Which States Have the Highest Radon Levels?

Radon Risk Is Geological

Radon comes from uranium in soil and rock. States with granite bedrock, uranium-bearing shales, or glacial soils deposited from uranium-rich source rock tend to have the highest indoor radon levels. It's not about climate or home age — it's about what's in the ground beneath your foundation.

The EPA has mapped the US into three zones based on predicted average radon potential:

• Zone 1 (Red): Average indoor radon above 4 pCi/L — highest risk
• Zone 2 (Orange): Average 2–4 pCi/L — moderate risk
• Zone 3 (Yellow): Average below 2 pCi/L — lower risk

But these are county-level averages. Radon can vary dramatically from house to house on the same street. The only way to know your home's level is to test.

Highest-Risk States

Iowa — Highest in the Nation

Iowa sits on uranium-bearing glacial till deposited by glaciers that scraped through uranium-rich Canadian Shield rock. The state average indoor radon level is the highest in the country — around 8 pCi/L, with some areas routinely testing above 20 pCi/L.

Pennsylvania

Pennsylvania's Reading Prong — a band of uranium-rich metamorphic rock that runs through the southeast — creates some of the highest radon concentrations in the Eastern US. The Reading Prong extends into New Jersey and New York.

Colorado

Colorado's Front Range and mountain counties sit directly on uranium-bearing granite and shale. Boulder County, Jefferson County, and El Paso County are among the highest-testing counties in the country.

Minnesota

Similar to Iowa, Minnesota has uranium-bearing glacial soils across much of the state. The Minneapolis-Saint Paul metro area has high average radon levels, and the state has one of the most aggressive radon mitigation programs in the country.

Ohio

Ohio has highly variable radon — some counties test very high, others are moderate. The geological boundary between glaciated and unglaciated portions of the state creates pockets of elevated risk.

Other High-Risk States

Montana, North Dakota, South Dakota, Wyoming, Kansas, and Nebraska all have significant high-radon zones due to uranium-bearing geology. Indiana, Illinois, and Michigan have large areas of glaciated terrain with elevated risk.

Lower-Risk Areas

The Southeast (Florida, Alabama, Georgia, Louisiana) generally has lower radon levels due to limestone-dominated geology and sandy soils with poor uranium content. Coastal states often test lower than inland states.

But "lower average" doesn't mean no risk. Even in low-risk states, individual homes can test high depending on local geology, construction type, and foundation.

Does Building Age Matter?

Older homes aren't inherently riskier — a well-sealed new home on uranium-bearing soil can test higher than a drafty old farmhouse in a low-risk area. What matters is:

• Foundation type: Basements and crawl spaces concentrate radon more than slab-on-grade
• Foundation condition: More cracks and gaps mean more entry points
• Ventilation: Tighter, more energy-efficient homes can accumulate radon faster
• Geology: The underlying soil and rock

What To Do If You're in a High-Risk State

Test first. Always. A $17 short-term charcoal test gives you your actual number within a week. Many states offer free or discounted test kits — check your state health department.

If you're above 4 pCi/L: get a certified mitigator. In high-radon states like Iowa, Colorado, and Pennsylvania, mitigation is common enough that contractors are experienced, competitive, and sometimes faster to schedule than in lower-risk states.

Find a certified mitigator in your state →