House Radon Test: What Homeowners Need to Know
A house radon test measures the concentration of radon gas in the living space of a single-family home, condominium, or townhouse. Radon is a naturally occurring radioactive gas that enters homes through soil contact at the foundation, and Colorado is one of the most radon-affected states in the country. This guide summarizes EPA and CDC guidance current as of 2026 — consult a certified professional for testing or mitigation and your physician for symptom evaluation. The discussion covers what a house radon test actually measures, where to place the device, how long to test, what the result means, and what to do at each action level.
What a house radon test actually measures
A house radon test measures the average radon concentration in indoor air over a defined period, expressed in picocuries per liter (pCi/L) in the United States or becquerels per cubic meter (Bq/m-cubed) internationally. Radon is a radioactive gas produced by the natural decay of uranium in soil and bedrock. Outdoor concentrations average about 0.4 pCi/L; indoor concentrations vary widely depending on soil conditions, foundation type, building tightness, and pressure dynamics.
The EPA action level is 4 pCi/L. Homes that test at or above 4 pCi/L are recommended for mitigation. Homes testing between 2 and 4 pCi/L sit in an intermediate zone where the EPA recommends considering mitigation, especially for occupants who spend significant time in the lowest livable level of the home. Homes testing below 2 pCi/L have a lower priority for action, though no level of radon is medically established as completely safe.
Why a house radon test matters in Colorado
All sixty-four Colorado counties are EPA Radon Zone 1, the highest priority category. Colorado Department of Public Health and Environment (CDPHE) data shows that approximately half of Colorado homes test above the 4 pCi/L action level. The state’s combination of high-altitude geology, uranium-bearing soil and bedrock in many regions, and the typical Front Range home design (with full or partial basements that often serve as living space) creates a strong setup for elevated indoor radon.
Every Front Range home should be radon-tested at least once. Many real estate transactions include radon testing under the inspection contingency. Long-term residents who have never tested should commission a house radon test even if they have lived in the home for decades, because long-term cumulative exposure is what drives the public-health risk. The CDPHE radon program reference publishes Colorado-specific guidance for homeowners.
House radon test methods
Three categories of radon test methods are available. Short-term tests measure radon for 2 to 7 days using a charcoal canister, alpha-track detector, or continuous radon monitor (CRM). Long-term tests measure for 90 days to a year using alpha-track or electret-ion-chamber devices. Continuous radon monitors used by certified professionals provide hour-by-hour readings during a 48-hour test window.
For initial testing, the EPA recommends either a short-term test or a continuous radon monitor deployment. Short-term test kits (charcoal canister or alpha-track) cost $15 to $40 plus lab fees and are available at hardware stores, online, and through state radon programs. CRM testing by a certified professional costs $100 to $300 in Front Range Colorado but produces immediate results with hour-by-hour data. The sibling article on home radon test guide covers the test-type comparison in detail.
Where to place the test device
Placement determines whether the result reflects actual exposure conditions. Place the test device in the lowest livable level of the home that is regularly occupied. For homes with basements used as living space (finished basements, basement bedrooms, basement family rooms), place the device in the basement. For homes with unfinished basements that no one regularly uses, place the device on the first floor.
Within the chosen level, place the device in a regularly occupied room (bedroom, family room, living room) at least 20 inches above the floor and 12 inches from exterior walls. Avoid placement near windows, doors, ventilation registers, kitchens, or bathrooms with strong air-flow patterns. Avoid placement within 3 feet of heaters, fans, or electronic equipment that may affect local air movement.
Closed-house conditions
Short-term radon tests require closed-house conditions during the test. Windows and exterior doors should be closed except for normal entry and exit. Mechanical ventilation systems (whole-house fans, HRV/ERV systems) should operate on normal settings. The 12 hours before the test begins should also follow closed-house conditions, allowing radon concentrations to stabilize.
Real estate transaction tests often follow a stricter 48-hour closed-house protocol because the buyer needs a reliable measurement of typical occupied conditions. Long-term tests do not require closed-house conditions because the longer duration averages out short-term variation. For testing during transactions, the sibling article on radon test duration by test type covers timing in detail.
Action levels and what to do at each
The EPA action level structure provides clear guidance at each result range:
Below 2 pCi/L: Lower priority for action. Re-test every two to five years or when significant changes occur (foundation modifications, basement finishing, HVAC replacement).
2 to 4 pCi/L: Intermediate zone. EPA recommends considering mitigation, especially when occupants spend significant time in the lower levels of the home. Re-test with long-term measurement to confirm short-term result before deciding on mitigation.
4 to 8 pCi/L: EPA action level. Mitigation is recommended. Many radon mitigation contractors take a confirmation-test step before quoting a system, but the first short-term test result alone is often sufficient justification.
8 pCi/L and above: Higher-priority action level. Mitigation is strongly recommended; some homeowners in this range pursue mitigation directly without a confirmation test, especially when the result is well above 4 pCi/L.
For the action-level framework, the EPA radon program reference publishes guidance for homeowners. The parent guide on Front Range radon testing covers Colorado-specific action-level interpretation.
