What Is a Radon Detector: A Plain-Language Guide
A radon detector is any device that measures the concentration of radon gas in indoor air. The category includes electronic continuous monitors (the kind a homeowner sets on a shelf and reads on an app), professional inspector-grade continuous monitors (used in real-estate transactions), passive charcoal canisters (mail-in lab tests), alpha-track long-term detectors (months-long lab tests), and electret ion chamber detectors (used by some inspectors). Each detector type works differently and serves a different testing situation. This guide summarizes EPA and CDPHE guidance current as of 2026 and walks through what each detector is, how it works, and when it is the right choice. Nothing here is medical advice or a substitute for professional radon assessment.
What Is a Radon Detector at the Most Basic Level
A radon detector measures radon-222, a naturally occurring radioactive gas that seeps into homes from soil and rock. The gas is colorless, odorless, and chemically inert, so the only way to know it is present is to measure it. Detectors quantify the concentration in picocuries per liter (pCi/L) and compare it to the EPA action level of 4.0 pCi/L. Levels at or above 4.0 pCi/L warrant mitigation. EPA also recommends consideration of mitigation between 2.0 and 4.0 pCi/L because no level of radon is risk-free.
The detector category breaks into two big buckets. Active detectors are electronic devices that measure radon in real time and either display the result on the device or send it to a phone app. Passive detectors are physical media (charcoal, polycarbonate film, electret) that absorb or record radon decay over an exposure period and require lab analysis to produce a result. Active detectors deliver immediate readings; passive detectors produce a measurement averaged across the exposure period.
The right detector depends on what the homeowner needs to know and why. The radon testing pillar for Colorado homeowners covers the broader testing decision and how to choose between detector types.
Continuous Radon Monitors
A continuous radon monitor (CRM) is an active electronic detector that samples air continuously and reports radon concentration on a rolling basis. Consumer models like the Airthings Wave Radon, Corentium Home, and Ecosense RadonEye sit on a table or plug into an outlet and report current readings, 24-hour averages, and longer-term trends. Most consumer CRMs also display short-term and long-term averages on a phone app. Cost runs $150 to $400 for a one-time purchase.
Professional inspector-grade CRMs from Sun Nuclear, Femto-Tech, and Radalink are larger devices used for real-estate transactions. They are calibrated annually, tamper-resistant, and produce a 48-hour test report with an hourly chart suitable for closing documentation. Inspector CRMs cost $1,000 to $3,000 and are owned by the inspector. The per-test cost on an inspector CRM is rolled into the radon test add-on fee, typically $135 to $200 on the Front Range.
CRMs use ionization chambers, photodiode detectors, or alpha-spectrometry sensors depending on the model. The technology converts radon decay events into electronic signals that the device counts and translates into a pCi/L reading. Modern consumer CRMs are accurate within roughly ±20% when properly calibrated, sufficient for most homeowner decisions. Inspector CRMs hit tighter accuracy bands because of regular calibration. The Airthings vs Corentium comparison walks through the consumer CRM choice in detail.
Why CRMs Dominate Real Estate
CRMs dominate real-estate testing because they produce defensible 48-hour results within a typical inspection contingency window, log hourly data that allows the inspector to verify closed-house protocol, and require no shipping or lab processing. A charcoal kit can take 7 to 14 days end-to-end; a CRM produces a written report within 2 to 4 days. For buyers under contingency time pressure, the CRM’s speed is decisive.
Charcoal Canister Detectors
A short-term charcoal canister is a passive detector. The container, roughly the size of a hockey puck or small soup can, holds activated charcoal that adsorbs radon decay products during the 48-hour to 7-day exposure period. After exposure, the canister is sealed and shipped to an accredited lab for gamma spectroscopy analysis. The lab returns a written result with a single pCi/L number.
Cost runs $15 to $35 including lab fee, making the charcoal canister the cheapest test option. The end-to-end timeline is the slowest, however — 7 to 14 days from start of exposure to result delivered. Mail transit and lab processing each add several days. Charcoal kits are appropriate for baseline measurement when the homeowner is not under time pressure and wants the cheapest possible answer.
Charcoal canisters are widely sold in big-box retail and online. Quality varies; the most important variable is the lab. Use only kits whose lab is accredited by the National Radon Proficiency Program (NRPP) or the National Radon Safety Board (NRSB). Common accredited labs that distribute through retail channels include AccuStar, Air Chek, and RadonZone. Kits without accreditation listed on the box should be skipped.
Alpha-Track Long-Term Detectors
An alpha-track detector is a passive long-term detector. The device — a small plastic disc or container holding a piece of polycarbonate film — exposes for 90 days to a year. Alpha particles from radon decay leave physical tracks in the film. After the exposure period, the lab etches and counts the tracks, then translates the count into an average pCi/L for the exposure period.
Cost runs $25 to $50 per detector including lab fee. Total turnaround is the exposure period plus 7 to 10 business days of lab processing. Alpha-track tests are inappropriate for real-estate transactions because the timeline is too long, but they are the gold standard for representative annual averages. Radon levels vary substantially across seasons, and short-term tests can over- or underestimate the typical year-round level.
Long-term testing is the right choice in three situations. First, baseline measurement when the homeowner is not under time pressure and wants the most representative number. Second, follow-up to a borderline short-term result where the homeowner wants a more accurate measurement before deciding on mitigation. Third, post-mitigation verification across a full season to confirm a system is working year-round.
