Notes: In addition to the national survey, the US EPA developed The Map of Radon Zones in 1993 to identify areas of the U.S. with the potential for elevated indoor radon levels. The map is intended to help governments and other organizations target risk reduction activities and resources. The Map of Radon Zones was not intended to be used to determine if individual homes need to be tested. Public health messages stressed that all homes be tested, homes be fixed if the level is 4 picocuries per liter (pCi/L) or higher, and that homeowners consider fixing if the level is between 2 and 4 pCi/L. The Map of Radon Zones was developed using data on indoor radon measurements, geology, aerial radioactivity, soil parameters, and foundation types. EPA recommended that the map be supplemented with any available local data in order to further understand and predict the radon potential for a specific area.  