How to Measure Radon: A Plain-Language Guide for 2026
Measuring radon is a methodology question more than a product question — the right device depends on what the result will be used for and how much time is available. This guide summarizes EPA and CDC measurement guidance current as of 2026, walks through the three main device categories (activated charcoal, alpha-track, continuous radon monitor), and explains the placement and closed-house protocol that makes any measurement valid. The content is informational and is not a substitute for a certified radon measurement professional for results that will inform real-estate transactions, lender requirements, or mitigation-system design.
The three device categories
Activated charcoal canisters are passive short-term devices that absorb radon over 48 hours to 7 days, then go to a lab for gamma counting. Alpha-track detectors are passive long-term devices that record alpha particle tracks on a plastic film over 90 days to 12 months, then go to a lab for track counting. Continuous radon monitors (CRMs) are active devices that record hourly concentrations on internal sensors and produce a real-time digital report. Each category has its place.
Activated charcoal: cheap, short-term, lab-dependent
A typical charcoal kit costs $25 to $50 from a hardware store or online vendor, includes return shipping to the lab, and reports a single-number result in pCi/L within 5 to 10 business days of lab receipt. The kit is exposed for 48 hours under closed-house conditions, then sealed and mailed. Charcoal is the standard for first-pass homeowner screening — appropriate when the result will inform a low-stakes decision rather than a contractual one.
Alpha-track: best annual-average accuracy
Alpha-track detectors use a small piece of nitrocellulose film inside a vented housing. Alpha particles from radon decay strike the film and leave permanent etched tracks. The lab counts the tracks after development and calculates an integrated average concentration over the deployment period. Costs run $30 to $80 per device with multi-device discounts. Alpha-track is the gold standard for annual-average concentration because it integrates over the full deployment period rather than sampling a short window.
Continuous radon monitors: real-time data with tamper detection
Professional CRMs cost $1,000 to $3,500 to purchase but are typically rented to homeowners as part of a $125 to $200 measurement service. The device records radon concentrations hourly, logs temperature and humidity, and uses motion sensors to detect any tampering during the test period. CRMs are the standard for real-estate transactions because the hourly data and tamper logs prove the closed-house protocol was maintained.
The EPA placement protocol
Devices go in the lowest livable level of the home — basement if finished and used, first floor in slab-on-grade homes. Placement is 20 inches to 6 feet above the floor, at least 3 feet from doors and windows, at least 1 foot from exterior walls, away from HVAC supply registers, away from drafts from fans or vents, and away from high humidity sources (laundry rooms, bathrooms). The device sits in the open, not in a closet or behind furniture.
Closed-house conditions
Closed-house conditions must be maintained for at least 12 hours before the test begins and through the entire test period. “Closed” means windows and exterior doors stay shut except for normal entry and exit. HVAC operates normally — heating and cooling cycles continue as usual. Whole-house fans, attic fans, and exhaust fans should not run continuously. Failing to maintain closed-house conditions invalidates short-term test results.
How to set up a DIY charcoal test correctly
Order a kit from an AARST-NRPP-certified lab brand. On arrival, check the deployment-window date on the canister — it must be deployed before the expiration date. Place per EPA protocol (lowest livable level, height and clearance rules above). Open the kit per the included instructions, log the start date and time on the chain-of-custody card, leave undisturbed for 48 hours, seal the canister, log the end date and time, and mail same-day in the prepaid mailer. Late mailing degrades the charcoal sample.
Why measurement professionals exist
A certified radon measurement professional brings calibrated CRMs, NRPP/NRSB certification, written protocol, chain-of-custody documentation, and same-day reporting. The professional’s report carries weight in transactions, with lenders, and in any future property-sale disclosure. DIY testing produces results that are valid for homeowner decision-making but generally not accepted in formal transactional contexts. For deeper context on the test types, see our charcoal vs alpha-track vs CRM comparison.
