How to Do a Radon Test: A Plain-Language Guide
Radon testing is procedurally simple but unforgiving of shortcuts. Place the detector in the wrong spot, leave windows open during the test, or seal the canister late and the result is unreliable. This guide walks through how to do a radon test step by step using the EPA Protocol for short-term measurement, names the common mistakes that invalidate results, and explains when to repeat versus when to act. This guide summarizes EPA Indoor Air Quality guidance and Colorado Department of Public Health and Environment guidance current as of 2026; for real-estate transactions and elevated results, consult a NRPP-certified or state-licensed radon professional.
How to do a radon test: the short version
Buy a short-term test kit (charcoal canister, alpha-track, or electret) from a hardware store or AARST-PROFICIENCY-listed online vendor. Choose the lowest occupied level of the home for placement (typically the basement). Close all windows and exterior doors for at least 12 hours before starting the test, and keep them closed throughout (closed-house conditions). Place the detector per the included instructions: 20 inches to 6 feet above the floor, away from drafts, exterior walls, kitchens, bathrooms, and HVAC vents. Start the test and record the time. Leave for the duration specified by the kit (48 hours minimum for short-term protocol; some kits run 90 days for an alpha-track). Seal the detector and mail it to the lab on the same day the test ends. Wait for results.
That is the whole workflow. Most failures come from violating closed-house conditions, placing the detector poorly, or delaying the mail-in. Each step is covered in detail below. For background on what the result means, see the radon testing guide for Front Range homeowners.
Step 1: Choose the right test type
Three categories of DIY radon detector exist. Charcoal canister kits ($15 to $35) absorb radon onto activated carbon during a 2-day to 7-day exposure. They are the most common and cheapest option. Results return within five to ten business days after lab receipt. Alpha-track detectors ($25 to $55) use a piece of plastic that records etched tracks from alpha particles over a longer exposure (90 to 365 days). Long-term alpha-track results better capture annual average radon, which is the EPA recommended measurement basis for action decisions. Electret ion chambers ($35 to $75) are the prosumer option; they measure ionization current change over the exposure window and can be read locally with a calibration reader.
Continuous radon monitors (CRM) are the professional-grade option. CRMs run $150 to $400 and display real-time readings; they are used by certified radon professionals for real-estate transactions and by serious DIYers who want repeatable longitudinal measurement. For a first-pass screening test, a charcoal canister is the right choice; for real-estate, hire a professional with a CRM.
Step 2: Establish closed-house conditions
Closed-house conditions are the single most-violated protocol step. EPA’s short-term protocol requires that all windows and exterior doors be closed except for normal entry and exit for at least 12 hours before the test starts and throughout the test duration. Interior doors may remain open. HVAC may operate normally. Exhaust fans should be used only briefly when needed.
What this means practically: do not open windows for fresh air during the test. Do not run whole-house fans. Do not leave the front door propped open while moving furniture. Do not start the test the same hour you arrived from outdoors with all the doors open. Schedule the test for a stretch when the home will be in normal closed-house operation.
Step 3: Pick the right location
Place the detector on the lowest occupied level of the home. For most homes that is the basement. For homes without a basement, the lowest occupied level is the main floor. For homes with a daylight or walkout basement, the basement counts as occupied if it has finished living space. The reason: radon enters from soil gas through the foundation, and concentration is highest at the lowest level closest to the source. The radon test in basement placement guide walks through basement specifics.
Within the chosen level, the detector should be 20 inches to 6 feet above the floor, at least 3 feet from exterior walls, at least 3 feet from windows and doors, away from heating and cooling vents, away from kitchens (humidity and cooking fumes interfere with charcoal canisters), and away from bathrooms (same reason). A shelf, a piece of furniture, or a small platform works fine for placement. Do not place on the floor directly; do not place inside a closet or cabinet where airflow is restricted.
