Radon is a gas which comes from rocks and soil.  We cannot see, smell or taste it but when it is in enclosed places, like our homes, it can pose a danger.  It is believed to be the second greatest cause of lung cancer, after smoking.  A quick check online on the Northern Ireland Interactive Radon Map will allow you to see at a glance the areas of Northern Ireland affected.  However, the map only shows the estimated likelihood that a property will contain radon gas.