Radon Testing in Colorado: Complete Homeowner Guide (2026)
Why Colorado Has One of the Nation’s Highest Radon Rates
Colorado is classified as EPA Zone 1, the highest-risk category, meaning predicted average indoor radon levels exceed 4 pCi/L. The reason is geology. The Front Range and Rocky Mountains sit on some of the most uranium-rich granite bedrock in North America. As uranium in that rock slowly decays, it produces radium, which in turn breaks down into radon gas. That gas travels upward through soil and enters homes through cracks in foundations, gaps around pipes, and porous concrete blocks.
The Front Range corridor from Fort Collins through Denver and Colorado Springs down to Pueblo has some of the highest average residential radon levels in the country. Studies and state testing data consistently show that roughly half of Colorado homes test above the EPA action level of 4 pCi/L. Mountain communities sit on the same granite formations, and ski towns like those in Summit, Eagle, and Pitkin counties see elevated readings regularly. This is not a localized problem in a few neighborhoods; elevated radon is the baseline expectation for most of the state.
Radon is a colorless, odorless radioactive gas. It causes no immediate symptoms. Long-term exposure is the leading cause of lung cancer in non-smokers in the United States, responsible for about 21,000 deaths annually. The only way to know your exposure level is to test.
What Colorado Homeowners Need to Know Before Testing
Test During the Heating Season
Colorado’s climate creates testing conditions that differ from most other states. The combination of cold winters, low humidity, and high altitude means homes seal up tightly from October through March. During this period, the “stack effect” is strongest: warm air inside the house rises and escapes through the upper structure, creating negative pressure at the foundation that pulls soil gas (including radon) into the living space.
Testing during the heating season captures peak radon conditions. A test run in July with windows open and air conditioning cycling will likely read lower than the actual long-term average. For the most accurate and conservative picture of your exposure, schedule professional testing between October and March.
Closed-House Conditions Are Required
All radon tests require closed-house conditions: windows and exterior doors kept closed (except for normal entry and exit) for at least 12 hours before the test begins and throughout the test period. At Colorado’s altitude, this matters more than in lower-elevation states. The thin air and low atmospheric pressure at elevations above 5,000 feet mean even small pressure differentials between inside and outside move more soil gas through foundation gaps. Opening windows during a test can drop readings by 30 to 50 percent, producing a falsely low result.
Altitude and Elevation Adjustments
Some radon measurement professionals apply an elevation correction factor when interpreting results at Colorado’s higher elevations. Because air pressure decreases with altitude, a radon concentration of 4 pCi/L at 7,000 feet represents a slightly higher biological risk than the same reading at sea level. CDPHE-certified testers are trained in these adjustments. Ask your tester whether they apply a correction factor for your elevation.
Types of Radon Tests Available in Colorado
Short-Term Tests: Charcoal Canisters and CRMs (2 to 7 Days)
Short-term tests are the most common choice for initial screening and real estate transactions. Two types are used:
- Charcoal canisters: Passive devices that absorb radon from the air over 2 to 7 days, then get mailed to a lab for analysis. Widely available as DIY kits. Acceptable for initial screening but not valid for real estate purposes unless placed by a certified professional.
- Continuous radon monitors (CRMs): Electronic devices that record radon concentrations every hour throughout the test period. CDPHE-certified testers use these for real estate transactions. They are tamper-evident and provide an hour-by-hour log that reveals any closed-house violations. Results are available within 24 to 48 hours of test completion.
A short-term test gives you a snapshot, not a full-year average. If results fall between 4 and 8 pCi/L on a short-term test, EPA guidelines recommend a second confirmatory test before making a mitigation decision (unless the first result was taken by a professional under strict closed-house conditions, in which case action is warranted).
Long-Term Tests: Alpha Track Detectors (90 Days or More)
Alpha track detectors are small passive devices left in place for 90 days to one year. They measure cumulative radon exposure across seasons and give a more accurate annual average than any short-term test can. In Colorado, where radon levels can swing significantly between summer and winter, a long-term test is the gold standard for understanding your actual year-round exposure.
Alpha track kits cost $20 to $50 for DIY use. A certified professional can also place and retrieve them. Long-term tests are not suitable for real estate transactions (the timeline does not fit contract contingency periods) but are ideal for confirming the need for mitigation after an elevated short-term result, or for verifying mitigation system performance over a full heating season.
Post-Mitigation Verification Tests
After a mitigation system is installed, a follow-up test confirms the system is working. Most Colorado mitigation contractors include a post-mitigation short-term test in their service. CDPHE recommends testing again 24 hours after installation and then annually to catch any changes in system performance.
Cost of Radon Testing in Colorado
| Test Type | Cost Range | Best For |
|---|---|---|
| Professional short-term (CRM) | $125 to $300 | Real estate transactions, initial screening |
| DIY charcoal canister | $15 to $40 | Personal screening, routine re-testing |
| DIY alpha track (long-term) | $20 to $50 | Annual average, seasonal accuracy |
| Professional long-term placement | $75 to $150 | Mitigation verification, legal accuracy |
Pricing varies across the state. Denver metro and Colorado Springs have enough certified testers to keep prices competitive. Mountain towns and the Western Slope often see higher costs because professionals travel farther. Breckenridge, Aspen, and Telluride homeowners should expect to pay at or above the top of these ranges.
CDPHE periodically offers free test kit programs for Colorado residents. Check the CDPHE radon program page for current availability. These free kits are charcoal canisters suitable for personal screening but not for real estate use.