Costs at a glance
DIY charcoal kit: $25 to $50. DIY alpha-track detector: $30 to $80. Consumer continuous radon monitor for ongoing surveillance (Airthings, Corentium): $150 to $300. Professional CRM measurement service: $125 to $200. Add-on to a home inspection during a real-estate transaction: typically a $25 to $75 discount versus standalone professional pricing. Long-term alpha-track for annual averaging in a high-radon area: $30 to $80 per device, two to four devices recommended.
What “measurement” actually counts
Charcoal canisters use the canister mass and the gamma counts to derive a concentration assuming a standard exposure period. Alpha-track devices count etched tracks on the plastic film, with each track representing a decay event. CRMs use a small ionization or scintillation chamber that detects alpha particles directly and reports counts per minute, converted to pCi/L based on the device’s calibration. All three methods ultimately measure radioactive decay, with the differences being measurement duration and data resolution.
When two measurements are better than one
EPA recommends a follow-up test when an initial short-term test produces a borderline result (2 to 6 pCi/L). The follow-up can be a second short-term test (faster decision) or a long-term test (more accurate annual average). For clearly elevated initial results (8 pCi/L and up), most homeowners proceed directly to mitigation without a second test. For clearly low initial results (below 2 pCi/L), no retest is typically needed unless conditions change.
Variables that affect radon concentrations
Concentrations vary with weather (rising barometric pressure tends to pull more soil gas into the home), HVAC operation (return-air leaks in crawlspaces can pull soil gas), season (winter heating amplifies the stack effect), occupancy (closed-house conditions concentrate gas), and time of day. A 48-hour test captures a snapshot influenced by these variables; an alpha-track test integrates over 90+ days and smooths them out.
Post-mitigation measurement
After a sub-slab depressurization system is installed, EPA recommends a post-mitigation short-term test within 30 days of installation to confirm the system reduces radon below 4 pCi/L. Many homeowners then leave a long-term alpha-track detector in place for the first year to confirm sustained reduction. After year one, a consumer continuous monitor like an Airthings Wave provides ongoing surveillance at low cost. The Front Range radon testing hub covers the post-mitigation cycle.
Multi-level homes: where to measure first
Radon concentrations are typically highest in the lowest livable level and decrease with each floor up. Measure in the basement first if the basement is used. If the basement is unfinished and unused, measure on the first floor. For a two-story slab-on-grade home, the first floor is the test location. Some homeowners run simultaneous tests on multiple levels to map vertical distribution before mitigation design.
Measurement errors and their fixes
Common errors: device placed in a closet or behind furniture (fails the open-placement rule), windows open during deployment (fails closed-house), device near a humidifier or laundry room (humidity error on charcoal devices), device near an HVAC register (airflow error), late mailing of a charcoal canister (charcoal continues to release radon after deployment ends, degrading the result). Each of these invalidates the test result and requires a retest.
What a measurement professional’s report includes
A complete report identifies the property address, the test device by serial number, deployment start and end dates and times, average concentration in pCi/L, measurement uncertainty (typically ±10 to 20 percent), closed-house compliance status with any tamper-event flags from the CRM, the certified measurement professional’s name and certification number, and a comparison to the EPA 4 pCi/L action threshold. CRM reports include a graph of hourly concentrations across the test period.
When to call a professional vs. DIY measurement
DIY for general homeowner screening, baseline assessment before a renovation, or post-mitigation surveillance. Hire a certified professional for any real-estate transaction, any lender-required test, any pre-mitigation assessment that will inform system design, and any test where the result will be cited in a disclosure or insurance context. For Front Range homes specifically, the high regional radon prevalence makes initial measurement a high-priority due-diligence item.
References
- EPA Radon Measurement Protocols — U.S. Environmental Protection Agency
- CDC Radon Testing Guidance — Centers for Disease Control and Prevention
- CDPHE Test Your Home for Radon — Colorado Department of Public Health and Environment
- American Lung Association Radon Page — American Lung Association
Front Range homeowners ready to measure their radon levels can get in touch through our contact page for a vetted local measurement professional.