Does Colorado Require Radon Testing in Denver?

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Colorado has no statewide law requiring homeowners to test for radon. But Denver sits in EPA Zone 1 — the highest-risk category — and the state strongly recommends testing every home, regardless of age or foundation type.

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Why Does Denver Have Such a High Radon Risk?

Denver’s geology is the core issue. The granite and uranium-rich bedrock beneath the Front Range naturally produces radon gas, and that gas moves upward through soil and into homes through foundation cracks, slab joints, and utility penetrations.

Colorado’s statewide indoor radon average sits around 6.3 pCi/L — well above the EPA’s action level of 4.0 pCi/L. In the Denver metro specifically, roughly half the homes we test come back elevated. That’s not a scare statistic — it’s just what the geology produces here.

Denver’s climate makes it worse. Homes stay sealed through long heating seasons, and the stack effect — natural air pressure differences between indoors and outdoors — pulls soil gases in more aggressively when windows are shut for months at a stretch. Summer readings with open windows can look reassuringly low, then climb significantly once heating season starts.

Understanding what Denver homeowners need to know about radon testing is a good starting point before you decide which test type makes sense for your situation.

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Is Radon Testing Legally Required in Colorado?

No mandatory testing law exists for most homeowners in Colorado. The state recommends testing but does not require it for owner-occupied residences outside of real estate transactions.

That said, Colorado passed legislation in 2023 requiring radon disclosure in real estate transactions. Sellers must now disclose any known radon test results. This doesn’t force a test, but it creates a practical incentive — buyers increasingly expect to see radon data, and many real estate contracts now include radon testing contingencies as standard practice.

For rental properties, Colorado law as of 2023 also requires landlords to disclose known radon test results to tenants. If a landlord has tested and results exceed 4.0 pCi/L, they’re required to inform tenants and take remedial action.

New construction is a separate matter. Colorado’s building code encourages radon-resistant new construction (RRNC) practices in high-risk zones, but adoption varies by jurisdiction and builder. Some Denver-area municipalities have gone further with local code requirements, so checking with your specific county or city building department is worth doing if you’re building new.

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What Radon Levels Actually Trigger Action in Denver?

The EPA sets the action level at 4.0 pCi/L — meaning mitigation is recommended at or above that threshold. Colorado health officials also suggest considering mitigation for homes testing between 2.0 and 4.0 pCi/L.

In practice, the radon levels that require mitigation in Denver homes depend on more than just the number. How much time your family spends in the lowest occupied level, whether children are in the home, and how well-sealed the house is all factor into the decision. We typically advise homeowners in the 2.5–3.9 pCi/L range to at least conduct a long-term test before deciding — a single short-term reading during summer months can underrepresent annual exposure.

The background level of radon in outdoor air averages around 0.4 pCi/L. Colorado’s average indoor level of 6.3 pCi/L is more than 15 times that — which gives you a sense of how much the local geology elevates indoor concentration compared to the national baseline.

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What Types of Radon Tests Are Available — and Which Should You Use?

Short-term charcoal canister tests run 2–7 days and cost between $15 and $40 as DIY kits, or $150–$250 when conducted by a professional. They’re EPA-approved screening tools and a reasonable starting point for most homeowners.

Long-term tests run 90 days or more and give a more accurate picture of annual average exposure. DIY long-term kits typically cost $25–$60. If your short-term result is near the action level, a long-term follow-up test is the right move before committing to mitigation.

Continuous radon monitors (CRMs) — devices like those from Airthings or Safety Siren — cost $150–$300 and provide ongoing measurement. These are useful after mitigation to verify the system is working, or for homeowners who want real-time data year-round.

One thing to keep in mind: summer testing with open windows can mask elevated levels. Closed-house conditions — windows and exterior doors kept shut for 12 hours before and during the test — are required for short-term tests to produce accurate results.

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How Do Mitigation Costs Compare to Testing Costs in Denver?

Most residential mitigation jobs in the Denver area take a single day to complete. A licensed contractor cores through the foundation slab, installs a PVC pipe routed to the exterior, and connects a continuously running fan that draws radon from beneath the slab before it can enter the living space. Post-installation testing is typically done 24 hours after the system is running.

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When Should You Call a Professional Instead of DIY Testing?

DIY kits work fine for initial screening. But there are situations where professional involvement isn’t optional.

If your result comes back at or above 4.0 pCi/L, a licensed radon mitigator should assess your home before any remediation work begins. Colorado requires radon professionals to hold active licensure through the Department of Regulatory Agencies (DORA) and carry professional liability insurance. That credential matters — improper mitigation can redistribute radon rather than remove it, or create negative pressure problems in the home.

Real estate transactions are another clear signal. Buyers and sellers both benefit from professional testing with documented chain of custody — a professionally conducted test is harder to dispute than a DIY result during negotiations.

Homes with complex foundations — crawl spaces combined with a partial basement, for example, or slab-on-grade with multiple zones — often need professional assessment to design a system that actually addresses all entry points.

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What Should Denver Homeowners Do Next?

Start with a short-term test if you haven’t already. Place it in the lowest regularly occupied level of your home — a finished basement counts, but so does a bedroom above a crawl space. Follow closed-house conditions for the full test duration.

If results come back below 2.0 pCi/L, retest every two years and after any major renovation that affects the foundation or ventilation. If results fall between 2.0 and 4.0 pCi/L, conduct a long-term follow-up test. If results are at or above 4.0 pCi/L, contact a DORA-licensed radon mitigator to schedule an assessment — most can schedule within a few days, and installation typically happens within one to two weeks of the initial assessment.

Keep records of every test. Colorado’s real estate disclosure requirements mean documented results become relevant the moment you consider selling.

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FAQ

Q: Does Colorado law require radon testing before selling a home?

No mandatory test is required, but sellers must disclose any known radon test results. Buyers frequently request testing as a contract contingency.

Q: How long does a professional radon mitigation installation take in Denver?

Most standard sub-slab depressurization systems are installed in a single day, with post-installation testing done 24 hours later.

Q: Can I test for radon myself, or do I need a professional?

DIY short-term kits are EPA-approved and accurate for screening. Professional testing is advisable for real estate transactions or results near the action level.

Q: Does Denver’s altitude affect indoor radon levels?

Altitude doesn’t directly raise radon levels. The uranium-rich bedrock and soil geology beneath the Front Range is the primary driver of elevated readings.

Q: Are summer radon test results accurate?

Open windows during summer can lower readings temporarily. Always test under closed-house conditions for results that reflect typical annual exposure.

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Sources

• U.S. Environmental Protection Agency: EPA Map of Radon Zones and Radon Action Level Guidance
• Colorado Department of Public Health and Environment: Radon Program — Testing and Mitigation Recommendations
• Colorado General Assembly: HB23-1267, Radon Disclosure Requirements for Real Estate and Rentals
• Colorado Department of Regulatory Agencies (DORA): Radon Mitigation Contractor Licensing Requirements
• Colorado Geological Survey: Radon Occurrence and Colorado Geology

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RadonSweep connects Denver homeowners with DORA-licensed radon professionals who know the Front Range’s geology and building stock. If your test results are back and you’re not sure what they mean — or you’re ready to schedule mitigation — RadonSweep can match you with a qualified contractor in the Denver metro area, usually within a day or two of your inquiry.