Radon Gas Map: What Homeowners Need to Know
A radon gas map is one of the first things many homeowners reach for when they hear about radon, hoping a quick glance at their county’s color will tell them whether to worry. The maps are genuinely useful for understanding regional risk, but they carry a crucial limitation: they describe the average potential across a whole county, not the actual level inside any one home. A house in a “low” zone can test dangerously high, and a house in a “high” zone can test clean. This guide explains what radon maps show, how the EPA zones work, where Colorado falls, and why a map can never substitute for testing.
This guide summarizes EPA and Colorado state guidance on radon current as of 2026. It is general information, not professional advice; the EPA recommends testing every home and acting at or above 4 pCi/L. Consult a certified radon professional for testing decisions and your physician about health concerns.
What a radon gas map shows
A radon gas map displays the geographic potential for elevated indoor radon, usually broken into zones by county. The best-known example is the EPA’s Map of Radon Zones, which classifies counties into three categories based on predicted average indoor radon levels. The map draws on indoor radon measurements, geology, soil characteristics, aerial radioactivity data, and foundation types to estimate the radon potential of an area.
The purpose is planning and awareness, not diagnosis. The EPA developed the zone map to help governments and builders prioritize radon-resistant construction and to raise public awareness in higher-risk regions. It tells a homeowner whether they live in an area where elevated radon is statistically more likely, which is valuable context — but it is a regional average built from many homes, not a measurement of any single house. Understanding that distinction is the whole point of reading the map correctly, and it ties into the broader picture in the radon testing pillar.
How the EPA radon zones work
The EPA’s map sorts counties into three zones based on predicted average screening levels, and knowing what each represents helps a homeowner interpret their location.
Zone 1: highest potential
Zone 1 counties have a predicted average indoor radon screening level at or above 4 pCi/L — the EPA action level. These are the highest-potential areas, where elevated readings are statistically most common. Many Colorado Front Range counties fall in Zone 1, reflecting the uranium-bearing geology of the region. A Zone 1 designation is a strong prompt to test, though it does not guarantee a high reading.
Zone 2 and Zone 3
Zone 2 counties have a predicted average between 2 and 4 pCi/L, representing moderate potential. Zone 3 counties have a predicted average below 2 pCi/L, the lowest potential. A lower zone does not mean a home is safe; it means elevated radon is statistically less common across the county. Individual homes in Zone 2 and Zone 3 still test high with enough frequency that the EPA recommends testing everywhere, regardless of zone.
Why a map cannot replace testing your home
The central message of every radon authority is that a map predicts regional averages, while only a test reveals a specific home’s level. Radon entry depends on factors that vary house to house: the soil directly beneath the foundation, the number and size of cracks and penetrations, the foundation type, how the home is sealed, the HVAC operation, and even how the occupants live. Two identical-looking homes on the same street can have very different radon levels.
This is why a low-zone designation cannot reassure and a high-zone designation cannot condemn. Homes in EPA Zone 3 have tested well above the action level, and homes in Zone 1 have tested clean. The map narrows the probability; the test delivers the fact. The factors that drive entry into a particular home are explained in this guide on how radon gets into homes, and they are precisely what a map cannot capture.
Colorado and the Front Range on the radon map
Colorado is a high-radon state, and much of the Front Range sits in EPA Zone 1. The region’s geology — uranium-rich soils and rock formations along the foothills and plains — produces elevated radon potential across Denver, Boulder, Jefferson, Douglas, El Paso, and neighboring counties. State data consistently shows a large share of tested Colorado homes reading at or above the action level, which is why state health authorities push testing hard.
For a Front Range homeowner, the map’s message is simple: the area is high-potential, so testing is essential, not optional. But the same caveat holds locally — a Front Range home is more likely to test high, yet the only way to know its actual level is to measure. Colorado-specific guidance and resources are available through the state health department, and the practical testing steps mirror those described in the radon testing pillar. The map tells a Front Range owner to test; it does not tell them their result.
Using a radon map the right way
A radon gas map is best used as a prompt and a planning tool, not a verdict. For a homebuyer, a high-zone designation reinforces the value of including a radon test in the inspection. For a homeowner, it signals whether radon deserves attention even if they have never thought about it. For a builder, it indicates whether radon-resistant new construction techniques are advisable in the area.
What a map should never do is end the inquiry. Treating a low-zone location as permission to skip testing is the most common misuse, and it leaves families exposed when their particular home happens to read high. The correct workflow is: check the map for context, then test the home regardless of zone. If the test reads at or above 4 pCi/L, mitigation follows. The map informs; the test decides.
