High radon levels are usually caused by radon gas that enters the home through cracks in the foundation. The main source of radon is soil gas, but it can also come from well water and building materials, such as granite, concrete and sheet rock.
About one in 15 homes have high radon levels, estimates the United States Environmental Protection Agency. Homeowners can test for radon by using radon testing kits or hiring a professional testing service. Radon levels can be lowered by sealing foundation cracks and openings. The EPA also recommends installing a vent pipe and fan system, which pulls and filters radon out of the house.