EPA Radon Map: What Homeowners Need to Know
When homeowners start researching radon, one of the first tools they find is the EPA radon map. It color-codes the country by predicted radon potential and looks authoritative, which leads many people to check their county and decide whether to worry. The map is genuinely useful for understanding regional risk, but it has important limits that are easy to miss. This guide explains what the EPA radon map shows, how its zones work, and why even the lowest-risk zone never replaces testing your own home. It summarizes EPA and Colorado Department of Public Health and Environment guidance current as of 2026 and is general information, not a substitute for a measurement.
What the EPA radon map shows
The EPA Map of Radon Zones divides U.S. counties into three categories based on predicted indoor radon levels. Zone 1 counties have the highest predicted average, above 4.0 pCi/L. Zone 2 counties fall in a moderate range, roughly 2 to 4 pCi/L. Zone 3 counties have the lowest predicted average, below 2.0 pCi/L.
The colors translate to a quick risk impression: Zone 1 is the area of greatest concern. The map was built from indoor radon measurements, geology, soil data, and other factors to help governments target outreach and shape building codes. It was never meant to predict any single home’s level.
How to read your county’s zone
To use the map, find your state and locate your county’s assigned zone. A Zone 1 designation means the county’s predicted average exceeds the action level, so homes there are more likely to test high. A Zone 3 label means the predicted average is low, but high-radon homes still exist in every zone.
Much of Colorado’s Front Range falls in Zone 1, reflecting the region’s uranium-bearing geology. Counties such as Denver, Douglas, Jefferson, Boulder, and El Paso carry that highest designation. Our companion guide on the radon map of the USA puts the national picture in context.
The map’s biggest limitation
The single most important thing to understand is that the EPA radon map predicts county averages, not individual homes. Two houses on the same street can have wildly different radon levels because of differences in foundation type, construction, soil directly beneath them, and how the home is operated. The map cannot capture that.
The EPA is explicit on this point: the map should not be used to determine whether a specific home should be tested. Every home should be tested regardless of its zone. A Zone 3 address with a basement and the right soil conditions can still test well above 4.0 pCi/L.
Why low-zone homes still test high
It feels counterintuitive that a low-risk county would produce a high-radon home, but it happens regularly. Radon entry depends on localized soil chemistry, cracks and gaps in the foundation, sump pits, crawlspace conditions, and the pressure difference that draws soil gas indoors. None of those are visible on a county-level map.
Newer, tightly sealed homes can actually concentrate radon because they exchange less air with the outdoors. So a modern, energy-efficient house in a Zone 3 county may read higher than an older, drafty home in a Zone 1 county. The only way to know is to test. See how to test for radon for the practical steps.
How the map is actually useful
Despite its limits, the map serves real purposes. It helps policymakers prioritize radon programs and resistant-construction codes in high-zone areas. It gives homebuyers a regional heads-up, especially when relocating to an unfamiliar area. And it underscores why radon testing is standard practice in places like Colorado’s Front Range.
Think of the map as setting expectations rather than answering questions. If you are moving to a Zone 1 county, you should expect radon to be a real consideration and budget for testing and possible mitigation. The Colorado radon testing hub explains how that plays out locally.
Map zones and Colorado specifics
Colorado is one of the more radon-prone states, and the map reflects that with widespread Zone 1 designations. The CDPHE reports a large share of tested Colorado homes exceed the EPA action level, consistent with the map’s predictions. For Front Range buyers, a Zone 1 county is the norm, not the exception.
That said, the within-county variation still applies. A Zone 1 county average does not tell you whether your specific home reads 3 pCi/L or 15 pCi/L. Local geology shifts block to block, which is exactly why state and federal agencies push universal testing rather than map-based decisions.
From map to action
Use the map as a starting point, then test. Place a short-term test in the lowest livable level for two to seven days, or use a long-term test for a fuller annual picture. If the result is at or above 4.0 pCi/L, plan mitigation; if it is borderline, confirm with a second test. The map informed your expectations, but the test makes the decision.
For homes that test high, our guide to radon mitigation cost in Colorado outlines realistic budgets. The map tells you the neighborhood’s odds; the test tells you your home’s reality.
How the EPA built the map
The Map of Radon Zones was not guesswork. The EPA combined several data layers to predict county radon potential: indoor radon measurements collected through state surveys, the underlying geology, soil characteristics, aerial radiometric surveys of natural radioactivity, and building foundation types. Each county was scored and assigned to one of the three zones based on its predicted average indoor level.
