States With Highest Radon Levels: A Clear Guide
Radon risk is not spread evenly across the country. Some states sit on geology that produces dramatically higher indoor levels, while others run comparatively low. Knowing the states with highest radon levels helps homebuyers, relocators, and homeowners understand the odds they face, and it explains why testing is routine in places like Colorado but easy to overlook elsewhere. This guide ranks the higher-risk regions, explains the geology behind them, and stresses the one point every agency makes: state rankings predict averages, not your home. It summarizes EPA and state guidance current as of 2026 and is general information, not a substitute for testing your own house.
Which states have the highest radon levels?
The states consistently flagged as highest-risk cluster in the upper Midwest, the northern plains, the Appalachian region, and the Mountain West. Iowa is frequently cited as the state with the highest average indoor radon, with a large majority of homes testing above the EPA action level. Other states routinely near the top include North Dakota, South Dakota, Minnesota, Pennsylvania, Ohio, and Colorado.
These rankings come from aggregated indoor test data and predicted-potential models. The exact order shifts depending on the data set, but the same states reappear. What they share is geology rich in uranium, the parent material that decays into radon gas.
Why geology drives the pattern
Radon is produced by the natural breakdown of uranium and radium in soil and rock. States sitting on uranium-bearing bedrock, certain granites, shales, and glacial deposits, generate more radon. The gas migrates up through soil and enters homes through foundation cracks, sump pits, and gaps around plumbing.
The northern plains and upper Midwest owe their high levels partly to glacial deposits rich in radon-producing minerals. The Mountain West, including Colorado, sits on uranium-bearing rock from its mining-era geology. Appalachia’s granites and shales drive its readings. The map of high-radon states is essentially a map of certain rock types.
Colorado’s place in the ranking
Colorado consistently ranks among the higher-radon states, and the Front Range is a clear example of geology at work. The Colorado Department of Public Health and Environment reports that a large share of tested homes exceed the EPA action level. Counties including Douglas, Jefferson, El Paso, and Boulder routinely produce elevated readings.
For Front Range homeowners, this ranking is not abstract. It means radon testing is a normal part of buying and owning a home here. The Colorado radon testing hub explains the local situation, and our guide on where radon is most common places Colorado in national context.
How states are ranked
State rankings combine two kinds of information. One is actual indoor measurements collected over years through state and federal programs. The other is predicted radon potential based on geology, soil, and aerial radiometric data, which feeds tools like the EPA Map of Radon Zones. Because methodologies differ, you will see slightly different “top ten” lists from different sources.
What does not change is the underlying message: high-radon states have geology that produces more radon, and the rankings flag where the odds of an elevated home are greatest. Our guide to the EPA radon map explains how the zone system relates to these state-level patterns.
Why rankings never replace testing
Here is the limit of any state ranking: it describes averages, not individual homes. A house in a low-ranking state can test high because of its specific soil, foundation, and construction. A house in Iowa or Colorado can test low for the same reasons in reverse. Radon entry is intensely local.
The EPA is unambiguous that every home should be tested regardless of its state’s ranking. Treating a low-ranking state as a reason to skip testing is the central mistake the rankings can encourage. They set expectations; they do not measure your house.
What high-ranking states mean for buyers
If you are buying or relocating to a high-radon state, the ranking is a practical heads-up. Budget for a radon test as part of your inspection, and be prepared for the possibility of mitigation. In states like Colorado, many real-estate contracts already include radon contingencies because elevated results are so common.
Conversely, moving from a high-radon state to a lower one does not mean you can ignore radon. The new home still needs its own test. The ranking changes the odds, not the recommendation. Our guide on how to test for radon walks through the steps wherever you live.
From ranking to action
Use state and county rankings to calibrate expectations, then test your specific home. Place a short-term device in the lowest livable level under closed-house conditions, or use a long-term test for a fuller annual picture. If the result reaches 4.0 pCi/L or more, plan mitigation; if it is borderline, confirm with a second test.
For homes that test high, our guide to radon mitigation cost in Colorado gives realistic figures, and the same mitigation approach works in any high-radon state. The ranking tells you the regional odds; the test tells you your home’s actual number.
The upper Midwest and northern plains
States like Iowa, North Dakota, South Dakota, and Minnesota dominate the high end of radon rankings, and glacial geology is the reason. Glaciers ground up and deposited mineral-rich material across this region, including rock that produces radon. Combined with widespread basements and cold winters that intensify the stack effect drawing soil gas indoors, these states see a large share of homes above the action level.
