Average Radon Levels in Colorado: EPA Zone Data and What It Means
Colorado Is Zone 1 Statewide
Every county in Colorado sits in EPA Radon Zone 1, the highest risk classification. Zone 1 means the predicted average indoor radon screening level exceeds 4 pCi/L, the EPA's action threshold. Colorado consistently ranks among the top three to five states for radon in the nation, and CDPHE data shows roughly 1 in 2 Colorado homes tests above that 4 pCi/L action level.
That statistic makes Colorado an outlier. Nationally, about 1 in 15 homes exceeds the EPA action level. In Colorado, you are flipping close to a coin. Being in Zone 1 means testing is not optional advice; it is genuinely necessary for every home in the state, including new construction.
But Zone 1 is a county-level average, not a guarantee. Two houses on the same street can read 2 pCi/L and 25 pCi/L depending on foundation type, local soil permeability, and construction details. Testing is the only way to know your specific number. Find a certified radon tester in Colorado through our directory.
The Geology Behind Colorado's High Radon
Radon does not appear randomly. It is a decay product of uranium-238, which is concentrated in certain rock types. Colorado's geology creates multiple overlapping conditions that drive radon higher than most states.
The Front Range Batholith
A batholith is a massive body of granite that intruded into the Earth's crust and cooled slowly, allowing uranium-bearing minerals to concentrate. Colorado's Front Range Batholith runs roughly 200 miles from Fort Collins south through Denver and on to Colorado Springs. This formation includes the Pikes Peak Granite (dominant in El Paso County), the Boulder Creek Granite (Boulder and Gilpin counties), and the Idaho Springs formation (Clear Creek County).
Uranium-235 and uranium-238 in these granites decay through a chain of isotopes. Radium-226 is the immediate precursor to radon-222, the isotope that accumulates in homes. When radium decays, the resulting radon gas migrates through soil pores and fractures toward the surface. Over granite bedrock with thin soil cover, that migration path is short and radon entry rates into homes are high.
The Colorado Uranium Belt
Beyond the granite batholith, Colorado has a distinct uranium mineral belt that runs through the central part of the state. Clear Creek, Gilpin, Boulder, and Jefferson counties all sit within or adjacent to this belt. The mining history of these counties is not incidental: Gilpin County was Colorado's first major mining district, and uranium ore was found alongside gold and silver across the region. Uranium-bearing minerals in the shallow crust here are more concentrated than in surrounding areas, translating directly to higher radon potential in the soil.
Mountain Glaciation and Glacial Till
During Pleistocene glaciation, mountain valleys in Summit, Eagle, and Pitkin counties were filled with glacial till: a mixture of ground-up rock, sand, and clay left by retreating ice. Glacial till derived from granite contains elevated uranium content, and its relatively loose, high-permeability structure allows radon gas to migrate toward the surface more freely than through dense clay soils. Mountain valleys in these counties, including the areas around Breckenridge, Frisco, Vail, and Aspen, can have high radon even in locations where residents assume the cold, open environment would be protective.
High Altitude and Fractured Rock
At elevation, two additional factors come into play. Granitic bedrock in mountain areas is heavily fractured by freeze-thaw cycles, creating a network of channels radon can travel through quickly. And thin or absent soil cover above fractured rock means there is little buffer between the radon source and the foundation. Mountain homes, especially those built directly on or into rock, can see very high radon entry rates.
Radon Levels by County and Region
While all Colorado counties are Zone 1, there is meaningful variation within that category. Here is what the data and geology show for major regions:
| County / Area | Major Cities | Risk Level | Primary Driver |
|---|---|---|---|
| El Paso County | Colorado Springs, Fountain, Manitou Springs | Very High | Pikes Peak Granite bedrock dominates the region |
| Jefferson County | Lakewood, Arvada, Golden, Wheat Ridge | Very High | Front Range Batholith; uranium belt proximity |
| Boulder County | Boulder, Longmont, Louisville (foothills areas) | High to Very High | Boulder Creek Granite; foothills homes above thin soil |
| Clear Creek County | Idaho Springs, Georgetown | Very High | Idaho Springs formation; historic uranium mining area |
| Gilpin County | Central City, Black Hawk | Very High | Uranium mineral belt; heavily mineralized bedrock |
| Arapahoe County | Aurora, Centennial, Englewood, Littleton | High | Denver Basin sediments over granite substrate |
| Denver County | Denver | High | Urban mix; older homes with basement foundations over alluvial soils |
| Larimer County | Fort Collins, Loveland, Estes Park | High | Granite foothills; Rocky Mountain National Park geology extends into suburbs |
| Summit County | Breckenridge, Frisco, Dillon, Silverthorne | High | Granite mountains; glacial till valleys; many seasonal homes left closed in winter |
| Eagle County | Vail, Edwards, Avon | High | Glacial till; mountain granite; high-end homes often built into hillsides |
| Pueblo County | Pueblo, Pueblo West | Elevated | Transition zone between granite and sedimentary geology |
| Mesa County | Grand Junction, Fruita | Moderate | Sandstone and shale geology of the Colorado Plateau; lower uranium content than Front Range |
Mesa County and the Western Slope generally read lower than the Front Range because the underlying geology shifts from uranium-bearing granite to the sedimentary sandstone and shale of the Colorado Plateau. This does not mean Grand Junction homes are safe without testing, but the statistical likelihood of a very high result is lower than in, say, Jefferson or El Paso counties.
