Radon in France: which areas, and does it matter for an old house?

By Phil, French House Check. 16 September 2026.

Every French commune has a radon category from 1 (lowest) to 3 (highest), set by the geology. Category 3 is granite country: Brittany, the Massif Central, the Vosges, Corsica and parts of lower Normandy. The category describes the rock, not the house. For an old house with a cellar and no mechanical ventilation, it is a measurement to take, not a defect found.

What radon is

A naturally occurring radioactive gas released by granite and similar rock. It disperses outdoors and collects indoors, mostly in ground floors, cellars and poorly ventilated rooms in contact with the ground. The health effect is a raised lung cancer risk over long exposure, substantially worse for smokers.

How to check a commune

The category is on the official Géorisques address report for any French address, and it appears in the état des risques the seller must give you. In a category 3 commune that document must mention radon. Some address reports go further and state the risk as existing and significant at the address itself.

What it means for a particular house

The arrangement in which radon collects is a basement or ground floor in contact with the ground, and no mechanical ventilation. Old stone houses with a cellar and natural-draught ventilation are the case in point. Many such houses in category 3 communes still measure low. The only way to know is to measure.

• Use passive detectors over a full heating season, in the main living room and any room in contact with the ground.
• The French reference level is 300 becquerels per cubic metre.
• Where a reading is high, the remedy is normally ventilating the cellar and underfloor space and sealing the floor slab and its penetrations, not structural work.

Why it belongs in the purchase arithmetic

If the house also needs mechanical ventilation for its energy rating, which most F and G rated houses do, the two jobs belong together and the cellar is the place to start. Measure before planning any floor or basement work, because the answer changes what that work is.