What Is a Radon Inspection: A Plain-Language Guide
A radon inspection is a controlled measurement of the concentration of radon gas inside a building, typically expressed in picocuries per liter of air (pCi/L). The inspection answers a single question: how much radon is currently in this home, and is it above the EPA action level of 4 pCi/L? Radon is a naturally occurring radioactive gas released from uranium-bearing soil and rock; it is invisible, odorless, and the second leading cause of lung cancer in the United States according to EPA and CDC estimates. This guide summarizes EPA, CDC, and CDPHE guidance current as of 2026 on what a radon inspection involves, which test types apply, what the numbers mean, and how mitigation works. Consult your physician for symptom concerns and a certified mitigator for system design decisions.
What is a radon inspection in plain terms?
A radon inspection deploys one of several measurement devices in the lowest livable level of a home (typically the basement or ground floor) for a defined time period under closed-house conditions, then either reads the device on-site or sends the device to a lab for analysis. The result is a numerical concentration that classifies the home as below, at, or above the EPA action level. Short-term tests run 48 hours to 7 days; long-term tests run 90 days to a full year. Continuous radon monitors give hour-by-hour data.
The broader picture of radon testing in Colorado, where most counties are EPA Radon Zone 1 (highest predicted indoor levels), sits on the radon testing pillar for Front Range homeowners.
Why radon inspections matter
The EPA estimates radon exposure causes approximately 21,000 lung cancer deaths per year in the United States, second only to smoking and substantially exacerbated by smoking. The risk is linear and cumulative: longer exposure at higher concentrations creates higher cancer risk over decades. EPA, CDC, and the U.S. Surgeon General all recommend testing every home below the third floor at least once, with re-testing after major construction, after radon system installation, and on the schedule of every 2 to 5 years for ongoing monitoring.
Colorado context matters. Most of the Front Range sits on uranium-bearing geology that produces above-average indoor radon levels. CDPHE estimates that approximately half of Colorado homes test above 4 pCi/L; some Front Range counties run substantially higher.
Types of radon inspection
Short-term test (48 to 96 hours)
The most common type used in real-estate transactions. Two methods dominate:
Continuous radon monitor (CRM): A digital device deployed by a certified radon professional that records hourly radon concentration for 48 hours under closed-house conditions. Results are displayed on the device and printed in a report. Tamper detection (movement, temperature, humidity logging) makes CRMs the standard for real-estate transactions. Front Range professional CRM testing runs $135 to $200.
Charcoal canister: A small sealed canister deployed for 2 to 7 days, then shipped to a lab. The canister adsorbs radon, and the lab measures the gamma signature to calculate concentration. Inexpensive ($15 to $40 retail) and accurate when handled correctly, but vulnerable to tampering and ineffective in high-humidity environments.
Long-term test (90 days to 1 year)
Alpha-track detector: A small plastic device containing a polymer film that records alpha particle hits from radon decay. Deployed for 90 days or longer, then sent to a lab. Long-term tests give the most accurate picture of annual average exposure but take too long for real-estate timelines.
Electret ion chamber: A chamber containing an electrostatically charged disk. Long-term electrets run 1 to 12 months. Less common in consumer markets.
Continuous consumer monitors
Devices like the Corentium Home and AirThings Wave Plus provide ongoing radon readings over days and months. They are not currently accepted as the sole basis for real-estate transactions but are valuable for ongoing household monitoring. Consumer monitors run $130 to $250.
How a professional radon inspection works
A certified radon professional (NRPP or NRSB credentialed) follows a structured protocol:
1. Pre-inspection
The inspector confirms the home is suitable for closed-house testing: all doors and windows closed except for normal entry and exit, HVAC operating normally, no exhaust fans running continuously. The home should be in closed-house conditions for at least 12 hours before testing begins.
2. Device placement
The monitor is placed in the lowest livable level (typically a basement or ground floor finished living space, not a crawlspace or unfinished basement that is not occupied). It is positioned away from exterior walls, drafts, and direct sun. Standard placement is 2 to 6 feet above the floor and at least 3 feet from exterior walls and windows.
3. Test period
For a CRM, the standard test runs 48 hours under closed-house conditions. The monitor logs hourly data and total integrated concentration.
4. Retrieval and reporting
The inspector retrieves the monitor, prints or downloads the report, and delivers a written summary noting the measured concentration, the test conditions, and any anomalies (HVAC interruptions, door openings logged by the device).
