Colorado has higher indoor radon than almost any state in the country. The EPA classifies 53 of Colorado's 64 counties as Zone 1 — the highest indoor radon potential category — and the other 11 as Zone 2 (moderate). Zero are Zone 3. CDPHE estimates roughly half of Colorado homes test above the EPA action level of 4.0 pCi/L. In El Paso County, more than 40% of homes tested between 2005 and 2023 came back elevated. (See the Colorado Radon Map for the full zone breakdown.)[1][2]
This isn't a fluke. It's geology, climate, and construction patterns combining to make Colorado one of the most radon-prone places to live in North America. Here's why.
The geology
Radon enters homes from the soil and rock beneath the foundation. The amount that enters depends on how much uranium is in that soil and rock — because uranium is the decay-chain parent that ultimately produces radon-222.
Colorado sits on bedrock with elevated uranium content. The geology breaks down by region:
The Front Range
The Front Range — the eastern slope of the Rockies that includes Denver, Colorado Springs, Boulder, Fort Collins, and the I-25 corridor — is built on Precambrian granitic basement rock. Two formations matter most:
- Pikes Peak Granite. The dominant bedrock under Colorado Springs and much of the southern Front Range, dated to roughly 1.08 billion years ago. Pikes Peak granite contains uranium-bearing accessory minerals (zircon, monazite, allanite) that release radon as they slowly decay over geological time.[3]
- Pierre Shale. A Cretaceous marine shale that outcrops in many parts of the Colorado Springs area. Pierre Shale has elevated organic-bound uranium and is a notable radon source where it's near or at the surface.
The Colorado Plateau
Western Colorado sits on sedimentary rocks of the Colorado Plateau, including the Morrison Formation and the Dakota Sandstone — both known for historical uranium mining (Cold War era). These rocks still contain uranium in trace amounts and contribute to elevated indoor radon in towns like Grand Junction, Montrose, and Durango.
Mountain bedrock
Higher-elevation Colorado bedrock is a mix of granites, gneisses, and metamorphic rocks. Many of these formations have uranium-bearing minerals as well, contributing to elevated radon in mountain communities — Estes Park, Aspen, Crested Butte, Telluride, and others.
Why El Paso County is on the higher end
El Paso County — which includes Colorado Springs, Monument, Fountain, and Manitou Springs — sits directly on Pikes Peak granite, with Pierre Shale outcrops in parts of the eastern county. The combination produces some of the highest indoor radon levels in Colorado:
- El Paso County Public Health: over 40% of homes tested 2005–2023 had elevated radon.[2]
- Many neighborhoods in Colorado Springs (Briargate, Stetson Hills, Black Forest, Falcon, Mountain Shadows, Old Colorado City) consistently see test results above 4.0 pCi/L.
- Test results vary significantly even within a single neighborhood — soil composition, foundation type, and house design all matter. Your neighbor's reading isn't your reading.
The housing stock
Geology gives you a high baseline of radon in the soil. The housing stock determines how much of it gets into your living space.
Colorado homes are particularly prone to indoor radon accumulation because:
- Basements are common. Approximately one in three Colorado Springs homes has a basement. Basements have the largest soil-contact area of any foundation type, so they capture more soil gas.
- Crawlspaces are common in older neighborhoods. Crawlspaces (especially unsealed ones) provide an open soil-air pathway directly into the home.
- Tri-level and split-level designs are common. These create multi-zone foundations where soil gas can enter through any of several pressure points.
- Cold winters mean tight homes. When you seal a home up against Colorado winters, you reduce air exchange with the outdoors. That makes the stack effect stronger and concentrates whatever radon enters.
Altitude — the part that surprises people
Colorado's high elevation doesn't increase indoor radon directly — the gas comes from the soil under your house, not from the air above it. But altitude does affect mitigation. Radon fans lose roughly 4% of their airflow capacity per 1,000 feet of elevation. At Colorado Springs altitude (about 6,000 feet), a fan that's sized for a sea-level install loses roughly 24% of its rated capacity. More on altitude correction →
What this means for Colorado homeowners
If you live in a Colorado home and you haven't tested for radon, the statistical expectation is that you're at or above the EPA action level. That doesn't mean you definitely are — Colorado neighborhoods and even adjacent houses can read very differently — but it means testing isn't optional. The geology and the housing stock combine to make testing essential.