Radon exposure: all companies are concerned

Radon, a carcinogenic radioactive gas, imposes new obligations on companies. The reference level has been lowered to 300 Bq/m3, and a risk assessment is required.

Marie Faucon
EHS Consultant
Update : 
12.09.2025
Publication: 
06.12.2018
Contents
Example H2
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Radon is a naturally-occurring radioactive gas, produced by the decay of uranium and radium, which forms in the subsoil (particularly in granitic and volcanic soils). It is present throughout France, with high concentrations in granitic and volcanic regions (Massif Armoricain, Massif Central, Corsica, Vosges, etc.). The concentration of radon in the air of a building depends not only on the characteristics of the soil, but also on the building and its ventilation. This gas is classified as carcinogenic. Studies show thatradon exposure is the second leading cause of lung cancer in France.

Since the publication of the decree of June 4, 2018, regulatory requirements have become more stringent for companies. We offer an overview of the main legal obligations in this area.

Reference level

The reference level is the level of radon activity concentration above which it is considered inappropriate to allow exposure of workers to ionizing radiation, even if it is not a limit that cannot be exceeded.

Since July1, 2018, the reference level for radon activity concentration in air has been set at 300 Bq/m3 annual average (previously 400).

Companies concerned

All businesses must take into account the risk of radon exposure. In particular, this applies to :

  • in the case of professional activities carried out in basements or on the first floor of buildings located in areas where radon exposure is likely to be harmful to workers' health,
  • in certain specific workplaces. A decree (not yet published) sets out the list of these places.

Risk assessment of radon exposure

Employers must assess the risks of radon exposure to workers, in order to determine whether the radon reference level is likely to be exceeded. To do this, they must take into account the radon potential of their locality and any measurements already carried out.

The radon potential of communes is set by the order of June 27, 2018 and listed on a map available on the IRSN (Institut de Radioprotection et de Sûreté Nucléaire) website. Communes are classified into 3 categories:

  • Zone 1: areas with low radon potential ;
  • Zone 2: areas with low radon potential but where particular geological factors may facilitate the transfer of radon to buildings;
  • Zone 3: areas with significant radon potential.

A ministerial instruction dated October 2, 2018 specifies that, for workplaces located in Zone 1 (low-potential zone), if the employer is not aware of any other elements, the risk can be considered negligible.

In zone 2, a more thorough investigation is required, and may lead to the need for measurements. If the company is located in zone 3, measurements are strongly recommended.

Radon concentration in air is measured using radon dosimeters, which are inexpensive measuring devices. The dosimeter is a device that measures the radon concentration present in the room, for the entire period during which it was deposited there. These dosimeters are analyzed by COFRAC-accredited organizations.

If the employer concludes that the assessment is likely to exceed the reference level, he must measure the radon concentration in the air and implement preventive measures. These include improving the airtightness of the building to limit radon ingress, or improving ventilation to dilute radon in the air. If preventive measures do not result in a radon activity concentration in the air below the reference level, the employer communicates the results of the measurements to IRSN.

Note: risk assessment must be recorded in the company's single document (DU), and this assessment and the results of measurements must be forwarded to health professionals (occupational physician, collaborating physician, nurse, etc.) and to the Social and Economic Committee(CSE).

Identifying radon zones

Employers must identify areas where workers are likely to be exposed to ionizing radiation in excess of 6 mSV/year in terms of radon concentration in air, measured in effective dose. These zones are known as "radon zones". Note: these "radon zones" are different from the "radon potential zones" mentioned above, which are geographical areas identified by decree.

If a radon zone is identified, the employer must :

  • appoint a radiation protection advisor ;
  • mark these areas;
  • limit access to radon zones to classified workers, or authorize unclassified workers on the basis of an individual assessment;
  • carry out initial verification by measuring the radon activity concentration in air in radon zones when the facility is commissioned, or following any significant change in working conditions that may affect worker health and safety. This verification is carried out by an accredited organization.
  • periodically renew this verification, which is then carried out by the radiation protection advisor;
  • provide workers with personal protective equipment when collective protection is not sufficient;
  • inform workers in radon zones about the risks associated with ionizing radiation and the organizational rules in place.
  • assess the personal exposure of workers accessing these areas prior to assignment to the workstation.
  • set up dosimetric monitoring of workers (using dosimeters with appropriate delayed reading) when the effective dose is likely to exceed 6 mSv over 12 months, and provide them with reinforced individual health monitoring.

Radon exposure: conclusion

All companies need to consider the risk of radon exposure to their workers. For most, however, a simple risk assessment will suffice. If you haven't already done so, we recommend that you first identify the potential of the municipality in which your workplaces are located:

  • if you are in zone 1 and have no other knowledge of the presence of radon, you can conclude that the risk of exposure is negligible.
  • If you're in zone 2 or 3, or in the case of specific activities, further research is required, and in category 3 zones in particular, dosimeter measurements can help.

If necessary, actions such as improving ventilation or sealing should be implemented to reduce the risk of exposure and remain below the reference level.

Where radon is very present, areas where exposure exceeds 6 mSv/year need to be identified and various organizational and preventive measures put in place (radiation protection advisor, dosimetric monitoring, etc.).

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