How a house radon test fits into a real estate transaction
Buyers under contract in Colorado typically commission a radon test alongside the general home inspection. The test takes 48 hours, with placement on day one of the inspection contingency and pickup or device-retrieval on day three. Continuous radon monitor results are often available immediately at pickup; lab-analyzed charcoal canisters or alpha-track devices return results within 3 to 7 business days.
A result at or above the EPA action level becomes part of the request-for-repairs negotiation. Common outcomes include the seller installing a mitigation system before closing (typical cost $1,000 to $2,500 in Front Range Colorado), the seller crediting the buyer to install the system after closing, or the parties splitting the cost. The result rarely terminates the transaction by itself because mitigation is effective and well-established.
What to look for in a certified radon measurement professional
Two relevant certifications exist for residential radon measurement. The federal National Radon Proficiency Program (NRPP) and National Radon Safety Board (NRSB) issue radon measurement certifications. In Colorado, CDPHE requires state certification for radon measurement professionals operating in the state. Buyers commissioning a radon test should verify both the federal and state credentials.
Verification is straightforward. The NRPP and NRSB websites list current certified professionals, and CDPHE publishes the Colorado-certified registry. Many Front Range home inspectors hold dual certification (general inspection plus radon measurement) and offer combined inspection plus radon testing as a packaged service. The sibling article on Colorado radon testing and the CDPHE program covers the credential framework.
DIY house radon test kits
DIY radon test kits cost $15 to $40 plus laboratory fees (often $25 to $40 for postal-return analysis). The kits are widely available at hardware stores, online retailers, and through some state radon programs that offer subsidized or free kits. Common DIY options include short-term charcoal canisters and long-term alpha-track detectors.
DIY testing is appropriate for non-transactional purposes: homeowners doing initial assessment, periodic re-testing, post-mitigation verification (though most mitigators recommend professional re-test), or testing in new locations after a remodel. Real estate transactions typically use professional CRM testing because the result needs to be defensible against potential challenge and the immediate result accelerates the negotiation timeline.
Variability in radon test results
Indoor radon concentrations vary substantially over time. Daily variation can be a factor of two; seasonal variation can be a factor of three to five, with winter typically showing higher concentrations than summer because building tightness and the stack effect both increase. A single short-term test result represents a snapshot of conditions during a specific 48-hour window.
This variability is why the EPA recommends confirming an initial short-term result with a follow-up test. The follow-up can be another short-term test (taken at a different season ideally) or a long-term test that averages out short-term variation. For homes with results just above or below the 4 pCi/L action level, the confirmation step matters most because the action decision turns on a result near the threshold.
What happens if the test result is high
A high house radon test result is not an emergency. Radon-related health risk is a function of long-term cumulative exposure, not short-term peak. Even at 10 or 20 pCi/L, a few additional days or weeks of exposure while planning mitigation does not materially change the lifetime risk profile.
The next step is engaging a radon mitigation contractor for assessment and quote. The mitigation system (typically sub-slab depressurization, or SSD) addresses the soil-gas entry pathway directly and is highly effective: most installed systems reduce indoor radon to below 2 pCi/L. Post-mitigation testing verifies the result. The sibling article on what is radon mitigation walks through the mitigation system in detail.
How often to re-test a Colorado home
The EPA recommends re-testing every two years for homes near the action level, every five years for homes well below it, and immediately after significant home changes. Major HVAC replacement, basement finishing, foundation work, and addition construction can all change radon dynamics by altering the building’s air pressure profile or creating new soil-gas entry points.
Homes with installed mitigation systems should be tested every two years to confirm the system continues to function as designed. The mitigation system fan has a typical service life of five to eight years; an aging fan that begins to under-perform may not be visible from the manometer alone, and periodic re-testing catches the problem before indoor concentrations climb back to the action level. The sibling article on radon system service and maintenance covers the maintenance schedule.
House radon test myths
Several common misconceptions persist about house radon testing. Some homeowners believe new construction is immune from radon because newer homes are built to current code. New construction is not immune; in fact, modern tighter building envelopes can concentrate soil-gas entry more efficiently than older leaky structures. Many newer Colorado homes test above the action level despite radon-resistant new construction techniques used during build.
Another myth is that radon is only a basement problem. Indoor radon distributes throughout the home through normal air circulation; concentrations on upper floors are typically 50 to 75 percent of basement concentrations rather than zero. A first-floor reading of 3 pCi/L in a home with a basement office likely reflects basement concentrations near or above the action level.
A third myth is that opening windows handles radon. Open windows do dilute indoor radon, but the dilution is only effective when windows are open, and Colorado weather makes open-window living impractical for much of the year. Active soil-gas mitigation through a sub-slab depressurization system addresses the source pathway directly and is the EPA-recommended response for homes above the action level.
References
- EPA Radon Program — Environmental Protection Agency
- Colorado Department of Public Health and Environment — CDPHE
- CDC Radon Information — Centers for Disease Control and Prevention
- American Lung Association radon guidance — American Lung Association