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Is a radon detector accurate?

By Radon411 Editorial TeamUpdated How we research

Short answer

Yes, a radon test from a qualified laboratory gives a reliable reading when it is used as its instructions say. Most wrong results come from how the test was run, not the device: improper testing may give an inaccurate result, and one short test only captures a few days of a level that changes.

Next step: pick the right test for your home. Which test to use

Reliable devices are sold in hardware stores and by mail, and qualified testers use them too. What makes the number trustworthy is a qualified laboratory, , the right room and a device nobody moves for at least 48 hours.

Results close to the are the least certain. If two average 4.1 , there is about a 50 percent chance the year-round average is somewhat below or above 4 pCi/L; a narrows that.

What to do next: use a device from a qualified laboratory, follow the testing checklist, and confirm any high short-term result with a second test.

What makes a radon reading reliable

Radon cannot be seen or smelled, so a device has to measure it, and reliable devices are easy to find: from qualified testers and companies, by phone or mail order, and in hardware stores and other retail outlets. Whether the number is right depends less on the device than on two other things: the laboratory behind it, and how the test was run.

What a reliable result depends on
FactorWhat it takes
The laboratoryA device from a qualified laboratory, used as its instructions say
Test lengthAt least 48 hours; some devices need longer
Closed houseWindows and outside doors shut from at least 12 hours before a 2 to 4 day test, and for the whole test
PlacementThe lowest level that could be used regularly, at least 20 inches above the floor, away from drafts, high heat, high humidity and exterior walls
Rooms to avoidKitchens, bathrooms, laundry rooms, furnace rooms, closets, stairways, hallways and crawl spaces
Hands offThe device is not disturbed at any time during the test
After the testThe device goes back to the laboratory promptly, with the start and stop times and the location filled in

Skip one of these and the reading can be off: improper testing may give an inaccurate result and mean another test. If a seller or tester cannot confirm every item was done, the answer is another test.

Passive and active detectors

Radon devices come in two families. Passive devices need no power: they sit in the home for a set time and are then sent to a laboratory for analysis. Active devices need power and record the level as they go, usually hourly, with an average for the test period.

Passive and active radon devices
AspectPassive devicesActive devices
ExamplesCharcoal canisters, alpha-track detectors, charcoal liquid scintillation devices, electret ion chamber detectorsContinuous radon monitors and continuous working level monitors
PowerNoneNeeded
ResultFrom the laboratory after the device is mailed backA record over time, usually hourly readings and an average
CostGenerally inexpensive, short-term and long-termMay cost more
InterferenceSome types resist disturbance better than othersThe record can reveal unusual swings; some models deter and detect interference
Table comparing a short-term radon test, a long-term radon test and a continuous radon monitor: how long each runs and what each is for.
Read this figure as text
TestHow long it runsWhat it is for
Short-term testStays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term testStays in the home for more than 90 days.It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitorLogs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.Logs a graph over time, so you can see trends and spot anomalies rather than a single number.
Short-term tests, long-term tests and continuous monitors, and how long each stays in the home.

Neither family is the accurate one and the other the rough one. The active devices with anti-interference features may give a more reliable result where interference is a risk, as in a home sale. Devices keep changing, so a state radon office can tell you which suit your needs before you test.

How certain is one result?

Even a perfect device only measures the days it was in the house. Radon varies within a building from day to day and even hour to hour, and from season to season, so a is less likely than a long-term one to show the year-round average. A measurement of at least three months is the better estimate of the annual level.

Getting more certainty from short-term tests
ApproachWhat it improves
Two tests at the same time, side by sideThe precision of the measurement
One test followed by anotherA better picture of the seasonal average
A long-term test, more than 90 daysThe best picture of the year-round level

Mistakes that throw a reading off

Most wrong results trace back to the test, not the detector. Opening up the house is the big one: fresh air dilutes the radon while windows are open, and levels most often return within about 12 hours of closing up, so a test run with windows open reads low.

Common testing errors and their effect
ErrorEffect on the reading
Windows or outside doors left openOutdoor air dilutes the radon, so the result is likely too low
Fans that bring in outside air runningThe same dilution; only a radon system's fan and small exhaust fans for short periods may run
Testing in a kitchen, bathroom or other humid or drafty roomOutside the places the guidance allows for a test device
Moving or disturbing the deviceCan invalidate the result, and is illegal in some states
A short test during a severe storm or high windsShort-term tests of under four days are not run in that weather
Mailing the device back lateThe laboratory needs it promptly, with times and location recorded

Professional testers in a sale guard against interference by recording barometric pressure and temperature during the test. Deliberate tampering is covered in how a radon test can be manipulated, and everyday influences in what can affect a radon test.

Standards and the people behind the test

Accuracy also rests on consistency. Measurements need to follow national protocols so they are consistent and reliable enough to make decisions on, and short-term tests done that way are useful when time is short, as in a home sale. Residential radon can be measured simply and cheaply with small passive detectors.

Checking who stands behind a result
CheckWhy it matters
A device from a qualified laboratoryPassive devices are analyzed by the laboratory, so its quality counts
A list from your stateYour state may list do-it-yourself devices from qualified laboratories
A tester's state license or credentialMany states license radon professionals; elsewhere, ask for a proficiency credential
A seller's earlier resultA buyer may ask for a new test if the checklist was not met, the test is more than about two years old, or the home was renovated since

Questions about radon detector accuracy

Are home radon test kits accurate?

Yes, when the kit comes from a qualified laboratory and the test follows its instructions and the closed-house rules. Most bad results come from how the test was run. See can I test for radon myself.

Are continuous radon monitors accurate?

They are active devices that measure and record the level as they run, which can reveal unusual swings during the test. Like any device, they need the full test conditions and at least 48 hours.

Why did two radon tests give different results?

Radon varies from day to day and season to season, so two tests on different days measure different air. Running two tests side by side at the same time is what improves precision.

Can a radon test be wrong?

Yes, if the test is done improperly: open windows, a bad location or a disturbed device can give an inaccurate result. If the checklist was not met, take another test.

Is a 4.1 pCi/L result really high?

It is right at the line, where a short-term result is least certain: there is about a 50 percent chance the year-round average is somewhat below or above 4 pCi/L. See is a radon level of 4.1 considered high.

Should I trust a seller's radon test?

Check it against the testing checklist. A buyer may ask for a new test if the checklist was not met, if the test is not recent, within about two years, or if the home has been renovated since.

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