How Is a Radon Test Done: A Plain-Language Guide
How is a radon test done in a typical Colorado home? The procedure follows EPA’s residential measurement protocol: choose the lowest livable level of the home, set up closed-house conditions, place the detector at the right height and distance from walls and openings, run the test for the required duration, retrieve the device, and process the result via lab analysis or device download. The full workflow takes 2 to 7 days for short-term testing and 90 days to 12 months for long-term integrated measurement. This guide walks through each step and the rationale behind it. Content reflects EPA, CDPHE, and lung-health guidance current as of 2026 — engage a certified radon professional for transaction or mitigation decisions.
How Is a Radon Test Done: The Five-Step Procedure
Five steps cover any radon test regardless of device type. Step one: choose the placement location. Step two: establish closed-house conditions. Step three: place the device at the correct height and clearance from openings. Step four: run the test for the required duration. Step five: retrieve the device and process the result.
The steps look simple but each carries protocol requirements that determine whether the result is valid. Skipping or shortcutting any step compromises the data. EPA’s residential protocol and the National Radon Proficiency Program (NRPP) and National Radon Safety Board (NRSB) certification programs codify the procedure for professional use.
Step 1: Choose the Placement Location
EPA’s protocol calls for placement in the lowest livable level of the home — defined as the lowest level where occupants spend at least four hours per day. For most Front Range homes that means a finished basement if one exists, otherwise the first floor. An unfinished basement used only for storage does not count as a livable level under the protocol, but a basement with a family room, home office, or guest bedroom does.
Within the chosen level, select a room representative of normal occupancy. Avoid kitchens (cooking creates aerosol artifacts), bathrooms (high humidity affects some devices), and laundry rooms (HVAC effects from dryers). Bedrooms, family rooms, and home offices are appropriate. The chosen room should be one where the doors and windows can remain closed for the entire test period.
Step 2: Establish Closed-House Conditions
The closed-house protocol requires keeping all windows closed for the entire test period plus 12 hours before the test begins. Exterior doors should remain closed except for normal entry and exit (no holding doors open for moving furniture or extended ventilation). The HVAC system should run in its normal mode for that time of year — heating in winter, cooling in summer — without manually adjusting the thermostat to extreme settings.
Whole-house fans, attic fans, and air-exchanger systems should not run during the test if they normally would not run for extended periods. Window air conditioners are allowed if they would normally be running. The objective is to measure radon under the conditions occupants typically experience, not to artificially raise or lower the reading.
The 12-hour pre-test closure matters. Air exchange from leaving windows open the night before sampling can take 12 hours to clear from the building, so the protocol bakes that delay into the procedure.
Step 3: Place the Device Correctly
Three placement rules apply to any detector. First, height: at least 20 inches above the floor and ideally 2 to 6 feet up — a small table or shelf works. Second, clearance from walls and openings: at least 3 feet from exterior walls, exterior doors, and windows. Third, no environmental interference: away from direct sunlight, heat sources, drafts from HVAC registers, fans, and high-humidity areas.
The reason for these rules is consistency. A device sitting on the floor next to a sump pit will read higher than a device sitting on a table in the middle of the room. EPA’s protocol standardizes the placement so results from different homes and different inspectors are comparable. Some inspectors use two detectors placed several feet apart to cross-check the result.
Step 4: Run the Test for the Required Duration
Short-term tests run 2 to 7 days. The minimum for a defensible real-estate transaction result is 48 hours. Longer durations within the short-term window improve precision because the result averages over more atmospheric variation. Most professional CRM tests run exactly 48 hours to match the protocol and keep the inspector’s schedule predictable.
Long-term tests run 90 days minimum, with 12-month tests preferred for the most representative annual average. Long-term tests are appropriate when the goal is chronic exposure assessment rather than transaction documentation. The two test types answer different questions and the results are not directly interchangeable.
During the test period, occupants should not move the device, alter the closed-house conditions, or change the HVAC operating mode beyond normal use. A test invalidated by mid-test interference must be restarted from the beginning.
Step 5: Retrieve and Process the Result
Retrieval procedure depends on device type. Charcoal canisters get sealed in their original packaging and mailed to the lab specified in the kit; analysis turnaround is 5 to 10 business days. Alpha-track detectors mail to the lab for chemical etching and microscopy; turnaround is 1 to 4 weeks. Continuous radon monitors download to a computer or smartphone app and produce the report immediately upon retrieval. Electret ion chambers are read with a hand-held meter that calculates the result on the spot.
The result includes the average concentration in picocuries per liter, the sampling duration, the location of placement, and any anomalies during the test. Professional inspections add an inspector certification number, chain-of-custody documentation, and a recommendation based on the EPA action levels. See how do they test for radon for the device-specific breakdown.