What the numbers mean
EPA published action and consideration levels:
4 pCi/L: The EPA action level. At or above this level, EPA recommends mitigation. The level was set as a balance between achievable mitigation technology and health risk reduction.
2.7 pCi/L: The World Health Organization reference level for residential radon. WHO recommends consideration of mitigation at this lower threshold.
Below 2 pCi/L: Generally considered low. EPA still notes that no level is fully risk-free, but routine mitigation is not typically recommended below 2 pCi/L.
For homes between 2 and 4 pCi/L, EPA recommends “consider mitigation” because the risk reduction is meaningful even though not strictly required. The radon mitigation levels guide covers the decision framework in more detail.
When to schedule a radon inspection
Five common triggers:
Real-estate transaction. The standard add-on to a general home inspection on the Front Range. Colorado’s contract timelines accommodate a 48-hour CRM test within the inspection objection window.
First-time test of a long-occupied home. Any home that has never been tested below the third floor should be tested at least once.
Re-test after mitigation. Post-mitigation testing confirms the system reduced radon below 4 pCi/L. EPA recommends re-testing 30 days after system installation and then every 2 years.
Re-test after major construction. Foundation modifications, basement finishing, or HVAC replacement can change radon entry. Re-test after any work that affects the foundation.
Periodic ongoing testing. Every 2 to 5 years even for previously tested homes, because radon entry can shift over time due to settling, sump pump changes, or seasonal variation.
What mitigation looks like
If the inspection result is at or above 4 pCi/L, mitigation typically involves sub-slab depressurization (SSD), which uses a fan and PVC pipe to vent radon-bearing soil gas from below the foundation slab to outside above the roofline. SSD reduces indoor radon by 50 to 99 percent in most homes. Installation runs $1,500 to $3,500 in Front Range markets in 2026 for a typical single-family home. The system requires a small ongoing electrical cost (an exhaust fan running continuously) and periodic monitoring of the fan and pressure indicator.
Choosing a radon inspector
Two professional credentials matter most: the National Radon Proficiency Program (NRPP) and the National Radon Safety Board (NRSB). Both offer measurement and mitigation certifications. CDPHE maintains a radon program that lists certified providers in Colorado. ASHI and InterNACHI general home inspectors often hold one of these radon certifications as an add-on; verify the credential is current.
Conditions that can skew radon test results
Three sets of conditions can produce misleading short-term radon readings:
Closed-house violation
Doors and windows left open during testing, continuous exhaust fan operation, whole-house fan running, or basement walkout doors propped open all reduce indoor radon by venting it. A test conducted under these conditions will underestimate the true average concentration. CRMs with environmental sensors typically log these violations and flag the report.
Weather extremes during the test
Major storm systems with rapidly changing barometric pressure can drive soil gas into the home and produce abnormally high short-term readings. Sustained high winds can produce variable results depending on direction. The 48-hour CRM averaging smooths most of this, but extreme weather events occasionally warrant a re-test.
HVAC variation
Continuous HVAC operation (typical winter heating) generally produces lower radon levels than periods with HVAC off (typical mild spring or fall). A 48-hour test in transitional weather can read differently from a test during heating season. Long-term tests average these variations out.
What happens after the inspection
The inspector delivers a written report typically within 24 to 48 hours of test retrieval. The report includes the average concentration, the test period, any environmental anomalies (HVAC interruptions, door openings), and a recommendation. The homeowner then decides whether to mitigate, follow up with a long-term test, or accept the result as low risk. For real-estate transactions, the report is shared with the buyer’s agent and used to inform the Inspection Objection or Inspection Resolution negotiation.
For homes already mitigated, post-mitigation testing 30 days after system installation is the standard verification step. Many Front Range homeowners also install a consumer continuous monitor for ongoing peace of mind once the professional measurement confirms the mitigation worked.
References
- EPA radon overview — U.S. Environmental Protection Agency
- CDC radon facts — Centers for Disease Control and Prevention
- CDPHE radon program — Colorado Department of Public Health and Environment
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
Front Range homeowners ready to schedule a radon inspection in Denver, Boulder, Colorado Springs, or elsewhere along the Colorado Front Range can reach out through our contact page for a vetted NRPP- or NRSB-certified inspector referral.