This methodology was designed for a specific purpose, helping governments target radon programs and building codes, not for diagnosing individual homes. Knowing how the map was made clarifies its strengths and limits. It is reliable for describing regional tendencies because it rests on real measurements and geology, but it averages across entire counties, which is exactly why it cannot speak to a single address. The data behind it is sound; the resolution is simply county-level.
What the three zones mean in practice
It helps to translate the zone colors into practical expectations. A Zone 1 county, predicted average above 4.0 pCi/L, means a high likelihood that homes there test above the action level; budget for testing and possible mitigation as a near-certainty. A Zone 2 county, the moderate band, means elevated homes are common but less universal. A Zone 3 county, predicted average below 2.0 pCi/L, means lower odds, though by no means zero.
None of these zones removes the need to test, but they do shape how you plan. A buyer relocating to a Zone 1 county should expect radon to be part of the conversation and set aside funds accordingly. A buyer in a Zone 3 area can be cautiously optimistic while still testing. The zones are a planning lens, calibrating expectations and budgets before the real measurement tells you your home’s actual number.
Why averages hide so much variation
The deepest limitation of any zone map is that a county average can mask enormous variation. Within a single Zone 1 county, individual homes may range from under 1 pCi/L to well into the double digits. The average that placed the county in Zone 1 is the midpoint of a wide spread, not a description of every home. The same is true in reverse for Zone 3 counties, where some homes still test high.
This variation arises because radon entry is governed by hyper-local factors: the soil immediately beneath a foundation, the specific cracks and openings in that foundation, and how the individual house exchanges air. A map drawn at county resolution cannot capture block-to-block, let alone house-to-house, differences. Understanding this is what keeps a homeowner from mistaking a favorable county average for a guarantee about their own address.
Newer maps and data sources
The original EPA zone map is now decades old, and many states have since collected far more indoor radon data. As a result, some state health departments publish their own, more detailed radon-potential maps drawn from larger and more recent test databases. These can offer finer geographic detail than the federal map, sometimes down to smaller sub-county areas, though they still describe averages rather than individual homes.
For Colorado, the state’s own resources complement the federal map and reflect the Front Range’s well-documented high radon. When researching your area, it is worth checking both the EPA zone and any state-level map, since the state data may be newer. Our broader guide to the radon gas map explains how these various map products relate and where each is most useful.
Using the map when relocating
The map earns its keep most clearly during a move. Someone relocating to an unfamiliar region can check the destination county’s zone to set expectations before house-hunting. A move to a Zone 1 area, such as much of Colorado’s Front Range, signals that radon will be a routine part of the home search, that testing should be budgeted, and that mitigation may be needed on a given home.
This advance knowledge changes how a buyer approaches inspections and negotiations. In a high-zone area, factoring radon testing into the inspection plan and reserving funds for potential mitigation is simply realistic. Buyers who understand the zone ahead of time are not blindsided when a test comes back high; they treat it as the expected, fixable condition it is.
Conversely, relocating to a low-zone area should not breed complacency. The map lowers the odds but never to zero, so the destination home still warrants its own test. Used as a relocation planning tool, the map informs budgets and expectations while leaving the actual decision to the home-specific measurement that follows.
Common ways people misuse the map
The most frequent mistake is using the map to decide whether to test. Homeowners in Zone 3 counties sometimes conclude they can skip testing, which is exactly the conclusion the EPA warns against. High-radon homes exist in every zone because foundation cracks, local soil, and construction vary house to house. A favorable zone is reassuring at the regional level and meaningless at the individual level.
A second misuse is treating the map as current and precise when it is neither at the address scale. Relying on a decades-old county average to judge a specific modern home invites error. The map answers “what are the regional odds?” It does not answer “what is my home’s level?” Keeping those two questions separate prevents the false confidence that leads people to forgo a simple, inexpensive test.
Using the map alongside a real test
The map and a home test are complementary, not competing, tools. Use the map early, when researching an area, relocating, or setting expectations and budget. Then test your specific home to get the number that actually drives decisions. A Zone 1 designation tells you to expect radon and plan for testing and possible mitigation; the test confirms whether your home needs a fix.
This two-step approach is how the EPA intends the map to be used. For Front Range buyers, the practical sequence is simple: expect a high-zone county, budget accordingly, and test the home during the inspection period. Our guide on how to test for radon covers the measurement, and the Colorado radon testing hub ties the regional and individual pictures together.
References
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- Radon in Colorado — Colorado Department of Public Health and Environment
- Radon and Indoor Air — American Lung Association
Front Range homeowners who want help moving from map research to an actual radon test, or connecting with a vetted local professional, can reach out through our contact page.