EPA Radon Maps 

Sections 307 and 309 of the Indoor Radon Abatement Act of 1988 (IRAA) directed EPA to list and identify areas of the U.S. with the potential for elevated indoor radon levels. EPA's Map of Radon Zones assigns each of the 3,141 counties in the U.S. to one of three zones based on radon potential:

Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (picocurries per liter) (red zones)
Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones)
Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones)

MAP DEVELOPMENT: The Map was developed using five factors to determine
radon potential:
  * indoor radon measurements
  * geology
  * aerial radioactivity
  * soil permeability 
  * foundation type

Radon potential assessment is based on geologic provinces. Radon Index Matrix is the quantitative assessment of radon potential. Confidence Index Matrix shows the quantity and quality of the data used to assess radon potential. Geologic provinces were adapted to county boundaries for the Map of Radon Zones.

National map (state by state breakdown)  (28k)

South Carolina (breakdown by county)  (26k)

North Carolina (breakdown by county)  (22k)

Georgia (breakdown by county)  (33k)

The data used to generate these maps was obtained based on studies which "averaged" the likelihood for radon in an area based on "reported incidences".   Using the maps should only be used as a "guide" for assessing potential radon risks  The EPA still recommends that every house be tested because elevated levels of radon have been encountered in all 50 states!  Because the building envelope for every house is different, there is no way to determine whether ANY individual home  is a candidate for problems with radon without testing!