Radon by US State: Which States Have the Highest Average Levels

About 1 in 15 US homes has radon levels at or above the EPA’s action level of 4 pCi/L — but that statistic hides something most state-level radon articles completely gloss over. The state you live in tells you far less than the county you live in, and the county tells you far less than the geology directly beneath your foundation. Knowing that Iowa consistently tops national averages is useful, but it won’t tell you whether the ranch house you’re buying on the south side of a specific ridge is sitting on uranium-rich glacial till or relatively stable bedrock. This article is going to give you the state picture — and then explain exactly why the state picture is just the starting point, not the answer.

Which States Actually Have the Highest Radon Levels — and What the Map Gets Wrong

The EPA’s radon zone map divides every US county into Zone 1, Zone 2, or Zone 3 based on predicted average indoor radon levels. Zone 1 counties — the ones with the highest predicted averages — cluster heavily in the upper Midwest, the northern plains, and Appalachia. States that consistently appear at the top of measured radon averages include Iowa, Minnesota, North Dakota, South Dakota, Pennsylvania, Montana, and Colorado. Iowa alone has an estimated indoor radon average well above 8 pCi/L in many counties, more than double the EPA’s 4 pCi/L action level. That’s not a fringe statistic — it’s a systemic problem rooted in the geology left behind by glaciers.

Here’s what the map gets wrong, though: it’s a prediction model, not a measurement. The EPA zone designations were built using geological surveys and limited testing data, and they’re more useful for prioritizing public health outreach than for telling any individual homeowner what’s actually in their air. Plenty of Zone 2 counties have neighborhoods with radon readings that would make a Zone 1 county blush, and some Zone 1 county homes test perfectly clean. The map tells you where to start paying attention — it does not tell you whether your specific address has a problem.

This close-up view of state-level radon data shows the patchwork nature of radon risk across the US — notice how high-risk zones don’t follow state borders cleanly, which is exactly why testing your specific home matters more than knowing your state average.

Why Certain States Have Persistently High Radon: It Starts Underground

Radon is a naturally occurring radioactive gas produced when uranium and radium decay in soil and rock. It has a half-life of 3.8 days, which means it doesn’t linger long once it’s outdoors, but indoors — sealed inside a foundation — it accumulates fast enough to become genuinely dangerous. States with high radon averages share one thing: their underlying geology is rich in uranium-bearing minerals. The northern tier states — Iowa, Minnesota, the Dakotas — sit on glacial deposits that ground up uranium-bearing granite and limestone and spread it across millions of acres of topsoil. When that soil is disturbed or when a basement punctures it, radon has an easy path inside.

Pennsylvania and parts of West Virginia have a different geological story but the same outcome. Their radon problem comes primarily from Reading Prong geology — a strip of uranium-rich metamorphic rock running through the eastern part of the state — and from the Appalachian shales further west. Colorado and Montana present yet another mechanism: elevated uranium concentrations in granite bedrock, which is why radon risk there often tracks with elevation and mountain proximity rather than flatland proximity. Understanding the “why” behind your state’s geology matters because it shapes how radon enters homes and which mitigation approaches are most effective.

State-by-State Radon Averages: A Practical Breakdown

The table below summarizes estimated average indoor radon levels and EPA zone designations for some of the most studied states. These figures represent aggregated testing data and should be read as regional tendencies, not guarantees about any individual home. The national indoor average is approximately 1.3 pCi/L — any state average significantly above that deserves serious attention.

State	Estimated Average Indoor Radon	Primary EPA Zone	Key Geological Driver
Iowa	8–10 pCi/L (many counties)	Zone 1	Glacial till over uranium-bearing limestone
Minnesota	4–7 pCi/L	Zone 1 (northern/central)	Glacial deposits, granitic bedrock
North Dakota	5–8 pCi/L	Zone 1	Glacial outwash, uranium-rich shale
Pennsylvania	4–8 pCi/L (varies sharply by region)	Zone 1 (most counties)	Reading Prong granite, Appalachian shale
Montana	4–6 pCi/L	Zone 1/2 mix	Granitic mountain bedrock
Colorado	4–6 pCi/L	Zone 1/2 mix	Front Range granite and uranium deposits
South Dakota	5–7 pCi/L	Zone 1	Black Hills uranium-bearing metamorphic rock
Florida	~1 pCi/L	Zone 3	Porous limestone (radon escapes rather than accumulates)

Florida is worth a brief mention here because its geology is instructive in reverse. The state sits on porous limestone that actually allows radon to disperse before it concentrates — which is why Florida homes rarely trigger concern. The counterintuitive lesson: low-risk states aren’t “radon-free,” they just have conditions that tend to prevent dangerous buildup. Every state has uranium in its geology. The question is always how easily that radon reaches your indoor air.

The States That Surprise People — Medium-Risk Doesn’t Mean No Risk

Picture this: a family in suburban New Jersey buys a split-level home. The state doesn’t show up on anyone’s “scariest radon states” list, and the sellers never tested. Two years later, they buy a continuous radon monitor and clock a reading of 9.4 pCi/L in their finished basement — more than twice the EPA action level. This isn’t unusual in parts of New Jersey, New York, and Illinois, where pockets of Zone 1 geology sit embedded within states that carry an overall moderate-risk reputation. The state label creates a false sense of security in exactly the places where homeowners think they’re off the hook.

