Radioactivity is present all over the world. It forms an ambient radiological background noise consisting mainly of radionuclides naturally present in the ground or in the materials surrounding us and of radiation of cosmic origin, but also artificially produced in different fields (industrial, medical, military...). In addition to this permanently present background, there are radiological accidents (Chernobyl, Fukushima...). We are therefore permanently exposed to ionizing radiation from natural and artificial.
The earth's crust contains radioactive atoms that are the source of natural radioactivity (thorium-232, uranium-235, uranium-238 and their descendants). With very long life spans of up to several billion years, they are still far from having disappeared and are therefore an integral part of our environment.
Depending on the nature of the rocks composing the subsoil, we are therefore more or less exposed to telluric radiation. In some granite regions, for example in France, Limousin or Brittany, the terrestrial radiation can be 4 times higher than in the Paris region. Some black sandy beaches in Kerala, India, rich in monazite (containing thorium), can reach radiation levels 10 to 100 times higher. In these regions, radiation exposure may also be higher inside a building due to the presence of radon, a radioactive gas resulting from the decay of radioelements present in soils or building materials.
Cosmic radiation is very different in nature from terrestrial radiation. It consists mainly of hydrogen nuclei (protons) and light nuclei (He for example) of very high energy. This radiation has two components, one is of galactic or extragalactic origin and comes from the disintegration of stars and the other from the sun, which has a continuous activity, with a cycle of about 11 years, to which are added sporadic eruptions. The particles accelerated during certain eruptions are so energetic that they produce effects at ground level.
Life on Earth is protected by barriers that filter some of these particles: the interplanetary magnetic field emitted by the sun, whose intensity varies during its cycle, and the Earth's magnetic field and atmosphere. The interaction of cosmic radiation with the constituents of the atmosphere produces a continuous "rain" of secondary particles (neutrons, muons, electrons...) which are largely absorbed before reaching ground level.
As a result, the intensity of cosmic radiation varies very strongly with altitude. At ground level, the dose received is about 100 times lower than at flight altitudes of long-haul aircraft.
It should be noted that radionuclides, known as cosmogenic, are also produced permanently by the interaction, in the upper atmosphere, of cosmic radiation with the atoms constituting our atmosphere. The best known is carbon-14.
Exposure to ionizing radiation in our environment is partly of artificial origin: medical field, consequences of nuclear accidents...