Updated November 2026 · Data: EPA Map of Radon Zones · Sources
Radon is a widespread concern in Colorado: 53 of its 64 counties are in EPA Zone 1, the group with the highest predicted indoor radon levels. The Colorado Department of Public Health and Environment (CDPHE) says about half of Colorado homes have radon above the EPA action level, and its test data show 46.4% of 237,408 pre-mitigation tests above 4 pCi/L. Find your county below, then learn how to test your home and what to do if the result is high.
Your county’s zone is not your home’s radon level. Homes with high radon are found in every zone. The EPA recommends testing every home, whatever its zone.
Map of radon test results by county
Radon test results by county (CDPHE data)
The Colorado Department of Public Health and Environment (CDPHE) publishes 237,408 pre-mitigation radon tests from 2005 to 2025. Statewide, 46.4% were above 4 pCi/L. The table is sorted by the share of high tests; click a county for details.
| County | Tests | Median (pCi/L) | Above 4 pCi/L |
|---|---|---|---|
| Kiowa | 18 | 4.6 | 66.7% |
| Morgan | 354 | 5.1 | 64.1% |
| San Juan | 108 | 5.3 | 63.9% |
| Pueblo | 6,260 | 5.7 | 63.2% |
| Jackson | 46 | 5.05 | 60.9% |
| Otero | 157 | 4.7 | 58.0% |
| Chaffee | 1,624 | 4.8 | 57.1% |
| Cheyenne | 7 | 6.1 | 57.1% |
| Washington | 51 | 4.5 | 56.9% |
| Yuma | 100 | 4.3 | 56.0% |
| Montrose | 2,491 | 4.5 | 55.2% |
| Gilpin | 636 | 4.6 | 54.9% |
| Teller | 2,727 | 4.7 | 54.9% |
| Fremont | 850 | 4.8 | 54.8% |
| Delta | 859 | 4.5 | 54.7% |
| Lake | 422 | 4.4 | 54.7% |
| Larimer | 20,368 | 4.6 | 54.6% |
| Costilla | 26 | 4.4 | 53.8% |
| Elbert | 1,031 | 4.2 | 53.4% |
| Hinsdale | 88 | 4.65 | 53.4% |
| Moffat | 266 | 4.1 | 52.3% |
| Routt | 2,753 | 4.2 | 51.7% |
| Rio Blanco | 193 | 4.1 | 51.3% |
| Jefferson | 30,229 | 4 | 50.3% |
| Ouray | 305 | 3.8 | 49.5% |
| Park | 1,064 | 3.8 | 49.0% |
| Kit Carson | 141 | 3.8 | 48.9% |
| Weld | 11,159 | 3.8 | 48.6% |
| Arapahoe | 21,419 | 3.7 | 48.0% |
| Adams | 12,383 | 3.5 | 46.5% |
| Douglas | 16,872 | 3.5 | 46.4% |
| Dolores | 39 | 3.8 | 46.2% |
| Summit | 3,724 | 3.4 | 46.1% |
| Pitkin | 2,767 | 3.5 | 45.8% |
| Crowley | 20 | 3.5 | 45.0% |
| Prowers | 209 | 3.5 | 45.0% |
| Eagle | 3,845 | 3.5 | 44.7% |
| Broomfield | 4,598 | 3.2 | 44.4% |
| Custer | 438 | 3.25 | 44.3% |
| Grand | 1,073 | 3.3 | 43.8% |
| Boulder | 19,914 | 3.3 | 43.5% |
| La Plata | 3,365 | 3.3 | 43.1% |
| Garfield | 3,243 | 3.3 | 42.9% |
| Phillips | 56 | 3.6 | 42.9% |
| Baca | 54 | 2.9 | 42.6% |
| Mineral | 80 | 3.1 | 42.5% |
| San Miguel | 241 | 3.1 | 42.3% |
| Las Animas | 946 | 3.1 | 41.1% |
| Denver | 21,979 | 3.1 | 39.8% |
| Gunnison | 1,693 | 2.7 | 39.5% |
| El Paso | 25,211 | 2.9 | 38.6% |
| Clear Creek | 710 | 2.4 | 37.3% |
| Rio Grande | 428 | 2.7 | 37.1% |
| Logan | 125 | 3 | 36.8% |
| Lincoln | 100 | 2 | 34.0% |
| Mesa | 4,318 | 2.7 | 33.5% |
| Archuleta | 1,030 | 2.5 | 33.3% |
| Conejos | 75 | 1.7 | 33.3% |
| Huerfano | 719 | 2 | 29.5% |
| Saguache | 203 | 1.8 | 27.6% |
| Alamosa | 168 | 2.05 | 27.4% |
| Montezuma | 784 | 2.4 | 26.9% |
| Sedgwick | 89 | 1.9 | 21.3% |
| Bent | 157 | 0.4 | 17.2% |
EPA radon zones by county in Colorado
The table lists the EPA radon zone for each of the 64 counties in Colorado. The zone shows the predicted average indoor radon screening level for homes in that county.
