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Owning and renting4 min read

When are radon levels highest?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon levels are usually highest in the colder months, when homes are closed up and heated and radon is most likely to be trapped and build up, and on the lowest floors, closest to the ground. Levels also rise and fall from day to day and hour to hour with weather and ventilation.

Next step: check the rules where you rent or let. Radon rules for rentals by state

Heating drives part of it: warm air rising out of the upper house is replaced by air drawn in low down, some of it from the soil. Rain or snow, barometric pressure and open or closed windows move the level too, and once windows are closed it most often returns within about 12 hours.

That variation is why a is less likely than a , which stays more than 90 days, to show the year-round average, and why testing in winter, with the heat running day and night, is more likely to give an accurate result.

What to do next: test in the lowest lived-in level with the house closed up, ideally during the heating season, and confirm any high result with a follow-up test.

The season: the colder months

Radon tends to build up most in the colder months, when homes and buildings are closed up and radon is most likely to be trapped. Heating adds to it: warm air rising and escaping through the upper part of the house is replaced by air drawn in low down, and some of that make-up air comes from the soil. Homes that are closed up or heated for much of the year can end up with higher levels.

What raises radon in the heating season
FactorWhy it matters
Closed windows and doorsRadon is trapped inside instead of diluting with outdoor air
Heating (stack effect)Warm air escapes upstairs and make-up air, partly from the soil, enters low in the house; taller homes have a greater stack effect
Appliances and exhaust fansRemoving indoor air can drive soil gas in as make-up air enters the lower part of the house
WindStrong winds over the house push and pull air in and out

From day to day and hour to hour

Within a single building, radon varies from day to day and even from hour to hour. Rain or snow, barometric pressure and other influences push the level up and down from month to month and day to day.

Short-term influences, and how testing handles them
InfluenceHow testing allows for it
Storms and high windsShort-term tests of under four days are not run during severe storms or periods of high winds
Barometric pressureProfessional testers record it to identify weather that may have affected a test
Open doors and windowsTesters record the temperature to see whether they were opened; levels most often recover within about 12 hours of closing up
Seasonal swingsTwo short-term tests one after the other give a better picture of the seasonal average than two at once; a test of at least three months estimates the annual mean

Where in the house radon is highest

Radon is usually highest in basements, cellars and living spaces in contact with the ground; basements and first floors typically carry the most because they are closest to it. Upper floors can still have considerable radon.

Cross-section drawing of a house with a basement and a crawl space, numbered at the seven ways EPA lists for soil gas to get in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply.
Read this figure as text
House in cross-section showing where soil gas entersGround levelSoil and rockBasementSolid floorCrawl spaceSuspended floor1234567
  1. Cracks in solid floors
  2. Construction joints
  3. Cracks in walls
  4. Gaps in suspended floors
  5. Gaps around service pipes
  6. Cavities inside walls
  7. The water supply
Radon enters from the ground, which is why the lowest floors usually have the highest levels.

That is where a test goes: the lowest lived-in level, the basement if it is used often, otherwise the first floor. If you start living on a lower floor, retest there. More on this is in where is radon found in homes.

What this means for your test result

Because levels peak and dip, one short test is a snapshot. A is less likely than a to show the year-round average, and a long-term test, more than 90 days, is more likely to show the year-round level.

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 and long-term tests, and how long each stays in the home.

A winter result that comes in high should be confirmed with a follow-up test before deciding on a fix, as any high result should. Lasting changes to the level are covered in can radon levels change over time, and sudden jumps in what causes radon spikes.

Questions about when radon is highest

What time of year is worse for radon?

The colder months, when homes are closed and radon is most likely to be trapped and build up.

In what month is radon highest?

No single month is named. The heating season, when homes are closed up and the heat runs, is when radon is most likely to build up.

Is radon higher in winter or summer?

The colder months, when the house is closed and heated, are when radon is most likely to build up. Opening windows lets outdoor air dilute indoor radon, though the level most often returns within about 12 hours of closing up.

Is radon higher at night?

Radon varies from hour to hour within a building. A short-term test runs for at least 48 hours, so its result spans both day and night.

Should I test for radon in winter?

Winter is a good time: testing while the heat runs day and night is more likely to give an accurate result.

Is radon higher in the basement?

Usually. Radon levels are typically highest in basements and other spaces in contact with the ground, and basements and first floors carry the most because they are closest to it.

Does rain affect radon levels?

Rain or snow, barometric pressure and other influences can make levels vary from month to month or day to day, which is one reason both short- and long-term tests exist.

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