How Is Radon Tested: A Plain-Language Guide
Radon is tested using one of four main detection methods, each suited to a different audience and time horizon. Colorado sits in EPA Zone 1 with CDPHE reporting about half of tested homes above the 4 pCi/L action level, so the choice of method matters for accurate decision-making. This guide compares charcoal canisters, alpha-track detectors, electret ion chambers, and continuous radon monitors — what each measures, how long each takes, and when each is the right tool. The summary reflects EPA and CDPHE guidance current as of 2026 — consult a licensed mitigation professional for elevated results and your physician for any health concerns.
How Is Radon Tested: The Four Main Methods
Charcoal canisters absorb radon decay products over 48 to 96 hours and return a single short-term reading via lab analysis. Alpha-track detectors record alpha particle hits on polymer film across 90 days or more, producing a long-term average. Electret ion chambers measure voltage drop across a charged Teflon disk over days to weeks. Continuous radon monitors use ionization chambers or scintillation cells to read radon hourly, integrating the time series into rolling averages. Each method targets a specific use case: short-term for screening and transactions, long-term for representative annual exposure, continuous for ongoing visibility.
Charcoal Canister Method
A small canister filled with activated carbon is opened and exposed to indoor air for 48 to 96 hours. The carbon absorbs radon gas; the decay daughters lodge in the medium. After the exposure window the canister is sealed and mailed to a lab, where gamma spectrometry counts the decay products and back-calculates the time-averaged radon concentration in pCi/L. Cost: $15 to $30 including lab analysis.
Strengths: cheap, fast, widely available at Front Range hardware retailers. Weaknesses: sensitive to weather and house conditions during the brief test window; charcoal can desorb if shipping is delayed; expired kits read low because the detection medium degrades on the shelf. Charcoal is the dominant short-term DIY method.
Alpha-Track Detector Method
A small piece of polymer film inside a plastic housing is exposed to indoor air for 90 days to one year. Alpha particles from radon decay create microscopic damage tracks in the polymer. The lab etches the film in a chemical bath and counts the tracks under a microscope, converting the count to a time-averaged pCi/L value. Cost: $25 to $50 including lab analysis.
Strengths: averages out daily, weekly, and seasonal variation, producing the most representative single measurement of true annual exposure. Weaknesses: slow — three months minimum. Not suitable for real-estate transactions, which need results inside a 7- to 10-day contingency window. Alpha-track is the dominant long-term DIY method. The pillar guide on radon testing for Colorado homeowners covers when long-term measurement is the right call.
Electret Ion Chamber Method
A small chamber holds an electrically charged Teflon disk. Radon and its decay products ionize the air inside the chamber; ion pairs migrate to the electret and reduce its surface charge. Before and after readings of the disk voltage convert to a radon concentration. Test windows range from 2 to 90 days depending on chamber size. Cost is comparable to alpha-track but the electret is typically reused with a fresh disk per test.
Electrets are more common in professional and research applications than in DIY use because the read-out requires a calibrated voltmeter. Some short-term electret protocols feature in EPA Protocols as acceptable methods for measurement specialists.
Continuous Radon Monitor Method
A continuous radon monitor (CRM) uses an ionization chamber, scintillation cell, or solid-state detector to read radon concentration on an hourly basis. Pro-grade CRMs — Sun Nuclear, RadonAway, AlphaGuard — are the standard for real-estate-transaction testing on the Front Range. They produce a tamper-evident hourly log, an integrated average, and meta-data on temperature, humidity, and pressure. Cost to deploy: $135 to $200 bundled with a home inspection, $150 to $250 standalone.
Consumer-grade CRMs — AirThings Wave Plus, Ecosense RadonEye, Corentium Home — operate on the same physics but at lower accuracy and without NRPP/NRSB calibration. They cost $150 to $300 as a one-time hardware purchase and are intended for personal ongoing visibility, not transactions. The in-batch in home radon test guide explores continuous-monitoring use cases.
Closed-House Conditions Across All Methods
Every short-term method — charcoal, electret, CRM — requires closed-house conditions 12 hours before testing and throughout the test window. Windows and exterior doors stay shut except for normal entry. Whole-house fans are off. HVAC runs normally. Open windows during the test dilute indoor radon and read artificially low. CRMs log condition breaches; charcoal and electret cannot detect tampering. Long-term alpha-track tests do not require strict closed-house conditions because the 90-day average swamps any short-term ventilation changes.
Placement Rules Across All Methods
Place the detector in the lowest livable level used at least eight hours per week. Position 20 inches above the floor, 12 inches from any exterior wall, four inches from objects. Avoid bathrooms, kitchens, laundry rooms, closets, fireplaces, HVAC vents, and direct sunlight. Front Range homes with walk-out basements should be tested on the level where occupants actually spend time, not the convenient upper-floor option. The in-batch how to test radon in home procedural guide covers placement in depth.