| County | EPA zone | Predicted average indoor level |
|---|---|---|
| Adams | Zone 1 | Above 4 pCi/L |
| Alamosa | Zone 2 | 2 to 4 pCi/L |
| Arapahoe | Zone 1 | Above 4 pCi/L |
| Archuleta | Zone 2 | 2 to 4 pCi/L |
| Baca | Zone 1 | Above 4 pCi/L |
| Bent | Zone 1 | Above 4 pCi/L |
| Boulder | Zone 1 | Above 4 pCi/L |
| Broomfield | Zone 1 | Above 4 pCi/L |
| Chaffee | Zone 1 | Above 4 pCi/L |
| Cheyenne | Zone 1 | Above 4 pCi/L |
| Clear Creek | Zone 1 | Above 4 pCi/L |
| Conejos | Zone 2 | 2 to 4 pCi/L |
| Costilla | Zone 2 | 2 to 4 pCi/L |
| Crowley | Zone 1 | Above 4 pCi/L |
| Custer | Zone 1 | Above 4 pCi/L |
| Delta | Zone 1 | Above 4 pCi/L |
| Denver | Zone 1 | Above 4 pCi/L |
| Dolores | Zone 1 | Above 4 pCi/L |
| Douglas | Zone 1 | Above 4 pCi/L |
| Eagle | Zone 2 | 2 to 4 pCi/L |
| Elbert | Zone 1 | Above 4 pCi/L |
| El Paso | Zone 1 | Above 4 pCi/L |
| Fremont | Zone 1 | Above 4 pCi/L |
| Garfield | Zone 1 | Above 4 pCi/L |
| Gilpin | Zone 1 | Above 4 pCi/L |
| Grand | Zone 1 | Above 4 pCi/L |
| Gunnison | Zone 1 | Above 4 pCi/L |
| Hinsdale | Zone 2 | 2 to 4 pCi/L |
| Huerfano | Zone 1 | Above 4 pCi/L |
| Jackson | Zone 1 | Above 4 pCi/L |
| Jefferson | Zone 1 | Above 4 pCi/L |
| Kiowa | Zone 1 | Above 4 pCi/L |
| Kit Carson | Zone 1 | Above 4 pCi/L |
| Lake | Zone 1 | Above 4 pCi/L |
| La Plata | Zone 1 | Above 4 pCi/L |
| Larimer | Zone 1 | Above 4 pCi/L |
| Las Animas | Zone 1 | Above 4 pCi/L |
| Lincoln | Zone 1 | Above 4 pCi/L |
| Logan | Zone 1 | Above 4 pCi/L |
| Mesa | Zone 1 | Above 4 pCi/L |
| Mineral | Zone 2 | 2 to 4 pCi/L |
| Moffat | Zone 1 | Above 4 pCi/L |
| Montezuma | Zone 1 | Above 4 pCi/L |
| Montrose | Zone 1 | Above 4 pCi/L |
| Morgan | Zone 1 | Above 4 pCi/L |
| Otero | Zone 1 | Above 4 pCi/L |
| Ouray | Zone 1 | Above 4 pCi/L |
| Park | Zone 1 | Above 4 pCi/L |
| Phillips | Zone 1 | Above 4 pCi/L |
| Pitkin | Zone 1 | Above 4 pCi/L |
| Prowers | Zone 1 | Above 4 pCi/L |
| Pueblo | Zone 1 | Above 4 pCi/L |
| Rio Blanco | Zone 1 | Above 4 pCi/L |
| Rio Grande | Zone 2 | 2 to 4 pCi/L |
| Routt | Zone 2 | 2 to 4 pCi/L |
| Saguache | Zone 2 | 2 to 4 pCi/L |
| San Juan | Zone 2 | 2 to 4 pCi/L |
| San Miguel | Zone 1 | Above 4 pCi/L |
| Sedgwick | Zone 1 | Above 4 pCi/L |
| Summit | Zone 1 | Above 4 pCi/L |
| Teller | Zone 1 | Above 4 pCi/L |
| Washington | Zone 1 | Above 4 pCi/L |
| Weld | Zone 1 | Above 4 pCi/L |
| Yuma | Zone 1 | Above 4 pCi/L |
What the EPA radon zones mean
- Zone 1 (highest potential): predicted average indoor screening level greater than 4 pCi/L. In Colorado: 53 counties (83%).
- Zone 2 (moderate potential): predicted average between 2 and 4 pCi/L. In Colorado: 11 counties (17%).
- Zone 3 (low potential): predicted average below 2 pCi/L. In Colorado: 0 counties (0%).
