Radon exposure poses a threat to health when the gas enters buildings through cracks, other openings in the foundation, or in some other manner, and is trapped inside at high concentrations. EPA estimates that six million homes in the United States contain high radon levels. The potential for high indoor radon is greater in areas with certain geological features. Areas known to have the potential for high radon levels are often referred to as "geographic hot spots." Although these geographic hot spots can be targeted for special attention to promote radon awareness and encourage remediation, high radon levels are found in many areas in all states. The presence of a building in a geographic hot spot does not guarantee an elevated radon level; at the same time, the presence of a building in an area of average radon potential does not indicate an acceptable radon level. The only way to know the radon level in a building is to test that building. EPA and the Surgeon General recommend that all homes below the third floor be tested.