Interpreting the Result
EPA’s residential action levels frame the interpretation. Above 4 pCi/L: mitigation recommended. Between 2 and 4 pCi/L: consider mitigation, particularly for occupants spending significant time on lower levels. Below 2 pCi/L: no action needed but periodic re-testing every two years is encouraged. The World Health Organization recommends a lower action threshold of 2.7 pCi/L, but for U.S. residential context the EPA’s 4 pCi/L is the operative number.
A result near the action level — say, 3.8 or 4.2 pCi/L — should trigger a confirmatory long-term test. Short-term tests carry inherent variability and a single reading near the threshold could fall either way on a retest. The protocol calls for a confirmation test before committing to mitigation when the initial result is borderline.
Why Closed-House Conditions Matter So Much
Radon concentration inside a building responds to air exchange rate. A house with windows open exchanges air rapidly with the outdoors, where radon concentrations are typically below 0.4 pCi/L. A closed house holds the radon that enters through the foundation and accumulates over time. The 12-hour pre-test closure plus 48 hours of closed-house conditions during the test captures the realistic concentration occupants breathe during heating season when the same conditions hold.
A test conducted with windows open underestimates the typical winter radon level. A test conducted with windows open before a buyer takes possession in winter will likely show higher readings once the new occupants close the windows for cold weather. The closed-house protocol exists to produce a result that predicts winter-time exposure.
Where Tests Go Wrong
Three failure modes account for most invalid tests. Closed-house breach: occupants forgot the test was running and opened windows for ventilation. Placement error: the device sat in a high-traffic area, on the floor, or near an HVAC register. Tampering: a seller in a transaction context disconnected the device or moved it to a different room. Professional inspections include tamper-evident features and document the placement to deter and detect tampering.
The inspector reviews the data for anomalies before issuing the report. Hourly CRM data makes anomalies visible — a sudden drop in radon concentration mid-test often signals a window opened or a door propped. The report can flag the issue and recommend a retest. The how to do a radon test guide walks through homeowner DIY procedure in more detail.
Front Range Considerations
Colorado homes test high frequently enough that most certified inspectors carry mitigation contractor referrals on hand. The Zone 1 baseline rate means tests are routinely above 4 pCi/L, with some Front Range counties showing exceedance rates above 60 percent. Buyers and homeowners should not be surprised by an above-action-level result, and the mitigation industry on the Front Range is mature with system installation typically running $1,200 to $2,000.
Winter testing on the Front Range often shows higher readings than summer testing because cold weather drives stack-effect ventilation that pulls more radon from below the slab. Inspectors note the season on the report. The radon testing in Colorado pillar covers seasonal patterns and mitigation strategy.
Two-Device Cross-Checks and When They Matter
Side-by-side device placement is a quality-control practice that some inspectors use routinely and others reserve for high-stakes inspections. Two devices placed 4 to 6 feet apart in the same room should produce readings within 10 to 15 percent of each other. A larger discrepancy signals device failure, a placement problem, or local variability that warrants investigation. The Tier 1 protocol many professional inspectors follow includes side-by-side placement for transactions involving homes with prior elevated readings or homes adjacent to suspected high-radon properties.
The cost of the second device on a professional inspection is small — typically $25 to $50 in additional fee — and the precision benefit is meaningful. Homeowner self-testing can replicate the practice by deploying two charcoal canisters in the same session. Both go to the lab, both report independently, and the average of the two is more reliable than either alone.
Pre-Test Preparation Steps for Occupants
The homeowner or seller in a transaction has a role to play before the inspector arrives. Twelve hours before the test starts, close all windows and exterior doors except for normal entry and exit. Set the HVAC system to operate normally for that season — do not turn it off, do not turn it up. Place a written note near each exterior door reminding occupants of the closed-house protocol. Avoid using the basement for laundry or other activities that change ventilation patterns during the test period.
The objective is to make the inspector’s arrival a non-event in terms of the data. The closed-house conditions should already be in place when the device is deployed, and they should persist for the entire sampling period. Sellers who do not understand the protocol sometimes inadvertently invalidate the test by opening windows for a real-estate showing or running attic fans during summer testing. Communicate the protocol clearly to anyone who has access to the home during the test period.
References
- EPA citizen’s guide to radon — U.S. Environmental Protection Agency
- CDPHE radon program — Colorado Department of Public Health and Environment
- American Lung Association radon resources — American Lung Association
- CDC radon information — Centers for Disease Control and Prevention
Front Range homeowners planning a radon test for a transaction or mitigation decision can reach out through our contact page for a connection to a certified radon professional.