Electret Ion Chamber Detectors
Electret ion chambers are professional-grade passive detectors used by some inspectors and consultants. The device contains an electrically charged Teflon disc (the electret) inside a small chamber. Radon ions reduce the charge during exposure. The inspector measures the charge before and after, calculates the difference, and translates it into a pCi/L reading. Electrets can run for 2 to 7 days for short-term testing or longer for long-term testing.
Electrets have advantages and limitations. They are accurate, produce results without lab processing (the inspector reads the electret on a calibrated voltage reader), and are temperature- and humidity-tolerant. They are also relatively fragile, require careful handling, and lose accuracy if exposure conditions vary widely. Some inspectors prefer electrets for specific testing situations; others prefer CRMs for the hourly logging capability. The best radon detectors guide covers the device-by-device comparison.
Electret cost is bundled into the inspector’s per-test fee. Homeowners do not typically buy electrets for personal use. The device is professional-grade and not sold through consumer retail channels.
Where to Place a Radon Detector
Placement matters for any detector type. The detector should sit in the lowest livable area of the home — a finished basement if the basement is used as living space, otherwise the lowest finished floor. Place it 20 inches above the floor, at least three feet from exterior walls, doors, and windows, and away from drafts, heat sources, high humidity, and direct sunlight. The location should not be disturbed during the test period.
Placing a detector in an unfinished basement, crawl space, or area not used for living gives results that do not represent typical occupant exposure. Placing it in a kitchen, bathroom, or laundry room — areas with high humidity or air movement — can interfere with some detector types. The lowest livable area on a stable interior surface is the standard recommendation.
For homes with multiple levels, EPA recommends testing the lowest level used regularly. A finished basement that hosts a home office or rec room is the right test location even if the family spends most of its time on the main floor. A walkout basement that is fully finished and used as living space gets tested even if the family spends little time there. The what is a radon test explainer covers placement decisions and closed-house protocol in detail.
Accuracy and Calibration
Detector accuracy depends on type, calibration, and protocol adherence. Inspector-grade CRMs calibrated annually typically deliver results within ±10% accuracy. Consumer CRMs run within ±20% accuracy. Charcoal canisters and alpha-track detectors from accredited labs run within ±15-25% accuracy. Electrets run within ±15% accuracy when properly handled. None of the detector types are perfectly accurate — the question is whether the accuracy band is tight enough for the homeowner’s decision.
For a real-estate decision based on EPA’s 4.0 pCi/L action level, a result of 5.5 pCi/L on a consumer CRM with ±20% accuracy is unambiguously elevated. A result of 4.2 pCi/L is borderline and warrants follow-up testing. Borderline results are common, and EPA recommends a second short-term test or a long-term test when the first result lands within the borderline band.
Calibration matters more than detector type for results within the accuracy band. An inspector-grade CRM that has not been calibrated in two years is less accurate than a recently calibrated consumer CRM. Buyers should ask the inspector when the CRM was last calibrated; reputable inspectors send their devices in annually and document the calibration date in the test report. The DIY radon test accuracy guide covers consumer CRM accuracy in detail.
How to Choose the Right Detector
Three rules of thumb. First, for real-estate transactions, use the inspector’s calibrated CRM. The 48-hour result fits the contingency window, the chain of custody is defensible, and the hourly log validates the test conditions. Cost is $135 to $200 on the Front Range as an add-on to the home inspection. Second, for homeowner baseline measurement without time pressure, use a charcoal canister at $15 to $35 or a consumer CRM at $150 to $400. The CRM has a higher upfront cost but pays for itself across multiple tests over the life of the home.
Third, for long-term representative averaging, use an alpha-track detector at $25 to $50. The 90-day to one-year exposure period produces the most accurate annual-average measurement. Alpha-track tests are also the right follow-up to a borderline short-term result. Consumer CRMs can also produce long-term averages by running continuously for 90 days or more.
Front Range homeowners doing thorough testing often combine methods. A buyer might use the inspector’s CRM during the inspection contingency, then install a consumer CRM for ongoing monitoring after closing, and run an alpha-track test in the second year for an annual average. Each detector type contributes a different angle on the same underlying question: what is the typical radon concentration in this home, and is it above the action level?
References
- EPA radon program and testing guidance — U.S. Environmental Protection Agency
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- CDPHE radon program for Colorado homeowners — Colorado Department of Public Health and Environment
- CDC about radon — Centers for Disease Control and Prevention
Front Range homeowners trying to choose between a DIY consumer CRM, a mail-in charcoal kit, or a professional 48-hour inspector test can connect with a vetted local inspector through our contact page for guidance based on the home and the testing goal.
Continuous radon detectors
A digital monitor gives you an always-on reading and a long-term average — better than a one-shot kit if you want to track levels over time.
| Product | Why | Buy |
|---|---|---|
 Airthings Corentium Home | Battery, portable; trusted long-term average. | Amazon — $138.83 |
 Ecosense RadonEye RD200 | Fast first reading (~1 hr); app logging. | Amazon — $169.00 |
 Airthings View Plus | Radon + CO2 + PM + VOC in one unit. | Amazon — $329.99 |