Radon is a radioactive gas that occurs naturally when the uranium in soil and rock breaks down. It is invisible, odourless and tasteless. The gas moves through the ground and can enter buildings, with the potential to accumulate to levels which pose a risk to the building occupants health.
Because there are so many factors, it is not possible to predict the radon level in a building. Therefore, only way to know for sure is to test and the only way to reduce the levels is through mitigation.
GeoPro Talks - Radon Awareness, Measurement and Mitigation