Radon maps, built from tests that actually happened
Most radon maps colour a county from a geological zone estimate. These are built from individual home tests that states recorded and published, and every area carries the number of tests behind it. Where a state has too few tests in an area to say anything honest, that area is grey rather than coloured.
Illinois
35.8% of tests at or above the EPA action level. The largest set of published home radon readings we have found anywhere.
Iowa
The highest radon levels in the country. Every Iowa county is an EPA Zone 1 county.
Minnesota
40.3% of tested homes at or above the action level. Census tract is finer than ZIP, so this is the most detailed map here.
More states as we find ones that publish real measurements with sample sizes attached. Plenty publish a map with no counts behind it, and those are not worth rebuilding.