EPA Map of Radon Zones: What Homeowners Should Know
When homeowners start researching radon, one of the first tools they encounter is the EPA map of radon zones — a color-coded map dividing U.S. counties into three risk categories. It is a useful planning resource, but it is also widely misunderstood. A “low” zone does not mean a home is safe, and a “high” zone does not guarantee a problem. This guide explains how the map works and summarizes EPA and CDC guidance current as of 2026. It is general information, not a medical diagnosis. Radon health figures are anchored to the EPA action level of 4 pCi/L, and any testing or mitigation decision should involve a certified professional.
What Is the EPA Map of Radon Zones?
The EPA Map of Radon Zones was developed with the U.S. Geological Survey to help governments, builders, and homeowners understand radon potential across the country. It assigns every county in the United States to one of three zones based on the predicted average indoor radon level for that area. The map was designed primarily to guide building codes and resource targeting, not to predict any individual home’s reading.
The three zones are defined by predicted average indoor radon concentration. Zone 1 counties have a predicted average at or above 4 pCi/L — the EPA action level. Zone 2 counties have a predicted average between 2 and 4 pCi/L. Zone 3 counties have a predicted average below 2 pCi/L. The colors typically run from red for Zone 1 down through orange and yellow for the lower-risk zones.
The critical caveat, which the EPA states plainly, is that the map is not a substitute for testing. It describes county-wide averages, and homes within a single county vary enormously based on local geology and how each house meets the ground.
What the Three Zones Actually Mean
Reading the zones correctly prevents two common mistakes: panic in a high zone and complacency in a low one.
Zone 1 — Highest Potential
Zone 1 counties have a predicted average indoor radon level of 4 pCi/L or higher. These are the areas where elevated radon is most likely, and where new construction codes often require radon-resistant features. A home in Zone 1 has a higher prior probability of an elevated reading, but the actual level still must be measured. Plenty of Zone 1 homes test low, and the only way to know is a test.
Zone 2 — Moderate Potential
Zone 2 counties have a predicted average between 2 and 4 pCi/L. The “moderate” label is deceptive, because the prediction is an average — individual homes can read well above 4 pCi/L. Many homes in Zone 2 require mitigation. The EPA recommends testing here just as firmly as in Zone 1.
Zone 3 — Lower Potential
Zone 3 counties have a predicted average below 2 pCi/L. Lower potential is not the same as no potential. The EPA has documented homes with dangerous radon levels in every zone, including Zone 3. A lower-zone designation should never be read as permission to skip testing.
Why Every Home Still Needs Testing
The single most important message about the map is that it does not replace a test. Radon levels are driven by the specific soil and rock directly beneath a house, the home’s foundation type, the number and size of entry points, ventilation, and even weather and the season. Two homes on the same street, in the same zone, can have very different readings.
The map predicts averages across large areas using limited indoor measurements, geology, aerial radiometric data, soil characteristics, and foundation types. It was never intended to estimate a single property. The EPA, the CDC, and the U.S. Surgeon General all recommend that every home be tested regardless of zone. A short-term test provides a quick snapshot; a long-term test better reflects year-round exposure. Homeowners can review our explanation of key radon facts to understand why testing matters more than mapping.
Where Colorado and the Front Range Fall
Colorado is one of the higher-risk states on the EPA map, with the majority of counties classified as Zone 1. Front Range counties — including Denver, Jefferson, Douglas, Arapahoe, Boulder, Adams, Broomfield, and El Paso — generally sit in Zone 1, reflecting the region’s uranium-bearing geology. The Colorado Department of Public Health and Environment reports that a large fraction of tested homes statewide come back at or above the action level.
Front Range homeowners should treat the Zone 1 designation as a prompt to test rather than proof of a problem. Cold winters can elevate readings as tightly sealed, heated homes draw soil gas upward through the stack effect, so winter testing often captures peak conditions. Even within a Zone 1 county, individual results range from very low to well above 4 pCi/L, which is precisely why the map cannot stand in for measurement.
How to Use the Map Correctly
The map’s real value is contextual. Builders and local officials use it to decide whether radon-resistant construction should be standard. Homebuyers and homeowners can use it to gauge baseline likelihood and to remind themselves that testing is warranted. It is a starting point that should always lead to an actual measurement.
To find a specific area’s zone, homeowners can consult the EPA’s map resources or their state radon program, which often maintains more granular local data than the national map. Many states publish county-level or even zip-code-level test result summaries that are more current than the original national map. For the practical steps that follow, our broader guide to radon testing in Colorado explains how localized data and testing work together. Whatever the map shows, the recommended next step is the same: test the home.
How the EPA Built the Map
Understanding how the map was assembled clarifies both its value and its limits. The EPA and the U.S. Geological Survey combined several data sources to predict county-level radon potential. They drew on indoor radon measurements collected across the country, geologic data describing the rock and soil types in each area, aerial radiometric readings that detect surface radioactivity, soil permeability characteristics that affect how easily gas moves, and information on the foundation types common in each region.
