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
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.
At a glance · 1 min read
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.
| Factor | Why it matters |
|---|---|
| Closed windows and doors | Radon 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 fans | Removing indoor air can drive soil gas in as make-up air enters the lower part of the house |
| Wind | Strong winds over the house push and pull air in and out |
At a glance · 1 min read
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.
| Influence | How testing allows for it |
|---|---|
| Storms and high winds | Short-term tests of under four days are not run during severe storms or periods of high winds |
| Barometric pressure | Professional testers record it to identify weather that may have affected a test |
| Open doors and windows | Testers record the temperature to see whether they were opened; levels most often recover within about 12 hours of closing up |
| Seasonal swings | Two 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 |
In a picture · 1 min read
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.
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.
In a picture · 1 min read
Because levels peak and dip, one short test is a snapshot. A
| Test | How long it runs | What it is for |
|---|---|---|
| Short-term test | Stays 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 test | Stays 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 monitor | Logs 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. |
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 · 1 min read
The colder months, when homes are closed and radon is most likely to be trapped and build up.
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.
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.
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.
Winter is a good time: testing while the heat runs day and night is more likely to give an accurate result.
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.
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.
Living with a home over the years: retests, a system to look after, and new builds.
Living with a radon mitigation system, from the manometer to maintenance and retesting.
Read the guideHow radon-resistant construction works, whether it is enough, and how to make it active.
Read the guideWhy a new home still needs a radon test, when to test after move-in, and what to do if the result is elevated.
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