Radon Home Test Kit: What Homeowners Need to Know
A radon home test kit produces a reliable result only when the testing protocol is followed correctly — and the protocol is more involved than the kit packaging usually conveys. The EPA’s standard short-term testing protocol requires closed-house conditions for at least 12 hours before the test starts and throughout the test period, placement in the lowest occupied level of the home, specific distance from walls and floors, and a minimum 48-hour exposure period. Skipping any step produces a number, but not a meaningful one. This guide summarizes EPA and Colorado Department of Public Health and Environment (CDPHE) guidance current as of 2026 and is written as a how-to-test rather than a buyer’s guide. It does not replace direct consultation with a certified radon professional, and any result above the EPA 4 pCi/L action level warrants mitigation planning.
The basic testing protocol
EPA’s short-term radon testing protocol — applied to charcoal canisters, electronic continuous monitors, and other short-duration devices — has five core requirements:
- Closed-house conditions established at least 12 hours before the test begins and maintained throughout the test period
- Minimum 48-hour exposure (most consumer kits run 48 to 96 hours)
- Lowest occupied level placement (basement if used as living space, first floor otherwise)
- At least 20 inches above the floor and at least 3 feet from exterior walls
- Away from drafts, direct sunlight, exterior doors, fans, heat sources, and high humidity
The kit packaging and instructions typically restate these in a condensed form. Reading them carefully before starting matters — re-running a test because of protocol error is expensive.
Closed-house conditions explained
The closed-house requirement is the most commonly violated part of the protocol. Closed-house means:
- Windows closed except for normal entry/exit (a few seconds of door opening is fine)
- External doors closed except for normal entry/exit
- Window fans, attic fans, and whole-house ventilation systems turned off or set to minimum
- HVAC running on normal operation (don’t change thermostat settings or HVAC mode during the test)
- No use of fireplaces or wood stoves during the test (they create negative pressure that draws additional radon)
- Bath fans, range hoods, and dryer used only as normal for short periods
Closed-house conditions must be established at least 12 hours before the test begins. For a 48-hour test, that means 60 hours of closed-house operation total. The protocol exists because radon levels vary substantially with home ventilation; without controlled conditions, results would be noise.
Short-term vs long-term testing
The EPA recognizes two protocol categories.
Short-term (48 hours to 90 days)
Used for fast results — most commonly real estate transactions and initial screening. Closed-house conditions required for any short-term test under 96 hours. Results reflect the specific time window of the test; radon levels fluctuate with weather, season, and home operation, so a short-term test may not match annual average exposure.
Long-term (90 days to a full year)
Used for annual or seasonal average measurement. Alpha-track detectors are the typical long-term device. Closed-house conditions are not required for long-term tests because the longer averaging period smooths out short-term variation. Long-term results are the most representative measure of actual radon exposure.
Where to place the test
Placement matters as much as protocol.
The lowest occupied level rule
If the basement is used as living space (bedroom, family room, finished entertainment area), test in the basement. If the basement is unfinished and only used for storage and utilities, test on the first floor. The principle is to measure radon where people actually breathe it.
Distance from walls and floors
At least 20 inches above the floor, at least 3 feet from exterior walls, and at least 1 foot from interior walls. The kit should sit on a stable surface that doesn’t get bumped during the test.
Avoid these locations
- Within 4 feet of a window or exterior door (drafts skew results)
- In direct sunlight or near a heat source (temperature affects some kit chemistries)
- In high-humidity areas like bathrooms or near washing machines
- Near fans, air vents, or HVAC supply registers (forced airflow affects radon concentration around the kit)
- Below countertops with hidden airflow or in closets with limited air exchange (the goal is representative room air)
Multi-point testing
For a comprehensive picture, EPA guidance suggests testing the lowest occupied level first. If that result is above 4 pCi/L, retest with a second short-term test OR a long-term test for confirmation. For larger homes or those with multiple living levels, additional tests on other floors can characterize the full exposure picture, but the lowest occupied level is the primary measurement point for mitigation decisions.
Timing the test
Several timing considerations matter.
Season
Radon levels are typically highest in winter, when homes are closed up and ground-level pressure differentials are largest. Front Range Colorado winter testing often produces higher readings than summer testing. The protocol works in any season, but be aware that a summer reading near the 4 pCi/L threshold may be higher in winter.
Weather
EPA guidance recommends avoiding tests during periods of unusually high winds or severe storms, which can cause pressure swings that affect short-term readings. Mild weather periods produce more representative results.
Real estate transaction timing
For a real estate transaction, the test typically starts within the first few days of the inspection contingency. A 48-hour test plus 24 to 48 hours of lab analysis time fits inside most contingency windows.
Sending the kit to the lab
Passive kits (charcoal canister, alpha-track) ship to an accredited lab after exposure. Critical steps:
- Note the exact start and end times of the exposure period on the chain-of-custody form
- Seal the device immediately when the exposure period ends
- Ship within 24 hours of test completion (sooner for charcoal canisters, which have a short shelf life after exposure)
- Confirm the lab is AARST-NRPP or NRSB accredited
Results typically come back within 3 to 7 business days of lab receipt. Some labs offer 24-hour rush turnaround for an additional fee.