Those inputs were modeled to produce a predicted average indoor radon concentration for each county, which determined its zone. The approach is sound for its intended purpose — guiding building codes and directing resources toward higher-risk areas — but it produces an average across a large geographic unit. A county can contain pockets of high-radon geology alongside areas with very low potential, and the single zone label cannot capture that internal variation. The map is a generalization built from generalizations, which is precisely why it points toward testing rather than substituting for it.
Common Misconceptions About the Zones
Several persistent myths surround the map, and correcting them helps homeowners act sensibly. The first is that a Zone 3 home is safe and needs no test. It is not; dangerous levels have turned up in every zone, and the only way to rule out a problem is to measure. The second is that a Zone 1 home is doomed to have high radon. It is not; many Zone 1 homes test below the action level, because an individual house’s level depends on its specific soil and foundation.
A third misconception is that a newer home is automatically safe. Age does not determine radon; both new and old homes can have elevated levels, though new construction in some areas now includes radon-resistant features. A fourth is that a neighbor’s low result means yours is fine. Adjacent homes can differ substantially because of variations in soil, foundation cracks, and entry points. The unifying correction is simple: the map and word-of-mouth are starting points, and a test on the specific home is the only reliable answer. Our overview of radon mitigation systems explains what happens after a test confirms elevated levels.
Using State and Local Data Alongside the Map
The national map is a coarse tool, and many states maintain finer-grained data that complements it. State radon programs often publish county-level or zip-code-level summaries of actual test results, which reflect more measurements and more recent data than the original national map. Colorado’s health department, for example, provides radon information and resources tailored to the state’s high-risk geology, and many counties have substantial local testing data.
For a homeowner, the practical workflow is to use the EPA zone to gauge baseline likelihood, check any available state or local data for a more localized picture, and then test the actual home regardless of what either source shows. Localized data can also inform decisions such as whether to include radon-resistant features in new construction or a remodel. But every layer of mapping ultimately reinforces the same conclusion: predicted risk informs expectations, while a measurement determines reality.
How to Test After Checking the Map
Once a homeowner has noted the zone, the next move is a test, and a few practices make the result trustworthy. Short-term tests, lasting from a couple of days to about a week, give a fast snapshot and are common during real estate transactions. Long-term tests, running more than 90 days, better capture the year-round average because radon levels fluctuate with weather, season, and how a home is operated.
Placement and conditions matter. Tests are generally conducted in the lowest livable level of the home, with doors and windows kept closed under “closed-house conditions” for short-term tests to avoid diluting the reading. Test kits should be kept away from drafts, exterior walls, and high-humidity areas, and used according to their instructions. If a short-term test reads at or above 4 pCi/L, the EPA suggests following up — either with a second short-term test or a long-term test — before committing to mitigation, since a single short measurement can be influenced by transient conditions.
Seasonal timing is worth a thought on the Front Range. Winter often produces the highest readings as heated, tightly closed homes draw soil gas upward, so a winter test captures something close to a worst case. A home that reads acceptably in winter is unlikely to spike in summer, which is one reason many homeowners choose the heating season to test. Whatever the zone the map assigned, this measurement is the figure that actually governs whether mitigation is needed.
When to Call a Professional
Homeowners can start testing on their own with a quality short-term kit. Bring in a certified radon professional when a test reads at or above 4 pCi/L, when results are inconsistent, during a real estate transaction, or when mitigation is being considered. A professional provides tamper-resistant testing, interprets borderline results in context, and designs a mitigation system suited to the specific foundation. For health concerns related to radon exposure, consult your physician. A professional is especially valuable in a real estate transaction, where a tamper-resistant or continuous-monitor test carries more weight than a consumer kit left unattended. In a high-potential area such as the Front Range, where the map assigns most counties to Zone 1, professional testing during a sale is common and gives both buyer and seller a defensible result. Beyond transactions, a professional can also advise on whether a borderline reading between 2 and 4 pCi/L warrants action given how the home is used, which is the kind of judgment the zone map alone can never provide. The map sets expectations; the measurement and a qualified interpretation determine what to do next. For homeowners who find the zones confusing, the simplest rule cuts through the complexity: regardless of color, test the home, confirm an elevated short-term result with a follow-up, and mitigate if levels reach the action level. That sequence holds in every zone in every state, which is exactly why the EPA pairs the map with an unambiguous recommendation to test rather than letting the map stand alone.
References
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- A Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- About Radon — Centers for Disease Control and Prevention
- Radon in Colorado — Colorado Department of Public Health and Environment
Front Range homeowners who want a definitive radon reading rather than relying on the zone map can connect with a vetted local inspector through our contact page.