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

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 in a category 3 commune, the category is a reason to measure. It does not show that the house has a problem.

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

Radon collects where a basement or ground floor is in contact with the ground and there is 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.

Radon and other planned work

If mechanical ventilation is also planned to improve the energy rating, the radon reading is relevant to its design. A measurement taken before any floor or basement work can change what that work involves.