Radon Levels by State: Average Indoor Concentrations Across the US

Why Radon Levels Vary by State

Radon comes from uranium in soil and rock. States built on granite, uranium-bearing shale, or glacially deposited material from uranium-rich sources tend to have higher average indoor radon. States with limestone-dominated coastal geology or thick sedimentary cover tend to run lower.

But state averages are exactly that — averages. A single home in Florida can test above 10 pCi/L. A home in Iowa might test below 4 pCi/L. The only way to know your home's level is to test.

The Highest-Radon States

These states consistently show average indoor radon above the EPA action level of 4 pCi/L:

North Dakota and South Dakota — among the highest in the nation, averaging 9.0 and 8.7 pCi/L respectively. Uranium-bearing shales across the plains, combined with glacial sediments, create widespread severe risk. Estimated 60-70% of homes exceed the action level.

Iowa — highest average in the contiguous US at approximately 8.5 pCi/L. Glacial till from uranium-rich Canadian Shield rock covers most of the state. Studies estimate roughly 70% of Iowa homes test above 4 pCi/L.

Montana — approximately 7.9 pCi/L average. Extensive uranium deposits throughout the Rocky Mountain regions and underlying eastern plains create widespread risk.

Pennsylvania — approximately 7.7 pCi/L average. The Reading Prong — a belt of uranium-bearing metamorphic rock running through Berks, Montgomery, and Northampton counties — creates exceptional concentrations. Pennsylvania has one of the most active state radon programs in the country.

West Virginia — approximately 6.7 pCi/L. Uranium-bearing Appalachian black shales create widespread risk, particularly in the Eastern Panhandle.

Colorado — approximately 6.6 pCi/L. Front Range and mountain counties sit on uranium-bearing granite. Boulder, Jefferson, and El Paso counties are among the highest-testing in the country.

Kansas and Nebraska — approximately 6.3 and 6.0 pCi/L. Uranium-rich glacial material and shale deposits across both states.

Wyoming — approximately 6.9 pCi/L. One of the most uranium-rich states geologically.

The Moderate-Risk States

These states average 3–6 pCi/L — lower than the highest-risk states but still requiring testing and often mitigation:

Indiana (5.6 pCi/L), Minnesota (5.5 pCi/L), Idaho (5.2 pCi/L), Illinois (5.1 pCi/L), Ohio (5.0 pCi/L) — all high-glaciated Midwest states with elevated risk.

Missouri (4.9 pCi/L), Wisconsin (4.8 pCi/L), Utah (5.0 pCi/L) — mixed geology with above-average risk.

Vermont (4.6 pCi/L), New Hampshire (4.6 pCi/L), Virginia (4.7 pCi/L), Kentucky (4.7 pCi/L) — Appalachian and New England granite geology.

Massachusetts (4.5 pCi/L), Maine (4.5 pCi/L) — granite bedrock throughout both states.

Michigan (4.3 pCi/L), Maryland (4.3 pCi/L), New Jersey (4.2 pCi/L) — above action level on average.

New York (4.4 pCi/L), Connecticut (4.1 pCi/L), Rhode Island (4.1 pCi/L) — New England and Hudson Valley granite geology.

Lower-Risk States

These states average below 4 pCi/L but still warrant testing:

North Carolina (2.8 pCi/L), Delaware (2.7 pCi/L) — moderate, particularly in Piedmont regions.

Georgia, Oregon, Washington, California — 2.3–2.5 pCi/L. Regional variation within each state is significant.

Tennessee, South Carolina, Oklahoma, Texas, Arizona — 2.0–2.2 pCi/L.

Nevada, New Mexico, Alabama, Mississippi — 1.7–1.9 pCi/L.

Louisiana (1.5 pCi/L), Florida (1.2 pCi/L), Hawaii (0.8 pCi/L) — lowest risk.

What These Numbers Mean for You

State averages are a starting point, not a verdict. Key factors that can push your home above or below the state average:

Foundation type: Basements accumulate radon more than slabs. Crawl spaces can go either way depending on vapor barriers.

Foundation condition: More cracks and gaps mean more entry points.

Home tightness: Energy-efficient homes can accumulate radon faster.

Local geology: County-level variation within a state can be extreme. Pennsylvania's Reading Prong counties average much higher than western Pennsylvania.

The only reliable way to know your home's radon level is to test it directly.

Short-term charcoal test kits cost $17–$25 from certified labs (see our shop). If your result is above 4 pCi/L, find a certified mitigator in your state:

Browse certified radon mitigators by state →