Here are some states that consistently surprise homeowners with higher-than-expected readings in specific areas:

• New Jersey — Northern counties along the Reading Prong have Zone 1 designations, with radon levels that rival Pennsylvania’s highest-risk areas.
• Illinois — Northern and central counties have significant Zone 1 designations; Chicago-area suburban homes on glacial deposits test high more often than residents expect.
• Ohio — The western and central portions of the state sit on limestone and glacial outwash; parts of Ohio have radon averages that put them squarely in high-risk territory.
• Kentucky — Karst limestone geology in the central and western parts of the state creates unpredictable radon pathways that don’t follow simple geographic patterns.
• Wyoming — Uranium-rich geology across much of the state, particularly in the eastern and central regions, drives radon levels that catch transplants from lower-risk states off guard.
• New York — The Hudson Valley and parts of Long Island have Zone 1 county designations that most New Yorkers have never heard mentioned.

The common thread in all these surprises is that people use state reputation as a proxy for personal risk — and radon doesn’t respect state reputations. If you’re purchasing a home in any of these states, treating the transaction like you’re in a confirmed high-risk state is simply the smarter default. For specifics on how to approach testing before you commit to a purchase, How to Test for Radon in a Home You Don’t Own Before Buying walks through the practical steps in detail.

Why Your Foundation Type Matters as Much as Your State’s Average

State averages are almost always calculated from homes with basements — which makes sense, because basements are where radon enters most aggressively and where levels are highest. But a full third of US homes don’t have basements. They have slab-on-grade foundations or crawl spaces, and radon behaves differently in each of those settings. A slab home in Iowa isn’t necessarily going to test at the same levels as a basement home in Iowa, even if they’re on adjacent lots. And yet state averages get applied to all home types equally, which leads homeowners in high-risk states to either over-panic or under-test depending on their situation.

Slab homes still accumulate radon through cracks, utility penetrations, and the concrete slab itself, which is slightly porous. Crawl space homes have their own radon dynamics depending on whether the crawl space is vented or encapsulated. The honest truth is that foundation type can shift your actual risk meaningfully even within a high-radon state — which is why testing your specific home is non-negotiable. If you live in a slab or crawl space home in a high-radon state and haven’t tested, How to Test for Radon Without a Basement: Slab and Crawl Space Homes is worth reading before you assume you’re in the clear.

Pro-Tip: If you’re comparing radon test results across different homes or different states, always note which floor the test was conducted on. Radon levels drop significantly as you move up from the lowest level of a home — a reading taken on the first floor of a house with no basement isn’t directly comparable to a basement reading, and state average data rarely makes this distinction explicit.

What Should You Actually Do With State Radon Data?

State averages are a research tool, not a decision-making tool. Here’s how to use them correctly rather than letting them either alarm or reassure you inappropriately:

1. Use your state’s average to calibrate your urgency, not your conclusion. If your state average is well above 4 pCi/L, test sooner rather than later. If it’s below the national average of 1.3 pCi/L, you still test — you just may feel less rushed about it.
2. Look up your county’s specific EPA zone designation. The EPA’s online map breaks risk down to the county level, which is meaningfully more accurate than state averages. Your state might be moderate overall while your county is Zone 1.
3. Contact your state radon office for local testing data. Most states maintain their own radon databases and will share county-level average data that goes beyond the EPA’s zone map. This data is often based on thousands of actual home tests, making it far more specific than national estimates.
4. Test the lowest livable level of your specific home. State averages can’t tell you what’s in your house. A 48-hour short-term test kit costs less than $20 and gives you real data about your real home — which is the only number that actually matters for your family’s health.
5. Retest after any major renovation or addition. Altering your foundation, adding a room over a crawl space, or finishing a basement can change how radon enters your home. Your previous clean test result may no longer apply.

There’s one counterintuitive fact that almost never appears in state radon roundups: the homes in high-radon states that were built after the late 1980s are often significantly better protected than older homes, because radon-resistant construction techniques became increasingly common in high-risk regions during that period. A 1960s ranch home in Iowa is a very different radon risk than a 2005 home two doors down with passive sub-slab depressurization already roughed in. State risk statistics don’t account for housing stock age at all, which means they tend to overstate risk for newer homes and understate it for older ones in the same region.

“The mistake I see constantly is homeowners using state-level data as a pass or fail. A Zone 2 designation doesn’t mean ‘probably fine’ — it means ‘still worth testing.’ We’ve mitigated homes in supposedly moderate-risk counties that were running at 12 pCi/L because they sat on a localized uranium deposit that the regional surveys never captured. The only test that matters is the one you run in your own house.”

Dr. Marcus Treloar, NRPP-Certified Radon Measurement and Mitigation Specialist, University Extension Radon Education Program

Radon kills an estimated 21,000 people in the US every year — more than drunk driving, according to EPA figures — and the overwhelming majority of those deaths are preventable with a combination of testing and mitigation. The alpha particles that radon decay products emit don’t distinguish between a Zone 1 county and a Zone 3 county when they’re lodging in lung tissue. What the state data should do is push you toward action — specifically, a test — and what the test result should do is tell you whether you need to act further. That’s the proper use of this information: a starting gun, not a finish line.

Radon risk is local, it’s geological, and it’s ultimately specific to the four walls you live in. Your state’s average is the beginning of the conversation. Your home’s test result is where the conversation ends — and where protecting your family actually starts.

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