Radon in France: the 3 zones and informing the buyer

What is radon?

Radon is a natural radioactive gas, colourless and odourless, produced by the decay of radium, itself produced by uranium in the Earth’s crust. It is found in soils everywhere, particularly in granitic or volcanic subsoils. It dilutes quickly in the open air, but builds up in enclosed spaces (Ministry for the Ecological Transition). Its concentration is expressed in becquerels per cubic metre (Bq/m³).

Three sets of factors determine the amount of radon in a building:

• the ground: concentration, permeability, moisture, cracks in the rock;
• the building: construction method, cracking of the surface in contact with the ground, ventilation;
• the occupants’ behaviour with regard to airing.

In the long term, inhaling radon increases the risk of lung cancer, in proportion to cumulative exposure, and combined exposure to radon and tobacco increases it sharply (ASNR).

The three radon potential zones

According to the ASNR, zone 2 municipalities are in particular those crossed by major faults or whose subsoil contains underground mine workings. The radon potential map’s maximum precision is the municipality: it cannot be used down to the individual plot. Above all, a house in zone 3 does not necessarily have a high concentration; it is merely “significantly more at risk” of having one than in a low-potential zone, which justifies screening (ASNR, FAQ).

Selling or letting in zone 3: what the law says

The significant radon potential zone, known as level 3, is one of the zones where the risk statement is compulsory (service-public.gouv.fr, checked on 28 September 2026). It is provided as an annex to the promesse de vente (preliminary sale contract) (failing that, to the deed) or to the tenancy agreement; for these properties, a fiche d’information des acquéreurs et des locataires sur le risque radon (information sheet for buyers and tenants on radon risk) is available on Géorisques (Ministry for the Ecological Transition).

Source: service-public.gouv.fr; code de l’environnement (Environmental Code), art. L125-5. For details of the document, see our guide on the état des risques (ERP) (risk statement).

The key point: disclosure is compulsory, measurement is not. “There are no regulations requiring a seller to measure radon” (ASNR, FAQ). Apart from disclosure, there is no regulatory obligation for homes, new or existing (Ministry for the Ecological Transition).

Measuring radon in a home

The only way to know your exposure is to measure with detectors (dosimeters), placed for at least 2 months during the heating season, in the lowest occupied living rooms (living room, bedroom). The ministry gives the period as mid-September to the end of April; the ASNR, in its press release of 7 November 2025, gives mid-September to the end of February.

When buying on a tight timetable, the ASNR mentions short-term electronic devices, while pointing out that only passive devices give the average exposure over the year. Measurements from a neighbouring house cannot be used to estimate those of the property you are looking at.

What to do depending on the result

The regulatory thresholds below apply to établissements recevant du public (premises open to the public) (nurseries, schools, health and social care establishments, spas, prisons), which must measure radon in zone 3 every 10 years. For a home, the ASNR uses them as benchmarks:

• below 300 Bq/m³: no particular action, apart from the good practices cited by the ministry (airing for 10 minutes a day, not blocking ventilation, maintaining it);
• from 300 to 1,000 Bq/m³: simple actions are often enough, such as checking the ventilation or sealing radon entry points (cellar door, pipe penetrations, cracks in the floor);
• above 1,000 Bq/m³, or if the concentration stays above 300 after these actions: have a building survey carried out by a professional.

In new construction, the ministry recommends limiting the surface in contact with the ground, ensuring an airtight seal between the building and its subsoil and ventilating the substructure well (crawl space, cellar).

Which professional to consult

The notaire (notary) checks that the risk statement, with the radon sheet, is annexed to the preliminary contract. The agent immobilier (estate agent) includes the Géorisques statement in the listing of a property concerned. The diagnostiqueur (property surveyor) carries out the other surveys in the file; for screening, detector suppliers are listed by the ASNR.