Radon exposure risk: strengthening the radon prevention approach

What is radon? And why is it a health hazard? We explain it all in this article.

Juliette Virly
EHS Consultant
Update : 
12.09.2025
Publication: 
22.07.2024
Contents
Example H2
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What is radon?

Radon is a naturally-occurring radioactive gas found throughout France: in air, soil and water, in concentrations that vary from place to place. High concentrations are found in granite and volcanic regions. Radon is rapidly diluted in outdoor air, but can accumulate strongly in enclosed spaces such as buildings.

Radon is a health hazard, classified as a carcinogen and the main source of natural exposure to ionizing radiation in France.

Radon potential zones have been defined on a municipal scale. Communes are divided into three zones for which radon exposure prevention measures must be implemented:

  • 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.

Zoning by commune is available on theIRSN website.

Employer's obligations

Radon represents an occupational exposure risk for workers, due to its potential accumulation in the air of enclosed spaces. Employers must therefore assess the risks of radon exposure to workers in their occupational risk assessment documents (DUERP). The risk assessment must enable the employer to determine whether the radon concentration in the air is likely to exceed the defined reference level of 300 becquerels per cubic meter (Bq/m3) in ground-floor and basement premises.

For establishments located in communities with radon potential zones 2 and 3, it is recommended to measure the radon concentration in the air to ensure that the reference level is not exceeded. In addition, measurement will be necessary whenever the results of a risk assessment do not allow us to conclude that there is no risk of exposure.

Depending on the results of the measurements, the employer must implement preventive measures to reduce the level of radon concentration in the air, such as improving ventilation or airtightness of the premises.

Reinforcing preventive measures

In order to improve prevention of the risks of exposure of workers to radon from the ground (i.e. radon generated directly by the rocks in the soil or secondarily by the water circulating in these rocks or the materials extracted from them), the Order of May 15, 2024 reinforces the preventive measures to be put in place by the employer.

Risk assessment and action plan

From now on, whenever the results of a risk assessment show that the radon activity concentration in the air is likely to reach or exceed 300 Bq /m3 on an annual average (reference level), the employer must measure the radon concentration in the air using integrated, delayed-reading measuring equipment supplied and operated by an accredited organization.

If the results of the measurements reveal a radon concentration in the air equal to or greater than the reference level, the employer must draw up a plan of action to reduce the radon concentration in the air to below the reference level, such as, for example, implementing work processes that do not expose or result in less exposure ; modifying the design and layout of workplaces and workstations to reduce exposure to ionizing radiation; improving the airtightness of buildings with regard to radon entry points, or air renewal in premises; choosing a work organization designed to reduce the duration and intensity of exposure, in particular by controlling access to demarcated areas.

This action plan must be implemented within 3 years. If the radon concentration in the air exceeds 1,000 Bq/m3, the action plan must be implemented within 1 year.

At the end of these periods, if the measures put in place are not effective and do not enable us to remain below the reference level on a permanent basis, the employer must set up a radon zone, along with reinforced preventive measures.

Setting up a radon zone

Setting up a radon zone involves :

  1. appoint a radiation protection consultant to define the radon zone;

  1. report and notify IRSN of this zone;

  1. have an initial check carried out by the radiation protection advisor to ensure that workplaces adjoining the radon zone do not contain an airborne radon concentration above the reference level. A new check must be carried out for any significant change in working methods, working conditions or workplace layout likely to affect workers' health and safety.

  1. draw up a periodic inspection program, taking into account activity in the radon zone and adjoining workplaces. Checks must be carried out at least every 5 years (every year if the radon concentration in the air exceeds 1,000 Bq/m3). When workers are present in the radon zone, the employer may choose to set up continuous radon measurement instead of the verification program.

  1. visibly signpost the "radon zone" in front of the zone entrance. In addition, a radon risk information sheet, accompanied by a diagram showing the zone's boundaries, and safety instructions for entering the zone must be prominently displayed at the zone's entrances.