The National Radiological Protection Board says the highest radon levels in Northern Ireland occur 'where the Mountains of Mourne sweep down to the sea - with one home in ten above the Action Level'. It has designated a stretch south and west of Belfast as a radon-affected area.
Measurements have been made throughout Northern Ireland, with the affected area running across Co Down from Strangford Lough to Carlingford Lough, and ending in Co Armagh. Radon is the largest source of radiation exposure to the population after X-rays.
The NRPB has been surveying the whole of the United Kingdom to try and identify radon 'hot spots'. The highest incidence has been in Somerset, Devon and Cornwall. The gas is produced as a result of radioactive decay in underlying rock - usually uraniferous granites. It filters up into basements of houses and can build up above official guidelines. Remedial action involves installing increased ventilation to draw the radon out of the house.
Lung cancers due to radon were detected by the Swiss physician and alchemist Paracelsus (1493-1541). He attributed miners' deaths from lung disease to their inhaling a gas, now known to be radon.