Common placement mistakes
Placing on a kitchen counter to keep it out of the way: invalid (kitchen humidity and cooking fumes). Placing in a bathroom: invalid (same). Placing in a closet to hide it: invalid (no airflow). Placing in front of an HVAC vent: invalid (forced air dilutes local reading). Placing on the floor: invalid (too low). Placing on a high shelf 10 feet up: invalid (too high, not representative of breathing zone).
Step 4: Start the test and record the time
Open the kit per the instructions. For a charcoal canister, peel back the foil seal and place the canister with the opening upward. Record the start date and time on the supplied card. Most kits also require recording the location of the detector, the home’s address, and contact information for results delivery. Some kits require selecting the test duration on the card (48-hour vs. 96-hour). Read the instructions carefully and do not skip the paperwork; the lab uses it to calculate the result.
Once started, the test runs unattended. Do not move the detector during the test. Do not interfere with it. Do not check on it more than briefly to confirm it is undisturbed.
Step 5: Maintain conditions during the test
Throughout the test duration, maintain closed-house conditions. Maintain normal HVAC operation. Do not run a whole-house fan, attic fan, or radon mitigation system during the test (if a mitigation system is installed and the goal is to verify its effectiveness, run the system normally; otherwise turn it off two days before testing for a baseline result). Avoid heavy weather during the test if possible: high winds and storms can affect short-term radon readings. The EPA notes that short-term tests during unusual weather may not represent normal conditions.
Step 6: End the test and ship immediately
At the end of the specified test duration, seal the detector per the instructions. For a charcoal canister, replace the foil cap and tape it. Record the end date and time on the card. Place the sealed detector and the completed information card in the lab’s mailing envelope. Ship the same day the test ends.
Delayed shipping is a major source of inaccurate charcoal canister results because radon continues to be absorbed (or desorbed) from the carbon during transit. The lab calculates the result assuming a specific time interval between exposure end and lab receipt. Same-day shipping minimizes this variable. Use first-class mail or expedited shipping if the lab is in a different region.
Step 7: Read and act on the result
Lab results return within five to ten business days of receipt. The result is reported in picocuries per liter (pCi/L). EPA’s action level is 4.0 pCi/L. A result at or above 4.0 pCi/L requires mitigation per EPA guidance. EPA also recommends consideration of mitigation for results between 2.0 and 4.0 pCi/L, especially for households with smokers or with people who spend extensive time in the lower level.
Results between 0.0 and 2.0 pCi/L are typically considered acceptable, with no mitigation recommended but periodic re-testing (every two to five years, and after any significant home modification) is good practice. Colorado homeowners should know that Colorado is in EPA Zone 1 with a high background prior probability of elevated readings; CDPHE recommends every Colorado home be tested.
When to repeat: short-term confirmation protocol
EPA recommends that any short-term test result at or above 4.0 pCi/L be confirmed with either a long-term test (90+ days for annual average) or a second short-term test in the same location. Two short-term tests can be averaged; if either is at or above 4.0, mitigation is recommended. For real-estate transactions, confirmatory testing typically takes the form of a 48-hour CRM measurement performed by a certified radon professional per EPA Protocol B with chain-of-custody documentation. The how to test for radon in your home guide covers the broader testing process including pre-test preparation in detail.
What can go wrong: troubleshooting bad results
Implausibly low result on a known-high home: closed-house conditions were not maintained, the detector was placed near a window or vent, or the test was run during a high-wind weather window.
Implausibly high result (above 20 pCi/L): possible but worth confirming with a second test or a CRM. Some basements with cracked slabs over uranium-rich soil do produce very high readings.
Lab reports an invalid result: usually due to mail-in delay, kit damage during shipping, or improperly sealed canister.
Results from two simultaneous tests in different rooms differ significantly: radon concentrations do vary room to room. The lowest occupied level result is the reference for mitigation decisions per EPA protocol.
References
- EPA Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- Colorado radon program — Colorado Department of Public Health and Environment
- CDC radon information — Centers for Disease Control and Prevention
- EPA radon zones map — U.S. Environmental Protection Agency
Front Range homeowners who want a NRPP-certified professional radon test or mitigation referral can reach out through our contact page.