Radon is a chemically inert, naturally occurring, radioactive gas. It has no smell, colour, or taste, and is produced from the natural radioactive decay of uranium which is found in rocks and soil. Radon gas escapes easily from rocks and soils into the air and tends to concentrate in enclosed spaces, such as underground mines, houses, and other buildings. Soil gas infiltration is recognized as the most important source of residential radon. Other sources of radon include building materials and water extracted from wells, but are of less importance.