Radon Levels by State: Interactive Map and Rankings

Here’s what most homeowners get wrong about radon risk: they assume that living in a “low-risk state” means they don’t need to worry. That assumption gets people killed. State-level radon data shows you where the odds are stacked against you — but radon doesn’t read maps, and your neighbor’s basement can test clean while yours registers three times the EPA’s action level of 4 pCi/L. The state rankings are a starting point, not a verdict.

That said, the geographic patterns are real, they’re driven by actual geology, and understanding them can help you make smarter decisions about testing and mitigation. About 21,000 Americans die from radon-induced lung cancer every year — more than drunk driving — and the distribution of that risk isn’t random. Let’s look at what the data actually tells you, and more importantly, what it doesn’t.

Why Your State’s Radon Zone Doesn’t Mean What You Think It Does

The EPA divides the country into three radon zones. Zone 1 states (high-risk) have predicted average indoor radon levels above 4 pCi/L. Zone 2 states fall between 2 and 4 pCi/L. Zone 3 states average below 2 pCi/L. These zones are built on county-level data from soil surveys, aerial radioactivity measurements, and indoor radon measurements — and they’re genuinely useful for policy decisions, like requiring radon-resistant new construction in high-risk areas.

The problem is that these zones are averages, and averages hide enormous local variation. Iowa — firmly Zone 1 — has counties where the majority of tested homes exceed 4 pCi/L, but it also has pockets where homes test much lower. Meanwhile, Florida, often listed as a “low-risk” Zone 3 state, has specific counties in the panhandle and phosphate-rich areas of the central region where elevated radon is well-documented. The zone tells you about statistical likelihood, not about your specific house sitting on your specific patch of bedrock.

This close-up view of the state-by-state radon data illustrates exactly why broad geographic categories can be misleading — the county-level variation within a single state is often larger than the difference between two neighboring states.

Which States Actually Have the Highest Radon Levels?

Based on EPA and state radon program data, the states with consistently elevated indoor radon measurements are concentrated in the Northern Plains, Midwest, and Appalachian corridor. The geology driving this is primarily uranium-bearing granite, glacially deposited soils with high radium content, and phosphate rock formations. Radon-222 — the isotope you’re dealing with in your home — is a direct decay product of radium-226, which itself comes from uranium-238 decay chains. Where uranium is abundant in the ground, radon tends to follow.

Here’s a snapshot of states that consistently show the highest percentage of homes testing above 4 pCi/L, based on compiled state radon program screening data:

State	EPA Zone	% Homes Above 4 pCi/L (approx.)	Primary Geologic Driver
Iowa	Zone 1	~71%	Glacial till, uranium-rich soils
North Dakota	Zone 1	~65%	Lignite coal deposits, glacial soils
Pennsylvania	Zone 1 (most counties)	~40%	Reading Prong granite, Appalachian geology
Montana	Zone 1	~52%	Granitic bedrock, thin soil cover

Iowa’s numbers are striking enough that radon testing is essentially considered mandatory by anyone selling a home there. But notice Pennsylvania on that list — a state many people associate more with coal than with radon, yet the Reading Prong geological formation cutting through the southeastern part of the state has produced some of the highest residential radon readings ever recorded in the US.

Does Living in a “Low-Risk” State Mean You Can Skip Testing?

No — and this is the single most dangerous misconception in the radon conversation. Most homeowners don’t think about radon testing until they’re buying or selling a home, which already means they’ve been living with unknown exposure for years. But homeowners in Zone 2 and Zone 3 states are especially likely to skip testing entirely because they’ve been told their region is “low risk,” and that logic has real consequences.

The counterintuitive fact that most radon articles skip over: the national average indoor radon level is just 1.3 pCi/L, but that number includes millions of homes that are effectively at zero. What it masks is that a meaningful percentage of homes in every state — including Hawaii and Florida — test above 4 pCi/L. The EPA’s own data suggests roughly 1 in 15 US homes has elevated radon regardless of state. Your risk isn’t just about where you live on a map; it’s about your soil, your foundation type, your ventilation patterns, and whether there are pathways for soil gas to enter your living space.

“State averages are useful for prioritizing public health resources, but they’re a poor substitute for an actual test. I’ve measured homes in Zone 3 counties with radon levels above 15 pCi/L, and I’ve tested Zone 1 homes that came back at 1.8 pCi/L. The geology sets the stage, but the house determines the performance.”

Dr. Marcus Ellroy, NRPP-Certified Radon Measurement Professional and environmental health researcher, University of Minnesota Extension

What Drives Radon Levels Within a State — the Factors That Actually Matter

Understanding why radon varies so dramatically — sometimes house to house on the same street — requires understanding the mechanism. Radon-222 has a half-life of just 3.8 days, which means it decays quickly into short-lived radioactive “progeny” (polonium-218, lead-214, bismuth-214, and polonium-214). Those progeny attach to airborne particles and, when inhaled, deposit alpha particles directly onto lung tissue. The damage isn’t from radon gas itself sitting in your lungs — it’s from those alpha-emitting decay products lodging in bronchial tissue and causing cellular DNA damage over years of exposure.

