The U.S. Environmental Protection Agency (EPA) has issued a radon risk assessment map that shows which areas of the United States are more likely to have dangerously high radon levels.
The map is divided into three types of "zones": high potential, moderate potential, and low potential. While this is fine for people living in the high and low zones, those in the moderate zones will probably want to know whether they're just above the low or just below the high!
Still, it's a useful visual guide. And all Americans are encouraged to have a look.
The radon gas map of the USA is available here.
If there are general conclusions to be drawn from the data, they're very basic: the Southeast (most of Texas, Florida, etc.) is the most radon free while the more dangerous radon areas are clustered toward the North, near the Canadian border.
Incidentally, if any Canadians are reading, though the map doesn't cover Canada, the results suggest that Canadians living near the US border (most of you!) are in a high risk radon area.
As for English-speaking Europeans, there are similar map available for England, Wales, Ireland, and Northern Ireland.
Radon Gas Map: USA