Home Radon Test: The 2026 Complete Homeowner Guide
A home radon test is the single most important environmental measurement a Front Range Colorado homeowner makes for their home. Colorado sits entirely in EPA Zone 1, the highest-risk classification, and 1 in 2 Colorado homes tests above the EPA action level of 4.0 pCi/L. This comprehensive guide summarizes EPA, CDPHE, and American Lung Association guidance current to 2026 and is informational; specific elevated test results should be discussed with a CDPHE-certified mitigation contractor and any health concerns reviewed with a physician. The information below covers short-term and long-term testing methods, proper placement, the 48-hour closed-house protocol, lab analysis, and how to interpret results.
What a home radon test actually measures
A home radon test measures the indoor air concentration of radon gas, expressed in picocuries per liter (pCi/L) of air. Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and bedrock. It enters homes through foundation cracks, slab penetrations, sump pits, and crawlspaces, and accumulates in lower levels of the home where ventilation is limited. The EPA action level of 4.0 pCi/L is the threshold at which mitigation is recommended for residential properties. The parent radon testing in Colorado pillar covers the broader testing program context.
The Front Range geological reality is that uranium-rich granite and shale formations underlie much of Denver, Boulder, Colorado Springs, and the foothills. Indoor radon concentrations in Colorado homes commonly run 4 to 15 pCi/L without mitigation, and concentrations above 20 pCi/L are not unusual in homes with strong soil-gas entry paths. Testing is the only way to know what is happening in any individual home; visual inspection and home age alone do not predict radon level reliably.
Short-term home radon test methods
Short-term tests run 48 hours to 7 days and are the most common option for real-estate transactions and initial homeowner screening.
Charcoal canister
A small metal or plastic canister filled with activated charcoal absorbs radon and its decay products during the exposure period. After 48 to 96 hours, the canister is sealed and mailed to a CDPHE-certified laboratory for gamma spectroscopy analysis. Cost runs $15 to $30 per canister including lab fee. Results return 5 to 10 business days later. Charcoal canisters are the most widely deployed short-term test for residential and real-estate use.
Alpha track (short-term variant)
A small detector containing a thin plastic film records alpha particle impacts from radon decay during the exposure period. The 48-hour or 96-hour alpha track variant is similar in cost ($20 to $35) and turnaround to charcoal canisters but uses a different detection physics that is less sensitive to humidity and temperature variation. Some testing programs prefer alpha track for higher humidity environments.
Continuous radon monitor (CRM)
An electronic device measuring radon concentration hourly throughout the exposure period and reporting a time-weighted average. CRMs are the gold standard for real-estate transactions because they record the closed-house compliance period and produce an immediate digital result without lab turnaround. CRMs cost $150 to $400 to purchase or $50 to $125 to rent for a 48-hour deployment. Most professional radon testers in Front Range real-estate transactions use CRMs.
Electret ion chamber
A chamber containing a charged Teflon disc records radon decay events by measuring voltage discharge over the exposure period. Less commonly used in residential settings but still available through some lab programs.
Long-term home radon test methods
Long-term tests run 90 days to 1 year and provide a more representative picture of typical occupancy exposure than short-term snapshots.
Alpha track (long-term)
The same alpha track technology used in short-term tests, deployed for 90 days, 6 months, or up to 1 year. Cost runs $30 to $80 per detector including lab fee. Long-term alpha track is the gold standard for understanding actual residential exposure across seasonal variation.
Long-term electret
Long-duration electret deployment, typically 90 to 180 days. Cost runs $40 to $100. Less common than alpha track for long-term residential use but available through some lab programs.
The choice between short-term and long-term depends on the use case. Real-estate transactions need answers in days, so short-term (preferably CRM) is the standard. Long-term occupancy assessment, particularly for a home where the family will live for many years, is better served by long-term alpha track that captures seasonal variation. A look at radon test duration covers timing considerations in more detail.
When to use each test type
Real-estate transactions
Short-term test (CRM preferred, charcoal canister acceptable) deployed for 48 hours under closed-house conditions, scheduled within the buyer’s contingency window. Results within 7 days of contingency deadline.
Initial homeowner screening
Short-term charcoal canister is the most cost-effective starting point. If the result is below 2.0 pCi/L, no immediate follow-up. If the result is between 2.0 and 4.0 pCi/L, a second short-term test or a long-term alpha track is the appropriate confirmation. If the result is above 4.0 pCi/L, proceed directly to mitigation planning.
Long-term occupancy assessment
Long-term alpha track deployed for at least 90 days, ideally 1 year, in the lowest occupied level. This is the best test for understanding what radon exposure the family is actually receiving across all seasons.
Post-mitigation verification
Short-term CRM or charcoal canister deployed 24 to 48 hours after the mitigation system starts operating, confirming the system achieved its designed reduction. A follow-up long-term alpha track confirms performance across seasonal variation. Companion piece on testing inspection covers post-mitigation verification in more depth.
The 48-hour closed-house protocol
Short-term radon test results are only valid when the home is in closed-house conditions during the exposure period. The standard EPA protocol requires the following.
For 12 hours before the test begins and during the entire exposure period: all exterior doors and windows remain closed except for normal entry and exit. Whole-house fans and exhaust fans (bathroom, kitchen, attic ventilation) are not used continuously. Window air conditioners and portable air conditioners are not operated with exterior venting open. HVAC systems on the main heating or cooling mode operate normally. The home is occupied normally otherwise.
Failure to maintain closed-house conditions invalidates the test result; the radon concentration measured during open-house conditions can be 30 to 70 percent lower than the actual long-term average. Real-estate testing relies on closed-house compliance and the CRM detector physically records open-window events, which is why CRMs are the preferred test in transactions.
