Radon levels in U.S. by County.

These average radon values were produced by the high radon project of Lawrence Berkley National Laboratory from a semi-empirical model that used short and long-term radon measurements and surface radon progeny concentrations. For details see http://eetd.lbl.gov/IEP/high-radon/hr.html .

This map was provided by Dr. Dan Steck and his students from "The Minnesota Radon Project." They are located in the Physics Department at St. John's University in Collegeville, Minnesota.