Why Zone 1 Does Not Tell the Whole Story
The Zone 1 designation is built on county-level data averages. Within any county, radon levels vary by neighborhood, by block, and by individual home. The factors that matter most at the house level are:
- Foundation type: Homes with basements pull more radon than slab-on-grade homes, because the basement sits closer to the soil and has more surface area in contact with the ground. Crawl spaces are also high-risk.
- Construction quality: Gaps around pipes, sump pits, floor cracks, and block foundation walls all create pathways for radon entry. Newer construction is not automatically safer; it is often better sealed against weather but that same sealing traps radon inside.
- Local soil permeability: Sandy or gravelly soils let radon move faster than dense clay. Two adjacent lots can have meaningfully different soil types depending on local fill history and natural deposits.
- Stack effect: Homes act like chimneys. Warm air rises and escapes from the upper stories, creating negative pressure at the foundation. This negative pressure pulls soil gas, including radon, into the house. The stronger the stack effect, the more radon enters.
Seasonal Variation: When Colorado Radon Is Highest
Radon levels in Colorado homes follow a seasonal pattern that homeowners should understand before scheduling a test.
Levels are highest in winter, typically November through March. Three factors combine to drive this:
- Homes are closed up for heating season, so there is less dilution from outside air
- The stack effect is strongest when there is a large temperature difference between indoors and outdoors; Colorado winters create extreme differentials
- Frozen ground seals surface pathways for radon to escape, pushing more gas toward buildings
Levels are lowest in summer, when windows are open and the stack effect is weaker. A test done in July with windows open can significantly understate the radon your family is breathing during the eight months of the year when the house is sealed.
CDPHE recommends testing October through March for the most representative results. If you test in summer and get a reading between 2 and 4 pCi/L, consider retesting in winter before assuming you are safe. That summer 3 pCi/L reading can easily be a winter 6 or 8 pCi/L.
Understanding the EPA Action Level in Colorado Context
The EPA action level of 4 pCi/L is the threshold above which the EPA recommends mitigation. But it is not a safety threshold in the strict sense. The EPA also states that radon levels between 2 and 4 pCi/L represent meaningful risk and that mitigation should be considered. There is no level of radon exposure that carries zero risk.
To put the numbers in perspective:
- 1.3 pCi/L is the average outdoor radon level nationally
- 4 pCi/L carries roughly the same lung cancer risk as smoking 8 cigarettes per day for a lifetime non-smoker
- 8 pCi/L is comparable in risk to smoking half a pack per day
- 20 pCi/L approaches the risk level of smoking one to two packs per day
Colorado homes regularly test above 10, 20, and even 30 pCi/L, particularly in El Paso and Jefferson counties. Radon is the second leading cause of lung cancer in the United States, responsible for approximately 21,000 deaths per year. In Colorado, where exposure rates are this high, it is the leading environmental health risk inside the home.
What to Do If Your Test Comes Back High
If your test shows 4 pCi/L or above, the next step is mitigation. The standard method is sub-slab depressurization: a licensed contractor installs a pipe through the foundation slab and a fan that draws radon from beneath the home and exhausts it above the roofline. This system typically reduces radon levels by 80 to 99 percent. Mitigation in Colorado costs $800 to $2,500 depending on foundation complexity, home size, and location.
After installation, a post-mitigation test confirms the system is working. Reputable contractors include this test as part of the job.
Find a certified radon tester or mitigator in Colorado through our directory of 428 licensed professionals.
Frequently Asked Questions
What EPA radon zone is Colorado?
All Colorado counties are classified as EPA Radon Zone 1, the highest risk category. Zone 1 means the predicted average indoor radon screening level exceeds 4 pCi/L. Colorado is consistently ranked among the top three to five states nationally for radon.
What percentage of Colorado homes have high radon?
Approximately 1 in 2 Colorado homes tests above the EPA action level of 4 pCi/L, according to CDPHE data. Nationally, the rate is about 1 in 15. Colorado's uranium-rich granite geology is the primary driver of this elevated rate.
Which Colorado counties have the highest radon levels?
El Paso County (Colorado Springs), Jefferson County (Lakewood, Arvada, Golden), Clear Creek County (Idaho Springs), and Gilpin County consistently show the highest readings. These areas sit directly on uranium-bearing granite bedrock and within Colorado's uranium mineral belt. Boulder County foothills are also very high.
When is the best time to test for radon in Colorado?
October through March gives the most representative results. Colorado radon levels peak in winter because homes are sealed, the stack effect is strongest when indoor-outdoor temperature differences are large, and frozen ground pushes more soil gas toward buildings. A summer test with windows open can significantly understate winter exposure.
Does altitude affect radon levels in Colorado mountain homes?
Yes. Mountain homes often face higher radon entry rates because granite bedrock is heavily fractured by freeze-thaw cycles, creating direct pathways for radon to migrate quickly into structures. Thin or absent soil cover above the rock removes the natural buffer that slows radon migration in lower-elevation areas. Seasonal homes in Summit, Eagle, and Pitkin counties that sit closed all winter can accumulate very high concentrations.