Ground Radioactivity

Terrestrial Exposure or Radioactivity Emanating from the Ground

2026 Editorial Revision

The Earth’s crust contains radioactive atoms that are a source of natural radioactivity: thorium-232, uranium-235 and uranium-238. With extremely long lifetimes (on the order of one billion to several billion years), they are still far from having disappeared and are therefore an integral part of our environment.

Together with their decay products, these three radionuclides are responsible for “terrestrial” radiation, the radiation emitted by rocks. This mainly results in low-level external exposure to gamma radiation, both inside and outside buildings. Radiation levels can in fact be higher indoors because of the presence of radionuclides (radioactive nuclei) in construction materials. This is not the case for wooden buildings, as wood contains very few radionuclides. However, wood provides only limited protection against gamma radiation coming from the ground.

In a country such as France, the average annual exposure due to terrestrial radiation is estimated at around 0.50 mSv. This estimate assumes that 80% of the time is spent indoors and 20% outdoors. Exposure can be four times higher in granitic regions, such as certain areas of Brittany, where the granite is rich in radioactive elements.

These figures remain far below the radiation levels observed on certain black-sand beaches in Brazil or southern India, due to their high concentration of monazite, a mineral containing nearly 10% radioactive thorium.

The world’s highest rate of exposure from the ground has been found in the Iranian city of Ramsar, where it reaches a maximum of 250 millisieverts per year near a volcanic hot spring. According to an Iranian publication from 2002, cytogenetic studies found no significant differences between people living in areas with high radiation levels and those living in areas with normal levels.

Analysis of the consequences of this excess natural radioactivity on certain communities in southern India has not revealed any marked effects, such as an increase in mortality (although chromosomal mutations have been observed). However, the analyses are not precise enough to draw firm conclusions. The highly exposed populations are small. Moreover, in the absence of individual monitoring, how can we estimate the amount of time a fisherman has spent over the course of a lifetime in the most radioactive locations?

In addition to this natural radioactivity emanating from the ground, there is radioactivity from deposits resulting from the nuclear weapons tests of the 1960s and, later, from the Chernobyl accident in 1986 and the Fukushima accident in 2011. The main source of radiation is the gamma rays emitted by caesium-137. The contribution from nuclear weapons testing, distributed across the globe, has now become small, whereas the contribution from the two nuclear accidents remains significant near the sites of those accidents.