—
For most Denver homeowners, start with a short-term test to get a quick reading. If results come back at or above 4.0 pCi/L, follow up with long-term testing or a continuous monitor before deciding on mitigation.
Why Does Radon Testing Matter More in Denver Than Most Cities?
Denver sits in EPA Zone 1 — the highest radon potential classification. Colorado’s bedrock is rich in uranium-bearing granite and gneiss, and as that uranium breaks down, it releases radon gas that migrates upward through the soil and into homes through foundation cracks, slab joints, and utility penetrations.
The statewide indoor radon average runs around 6.3 pCi/L — more than 50% above the EPA’s action threshold of 4.0 pCi/L. In the Denver metro, we typically see roughly half of homes test above that threshold on initial screening. With winter approaching, closed-up homes trap radon more effectively, and the stack effect — warm interior air rising and pulling soil gases upward — intensifies. Fall is one of the best times to test, because you get a realistic picture of what your home does during heating season when radon concentrations peak.
If you’re new to this topic, understanding the basics of radon testing for Denver homeowners is a good place to start before choosing a test type.
—
What Is a Short-Term Radon Test and When Should You Use One?
A short-term test gives you a radon reading over 2 to 7 days. It’s the fastest way to screen your home and the right first step for most situations.
The most common short-term device is a charcoal canister — a small passive collector you place in the lowest regularly occupied level of your home, away from windows, doors, and exterior walls. After the exposure period, you mail it to a lab. Results typically come back within a week. DIY kits cost around $15–$40 at hardware stores. Professional short-term testing, where a certified technician places and retrieves the device, runs $150–$250 and usually includes a written report.
Short-term tests require closed-house conditions — all windows and exterior doors kept shut for at least 12 hours before and during the test period. This is important in Denver, where homeowners are often tempted to open windows in mild fall weather. Breaking closed-house conditions skews results low and gives you a false sense of security.
Use a short-term test when you need results quickly — for a real estate transaction, as an initial screening, or when you simply want to know if further testing is warranted. It won’t capture seasonal variation, but it gives you actionable data within days rather than months.
—
What Is a Long-Term Radon Test and When Does It Make More Sense?
A long-term test runs for 90 days to 12 months and gives you a picture of average radon exposure across seasons and weather patterns.
Long-term tests use alpha track detectors — small devices that record cumulative radon exposure over time. Because Denver’s radon levels fluctuate significantly between summer and winter, a long-term test captures that full range and produces a more accurate annual average. DIY long-term kits cost around $25–$60. Many professional testing services include long-term follow-up monitoring as part of a broader assessment.
In our experience, long-term results and short-term results don’t always agree. A short-term test in July might show 2.8 pCi/L. The same home tested through the winter with a long-term device often comes back above 4.0 pCi/L once heating season is factored in.
Long-term testing is the right call when your short-term result lands in the 2.0–4.0 pCi/L range and you want a more complete picture before deciding whether to mitigate. It’s also useful after mitigation system installation to confirm the system is performing consistently year-round, not just at one point in time.
—
How Do Short-Term and Long-Term Tests Compare on Cost and Accuracy?
| Short-Term Test | Long-Term Test | Continuous Monitor | |
|---|---|---|---|
| Duration | 2–7 days | 90 days–1 year | Ongoing |
| DIY Cost | $15–$40 | $25–$60 | $150–$300 |
| Professional Cost | $150–$250 | Included with service | Included with service |
| Closed-House Required? | Yes | No | No |
| Best Use | Initial screening, real estate | Confirm borderline results | Post-mitigation monitoring |
| Captures Seasonal Variation? | No | Yes | Yes |
| Results Timeline | ~1 week | 3–12 months | Real-time |
Continuous radon monitors (CRMs) — devices like the Airthings Wave or Safety Siren Pro — fall into their own category. They cost $150–$300 and provide real-time radon readings on an ongoing basis. They’re not a substitute for a certified lab test in a real estate context, but they’re valuable for post-mitigation monitoring and for homeowners who want ongoing visibility into their radon levels across seasons.
—
What Radon Levels Should Trigger the Next Step?
If a short-term test comes back at 4.0 pCi/L or above, the next step is mitigation — not more testing.
