Radon Risk in Colorado
Why radon is a growing concern for Colorado homeowners, what it does to your health, why Colorado levels run high, and why newer energy-efficient homes can trap more of it.
Updated November 2026
A growing concern in Colorado homes
Radon is a colorless, odorless radioactive gas that forms naturally as uranium in soil and rock breaks down. It moves up through the ground and enters homes through cracks in foundations, gaps around pipes, sump pits and crawlspaces.
The Colorado Department of Public Health and Environment (CDPHE) reports that about half of Colorado homes have radon above the EPA action level of 4.0 picocuries per liter (pCi/L). Colorado's governor proclaims January National Radon Action Month each year, and state and county health departments give out free test kits to encourage testing.
Demand for testing and mitigation keeps growing as more buyers test during home sales, more new homes are built tighter, and more Coloradans learn that their home can test high even when a neighbor's does not.
Radon levels, in pCi/L
- 0.4
- Average outdoor air
- 1.3
- Average U.S. home
- 2.0
- EPA: consider fixing
- 4.0
- EPA action level: fix the home
About half of Colorado homes test above 4.0 pCi/L, according to CDPHE. The EPA says there is no known safe level of radon.
Health risks of radon
When you breathe radon, its radioactive particles lodge in the lining of your lungs and damage cells over time. Long-term exposure raises the risk of lung cancer. There are no early symptoms, so testing is the only warning.
The U.S. EPA identifies radon as the second leading cause of lung cancer in the United States, after smoking, and the leading cause among people who have never smoked. EPA estimates radon is responsible for about 21,000 lung cancer deaths nationwide each year.
In Colorado, CDPHE attributes more than 500 lung cancer deaths each year to radon. People who smoke and are exposed to high radon face a much greater combined risk.
Why radon levels are higher in Colorado
Geology. Much of Colorado sits on granite and other rock and soil that contain uranium. As uranium breaks down it produces radon, so the supply under many Colorado homes is higher than the national average.
Basements and crawlspaces. Colorado homes commonly have basements or crawlspaces, which put more of the home in contact with the soil where radon originates.
Long heating seasons. In cold months, homes stay closed and warm air rising through the house creates a slight vacuum at the foundation (the stack effect), drawing more soil gas inside.
Local variation. Radon levels change from lot to lot. Two homes next door to each other can test very differently, so a neighbor's result tells you little about your own.
Energy-efficient homes can trap more radon
Modern homes are built and upgraded to save energy: tighter windows, more insulation and sealed air leaks. These improvements lower heating bills, but they also reduce how much fresh outdoor air replaces indoor air. When less air is exchanged, radon that enters the home can build up to higher levels.
A large study of 470,689 homes in the United Kingdom, published in the journal Indoor Air in 2019, found that homes with energy-efficiency upgrades had higher average radon levels than homes without them. Homes with new windows, loft insulation and wall insulation together averaged about 79% higher radon. The authors linked the increase to reduced ventilation.
This does not mean you should skip energy upgrades. It means you should test before and after air sealing, new windows or insulation work, consider balanced ventilation such as an ERV or HRV, and install radon mitigation if levels rise. For new construction, ask your builder about radon-resistant construction features, which are far cheaper to install during the build.
Radon risk FAQ
Does radon cause lung cancer?
Yes. The EPA identifies radon as the second leading cause of lung cancer in the United States after smoking, and the leading cause among people who have never smoked.
How do I know if my home has radon?
You cannot see or smell radon. The only way to know is to test. Short-term test kits take 2 to 7 days; long-term tests run 90 days or more.
When is the best time to test for radon in Colorado?
Any time works, but winter tests often read higher because homes are closed up and heated air draws more soil gas inside. January is National Radon Action Month.
Do energy-efficient homes have more radon?
They can. Air sealing and insulation reduce the exchange of indoor and outdoor air, which can let radon build up. Energy upgrades are still worthwhile; test before and after, and plan for ventilation or mitigation.
How often should I retest?
CDPHE recommends retesting a mitigated home every two years and after major renovations, such as finishing a basement.
This page is general information, not medical advice. Talk with your doctor about personal health concerns.
Sources
- Colorado Department of Public Health and Environment — Radon program
- CDPHE press release: Colorado residents urged to test homes for cancer-causing radon
- Colorado DORA: Radon professional licensing (House Bill 21-1195)
- U.S. EPA — What is EPA's action level for radon and what does it mean?
- Symonds P. et al., Home energy efficiency and radon: an observational study, Indoor Air (2019)
- La Voz Colorado: Half of Colorado homes have elevated radon levels (January 2026)
- Larimer County Health Department: Radon Action Month (January 2026)
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