emanating from ground, one should add the radioactive deposits coming from 1960s nuclear tests and the 1986 and 2011 Chernobyl and Fukushima accidents. The main emanation is that of the cesium-137 gamma rays. The contribution of nuclear tests spreadover the whole Eearth are now weak, while that of the Chernobyl and Fukushima accidents remains high near the sites.
Acces to page in french