Iowa in particular is frequently cited as the highest-radon state, with the majority of homes testing above 4.0 pCi/L. That figure is striking, but it follows logically from the geology and climate. For residents of these states, radon testing is, or should be, completely routine, much as it is along Colorado’s Front Range. The combination of high prevalence and cold-weather concentration makes these among the most radon-prone places in the country.
The Mountain West and Colorado
The Mountain West, including Colorado, Wyoming, Montana, and parts of neighboring states, ranks high because of uranium-bearing rock tied to the region’s mining-era geology. Colorado’s Front Range is a textbook case: the same geology that once supported uranium mining now produces radon that seeps into homes. The CDPHE reports a large share of tested Colorado homes exceed the action level, placing the state firmly among the higher-radon ranks.
Elevation and climate reinforce the pattern. Cold winters keep homes sealed and intensify the stack effect that draws soil gas indoors, much as in the northern plains. For Front Range homeowners, the state’s high ranking translates into a simple practice: test the home and act on the result. Our guide on radon testing in Denver covers the metro, and the Colorado radon testing hub explains the regional picture.
Appalachia and the Reading Prong
The Appalachian region, especially areas underlain by a geologic formation known as the Reading Prong stretching through Pennsylvania, New Jersey, and New York, is another high-radon zone. The granites and uranium-bearing rock here produce significant radon, and the region’s history with elevated indoor levels helped spur national radon awareness in the first place. Pennsylvania consistently ranks among the highest-radon states.
These geologic patterns underscore the central lesson of state rankings: they track rock, not borders. A high-radon formation does not stop at a state line, and pockets of high and low radon exist within every state. Our guide on where radon is most common maps these patterns in more detail and explains why the same formations recur across the rankings.
How rankings can change over time
State rankings are not fixed. As more homes are tested and data sets grow, the precise order shifts, and methodological differences between sources produce slightly different lists. A state near the top in one analysis may appear a notch lower in another simply because of how the data was gathered or weighted. What stays constant is the cluster of high-radon states tied to specific geologies.
This is why fixating on whether a state ranks fifth or eighth misses the point. The rankings are a rough guide to regional risk, not a precise leaderboard. The geologic drivers, uranium-bearing rock, glacial deposits, certain granites and shales, are stable, so the same states recur near the top across analyses even as their exact positions move.
For a homeowner, the lesson is to read rankings as a category, high, moderate, or low risk, rather than an exact rank. Whether your state is officially first or fourth does not change the recommendation: test your home and act on the result. The ranking calibrates expectations; the test makes the decision.
Why low-ranking states still have high homes
It bears repeating because it is so often misunderstood: every state, even those near the bottom of the rankings, contains homes that test well above the action level. Radon entry depends on the soil directly beneath a specific home, the cracks and openings in that home’s foundation, and how the house exchanges air. None of those track the state average.
A homeowner in a low-ranking state who skips testing because the state “doesn’t have radon” is making the exact error the EPA warns against. The ranking shifts the odds, perhaps from a high chance to a lower one, but it never reaches zero. This is why the universal recommendation is to test every home regardless of its state’s standing. The ranking informs expectations; only a test informs decisions.
How rankings should shape a homebuyer’s plan
For someone buying or relocating, state rankings are a useful planning input. Moving to Iowa, Colorado, Pennsylvania, or another high-radon state means radon should be a budgeted, expected part of the home purchase, including a test during the inspection period and a contingency for possible mitigation. In high-radon states, many contracts already treat radon as a standard item.
Moving to a lower-ranking state does not eliminate the step; it just lowers the odds the test comes back high. Either way, the action is the same: test the specific home. For homes that test above the action level, mitigation works identically across all states, and our guide to mitigation cost in Colorado illustrates the typical investment in a high-radon market.
From national ranking to your front door
The journey from a national ranking to a real decision is short. Note your state’s and county’s standing to calibrate expectations. Then test your home, placing a device in the lowest livable level under closed-house conditions, or running a long-term test for a fuller annual average. Confirm borderline results, and act on a confirmed result at or above 4.0 pCi/L.
Rankings are a map of probabilities; your home is the actual territory. A high-ranking state raises the likelihood you will need to act, and a low-ranking one lowers it, but neither answers the only question that matters for your family’s exposure: what does your home actually read? The guide to testing for radon walks through getting that answer.
The states-with-highest-radon picture is ultimately a story about geology meeting climate and construction. It explains why some regions, including Colorado’s Front Range, treat radon testing as routine while others overlook it. Wherever you fall on that map, the practical conclusion is the same: respect the regional odds, then measure your own home and act on the number you find.
References
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- Radon in Colorado — Colorado Department of Public Health and Environment
- Radon and Indoor Air — American Lung Association
Front Range homeowners who want help testing for radon or connecting with a vetted local professional can reach out through our contact page.