La principale source de radioactivité naturelle : le radon

Radon is a natural radioactive gas, odorless and invisible, produced from uranium. It is present everywhere in soils but more strongly in granite and volcanic subsoils. Generally weak in outdoor air, radon concentrations can sometimes be high inside buildings and enclosed spaces.

Naturally occurring radioactive radiation accounts for two-thirds of the population's exposure to radioactivity in metropolitan France. This includes in particular the inhalation of radon gas, irradiation from space or Earth, as well as consumption of food or water naturally containing radioactive elements.

Radon alone represents one-third of the total exposure to radioactivity received each year by the French population.

Radon is present everywhere, but its concentration in subsoils is highly variable. The probability of radon presence inside buildings, or "radon potential", is estimated from the uranium content of subsoils.

• Nearly a quarter of municipalities are affected by medium or high radon levels in their subsoils due to particularly high uranium content. This affects at least one municipality out of two in 21 departments, notably in Auvergne, Brittany and Limousin. The phenomenon particularly affects Upper Corsica, Creuse and Upper Vienne, where at least 90% of municipalities are concerned. • Radon is predominantly present in large granite or rocky massifs such as Corsica, the Armorican and Central massifs and the Vosges, in Mayotte or in French Polynesia.

In the 7,000 municipalities affected which host 12.2 million inhabitants, more than 40% of buildings exceed 100 becquerels per cubic meter (Bq/m³). More than 10% of municipalities exceed 300 Bq/m³, the limit value beyond which it is necessary to reduce radon concentrations.

The IRSN (Institute for Radiation Protection and Nuclear Safety) provides citizens with an interactive map to find out the "radon potential" of their municipality:

In areas with high radon potential, residents of individual homes are more exposed than those in apartment buildings, due to more direct contact with the subsoil.

In France, more than 6 out of 10 French people live in individual homes. 21% of them are exposed to medium or high radon potential, 10% to low potential.

10 departments account for more than 80% of individual home residents in areas exposed to medium or high radon potential, totaling 2.5 million inhabitants: South Corsica (100%), Creuse (97%), Finistère (93%), Upper Vienne (91%), Morbihan (90%), Cantal (87%), Côtes-d'Armor (84%), Corrèze (83%), Lozère (83%), Aveyron (82%). For 14 departments, exposed residents represent between one-half and three-quarters of the territory's total population.

Radon has been recognized as a definite lung carcinogen for humans since 1987 by the International Agency for Research on Cancer.

In France, radon exposure is estimated to cause nearly 3,000 lung cancer deaths annually according to the IRSN, or 10% of lung cancer victims. The risk is significantly higher in smokers, as the interaction between radon and tobacco multiplies lung cancer risk by three.

• Yet according to France's Public Health Agency, fewer than 1 in 3 French people have heard of radon, and 8 in 10 consider themselves poorly informed about the risks associated with radon according to the IRSN's 2023 Barometer. • When they show distrust or suspicion on this topic, they do so in proportions significantly lower than those observed for most other environmental and health risks.

In the 31 most affected departments, survey respondents prove to be better informed about this risk than the rest of the population: 29% know what it is, compared to 18% in the rest of France. However, only 21% are aware of being exposed to this type of risk.

A French person's annual radon exposure can be doubled compared to the national average depending on their place of residence according to the IRSN.

Testing, using a dosimeter, allows measurement of average radon concentration in a home and shows the average exposure of its inhabitants. In case of high concentration, exposure can be reduced through renovation work such as improving the seal between soil and building, ventilation and heating system.

In France, certain categories of facilities are subject to special monitoring. Radon measurements must be performed every ten years in facilities open to the public: schools, hospitals, nursing homes, etc. If concentration exceeds the threshold, work must be undertaken to reduce exposures. For professionals working in underground locations (road or rail tunnels, caves, etc.), employers must perform measurements every 5 years. The 4th national action plan 2020-2024 for radon risk management was implemented to continue public information and awareness, improve knowledge and the consideration of this risk in buildings.