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Owning and renting5 min read

Can radon levels change over time?

By Radon411 Editorial TeamUpdated How we research

Short answer

Yes. Radon levels swing from day to day and season to season with weather, heating and ventilation, and they can change for good when the house changes: after a renovation, settling, new heating or cooling, or a radon fan that stops working. That is why one old test cannot stand in for a current one.

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

Short-term swings come from rain or snow, barometric pressure, the season and how the house is opened up or closed. Lasting changes come from the building itself: renovations can change the level in any home, normal settling opens new entry routes, and heating or ventilation changes might affect it.

Moving living space downstairs changes your exposure too, since floors in contact with the ground usually carry more radon.

What to do next: retest after any of these changes, and use a , more than 90 days, for the year-round picture.

Short swings and lasting changes

Radon levels change in two ways. They swing constantly, from day to day, season to season and even hour to hour within one building. And they can shift for good when the house changes: a renovation, settling, a new heating system, or a radon system that stops working. A single test captures neither fully.

Two kinds of change
KindWhat drives itWhat to do about it
Short-term swingsRain or snow, barometric pressure, the season, heating, and how much the house is opened upUse a long-term test, or read a short-term result with its limits in mind
Lasting changesRenovations, settling, heating and cooling changes, a failing radon fan, and starting to live on a lower floorRetest after the change

Why radon goes up and down

The level in a house depends on the soil under and around it and how easily radon gets in, and on how fast indoor air trades places with outdoor air, which depends on the building, its air-tightness and how the people in it ventilate. Weather and seasons move all of that.

Everyday influences on the level
InfluenceEffect
Season and heatingHomes closed up or heated for much of the year can have higher levels; colder months are when radon is most likely to be trapped and build up
WeatherRain or snow and barometric pressure make levels vary from month to month and day to day
Open windowsLevels drop while the house is open and most often return within about 12 hours of closing it
Appliances and exhaust fansRemoving indoor air can drive soil gas in as make-up air enters low in the house
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 catch a slice of these swings; long-term tests, more than 90 days, are more likely to show the year-round average.

When during the year the level tends to peak is covered in when are radon levels highest.

Changes to the house that change the level

Changes that call for a new test
ChangeWhy it matters
A major renovation, such as finishing a basementRenovations can change the level of radon in any home; test before and after
Normal settlingIt opens new entry routes and reopens old ones
Heating, ventilation or air-conditioning changesSuch changes might affect radon levels
Moving living space to a lower floorLower floors, nearer the ground, usually have more radon; retest on that level
A remodel in a home with a radon systemRetest to make sure the work did not reduce the system's effectiveness

Neighbors are no guide to any of this: levels differ considerably even between buildings next door to each other.

When a radon system's level changes

A working radon system holds the level down, but it depends on its fan running all the time. If the fan stops, the protection stops with it. Radon levels can change, so homes with a system are retested at least every two years, and the warning device shows in between whether the system is running.

Timeline after a radon mitigation system is installed: the fan runs continuously, test no sooner than 24 hours and within 30 days, then retest at least every two years.
Read this figure as text

  1. Day of installation

    The system goes in

    Keep the fan running at all times: it must run continuously for the system to work correctly.

  2. No sooner than 24 hours

    Earliest post-mitigation test

    Test no sooner than 24 hours after your system is in operation with the fan on, if it has one.

  3. Within 30 days

    Post-mitigation test done

    A two- to seven-day measurement is recommended, and EPA recommends an independent follow-up radon measurement as well as any test the contractor runs.

  4. At least every two years

    Retest

    Retest your home at least every two years to be sure radon levels remain low.

Test conditions: Windows and doors must be closed 12 hours before and during the test, except for normal entry and exit. Not drawn to scale.

After a remodel. After you remodel, retest in the lowest lived-in area to make sure the construction did not reduce the effectiveness of the radon reduction system.

Testing after a radon system is installed: the first check, retests at least every two years, and a retest after any remodel.

See how often should I test for the full schedule.

How to keep track of your level

Ways to follow radon over time
ToolWhat it tells you
A long-term test, more than 90 daysA result more likely to reflect the year-round average than any short test
A continuous monitorHourly readings and an average for the test period; its record can show unusual swings
Repeat short-term testsA fresh reading after a change to the house; a high result is followed up before any decision
A radon system's warning deviceWhether the system's fan is still running; it does not measure radon itself

Keep each result with the date, the floor and the kind of test. A record like that shows whether a change in the house moved the level, and gives a buyer what they will ask for if you sell.

Questions about changing radon levels

Do radon levels change over time?

Yes. They swing with weather and season and can shift for good when the house changes, which is why a retest after renovations or other changes is recommended.

Can radon levels go up after a renovation?

They can. Major renovations can change the level of radon in any home, so test before the work and again after it is finished.

Why did my second radon test come out different?

Levels vary from day to day and season to season, and a short-term test catches only a slice of that. The closer two short-term results average to 4 pCi/L, the less certain the year-round level is; a long-term test gives the better picture.

Can a new heating system change radon levels?

It might. Changes to heating, ventilation and air conditioning might affect radon levels, so retest after them.

Do radon levels change between floors?

Yes. Levels are usually highest on floors in contact with the ground, though considerable radon can be found above the ground floor too.

Should I retest if I start using my basement?

Yes. If you begin living on a lower level, such as a basement, retest on that level.

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