The US Radon Map: Where Radon Is Worst

By Steve Crews · Last updated June 27, 2026

How the EPA radon map works

The EPA’s Map of Radon Zones was first published in 1993. It looks like a simple color-coded picture, but there is real science behind it. Researchers drew on four sources of data to classify each US county: geology and mineral surveys (which rock and soil types lie beneath the county), airborne radioactivity measurements taken by the Department of Energy, soil permeability (how easily gas can move through the ground), and regional housing characteristics that affect how radon enters buildings.

Each county lands in one of three zones:

• Zone 1 (highest priority): predicted average indoor radon level above 4 pCi/L
• Zone 2 (moderate priority): predicted average between 2 and 4 pCi/L
• Zone 3 (lowest priority): predicted average below 2 pCi/L

The zone numbers track with the EPA action level. At or above 4 pCi/L, the EPA recommends taking action to reduce radon in your home. A Zone 1 designation means the average house in that county is expected to be at or above that threshold. Nationally, the EPA estimates roughly 1 in 15 US homes has radon at or above 4 pCi/L.

One limitation worth knowing: the map has not been substantially updated since 1993. Many state geological surveys have done more granular work since then, and some state-level maps now reflect actual test results from thousands of homes rather than predicted geology alone. When a state map disagrees with the federal one, the state version is usually the more current and precise source.

The states where radon runs highest

Zone 1 counties are not scattered evenly across the country. They cluster in specific regions tied to geology.

Iowa is the standout. The state sits atop glacial soils rich in uranium-bearing minerals, and its housing stock, with many full basements and older construction, gives radon easy pathways indoors. Some Iowa counties report average indoor levels well above 10 pCi/L. Statewide, a large share of tested homes exceed the 4 pCi/L action level. Iowa’s state radon program is among the most active in the country for exactly this reason.

The northern Plains and Upper Midwest carry similar risk. South Dakota, North Dakota, Minnesota, Nebraska, and Kansas all have substantial Zone 1 coverage. Granite and uranium-rich bedrock underlie much of this region. Cold winters keep houses buttoned up for months, which allows radon to build up rather than vent out with fresh air.

Pennsylvania is one of the most variable states in the country. The Reading Prong, a formation of metamorphic rock running through eastern Pennsylvania and into New Jersey and New York, ranks among the most radon-prone geological features in the US. Parts of eastern Pennsylvania record average indoor levels well above the action level, while western Pennsylvania is generally lower risk. This is a state where county-level data matters more than the statewide average.

Ohio, Indiana, Illinois, Michigan, and Wisconsin all contain significant Zone 1 territory tied to glacial deposits and local rock types. Out west, Montana, Wyoming, Colorado, and Idaho are high-risk in areas where granite and uranium geology are prevalent, particularly at higher elevations where that bedrock is closer to the surface.

The Southeast and most coastal states generally fall into Zone 2 or Zone 3. Florida, Louisiana, Mississippi, and Hawaii have low average levels. Florida’s geology is largely sandy limestone with little uranium, and a lot of the housing stock sits on slabs without basements, which limits how radon accumulates. Most of coastal California and the Gulf Coast of Texas are also lower risk, though the Mountain West pushes the numbers back up as you move inland and upward in elevation.

For a complete state-by-state breakdown, see our guides on states with the highest radon levels and acceptable radon levels by state, which also covers state-specific guidelines and program resources.

Why the map is a starting point, not a verdict

The radon zone map tells you about the average house in a county. It says nothing definitive about your house.

Radon levels vary for reasons that have little to do with regional geology: the specific soil composition directly under your foundation, construction materials, how well the slab is sealed, whether the basement has a sump pit (one of the more common entry points), how the house is ventilated, and how many hours a day windows stay closed. Two houses next door to each other can differ by a factor of five or more. A house in Zone 3 can test above 4 pCi/L. A house in Zone 1 can test at 1.5 pCi/L.

This is not a reason to ignore the map. If you live in a Zone 1 county, the base rate of elevated homes around you is genuinely higher, and testing becomes more urgent. But a favorable map cannot substitute for an actual measurement. The map also cannot tell you what is happening in your basement specifically, which is typically the highest-radon space in a home and often the most-used one.

The U.S. Surgeon General has specifically recommended that all Americans test their homes for radon, regardless of location or region. The EPA and AARST-NRPP (the national radon proficiency program) make the same recommendation. The message is not “everyone panic” but “everyone measure,” because the risk is cumulative and invisible and entirely fixable once you know your number.

What to do if you are in a high-radon region

If you live in a Zone 1 county or a high-radon state, treat testing as urgent rather than optional.

Many states with elevated radon rates offer free or reduced-cost test kits through their radon programs. Iowa, Minnesota, Pennsylvania, and several others run periodic promotions where a kit costs a few dollars or nothing at all. Your first stop should be your state health or environmental agency website to check current availability.

If your state does not offer a subsidized kit right now, a short-term charcoal canister test from a certified lab typically runs $15 to $50 and gives results within a week of mailing it back. For an ongoing picture rather than a single snapshot, a long-term alpha track test or a digital continuous monitor gives more robust data. See our roundup of the best home radon detectors for options at different price points and use cases. If you are ready to order a basic mail-in kit now, you can check prices on Amazon for certified short-term kits.

If your results come back at or above 4 pCi/L, that is the EPA action level, and the next step is to contact a certified mitigator. AARST-NRPP maintains a directory of credentialed contractors. Most mitigation systems, a fan and pipe that pull radon from under your floor and vent it above the roofline, bring levels down dramatically, often well below 2 pCi/L. If your result is between 2 and 4 pCi/L, the EPA suggests considering action, especially if someone in the household smokes. Below 2 pCi/L is considered low, though no level is completely without risk.

A note on timing: test sooner rather than later. Radon risk is a function of concentration and time. The years you spend in an untested home are the years that accumulate.

FAQ

Is Iowa really the most radon-prone state in the US? Iowa consistently ranks at the top of national radon surveys. The combination of uranium-rich glacial soil, cold winters that keep houses closed for months, and a housing stock full of older basements produces the highest average indoor levels in the country. Some counties average above 10 pCi/L. Iowa’s state radon program publishes county-level testing data for homeowners who want specifics for their area.

If I live in a Zone 3 county, do I still need to test? Yes. Zone 3 means the predicted county average is below 2 pCi/L, but individual homes within that county can still test well above the 4 pCi/L action level. Geology, construction, and ventilation all create wide variation within any county. The U.S. Surgeon General recommends testing all homes regardless of zone, and for good reason: the only way to know your level is to measure it.

Do state radon maps give better data than the EPA map? Often, yes. States including Pennsylvania, Iowa, Minnesota, Ohio, and Colorado have produced county-level radon data based on thousands of actual test results, not just predicted geology. State maps tend to be more current and more granular than the EPA’s 1993 publication. Check your state’s health or environmental agency website for local data before drawing conclusions from the federal map alone.

Does a finished basement in a low-radon state still need to be tested? Yes. Basements accumulate radon more than upper floors regardless of region, because radon rises from the soil and concentrates in the lowest levels of a home. A finished basement used as a bedroom or home office represents meaningful daily exposure over years. Even in lower-risk states, the basement reading is the one that matters most for your household’s health.