Radon is a naturally occurring radioactive gas that is produced by the radioactive decay of uranium present in all earth materials such as rocks, soils, brick and concrete. Outdoor air has radon in it (but not much). Indoors, however, radon concentrations can build up to much higher than outdoor levels, if ventilation is poor.
Breathing high concentrations of radon can cause lung cancer. More importantly, radon decays radioactively into other elements that are also radioactive, and unlike radon these other elements (such as polonium) stick in the lungs if inhaled. Additionally, there is evidence that radon is much more dangerous to smokers than to non-smokers.
The main source of radon is the ground below the home, particularly granitic geology. In the UK the highest levels have been found in some homes on or near granite in southwest England, but not all granites give high levels. Some other rocks, but not clays, also cause high radon levels. Radon is present in all parts of the UK, but in the most populous areas the levels are quite low.
Radon levels in homes vary during the day, from one day to the next, and from winter to summer, mainly because of temperature differences between indoors and outdoors. They are generally higher at night and during the winter. Although radon enters homes all the time, some is carried away by the natural ventilation. Even in a home with good draught proofing and double-glazing, the air changes several times a day. Increasing the ventilation, especially on the ground floor, will in most cases cause a moderate reduction in the radon level.
Wherever you live in the UK, if you want to have the radon level in your home measured, contact the National Radiological Protection Board.