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Are Radon Levels Higher in Winter? When to Test

Updated November 1, 2026 · General information, not medical advice

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Why winter readings run higher

• Pressure difference. Virginia says the larger difference between outdoor and indoor pressure in winter is likely to lead to more radon entering a house than in summer.
• Less ventilation. Virginia says homes are likely to be less ventilated in winter. University of Missouri Extension says energy-efficient or poorly ventilated homes are more likely to have higher radon levels.
• The test needs a closed house. Connecticut says to keep windows and outside doors closed as much as possible during the test. Missouri says winter is a good time for that reason.

When to test

Connecticut says the best time is the colder season, November through March, when your house is closed up. Virginia says to avoid taking your first test in the summer if possible.

Steps for a winter test

1. Close up the house. EPA says that for a 2 to 3 day short-term test you should close windows and outside doors at least 12 hours before you begin.
2. Place the kit correctly. Virginia says to put it in the lowest occupied level of the home, at least 20 inches off the ground, well away from fans and any source of high heat or humidity, and to run it at least 48 hours.
3. Keep the house closed for the whole test. Virginia says closed-house conditions apply for the duration of the test.
4. Read the result with the season in mind. EPA says a short-term test is less likely than a long-term test to tell you your year-round average. Enter your number in our result checker.
5. Follow up if it is 4 pCi/L or higher. EPA says to take a long-term test for a better picture of your year-round average, or a second short-term test if you need results quickly. EPA says that if the average of two short-term tests is 4 pCi/L or higher, consider fixing your home.

EPA gives one example of the uncertainty: if the average of your two short-term results is 4.1 pCi/L, there is about a 50% chance your year-round average is somewhat below 4 pCi/L.

Common questions

Are radon levels higher in winter?

Usually, yes. Virginia's health department says most homes test at their highest radon levels in the winter and their lowest in the summer. Connecticut's health department says radon levels vary seasonally and tend to be higher in the winter months.

Why is radon higher in winter?

Virginia gives two reasons: the larger difference between outdoor and indoor pressure in winter is likely to lead to more radon entering the house, and homes are likely to be less ventilated in winter.

Is winter a good time to test for radon?

Yes. Virginia says EPA recommends testing in the winter, and Connecticut says the best time is the colder season, November through March, when the house is closed up. University of Missouri Extension adds that doors and windows need to be kept closed as much as possible during a test, which is easier in winter.

Should I avoid testing in summer?

Virginia says to avoid taking your first test in the summer if possible, because winter readings are typically higher. If you already tested in summer, treat a low result with care and think about testing again in a colder month. That second suggestion is ours.

Does a winter test show my yearly average?

Not exactly. EPA says radon levels tend to vary from day to day and season to season, so a short-term test is less likely than a long-term test to tell you your year-round average. A year-long test accounts for seasonal variation (Virginia).

What if my winter result is 4 pCi/L or higher?

EPA says to follow up with either a long-term test or a second short-term test. If the first short-term result is more than twice the 4 pCi/L action level, EPA says to take a second short-term test immediately.

Related guides

• Radon test kits: how to choose and use one
• What is radon?
• Radon mitigation systems