The EPA sets 4.0 pCi/L as the action level, and Colorado health officials recommend considering mitigation even in the 2.0–4.0 pCi/L range, particularly in homes with children or occupants with a history of lung cancer. For a deeper look at what radon levels require mitigation in Denver homes, including the gray zone between 2.0 and 4.0 pCi/L, that’s worth reading before you make a final call.
If a short-term test lands between 2.0 and 4.0 pCi/L, a long-term follow-up test is the reasonable next move — it either confirms the borderline reading or reveals that winter levels push your home above the threshold. Either way, you’re making a decision based on complete information rather than a single data point.
Results below 2.0 pCi/L generally don’t require action, though retesting every two years is still reasonable given how much Colorado geology varies from one property to the next.
—
When Should You Call a Radon Professional Instead of Testing Yourself?
DIY kits are legitimate screening tools, but there are situations where professional testing is the better call.
Real estate transactions are the clearest example. Colorado’s radon disclosure requirements mean test results may become part of a formal record. A certified professional’s report carries more weight in negotiations than a mailed-in charcoal canister, and the testing process will be documented properly.
You should also call a professional if your initial DIY result comes back above 4.0 pCi/L and you want confirmation before committing to mitigation costs. A professional can place multiple devices across different areas of your home — something a single kit won’t capture — and identify where radon is entering.
In Denver, professional radon testers are certified through the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Colorado’s Division of Professions and Occupations (DORA) licenses radon mitigators separately. If you’re moving toward mitigation, the contractor needs both credentials.
Scheduling a professional test is typically a same-day or next-day appointment during fall and winter — the busiest testing season in Denver. Plan ahead if you’re working against a real estate deadline.
—
What Should Denver Homeowners Do Next?
If you haven’t tested your home, start with a short-term charcoal canister kit from a hardware store or order one online. Place it in the lowest regularly occupied level — finished basement, ground-floor bedroom, or main living area if there’s no basement. Maintain closed-house conditions for the full test period.
If your result comes back below 2.0 pCi/L, retest in two years. If it’s between 2.0 and 4.0 pCi/L, run a long-term test or install a continuous monitor to get a seasonal average. If it’s at or above 4.0 pCi/L, contact a certified radon professional. Mitigation system installation in the Denver metro typically runs $800–$2,500, with most residential jobs landing around $1,200. Most installations are completed in a single day.
Fall is the right time to do this. By the time heating season is fully underway, your home will be sealed up and radon concentrations will be at their annual high.
—
FAQ
Q: Can I use a short-term test for a real estate transaction in Denver?
Yes, but a professionally certified test carries more weight. Many Denver buyers and sellers require NRPP-certified testing for formal documentation during closing.
Q: How long does professional radon testing take?
Short-term professional tests run 48–96 hours with closed-house conditions. Results are typically delivered within a week of device retrieval.
Q: Do new Denver homes need radon testing?
Yes. New construction can have higher radon than older homes. Construction type matters more than age — slab-on-grade homes aren’t automatically safe.
Q: Can I open windows during a short-term radon test?
No. Closed-house conditions are required for at least 12 hours before and throughout the test. Open windows skew results low and invalidate the reading.
Q: How soon after a mitigation system is installed should I retest?
Wait at least 24 hours after installation, then run a short-term test. Annual retesting afterward confirms the system continues to perform correctly.
—
Sources
- U.S. Environmental Protection Agency: Home Buyer’s and Seller’s Guide to Radon
- U.S. Environmental Protection Agency: A Citizen’s Guide to Radon — The Guide to Protecting Yourself and Your Family from Radon
- Colorado Department of Public Health and Environment: Radon in Colorado — Frequently Asked Questions
- Colorado Geological Survey: Radon in Colorado — Sources and Geology
- National Radon Proficiency Program (NRPP): Radon Testing and Mitigation Certification Standards
- Colorado Division of Professions and Occupations (DORA): Radon Contractor Licensing Requirements
—
RadonSweep helps Denver homeowners connect with certified radon professionals who know Colorado’s geology and testing requirements. Whether you need a fast screening before a real estate deadline or a full assessment ahead of winter, the network includes licensed testers and mitigators serving Denver and the surrounding metro area.