The EPA built the map from indoor radon measurements, geology, soil permeability and type of house foundation. It was made to help governments and builders target resources, not to decide whether a single home needs testing.
Colorado counties with the highest radon potential
53 counties in Colorado are in Zone 1: Adams, Arapahoe, Baca, Bent, Boulder, Broomfield, Chaffee, Cheyenne, Clear Creek, Crowley, Custer, Delta, Denver, Dolores, Douglas, Elbert, El Paso, Fremont, Garfield, Gilpin, Grand, Gunnison, Huerfano, Jackson, Jefferson, Kiowa, Kit Carson, Lake, La Plata, Larimer, Las Animas, Lincoln, Logan, Mesa, Moffat, Montezuma, Montrose, Morgan, Otero, Ouray, Park, Phillips, Pitkin, Prowers, Pueblo, Rio Blanco, San Miguel, Sedgwick, Summit, Teller, Washington, Weld, Yuma.
How to test for radon in Colorado
Testing is the only way to know your home’s radon level. Short-term test kits (2 to 90 days) are sold at hardware stores and online, and some state programs offer kits at low cost; check with the Colorado Radon Program (CDPHE). Test in the lowest lived-in level of the home, with windows and doors closed, and follow the kit instructions.
- Do-it-yourself kits: a simple, low-cost first step for your own home.
- Long-term tests (more than 90 days): give a better picture of your home’s year-round average.
- Certified professionals: look for certification from the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB), and ask the state program whether Colorado requires a state license.
- Buying or selling a home: ask for a radon test as part of the home inspection.
CDPHE has offered free radon test kits during National Radon Action Month (January), while supplies last. Check the CDPHE radon page for current availability.
If your radon level is high
The EPA recommends fixing your home if the radon level is 4 pCi/L or higher, and considering a fix between 2 and 4 pCi/L. The most common fix is a radon mitigation system (sub-slab depressurization), which pulls radon from under the foundation and vents it outside, above the roof line.
- The EPA says most homes can be fixed for about the same cost as other common home repairs; the price depends on the size and design of the home.
- Get estimates from more than one qualified, certified mitigation contractor. See our radon mitigation cost guide for typical prices.
- Test again after the system is installed, and retest every few years.
Learn more: safe radon levels · how to test · test kits compared · radon and home sales · radon in well water
Colorado radon program
Radon rules and programs in Colorado
- Home sales: Colorado’s Seller’s Property Disclosure form asks sellers to disclose known radon, and brokers must disclose known adverse material facts (C.R.S. § 12-61-804).
- Low-income help: the state’s Low Income Radon Mitigation Assistance (LIRMA) program has provided up to $1,500 toward mitigation for qualifying owner-occupied homes (state fact sheet). Ask CDPHE about current eligibility.
- Child care: Colorado requires radon testing in licensed child care facilities.
Contact
The Colorado Radon Program (CDPHE) answers questions from residents about radon testing, results and certified professionals.
- Phone: 1-800-846-3986 · 303-692-2422
- Website: cdphe.colorado.gov/hm/radon
Contact details as listed by the state radon program directory. Please confirm on the program’s website before calling.
Frequently asked questions
How many Colorado homes have high radon?
CDPHE says about half of Colorado homes have radon levels above the EPA action level of 4 pCi/L, and 53 of Colorado’s 64 counties are in EPA Zone 1.
Is radon a problem in Colorado?
53 of Colorado’s 64 counties are in EPA Zone 1, where the predicted average indoor radon level is above 4 pCi/L, and 11 are in Zone 2 (2 to 4 pCi/L). High levels can be found in any home, so testing is the only way to know.
Which Colorado counties have the highest radon potential?
The 53 counties in EPA Zone 1 have the highest predicted average levels. You can find them in the table above and in the Zone 1 list.
What radon level is safe?
There is no known safe level of radon. The EPA recommends fixing your home at 4 pCi/L or higher and considering a fix between 2 and 4 pCi/L.
Who can I contact about radon in Colorado?
The Colorado Radon Program (CDPHE) answers questions about testing, results and certified professionals. Phone: 1-800-846-3986; 303-692-2422.
Sources
- CDPHE: Pre-Mitigation Radon Test Results 2005–2025 (retrieved November 2026)
- KKCO 11 News: Half of Colorado homes have high radon levels, free tests offered (January 6, 2026)
- SOSRadon.org (CRCPD/EPA): Colorado state radon program fact sheet (undated)
- U.S. EPA: EPA Map of Radon Zones (county data file, June 2024)
- U.S. EPA: A Citizen’s Guide to Radon (checked November 2026)
- U.S. EPA: How much can a radon mitigation system cost? (updated September 16, 2026)
- Colorado Radon Program (CDPHE) (checked November 2026)
- SOSRadon.org (CRCPD): State radon contacts (checked November 2026)