2020 No https://pubmed.ncbi.nlm.nih.gov/33130895/ Curguz, Z Europe Observational Radon Radon concentrations at the ground floor are generally higher than at first floor: indeed, they range from about 90 to about 4200 Bq m−3, and for more than 25% of these rooms, radon concentrations are higher than 300 Bq m−3. Notably, if rooms with radon levels higher than 300 Bq m−3 (or higher than 200 Bq m−3) were found at first floor, they were also found at ground floors. On the contrary, for buildings with the radon concentrations exceeding 300 Bq m −3 at the ground floor rooms, this concentration level was exceeded only in 1 of 10 cases at the first floor.