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What can affect a radon test?

By Radon411 Editorial TeamUpdated How we research

Short answer

Open windows, outside air, storms and high winds, the wrong spot, and a disturbed device can all skew a radon test. Closed-house conditions, correct placement in the lowest lived-in level and leaving the device alone for the whole test guard against each one, and a followup test confirms a high result.

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

Outside air dilutes the reading. Keep windows and outside doors closed except for normal entry and exit, and don't run fans or machines that bring in outside air; for a test of 2 to 4 days, close up at least 12 hours ahead. Once windows are closed again, radon most often returns to previous values within about 12 hours.

Weather and time matter: don't run a short test of 2 or 3 days during unusually severe storms or high winds, and because radon varies from day to day and season to season, a gives a better picture of the year-round average.

Placement and handling matter too. Use the lowest lived-in level, at least 20 inches up, away from drafts, high heat, high humidity and exterior walls, and leave the device alone; interference invalidates the result.

What to do next: run the test under for at least 48 contiguous hours, and read the result with the interpreter.

The main things that skew a radon test

A radon test measures the air under set conditions, and anything that changes those conditions changes the number. Most of what can go wrong falls into a few groups, and the test rules guard against each one.

What can affect a radon test
FactorHow it affects the resultThe rule that guards against it
Outside airOpen windows and outside air dilute radonKeep windows and outside doors closed; no fans bringing in outside air
WeatherStorms and high winds can skew a short testNo short test of 2 or 3 days in unusually severe storms or high winds
Season and timingRadon varies day to day and season to seasonUse a long-term test for the year-round average
PlacementDrafts, heat, humidity and the wrong room skew the readingLowest lived-in level, 20 inches up, a regularly used room
HandlingA moved or disturbed device gives an invalid resultLeave it alone; reseal and send it promptly

Ventilation and outside air

Opening windows, doors and vents on the lower floors increases ventilation, and once they are closed again, radon most often returns to previous values within about 12 hours. That is why a of 2 to 4 days starts with at least 12 hours before and keeps them throughout. Heating and cooling that recirculate indoor air are fine.

Weather and time

Radon concentrations vary within a building from day to day and hour to hour, and radon levels tend to vary from season to season, so a short-term test is less likely than a to tell you your year-round average. Don't run a short-term test of just 2 or 3 days during unusually severe storms or periods of unusually high winds.

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, long-term and continuous tests: how long each runs and what it tells you.

Placement

Place the device in the lowest lived-in level, at least 20 inches above the floor, away from drafts, high heat, high humidity and exterior walls, in a room used regularly but not the kitchen or bathroom. Do not test in a closet, stairway, hallway or crawl space, or in an enclosed area of high humidity or high air velocity, such as a laundry or furnace room.

Handling and interference

Leave the kit in place for as long as the package says, then reseal it and send it to the lab right away. Interference with the test or with closed-house conditions will invalidate the result, and it is illegal in some states. Improper testing may yield inaccurate results and require another test.

Ways to prevent or detect interference
MethodWhat it catches
A device that frequently records radon levelsUnusual swings during the test
A motion detectorWhether the device was moved or conditions changed
Devices with anti-interference featuresSome passive and active devices resist disturbance better

See can a radon test be manipulated.

Questions about what affects a radon test

Can a radon test be wrong?

A test run outside the rules can misstate the level, and even a good short-term test is a snapshot. If a first short-term result is high, take a followup test to be sure.

Does rain affect a radon test?

Don't run a short test of 2 or 3 days during unusually severe storms or periods of unusually high winds.

Does running the HVAC affect a radon test?

Heating and cooling that recirculate air may run normally. Fans or machines that bring in outside air should not.

How do I prepare for a radon test?

Close windows and outside doors at least 12 hours ahead, pick a regularly used room on the lowest lived-in level, and record the start time.

Does the season affect radon test results?

Radon varies from season to season, so a long-term test gives a better picture of the year-round average.

Does humidity affect a radon test?

Keep the device away from high humidity, and do not test in enclosed humid rooms such as bathrooms, kitchens and laundry rooms.

Do I need to follow the kit's instructions exactly?

Yes. Leave the kit in place for as long as the package says, then reseal it and send it to the lab right away.

What if someone moved my test device?

Disturbing the device invalidates the result, so run the test again.

Test

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