Detector placement rules
Where you place the test matters as much as what test you use. EPA placement guidelines specify the following for residential testing.
Test the lowest occupied level of the home. In Front Range homes with finished basements, this means the basement floor. In slab-on-grade homes, this means the main level. Test where people actually spend time, not in an unoccupied storage area.
Position the detector 20 to 72 inches above the floor, away from exterior walls and drafts. Detectors placed too close to floors or walls produce skewed readings. Detectors placed near interior closets or central locations away from exterior wall drafts produce the most representative readings.
Keep the detector away from heat sources, direct sunlight, and high-humidity sources (laundry rooms, kitchen counters, bathroom exhausts). These ambient conditions can interfere with detector physics, particularly for charcoal canisters.
Maintain the detector position throughout the exposure period. Do not move the detector mid-test, do not handle the detector unnecessarily, and do not place objects on top of it.
CDPHE-certified labs and the Colorado subsidized kit program
The Colorado Department of Public Health and Environment maintains a radon program that includes a subsidized home test kit available to Colorado residents for $15 (charcoal canister with lab analysis included). The CDPHE program is the most cost-effective option for initial homeowner screening and uses CDPHE-certified laboratories for analysis. Residents can request kits through the CDPHE website or pick them up at participating county public health offices.
Beyond the CDPHE subsidized program, several private radon test kit providers serve the Colorado market. AccuStar, AirChek, and RadonAway are common brand names for residential test kits. Cost from retail or direct ordering runs $20 to $30 per kit including lab fee. A look at residential exposure covers the public-health rationale for the subsidized program.
How to interpret results
Below 2.0 pCi/L
Low risk. No immediate action required. Consider re-testing every 2 to 5 years to confirm levels remain stable.
2.0 to 4.0 pCi/L
Mid-range. EPA recommends considering mitigation, particularly for homes with vulnerable residents (smokers, children, pregnant residents, elderly family members). A second short-term test or a long-term alpha track confirms the reading.
4.0 to 10.0 pCi/L
Elevated. EPA action level exceeded. Mitigation is recommended; a CDPHE-certified mitigation contractor can quote a system within 1 to 2 weeks.
Above 10.0 pCi/L
Substantially elevated. Mitigation is strongly recommended. Some homes in Colorado test above 20 pCi/L, which still responds well to standard SSD mitigation.
The companion piece on high radon level response walks through the mitigation decision pathway.
Common testing mistakes Front Range homeowners make
Three errors recur frequently in initial testing. Testing during open-window summer months in an unconditioned home produces low readings that misrepresent the closed-house heating-season reality; Colorado homes with the heating system running in winter typically test higher than the same home tested with windows open in summer. Testing only the main level when a finished basement exists understates the exposure of any family members using the basement as living space. Re-testing immediately after a borderline result without waiting 30 to 90 days allows short-term weather variation to dominate the second result and produces conflicting numbers that delay any actual response.
Homeowners with questions about test interpretation, follow-up testing, or mitigation planning can reach out through our contact page for a connection to a CDPHE-certified radon professional.
Seasonal patterns in Colorado radon
Colorado radon concentrations vary meaningfully by season because soil-gas pressure dynamics shift with temperature and snowpack. Winter heating-season radon is typically 15 to 40 percent higher than summer because warm indoor air rising through the home (the stack effect) creates negative pressure at the foundation that draws soil gas inward. Heavy snowpack also seals the ground surface temporarily, forcing soil gas to enter the home rather than vent naturally to the surface.
This seasonality has a practical implication for testing. A home that tests at 3.5 pCi/L in July, just below the EPA action level, may test at 5.5 to 7.0 pCi/L in January under the same conditions. Long-term alpha track tests capture this variation; short-term summer tests can produce false-negative reassurance. Homeowners commissioning a single initial test for actionable decisions should aim for the November to March heating-season window if possible, when the result is most representative of typical exposure conditions.
What to do if test result is borderline
A first test result between 2.0 and 4.0 pCi/L sits in the zone where EPA recommends consideration of mitigation but does not require it. The right response depends on residency context and household risk factors. For a household with current or former smokers (radon-attributable lung cancer risk multiplies dramatically with smoking history), mitigation in the 2.0 to 4.0 pCi/L range is medically defensible. For a household with young children who will spend decades in the home, mitigation at 3.0 pCi/L or above reduces lifetime cumulative dose meaningfully. For a household with healthy adults planning to sell within 3 to 5 years, mitigation at 3.0 pCi/L may not produce sufficient health benefit to justify the cost relative to other home-improvement priorities.
Re-testing with a long-term alpha track is the cleanest next step when the first result is borderline. The long-term result either confirms the borderline reading (suggesting baseline stable exposure) or shifts higher (suggesting the first test understated the typical exposure). Either result clarifies the mitigation decision.
Free and subsidized testing resources
Beyond CDPHE’s $15 subsidized kit, several Colorado counties operate periodic free testing campaigns. Boulder County Public Health, Jefferson County Public Health, Tri-County Health (Adams, Arapahoe, Douglas), and Denver Environmental Health each run seasonal radon test kit distributions, typically in October through January, coinciding with National Radon Action Month in January. Quantities are limited; the programs are first-come, first-served. Homeowners can check with their county public health department for current availability.
Some Colorado employers and HOAs distribute test kits as part of workplace wellness or community-safety programs. Real-estate brokerages and home-purchase support organizations occasionally subsidize kits during buyer education events. None of these alternative sources require purchase from a retailer; the kit physics and lab analysis are the same regardless of distribution channel.
References
- EPA home radon testing and mitigation — U.S. Environmental Protection Agency
- CDPHE Colorado radon program — Colorado Department of Public Health and Environment
- American Lung Association radon resources — American Lung Association
- CDC radon and lung cancer prevention — Centers for Disease Control and Prevention