States With the Highest Radon Levels

By Steve Crews · Last updated June 26, 2026

Which states rank highest

No national database tracks every US home, so state rankings come from EPA survey data, state radon programs, and academic studies that estimate the share of homes above the 4 pCi/L action level. Those sources point to a consistent group of leaders.

Iowa ranks first in virtually every analysis. State surveys run by the Iowa Department of Public Health have found that a majority of Iowa homes test above 4 pCi/L in at least some rooms, with statewide indoor averages far above the national figure of roughly 1.3 pCi/L. Iowa’s radon program has been active for decades, and its data make clear that homeowners there face a meaningfully higher baseline risk than most of the country.

North Dakota and South Dakota are close behind. Much of both states sits in EPA Radon Zone 1, and surveys show a high proportion of homes with elevated readings. The combination of uranium-bearing till soils left by retreating glaciers and tightly sealed homes built for harsh winters creates conditions where radon can accumulate to high concentrations.

Minnesota and Nebraska fall in the same tier. Minnesota’s geological survey has mapped high-uranium granitic bedrock across much of the state, and Nebraska’s sandy, uranium-bearing soils produce consistently elevated readings across many counties.

Pennsylvania is the standout eastern state. While most of the South and Southeast sit in lower-risk zones, Pennsylvania has one of the most active radon programs in the country for good reason. The Reading Prong, a belt of uranium-bearing metamorphic and igneous rock running through eastern Pennsylvania into New Jersey and New York, produces some of the highest naturally occurring indoor radon levels in the country. Pennsylvania has identified radon as a leading public health priority and offers extensive testing resources through its Department of Environmental Protection.

Other states with large numbers of Zone 1 counties include Colorado, Montana, Wyoming, Idaho, and Ohio. Every state has at least some elevated-radon areas, but the states above have more counties where above-4 readings are common rather than the exception.

States with generally lower levels include Hawaii, Mississippi, Louisiana, and much of the Gulf Coast, where geology produces less radon and warmer climates allow more air exchange with the outdoors. “Generally lower” does not mean zero risk: individual homes can still test high anywhere, and testing remains the only reliable answer regardless of location.

Why geology drives the map

Radon comes from uranium. The more uranium in the rock and soil beneath a home, the more radon that soil can generate. That is why the radon risk map and the geological map look so similar across the country.

The Upper Midwest sits on thick deposits of glacial till, soil scraped from uranium-bearing bedrock and redistributed across the landscape as glaciers retreated at the end of the last ice age. That till is fairly permeable, which lets soil gas move easily toward the surface. Pair permeable, uranium-rich soil with a house that acts as a slight vacuum relative to the ground below it, and radon flows in.

In the Mountain West, exposed granite outcroppings and mineralized zones contain elevated uranium, especially in Colorado, Wyoming, Montana, and Idaho. Parts of these states also have phosphate-bearing rock, which is a secondary radon source.

Pennsylvania’s Reading Prong is a band of old crystalline rock including gneisses and granites with high uranium content. Unlike the broad gradual pattern of the Midwest, the Reading Prong creates a more sharply defined high-risk corridor, which is why some Pennsylvania counties rank among the most elevated in the country while neighboring counties test considerably lower.

Building construction amplifies the geology. Tightly sealed homes built for cold climates trap more radon than loosely built older construction. Homes with basements provide a large soil-contact area and an enclosed lower level where radon can accumulate before anyone thinks to measure it. That is why cold-weather states with lots of basements appear on the high list even when uranium content alone would not fully predict it.

The EPA Zone Map and what it actually tells you

The EPA divides every US county into one of three radon zones based on predicted average indoor screening levels:

• Zone 1: predicted average above 4 pCi/L
• Zone 2: predicted average between 2 and 4 pCi/L
• Zone 3: predicted average below 2 pCi/L

The map is a planning and policy tool, not a home-by-home forecast. The EPA is explicit that even Zone 3 homes can test above 4 pCi/L, and the agency recommends testing all homes regardless of zone. The zone tells you what is statistically typical in your county; it does not tell you what is in your house.

You can look up your county’s zone and understand what the thresholds mean at our EPA radon action level guide. Many state radon programs also publish their own county-level data, which in some cases is more current and detailed than the federal map.

For a full breakdown of where each state stands and what state-specific programs are available, see our acceptable radon levels by state reference.

What to do if you live in a high-radon state

Living in Iowa, Pennsylvania, North Dakota, or any other high-radon state does not mean your home is dangerous. It means the odds are higher than average that your home has elevated radon, and that testing is especially important.

The first step is straightforward: test. A short-term charcoal canister kit costs $15 to $40 and can be ordered online or picked up at most hardware stores. Many states with high radon rates offer free or reduced-price kits through their radon programs, so check your state health department’s website before buying anything. Iowa, Minnesota, and Pennsylvania all have well-established programs with easy ordering. A long-term alpha track test gives a more accurate picture of your annual average, which is the better basis for any mitigation decision. Our DIY radon testing guide covers both types and how to interpret the results.

For a widely used mail-in option, the AccuStar long-term alpha track test is commonly used by home inspectors and referenced by several state programs, though the free kits your state may offer are equally valid.

If your test comes back at or above 4 pCi/L, the EPA recommends taking action. A professionally installed sub-slab depressurization system typically brings levels down to 2 pCi/L or below in the vast majority of homes. The EPA recommends hiring a contractor certified through AARST-NRPP (the Indoor Environments Association’s National Radon Proficiency Program), and most states let you search for certified mitigators by zip code through their health department websites.

If you are buying a home in a Zone 1 county, ask whether the seller has tested for radon or has a mitigation system already installed. Radon contingencies in purchase contracts are common and reasonable in high-radon states, and a documented, certified install with a clean retest is a selling point at resale.

The U.S. Surgeon General has issued a national health advisory urging all Americans to test their homes, with particular emphasis on people living in higher-risk areas. Knowing your level is the entire game. Radon you have measured and addressed is far less concerning than radon you have never thought about.

FAQ

Is it safe to live in Iowa or Pennsylvania given the high radon levels? Elevated average radon is a manageable risk, not a reason to avoid an entire state. Many homes in Iowa and Pennsylvania test well below the action level, and those that do test high can be mitigated effectively. The key is to measure first, then make decisions based on your actual result rather than regional averages.

Does living in a Zone 1 county mean my home definitely has high radon? No. Zone 1 means the predicted county average exceeds 4 pCi/L, but individual homes vary enormously within any county. Your specific soil, foundation type, and home construction all factor in. The EPA says to test every home regardless of zone because variation within zones is too large to predict from the map alone.

Can newer homes in high-radon states have low radon levels? Yes. Homes built in the 1990s and later in many high-radon states may include radon-resistant new construction features: a gravel layer under the slab, a polyethylene barrier, and a passive vent pipe that can be activated with a fan if testing shows it is needed. These features reduce but do not eliminate radon entry, so testing a newer home is still necessary.

What is the national average indoor radon level, and how do the top states compare? The EPA estimates the national average indoor radon level at roughly 1.3 pCi/L, well below the 4 pCi/L action level. States like Iowa and North Dakota have reported statewide averages several times higher than that national figure, which is why their state radon programs are among the most active and well-funded in the country.