Colorado-Specific Testing Situations
Real Estate Transactions
Colorado requires sellers to disclose known radon test results on the Colorado Real Estate Commission’s Seller’s Property Disclosure form. As a result, radon testing has become a standard part of nearly every home inspection in the Front Range market. Buyers typically request a professional radon test as part of the inspection contingency period. Results above 4 pCi/L give buyers grounds to negotiate for mitigation or a price reduction under standard Colorado contracts.
For real estate transactions, use a CDPHE-certified measurement professional using a CRM. The test runs 48 hours under closed-house conditions. Results are ready quickly, which fits the typical 10-day inspection window in Colorado purchase contracts.
New Construction
Colorado requires radon-resistant new construction (RRNC) features in new homes built in Zone 1 areas. These features (a gravel layer under the slab, a vapor barrier, and a PVC pipe rough-in) make it easier to install an active mitigation system later, but they do not guarantee low radon levels on their own. Test your new construction home after 90 days of occupancy, once the home has settled into normal living patterns. RRNC homes still test above 4 pCi/L often enough that CDPHE recommends testing as a standard step.
Mountain and Ski Town Homes
Vacation homes and part-time residences in Colorado mountain towns present a specific challenge: radon accumulates during periods when the home is unoccupied and closed up. When you return after weeks away, concentrations can be especially high. Test during a period of normal occupancy with heating running. If your mountain home sits on a crawl space or basement in Summit, Eagle, Garfield, or Pitkin County, treat elevated radon as likely until proven otherwise. Levels above 10 pCi/L are not unusual in these areas.
After Basement Renovations
Any significant work on a basement: finishing it, adding a bathroom, breaking through the slab for plumbing, or repouring sections of concrete, can alter radon entry points. Re-test within 90 days of completing major basement work, even if a previous test showed acceptable levels.
How to Interpret Colorado Results
The national context matters here. The U.S. average indoor radon level is about 1.3 pCi/L. Colorado’s average is significantly higher. Many Colorado homes test between 4 and 10 pCi/L on first screening, and readings above 20 pCi/L are not rare in high-uranium geology areas along the Front Range and in mountain communities.
| Result (pCi/L) | Risk Category | Recommended Action |
|---|---|---|
| Below 2 | Low | Re-test in 2 years. No action needed. |
| 2 to 3.9 | Moderate | Consider a long-term test to confirm. Mitigation optional but worthwhile. |
| 4 to 7.9 | High (EPA action level) | Mitigate. This range is common in Colorado. |
| 8 to 19.9 | Very High | Mitigate promptly. Comparable to smoking half a pack of cigarettes daily. |
| 20 or above | Extremely High | Mitigate as soon as possible. Levels this high are found in Colorado homes near high-uranium geology. |
The WHO recommends action at 2.7 pCi/L, lower than the EPA’s 4 pCi/L threshold. Given Colorado’s geology, getting to below 2 pCi/L post-mitigation is a realistic and worthwhile goal. A properly installed sub-slab depressurization system can reduce levels by 80 to 99 percent.
CDPHE Radon Program and Certification
The Colorado Department of Public Health and Environment (CDPHE) administers the state’s radon program. Colorado requires that radon measurement and mitigation professionals be certified through either NRPP (National Radon Proficiency Program) or NRSB (National Radon Safety Board). Before hiring anyone, verify their current certification on the CDPHE website or through NRPP’s online lookup.
Hiring an uncertified tester risks inaccurate results and leaves you without recourse if the testing was done improperly. For real estate transactions especially, always confirm certification before scheduling a test.
Find a Certified Radon Tester in Colorado
Ready to schedule a test? Browse our directory of radon professionals in Colorado to find CDPHE-certified testers near you. Whether you are in Denver, Fort Collins, Colorado Springs, or a mountain community, 428 listed professionals serve the state.
Frequently Asked Questions
How much does a radon test cost in Colorado?
Professional radon testing in Colorado typically costs $125 to $300. Prices vary by location: Denver metro testers tend to be on the lower end, while mountain towns and the Western Slope often run higher due to travel time. DIY charcoal canister kits cost $15 to $40 at hardware stores. CDPHE also periodically offers free or subsidized kits through its radon program.
Do I need a radon test if I live on the second floor?
If your home sits on a slab or has a basement or crawl space below it, radon testing is still recommended. Radon rises from the soil and can accumulate on any floor level, though concentrations are highest in the lowest occupied spaces. In Colorado, even upper floors in attached townhomes and condos over concrete can show elevated readings.
When is the best time of year to test for radon in Colorado?
The heating season, roughly October through March, is the best time to test. Colorado homes seal up tightly during cold months, and the stack effect (warm air rising and escaping) draws more soil gas upward through the foundation. Testing during heating season tends to capture peak radon concentrations and gives you the most conservative baseline.
What does a result above 4 pCi/L mean for a Colorado home?
The EPA action level is 4 pCi/L. In Colorado, given the uranium-rich granite geology across the Front Range and mountains, results between 4 and 10 pCi/L are common. A result above 4 pCi/L means a mitigation system is recommended. Mitigation in Colorado typically costs $800 to $2,500 and reduces levels by 80 to 99 percent.
Can I use a DIY test kit for a real estate transaction in Colorado?
No. Colorado real estate transactions require testing by a CDPHE-certified radon measurement professional using calibrated equipment. Buyer agents and lenders will not accept DIY kit results for contract purposes. After you close, DIY kits are fine for routine re-testing.