Lab Analysis Logistics
Short-term passive tests must be mailed the same day the test window closes. Radon-222 has a 3.8-day half-life, so absorbed signal in the medium decays continuously during shipping. A canister sitting on a counter for three days reaches the lab with about 60 percent of its true signal. Same-day overnight shipping using the kit manufacturer’s prepaid label is the operating norm. Labs typically return results by email within 5 to 10 business days. CRMs, by contrast, produce the result on retrieval — no shipping delay.
Picking a Method for Your Situation
First-Time Screening
Charcoal canister, $15 to $30. Result in 7 to 12 days. Confirms whether further action is needed.
Real-Estate Transaction
Pro CRM, $135 to $200 bundled. Result in 3 to 5 days. Satisfies EPA Protocols for Real Estate Transactions, lenders, and buyers’ agents.
Representative Annual Exposure
Long-term alpha-track, $25 to $50. Result in 90 to 365 days. Averages seasonal swing for a true annual reading.
Ongoing Monitoring
Consumer CRM, $150 to $300 one-time. Ongoing hourly readings via display or app. Best for households with prior elevated results, mitigation systems, or basement bedrooms.
The in-batch radon testing process overview walks through the end-to-end method selection in more detail.
How Accuracy Differs Across Methods
For a single integrated measurement, charcoal canister, electret, and CRM accuracy is broadly comparable when each is deployed under proper protocols. Long-term alpha-track is more representative of true annual exposure because it averages 90 days of variation. CRMs add value through hourly logging and tamper-evidence, not through single-point accuracy. The EPA does not consider any single method definitive — the standard for action is two short-term tests averaging above 4 pCi/L, or one long-term test above 4 pCi/L.
What Happens After an Elevated Result
A confirmed reading at or above 4 pCi/L prompts a call to a Colorado-licensed mitigator. The standard Front Range remedy is active sub-slab depressurization: a 4-inch PVC pipe drawn through the slab, an inline fan, and a vent terminating above the roofline. Install costs typically run $1,200 to $2,500. Post-mitigation testing within 30 days confirms the system brought levels below 4 pCi/L. CDPHE maintains a public list of licensed mitigators.
Why Method Accuracy Is Less Important Than Protocol Compliance
Two homeowners running identical 48-hour charcoal kits on the same Front Range street can get readings of 2.8 pCi/L and 6.5 pCi/L for unrelated reasons. The first homeowner placed the kit in the basement bedroom under proper closed-house conditions for the full 48 hours. The second placed the kit in the basement laundry room with the door cracked for a pet, ran a window fan one afternoon, and mailed the canister three days after the test ended.
Both used the same physics, the same lab, and the same kit brand. The difference is protocol compliance, not method accuracy. The most expensive pro CRM in the hands of a careless deployer can produce worse data than a careful homeowner’s $15 canister. The in-batch accurate radon testing guide expands on this point.
The Lab Side of Method Quality
NRPP and NRSB accredit the labs that analyze charcoal and alpha-track tests. Accredited labs participate in proficiency testing programs that compare their results against reference samples. A lab that consistently reads 25 percent low or 25 percent high on reference samples loses accreditation. Most major U.S. radon labs hold current accreditation; CDPHE maintains a list of Colorado-acceptable labs.
Homeowners buying a DIY kit at a hardware retailer should check that the kit’s lab is on the CDPHE list. Most branded kits — Air Chek, AccuStar, Radalink — use accredited labs. House-brand or off-brand kits sometimes use lower-tier analysis services with less rigorous quality control. The few dollars saved is rarely worth the accuracy uncertainty.
Future Methods on the Horizon
Several emerging technologies may change consumer radon testing within the next decade. Microelectromechanical sensors (MEMS) promise CRM-quality readings at $50 to $80 hardware cost. Smartphone-integrated radon detection has been demonstrated in research prototypes using the phone’s camera sensor as an alpha-particle counter, though commercial products are not yet stable. EPA Protocols for Real Estate Transactions will continue to require certified equipment regardless of consumer-side innovations.
None of this changes the action threshold. 4 pCi/L will continue to be the EPA’s recommended mitigation level for the foreseeable future based on the underlying epidemiology.
References
- Radon Measurement Methods — U.S. Environmental Protection Agency
- Testing Your Home for Radon — Colorado Department of Public Health and Environment
- Radon Testing — Centers for Disease Control and Prevention
- Radon Overview — National Institute of Environmental Health Sciences
Front Range homeowners who want a connection to a licensed radon measurement professional can reach out through our contact page.