1. The Short Answer
Yes, radon levels are typically higher in winter. A 2022 review published in the International Journal of Environmental Research and Public Health analyzed studies from across the globe and found that radon concentration peaks during cool winter months and drops to its lowest during warm summer months. The difference is significant: winter levels can be 2 to 5 times higher than summer levels in many homes.
But the picture is more complicated than "winter = high, summer = low." Recent research, including data from the Evict Radon National Study, suggests the seasonal pattern is becoming less predictable in modern homes. We will get into why in a moment.
2. The Stack Effect Explained
The primary reason radon levels rise in winter is something called the thermal stack effect. It works like this:
Your home is heated
In winter, you run your furnace or heat pump. The warm air inside your home is less dense than the cold air outside.
Warm air rises and escapes
Like a hot air balloon, warm air naturally rises. It escapes through the upper parts of your house: the attic, roof vents, upper windows, and any gaps in the building envelope.
Negative pressure forms at the bottom
As air escapes from the top, it creates a vacuum effect at the lower levels of your home. Your basement, crawl space, and ground floor develop negative air pressure relative to the soil outside.
Soil gas gets pulled in
That negative pressure literally sucks air from the soil into your home through every crack, gap, and opening in the foundation. And the air in the soil contains radon.
Think of your home as a chimney. Warm air goes up and out. Replacement air comes in from below. In winter, this cycle runs constantly because the temperature difference between inside and outside is large. In summer, the effect is weaker or may even reverse in air-conditioned homes.
3. Why Winter Drives Radon Up
The stack effect is the main driver, but several other winter factors pile on.
Closed-House Conditions
Nobody opens windows when it is 35 degrees outside. With windows and doors sealed shut for months, there is far less natural ventilation to dilute radon. The gas accumulates indoors instead of being flushed out.
Frozen Ground Surface
When the ground surface freezes (less common in Atlanta, but it happens), radon gas that would normally escape to the atmosphere through the soil gets trapped. It takes the path of least resistance, which is into your warm home through the foundation.
Stronger Temperature Differential
The bigger the gap between indoor and outdoor temperature, the stronger the stack effect. A 40-degree difference (70 inside, 30 outside) creates much more suction than a 5-degree difference in spring.
Heating System Effects
Furnaces and boilers consume indoor air for combustion, increasing the negative pressure in lower levels. Ductwork in crawl spaces and basements can also create localized pressure zones that pull in soil gas.
4. The Summer Surprise
Here is where it gets interesting. The traditional advice was simple: radon is highest in winter, lowest in summer. But newer data paints a more nuanced picture.
A large study of paired winter and summer radon tests in Canadian homes found that while 24.7% of buildings showed significantly higher radon in winter (as expected), 27.8% actually showed higher radon in summer. And 47.5% showed minimal difference between seasons.
Why would summer levels be high? Air conditioning. When you run your AC, you close up the house just like in winter. The HVAC system can also create pressure differentials. And in Atlanta, where AC runs from May through October, your home may be just as sealed up in July as it is in January.
The Bottom Line on Seasons
Do not assume your home is safe just because it is summer. While winter typically produces higher readings, modern homes with central air conditioning can have elevated radon year-round. The only way to know your home's actual radon levels is to test it.
5. What This Means in Atlanta
Atlanta has a climate that complicates the simple winter-high, summer-low model. Here is why.
Atlanta winters are relatively mild compared to the Midwest or Northeast. We get cold snaps, but extended freezes are rare. This means the winter stack effect is present but not as extreme as in Chicago or Minneapolis.
However, Atlanta summers are hot and humid. Air conditioning runs heavily from May through September, creating the same closed-house conditions that winter produces in colder climates. Many Atlanta homes may have relatively consistent radon levels across seasons, rather than the dramatic winter spike seen in northern states.
The Metro Atlanta area also sits on uranium-bearing granite bedrock, particularly in the northern counties. This geology means the soil produces significant radon regardless of season. Whether that radon accumulates inside your home depends on the pressure dynamics we have discussed, but the source is always there.
For Atlanta homeowners, the practical takeaway is this: do not wait for a "better" time to test. If you test in February and your levels are below 4 pCi/L, you are likely fine year-round. If your winter test shows elevated levels, you need mitigation regardless of season.
6. When Should You Test?
The EPA's guidance is clear: test your home regardless of season. The most important thing is to test, period. But here is a framework for thinking about timing.
Winter (December through February)
Testing now gives you the most conservative reading. If your home passes a winter test, you can be confident about year-round levels. This is also Radon Action Month season (January), so there is more awareness and often more availability from testing companies. Right now is an excellent time to test.
Spring (March through May)
A good time to test, especially before the real estate market heats up. Radon levels may be starting to decrease from winter peaks, giving you a moderate reading.
Summer (June through August)
Still a valid time to test. In Atlanta, with AC running, your house is closed up anyway. A summer test may read lower than winter, but if it shows levels above 4 pCi/L, you definitely have a problem because winter levels would likely be even higher.
Fall (September through November)
Shoulder season. Levels may be rising from summer lows. A good time to test before winter arrives, especially if you want to get mitigation installed before the holiday season.
The Best Time to Test
The best time to test for radon is now, whatever month that happens to be. If you have never tested your home, waiting for the "right" season means another season of exposure to a potential carcinogen. Test now. If levels are borderline, you can always retest in a different season for comparison.
