A radon area, often called a High Radon Area, is a part of the country where the EPA's radon map predicts that a significant proportion of homes exceed the national reference level for indoor radon. The designation is based on measurements and geology, and it affects the radon protection measures required in new buildings.
Radon is a naturally occurring radioactive gas produced by the decay of uranium in rocks and soils. It disperses harmlessly outdoors, but it can accumulate indoors, and long-term exposure to elevated levels increases the risk of lung cancer. The EPA maintains a national radon map that predicts the percentage of homes above the reference level in each grid square.
Being in a High Radon Area does not mean a particular house has high radon, and being outside one does not guarantee low radon. Levels vary from house to house depending on construction, ventilation and ground conditions, which is why measurement in the individual dwelling is the only way to know.