EPA Radon Zones: What Zone 1, 2, and 3 Really Mean
The EPA Map of Radon Zones sorts every U.S. county into Zone 1, Zone 2, or Zone 3 based on the predicted average indoor radon level for that county. People reach for it constantly when deciding whether to test a house, but it answers a narrower question than most assume. The map predicts where radon problems are statistically likely across a whole county. It cannot tell you what any single home will measure, because the house next door can read three times higher.
What the numbers behind each zone are
The zones are keyed to the same benchmark the EPA uses everywhere else: 4 picocuries per liter (pCi/L), the level at which the EPA recommends fixing a home. The agency also suggests considering a fix between 2 and 4 pCi/L.
- Zone 1 — predicted average indoor screening level above 4 pCi/L. Highest potential.
- Zone 2 — predicted average between 2 and 4 pCi/L. Moderate potential.
- Zone 3 — predicted average below 2 pCi/L. Lowest potential.
These are screening averages for a county, not thresholds any individual home is guaranteed to fall under. The EPA’s own map documentation states plainly that the map is not meant to determine whether a home should be tested, and that homes with elevated radon exist in all three zones.
Why the map cannot predict your house
Radon enters through the soil-gas pathway: uranium in soil and rock decays to radium, then to radon gas, which seeps in through foundation cracks, sump pits, slab penetrations, and floor-wall joints. Two houses on the same street can differ enormously because the variables that actually drive an indoor reading are local and structural, not county-wide:
- Soil permeability and the exact geology under the footprint
- Foundation type — a basement pulls in more soil gas than slab-on-grade
- Cracks, sumps, and unsealed penetrations that give gas an entry route
- How tightly the house is sealed, plus the winter stack effect that draws air (and radon) up from below
That is why the EPA and the CDC recommend the same thing regardless of zone: test the home. A Zone 3 designation lowers the odds; it does not clear a specific address. Nationally the CDC estimates about 1 in 15 U.S. homes has a radon level at or above the action level, and elevated homes turn up in every zone.
How the map was actually built
The current map dates to 1993 (EPA Report 402-R-93-071) and has not been formally revised since. The designations combined several data layers:
- U.S. Geological Survey radium and uranium data for near-surface geology
- Aerial radiometric surveys of natural background radiation
- Indoor radon measurements from the EPA’s State/EPA Indoor Radon grant surveys
- Soil permeability and regional foundation-type prevalence
Those inputs are good for allocating state program resources and setting code priorities. They were never built at a resolution fine enough for a per-home decision, and the underlying survey data is now more than three decades old.
Where Zone 1 clusters — and where Colorado sits
Roughly a third of U.S. counties fall in each zone. Zone 1 concentrates in a few predictable geologies: the Mountain West, the granite bedrock of New England, Pennsylvania’s uranium-bearing Reading Prong, and the glacial soils of the Upper Midwest such as Iowa and Minnesota.
Colorado’s Front Range is squarely in this high-potential band. Nearly every county along the I-25 corridor — Denver, Boulder, Larimer, El Paso, Jefferson, Adams — carries a Zone 1 designation, and the Colorado Department of Public Health and Environment reports that roughly half of Colorado homes tested come back at or above 4 pCi/L. For a Front Range buyer or homeowner, the practical takeaway is simple: the map already tells you to expect elevated readings, so the real work is the test itself, not the map lookup.
Where the map still matters: building codes
The one place a zone designation carries direct weight is new construction. Appendix F of the International Residential Code specifies radon-resistant construction techniques — a sub-slab gas-permeable layer, a sealed vent pipe routed through the roof, and sealed foundation penetrations — and jurisdictions in Zone 1 commonly adopt it as mandatory. Many Colorado municipalities require these passive rough-ins on new homes. A passive system can often be activated later with an inline fan if a post-construction test comes back high. Always confirm what your specific local amendment requires.
Using the map the right way
Treat the zone as a prior, then override it with a real measurement:
- Buying: put a radon test in the inspection contingency in any zone. In Zone 1 it is close to non-negotiable. See how the test fits an inspection in our radon testing overview.
- Interpreting a result: the zone is irrelevant once you have a number — compare it against what counts as a safe radon level, not against the map.
- Ongoing monitoring: radon fluctuates by season and even by day, so a continuous monitor gives visibility a one-time kit cannot. If a reading lands high, the fix is a mitigation system, covered in what radon mitigation involves.
Beyond the federal map, check your state health agency — Colorado, Pennsylvania, and several others publish finer county- or ZIP-level data, and some offer discounted or free test kits.
Radon is the second-leading cause of lung cancer in the U.S. and the leading cause among non-smokers, which is why the American Lung Association and the EPA push testing over map-reading. The zone tells you the odds. Only a measurement tells you the house.
InspectandTest is an independent editorial team, not certified radon measurement professionals. This guide summarizes EPA, CDC, and CDPHE guidance current as of 2026; confirm local code and thresholds with your state radon program.