So what actually controls how much radon gets from the soil into your home? The drivers are more specific than “geology,” and knowing them helps you understand why two houses in the same neighborhood can have wildly different readings. Here’s what the research consistently identifies:

• Foundation type: Homes with basements or slab-on-grade foundations directly contacting soil are most vulnerable. Homes on crawlspaces fall in the middle, depending on whether the crawl space is vented and how it’s sealed.
• Soil permeability: Coarse, permeable soils (gravel, sandy loam) allow radon to migrate quickly toward the house. Dense clay soils slow migration — though they don’t eliminate the source.
• Foundation cracks and penetrations: Every gap around a pipe, every crack in a slab, every uncovered sump pit is a potential entry point. Pressure differential between indoor air and soil gas pulls radon inward.
• Building depressurization: Exhaust fans, fireplaces, and HVAC systems can create negative pressure indoors, actively drawing soil gas through foundation gaps.
• Construction era: Homes built before radon-resistant construction techniques were commonly adopted tend to have fewer passive barriers against soil gas entry.
• Local bedrock depth: Where bedrock sits close to the surface — as in many parts of Appalachia — there’s less soil buffer between uranium-bearing rock and your foundation.

In most homes we’ve tested, foundation cracks and open sump pits are the two biggest contributors to elevated readings — even in homes where the surrounding soil isn’t particularly high in radium. Sealing those entry points is often the first step a mitigator takes before installing any active system.

How to Use State Radon Data to Make Smarter Decisions About Your Home

The most actionable use of state and county-level radon data isn’t to decide whether to test — you should always test, regardless of where you live. It’s to set realistic expectations about what you might find and to understand what mitigation might involve if you do find elevated levels. A homeowner in Iowa finding 8 pCi/L isn’t facing an unusual situation — but a homeowner in Georgia finding the same number should probably investigate their specific foundation situation more carefully, because something localized is likely driving that number.

Here’s a practical framework for using the state data intelligently, whether you’re buying, selling, or simply trying to protect the family you’ve had living in a house for the past decade:

1. Check your county-level data, not just your state. The EPA’s Map of Radon Zones goes down to the county level. A Zone 2 state can have Zone 1 counties, and that matters for your risk assessment.
2. Test regardless of zone. Long-term tests (90+ days, using an alpha track detector) give the most accurate picture of your actual annual average exposure — more reliable than short-term kits for a final decision.
3. If you’re in a high-radon state, test your lowest livable level. Radon concentrates in basements and on ground floors. A second-floor reading won’t tell you much about where the real exposure risk is.
4. If you find levels at or above 4 pCi/L, mitigation is worth it. Sub-slab depressurization systems — the most common fix — reduce radon by 50–99% in most homes. Understanding how a radon mitigation system works before you call a contractor helps you ask better questions and avoid upsells.
5. Get a cost estimate anchored to your specific situation. Mitigation costs vary significantly based on foundation type, home size, and local labor markets. Getting a sense of radon mitigation cost and what to expect before you get quotes prevents sticker shock and helps you spot outliers.
6. Retest after any major renovation or HVAC changes. Adding a finished basement, sealing old cracks, or installing a new HVAC system can all shift your home’s radon dynamics — in either direction.

Pro-Tip: If you’re buying a home and the seller provides a short-term radon test result from more than two years ago, don’t accept it as current. Radon levels can change meaningfully over time as foundation materials shift, soil conditions change, or ventilation patterns in the home evolve. Request a new test or negotiate for one as part of the inspection process — most testing labs can turn results around in under a week.

There’s one honest nuance worth naming here: the line between “safe” and “unsafe” radon levels is genuinely a continuum, not a hard threshold. The EPA’s 4 pCi/L action level is a policy decision that balances risk reduction with practical feasibility — it doesn’t mean that 3.9 pCi/L is fine and 4.1 pCi/L is dangerous. Even the EPA recommends considering mitigation at levels between 2 and 4 pCi/L, especially for heavy smokers or families with young children, since radon and tobacco smoke together dramatically amplify lung cancer risk. Where you draw your personal line is a legitimate individual decision, but it should be an informed one.

State rankings and EPA zone maps will keep getting updated as more homes are tested and more granular soil data becomes available. What won’t change is the underlying reality: your home’s radon level is a function of your specific geology, your specific structure, and your specific ventilation — and the only number that matters for your family’s health is the one from a test in your actual house. The map shows you the neighborhood; the test shows you the truth.

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