Radon Levels by State: Where Is Radon Most Common?

Some states have serious radon problems. Others... still have radon problems, just not as many people know about it.

The EPA classifies every US county into one of three zones based on predicted average indoor radon levels. Zone 1 is the highest risk -- predicted average above 4.0 pCi/L. Zone 2 falls between 2.0 and 4.0. Zone 3 is below 2.0. But those maps don't tell the whole story.

The Highest-Risk States

Iowa takes the crown. The entire state is Zone 1 -- highest average indoor radon concentration in the country. Not even close.

Pennsylvania is right up there, especially eastern and central PA. The Reading Prong -- a geological formation loaded with uranium-bearing rock -- runs through the Lehigh Valley and extends into New Jersey and New York. If you're in the Philly metro or anywhere in the Lehigh Valley, testing isn't optional. It's essential.

Ohio is mostly Zone 1, particularly the northeastern and central regions. Colorado's Front Range corridor -- Denver, Boulder, Colorado Springs, Fort Collins -- is heavily Zone 1, and Colorado has one of the highest testing rates in the country because of it.

And yeah, Minnesota, Indiana, Nebraska, and the Dakotas all have major Zone 1 coverage. The northern Great Plains got hit hard by glacial deposits that happen to be radon-friendly.

What About the Northeast?

Significant risk across the board. That Reading Prong we mentioned? It's the geological gift that keeps on giving. It runs from Pennsylvania through northern New Jersey, southeastern New York, and into Connecticut. Counties along that formation consistently show elevated levels.

New York, New Jersey, Massachusetts, Connecticut, New Hampshire, Vermont, and Maine all have substantial Zone 1 areas. The Northeast doesn't get as much attention as the Midwest for radon, but it probably should.

The Southeast: Lower on Average, but Don't Get Comfortable

Generally lower levels than up north. But the Appalachian corridor through Virginia, North Carolina, Tennessee, and northern Georgia includes plenty of Zone 1 counties. The piedmont region of the Carolinas also shows elevated levels in spots.

Florida? Lowest average radon levels of any state. But "average" is the operative word -- individual homes can still test high depending on the soil and construction.

Out West

Variable. Colorado and Montana have significant Zone 1 areas. Idaho, Utah, and parts of Washington too. California, Oregon, and the desert Southwest generally run lower, but individual results are all over the map.

So Here's What Actually Matters

Zone maps are county-level predictions. Your home isn't a county -- it's a specific building on specific soil with a specific foundation. Radon levels can vary wildly between neighbors. One house at 1.0 pCi/L, the house next door at 10.0. It happens all the time.

What drives individual home levels:

Soil composition directly under the foundation
Foundation condition -- cracks, joints, pipe penetrations
Building pressure dynamics (stack effect, how your HVAC operates)
Groundwater radon levels (matters if you're on well water)

This is why the EPA recommends testing ALL homes regardless of zone. A short-term test kit costs $15 to $30. That's nothing compared to the cost of not knowing.

State Testing Programs

A bit of context: many states offer free or discounted radon test kits through their health departments. Some maintain searchable directories of certified testing and mitigation professionals. Check your state environmental agency's website -- or just search for contractors in your state right here.

Look, radon doesn't care about zone maps. It cares about geology and building science. The only way to know your home's actual level is to test it. And if it comes back high, we've got a guide on exactly what to do next.