How radon maps are built
Understanding how a radon map is constructed clarifies both its usefulness and its limits. The EPA’s Map of Radon Zones drew on several data sources: indoor radon measurements collected across the country, the underlying geology and soil characteristics of each area, aerial radiometric surveys measuring natural radioactivity, and information on housing and foundation types. Combining these inputs let the agency estimate the average radon potential of each county and sort them into zones.
This methodology produces a reliable picture of regional tendencies but inherently averages over a large area. A county classified as high-potential contains many individual homes with widely varying levels, because the map cannot account for the specific soil beneath one foundation, the cracks in one slab, or the construction of one house. The map answers the question “is this area prone to elevated radon?” — not “does this house have elevated radon?” The two questions require different tools.
State and local agencies sometimes publish more granular radon data, including measurement summaries by zip code or neighborhood, which can refine the regional picture. Colorado’s health department, for example, provides radon information specific to the state’s counties. Even these finer datasets, however, remain summaries of many homes and cannot substitute for measuring a particular dwelling. The map and its refinements inform; the test measures.
Common misconceptions about radon maps
Several persistent misconceptions lead homeowners astray when reading radon maps. The first is that a low-zone designation means a home is safe — it does not, because individual homes in the lowest-potential zones still test above the action level with enough frequency that universal testing is recommended. Skipping a test because of a favorable map color is the most consequential misreading.
A second misconception is that a high-zone designation guarantees a problem. While elevated readings are statistically more common in high-potential areas, plenty of homes in Zone 1 test below the action level. The map raises the probability of a problem but does not confirm one, so a high-zone homeowner should test rather than assume the worst or, conversely, assume a neighbor’s high reading predicts their own. Radon varies house to house even on the same street.
A third misconception is that a one-time test settles the question permanently. Radon levels change with the seasons, with foundation conditions, and with home modifications like new HVAC equipment or basement finishing. A map cannot capture this temporal variation any more than it captures house-to-house variation, which is why periodic retesting — regardless of zone — is the prudent practice. Treating the map as context and the test as the recurring measurement avoids all three errors.
Using maps for new construction and prevention
Radon maps serve a valuable role beyond informing individual homeowners: they guide construction practices in high-potential areas. Builders and code officials use the zone designations to decide where radon-resistant new construction techniques are advisable. In a Zone 1 county like many along the Colorado Front Range, building a home with passive radon-resistant features — a gravel layer beneath the slab, a sealed vapor barrier, sealed foundation penetrations, and a roughed-in vent pipe — costs little during construction and makes later mitigation far easier if a test comes back high.
This preventive use of the map is one of its strongest applications. Installing the passive infrastructure during construction is much cheaper than retrofitting an active system later, and in a high-radon region it is a sensible default. If a post-construction test shows elevated radon despite the passive features, activating the system by adding a fan is straightforward because the piping is already in place.
For a Front Range homebuyer considering new construction, the map’s high-potential designation is a prompt to confirm whether the builder included radon-resistant features and to test the finished home regardless. The map informs the decision to build in radon-resistant ways; the test on the completed home confirms whether those measures, passive or active, brought the level below the action threshold. Prevention and verification work together, with the map guiding the first and the test settling the second.
From map to test to action
The sensible sequence starts with the map for awareness, moves to a radon test for the actual measurement, and proceeds to mitigation if the result warrants it. Testing is inexpensive and straightforward — short-term kits run a few days under closed-house conditions, while long-term tests over months capture seasonal variation. A reading at or above the action level calls for a mitigation system, typically active sub-slab depressurization, designed and verified by a certified professional.
For Colorado Front Range residents especially, where Zone 1 designations are common and a high share of homes test elevated, this sequence is not theoretical caution but practical health protection. A radon gas map is a useful starting point that puts a home’s region in context, but it is the test — not the map’s color — that determines whether a specific home needs action.
The broader value of radon maps lies in awareness. Many homeowners simply do not know radon exists as a hazard until they encounter a map or hear about it during a transaction, and the map’s stark zoning communicates that radon is a real, geographically driven risk worth taking seriously. In that sense the map succeeds even when it cannot diagnose an individual home: it prompts the testing that does. A homeowner who treats the map as the beginning of the inquiry rather than the end uses it exactly as intended, and ends up with the measured knowledge that actually protects their household.
References
- EPA Map of Radon Zones — U.S. Environmental Protection Agency
- Radon Information for Colorado Residents — Colorado Department of Public Health and Environment
- Health Risk of Radon — U.S. Environmental Protection Agency
Front Range homeowners who want help interpreting their area’s radon potential and arranging a test can reach out through our contact page for a connection to a certified local radon professional.