The EPA produced a map that classifies each county in the United States into one of three categories for radon concentration. The map contains the caveat that not every home in a given county will have radon levels consistent with the rating. You can live in an area with high radon concentrations and still have a home with low levels, just as you can live in an area with low radon levels and have a home with high levels. The amount of uranium in the rocks around your home is not the only factor. How that uranium decays affects the amount of radon released. Do not use the map as a substitute for an individual test of your home.
Zone 1, labeled by a red color on the EPA radon map, identifies counties where the predicted level of radon in a home is 4 picocuries per liter (pCi/L). This zone has the highest potential for radon. The EPA considers 4 pCi/L to be the highest acceptable level for safety, although acknowledging that some risk does exist below 4. Zone 2, the orange zone, identifies counties with moderate potential, with predicted radon levels between 2 and 4 pCi/L. The EPA considers levels of 2 and below to be the most desirable and safest for a home. Zone 3, colored in yellow, shows counties with low potential and predicted levels below 2 pCi/L.
Eastern United States
The areas in the eastern portion of the country with the most radon are in a belt of counties that stretch from Iowa through Illinois, Indiana, Ohio, Pennsylvania, New York and up into New England. Another band follows the mountains down from Maryland to Virginia, the Carolinas and into Georgia. The mountainous regions of Kentucky and Tennessee also are in the red zones. Coastal areas and the deep South fall out of the high-risk areas.
Western United States
Kansas, North Dakota, eastern South Dakota, Montana, Idaho, Wyoming and Colorado represent the states with the highest radon levels in the western part of the country. Counties in eastern Washington, Nevada, Utah, eastern Nebraska and northern New Mexico also fall within zone 1.