Radon is a naturally occuring radioactive gas, which is always present in our atmosphere. It occurs as a result of geologogical phenomena, such as cracked subterranean limestone and granite. Natural as it may be, it does have serious health implications, which to many are largely unknown. As part of its radioactive decay, Radon in air breaks down to solid particles, which can then attach to particles of aerosol gases. When breathed in, these solids can then attach to bronchial tubes. They then decay further, and fairly rapidly, to safe compounds, but for the time they are in contact with the bronchial tubes they irradiate them, and and can cause lung cancer.
While outside atmospheric levels are safe, Radon can accumulate in enclosed spaces, such as man-made structures, instead of dispersing into the air. In areas where Radon is more abundant due to the geological makeup of the land, it can reach unsafe levels, such as would be illegal within the nuclear industry. Studies have shown that the incidence of lung cancer is higher in these areas.
As Radon is an airborne phenomenon, ventilation of some description is an obvious solution. Simply leaving the windows open won't do the job though, as it's release is triggered by low temperatures, so open windows all night and all winter are not a practical solution.
Radon Mitigation Systems work on a simple vacuum basis. A pipe runs from a sump dug below the lowest level of the building up to the attic or roof space, where a radon mitigation fan runs continuously and exhausts through the roof. This creates low air pressure in the sump, drawing in the radon gas from the surrounding ground, which is then sucked up the pipe and expelled into the atmosphere.