One EPA number, county by county

Every county page reports EPA's own Zone rating (1, 2 or 3) from the Map of Radon Zones — built in 1993 from aerial radioactivity surveys, soil parameters, geology and foundation-type data, and republished unchanged in September 2025. Los Angeles County, CA and Cook County, IL show how two very different, very populous counties can share the same Zone 2 rating - the map groups by predicted average, not by any other similarity.

What Zone 1, 2 and 3 actually mean

Zone 1 counties have a predicted county-wide average indoor level above 4 pCi/L — EPA's own action level, the concentration at which it recommends fixing a home. Zone 2 falls between 2 and 4 pCi/L, and Zone 3 below 2 pCi/L. These are county averages from a geologic model, not measurements of any specific house — see the zone explainer for why that distinction matters.

How the national rank is built

EPA's zone is only 3 buckets, so this site ranks every rated county first by zone (highest potential first) and then by population, to give a genuine per-county position rather than three enormous tied groups. A within-zone rank (population only, among counties sharing the same zone) is also shown for the same reason.

Where the raw numbers come from

Radon zones are EPA's Map of Radon Zones spreadsheet (epa.gov/radon). County names, coordinates and population come from the Census Bureau's 2025 gazetteer and population estimates (POPESTIMATE2024). Every source is cited on the county page it feeds.

Where to go next

See the range this plays out across: Loving County, TX, Logan County, KY, Middlesex County, MA.

What Your County's Radon Zone Actually Means (and Doesn't) · What Radon Testing and Mitigation Actually Cost