Sellers of French real estate located in an area in France where there a potential risk of Radon gas as of July 1st 2017 will be required to have a diagnosis done to inform buyers of existence of this risk. Landlords will also be obliged to have the same diagnostic performed and share the result with renters.
Radon is a colorless, odorless radioactive gas.
Note again, the start date of this requirement is July 1st 2017 and the buildings concerned and the modalities of monitoring and levels of activity concentration risk will be defined by regulation. Contact your English speaking French notary for more details.