Interpreting results
EPA action level guidance:
- Below 2 pCi/L: Low; retest every 2 years
- 2 to 4 pCi/L: Consider mitigation; long-term retest before deciding
- 4 to 10 pCi/L: EPA recommends mitigation
- 10+ pCi/L: Mitigation as soon as practical
No level of radon exposure is considered fully safe — the relationship between exposure and lung-cancer risk is linear. The EPA attributes about 21,000 U.S. lung-cancer deaths per year to radon exposure, making it the leading cause of lung cancer among non-smokers.
Colorado and the Front Range protocol
Colorado is predominantly EPA Zone 1 (predicted indoor screening levels above 4 pCi/L). Front Range homes — Denver, Douglas, Elbert, Arapahoe, Jefferson, El Paso, Adams, Boulder, Broomfield counties — frequently test above 4 pCi/L in basements. CDPHE recommends every Colorado home be tested for radon, and many county health departments distribute free or low-cost test kits seasonally. The 48-hour closed-house protocol applies the same way in Colorado as elsewhere. Our broader guide to radon testing for Front Range homeowners walks through the regional picture in more depth.
Common protocol mistakes
- Opening windows during the test. Single biggest cause of invalid results. The protocol is unforgiving on this.
- Placing the kit in the wrong location. Attics, garages, crawl spaces, and bathrooms produce unrepresentative readings.
- Running fireplaces during the test. Fireplaces and wood stoves create negative pressure that draws additional radon; either lock them out or postpone the test.
- Bumping or moving the kit. Charcoal canisters are sensitive to handling; alpha-tracks and CRMs less so but still require stable placement.
- Delaying shipment. Charcoal canisters continue absorbing radon and re-emitting it after the planned exposure ends; ship within 24 hours of test completion.
Post-mitigation verification
If a mitigation system is installed in response to a high test result, run a follow-up test at least 24 hours after the system is activated. The follow-up test should use the same closed-house protocol and should be placed in the same location as the original test for direct comparison. EPA guidance recommends a long-term test or repeat short-term test annually to verify ongoing system performance. For more on what comes next, our guide on radon testing as part of home inspections covers the inspection-side considerations.
Seasonal considerations and when to retest
Radon levels vary across seasons in most climates and especially in Colorado, where temperature swings drive significant pressure differentials between heated interior spaces and cold soil gas. Winter readings are typically the highest because closed homes and stack-effect dynamics draw soil gas inward; summer readings tend to be lower as homes are more ventilated. The EPA recommends retesting every two years for low initial results and confirming any single short-term result above 4 pCi/L with either a second short-term test or a long-term test. For homes with mitigation systems installed, an annual short-term verification test confirms ongoing system performance. Homeowners who finish a basement, add an addition, or make significant changes to the home’s HVAC system should retest because the airflow dynamics may have changed substantially.
What to do if the result is borderline
Test results between 2 and 4 pCi/L sit in a gray zone — below the EPA action level but above the WHO 2.7 pCi/L reference. EPA guidance recommends considering mitigation in this range, with a long-term retest before committing. The decision-making factors include: whether children spend significant time in the lowest occupied level, whether the test was conducted in winter (lower-bound) or summer (likely higher in winter), the homeowner’s risk tolerance, and the cost of mitigation. A long-term alpha-track test over 3 to 12 months produces the most informative read for borderline cases because it smooths out seasonal variation and gives a closer estimate of actual annual average exposure. Front Range homeowners with borderline summer readings often find winter retesting pushes them clearly above the action level.
When to call a professional
DIY testing is the right entry point for most homeowners. Hire a certified radon measurement professional if you need a defensible test result for a real estate transaction (some jurisdictions require it), if your DIY test result was above 4 pCi/L and you want a verified second measurement before mitigating, or if you’re preparing to install or verify a mitigation system. CDPHE’s certified radon professional list is the starting point for Colorado.
References
- A Citizen’s Guide to Radon — U.S. Environmental Protection Agency
- EPA Radon Measurement Method Protocols — U.S. EPA
- Colorado Radon Program — Colorado Department of Public Health and Environment
- CDC Radon Information — Centers for Disease Control and Prevention
- Radon and Lung Health — American Lung Association
Front Range homeowners with elevated test results who need a connection to a certified radon mitigation professional can reach out through our contact page.
Radon test kits we recommend
Short-term charcoal kits are the cheapest way to get a first reading; mail the canister to the lab and results come back in days.
| Product | Why | Buy |
|---|---|---|
 AirChek Charcoal Radon Test Kit | Lab fee usually included; EPA short-term protocol. | Amazon — $21.95 |
 First Alert RD1 Radon Test Kit | Widely stocked; 48-hour charcoal test. | Amazon — $17.95 |
| Health Metric Radon Test Kit | Two-pack option for closed-house testing. | Amazon — $17.95 |