How Do You Know If Your House Has Radon: Testing Guide
The question how do you know if your house has radon is one of the most common in residential safety, and the answer has two parts. The first part is the same answer that applies to any indirect-detection radon question: you cannot know without testing, because radon has no detectable sensory signature and no observable symptom. The second part — the testing-frequency question that this guide focuses on — addresses why a single test isn’t enough, when to retest, and how Colorado homeowners should think about ongoing monitoring rather than one-time screening. The information here reflects EPA, CDPHE, and American Lung Association guidance current as of 2026 and is informational only.
You Cannot Know Without Testing
The short answer first. Radon is a colorless, odorless, tasteless, radioactive noble gas. It cannot be detected by any of the human senses, by smart-home air quality monitors that don’t include a specific radon sensor, by visual inspection of the basement, foundation, or crawlspace, or by any pattern-matching to neighbors, building age, or geography.
The only reliable detection methods are passive testing devices (charcoal canisters and alpha-track detectors) or continuous radon monitors (electronic devices with ionization chambers or solid-state detectors). Both measure the alpha-particle emissions from radon decay, with the concentration reported in pCi/L (US) or Bq/m³ (international). EPA’s action level is 4.0 pCi/L, above which mitigation is recommended.
For the foundational testing-only argument and why indirect indicators don’t work, see the how to know if you have radon in your home companion guide. The current guide picks up where that one ends — assuming you accept the testing-only answer, how often should you test, and why does the frequency matter?
Why One Test Isn’t Enough
A single radon test produces a single number representing the average concentration during the test window. That number is informative, but it doesn’t fully characterize the home’s radon exposure for several reasons:
Seasonal variation. Indoor radon concentration varies significantly across seasons. Winter heating drives the stack effect that pulls soil gas into the home, often doubling or tripling winter radon levels compared to summer. A test conducted in July may understate the wintertime exposure that drives most of the cumulative dose.
Weather and barometric variation. Storm fronts, sustained low-pressure systems, and rapid temperature changes alter the indoor-outdoor pressure differential and shift radon entry rates. A test conducted during stable weather may differ from one conducted during a frontal passage.
Foundation aging. Concrete slabs, basement walls, and foundation joints develop new cracks over time as the building settles, freezes and thaws, and ages. New cracks can change radon entry patterns dramatically. A house that tested at 2.5 pCi/L in 2015 might test at 6.0 pCi/L in 2026 with no visible building change.
Soil moisture and freeze-thaw cycles. Saturated soils block soil gas migration; dry soils allow it. Front Range homes can see radon concentrations swing by a factor of 2-3 between wet spring conditions and dry winter conditions.
Ventilation changes. A new HVAC system, a closed-off room that was previously open, a new bathroom exhaust fan, or a basement renovation can all change indoor air pressure patterns and radon entry.
Measurement uncertainty. Short-term passive tests have inherent measurement uncertainty of typically ±10-20%. A single result of 3.8 pCi/L could represent a true concentration anywhere from roughly 3.0 to 4.6 pCi/L.
For these reasons, EPA and CDPHE both recommend ongoing retesting rather than treating a single test as definitive. The should I test my house for radon guide walks through the threshold decision logic.
EPA-Recommended Retest Frequency
EPA’s testing-frequency recommendations adapt to the initial result and the home’s circumstances:
If the initial test is below 2.0 pCi/L: Retest every 5-10 years. This is the lowest priority for retesting but still important because foundation aging and weather variation can shift the result over time.
If the initial test is 2.0 to 3.9 pCi/L: Retest every 2-5 years. The result is below the action level but elevated enough that ongoing monitoring is worthwhile, especially given short-term measurement uncertainty.
If the initial test is 4.0 pCi/L or higher (no mitigation installed): EPA recommends mitigation. If mitigation has not yet been installed, retest with a long-term alpha-track or continuous monitor for confirmation, then move to mitigation.
If a mitigation system has been installed: EPA recommends post-mitigation testing 24-48 hours after the system is operating, then retesting every 2 years to verify ongoing performance.
After major renovation or foundation work: Retest within 3-6 months. This includes basement finishing, slab repair, sump pit work, foundation crack sealing, or HVAC system replacement.
After buying a previously tested home: Retest within the first 6 months. The previous owner’s test result is informative but doesn’t reflect your occupancy patterns, ventilation habits, or the home’s current condition.
Why Colorado Specifically Needs More Frequent Testing
Colorado’s EPA Zone 1 designation means the state-average risk is significantly higher than the national average, and CDPHE recommends testing frequency on the higher end of EPA’s range:
State average is 6-9 pCi/L across many Front Range counties — well above the national average of 1.3 pCi/L.
50% of Colorado homes test above 4 pCi/L, compared to roughly 6% nationally, per CDPHE estimates.
The freeze-thaw cycle is more severe in Colorado than in coastal states, creating more foundation crack development over time.
The heating season is longer in Colorado, increasing the cumulative stack-effect-driven radon entry over the year.
CDPHE therefore recommends retesting every 2-5 years in Colorado regardless of the initial result. For homes with mitigation systems, CDPHE recommends every 2 years to verify the system continues to perform.
Why You Should Test Even With No Symptoms
Radon exposure does not produce short-term symptoms. There are no headaches, no fatigue, no shortness of breath, no specific marker on the day of exposure. The health effect is cumulative lung cancer risk that manifests 10-20+ years after sustained elevated exposure.
This temporal disconnect is the central reason “I feel fine” is not evidence of low radon. A person living in an 8 pCi/L home for 20 years feels exactly the same on any given day as a person living in a 2 pCi/L home — until the lung cancer diagnosis arrives decades later. EPA’s emphasis on universal testing reflects this: every home should be tested at least once, period, regardless of how the occupants feel.
The Surgeon General’s office, EPA, CDC, and the American Lung Association all align on this recommendation. There is no medical examination, no blood test, no symptom check, and no environmental assessment short of direct measurement that substitutes for testing the air in the home.
Continuous Monitoring as an Alternative to Periodic Testing
The rise of consumer-grade continuous radon monitors since 2020 has changed the testing-frequency question. A home with an Airthings, RadonEye, or similar device installed continuously produces a real-time and rolling-average radon record that effectively replaces periodic retesting.
Consumer CRMs cost $150-$300 — comparable to the cost of two or three professional short-term tests. Over a 5-year period, the CRM is typically cheaper than periodic testing and provides:
Seasonal variation visible in the rolling average. The homeowner sees winter peaks and summer troughs without separate tests.
Real-time response to events. A heavy snowfall, a basement renovation, or a new HVAC system shows up in the data immediately rather than waiting for the next periodic test.
Mitigation system verification. If a mitigation system is installed, the CRM shows whether the system continues to perform at the target reduction level.
For Front Range homeowners who already know their home has elevated radon and have either mitigated or are monitoring proactively, a continuous monitor is often the right ongoing solution. For homeowners who have not yet tested at all, an initial passive or professional test followed by a CRM purchase is a reasonable two-step approach. The Colorado radon testing pillar covers the full range of measurement options.
References
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- CDC radon prevention and ongoing monitoring — Centers for Disease Control and Prevention
- CDPHE Colorado radon testing recommendations — Colorado Department of Public Health and Environment
- American Lung Association radon information — American Lung Association
Front Range homeowners ready to schedule periodic testing or invest in a continuous monitor can connect with us through the contact page for a referral to an NRPP-certified Colorado measurement professional.