What is Radon?
Radon is a naturally occurring gas, which may be hazardous to health, and can build up in poorly ventilated areas such as cellars or basements.
A local search will reveal if the property is in a Radon area. You can also buy test kits online.
See NHS and Public Health Wales advice as below...
https://phw.nhs.wales/services-and-teams/environmental-public-health/radon/