The radon is generated in the subsoil, ascends to the area and it enters the interior of the edifications depending on the type of construction, are already housing or workplaces.
The gas radon, when being denser than the air, once in the interior of the building it tends to accumulate in undergrounds and ground floors.
The Nuclear Safety Council (CSN) offers the map of the potential of radon with areas of Spain of priority activity, those with potential of radon higher than 300 Bq/m3: