Radon Levels in Winter: Why They Rise and What It Means for Your Test

If you ran a radon test over the winter and the number came back higher than you expected, you are not imagining a seasonal pattern. Radon levels genuinely tend to be highest in the coldest months, and for homes across the high-radon belt of the Upper Midwest and Mountain West, the winter reading is often the one that matters most. Understanding why turns a scary number into a useful one.

Yes, Radon Is Usually Higher in Winter

Indoor radon is not a fixed property of a house. It rises and falls with weather, soil conditions, and how the home is being lived in, and for most homes in heating climates the seasonal low is summer and the seasonal high is deep winter. Swings of two times or more between the warm and cold seasons are common, and some homes swing even harder.

That variability is exactly why a single short-term reading should be understood in context, and why the season you test in changes what the result means. A 6 pCi/L reading in January and a 6 pCi/L reading in July are not telling you the same thing about your annual exposure. For a sense of how to read any given number, our guide on safe versus dangerous radon levels lays out what the EPA action level of 4 pCi/L does and does not mean.

The Three Reasons Winter Drives Radon Up

Three things happen in winter, and they stack on top of each other.

The Stack Effect

This is the big one. Warm air inside a heated home is buoyant, so it rises and escapes through the upper floors, the attic, and gaps around the roofline. As that warm air leaves the top of the house, it has to be replaced, and the replacement air gets pulled in low: through the basement, the slab, the foundation walls, and any crack or penetration near the ground. Your house, in other words, behaves like a chimney all winter, and the suction at the bottom actively draws soil gas (radon included) up out of the ground and into the living space. The bigger the temperature difference between inside and outside, the stronger the stack effect, which is why the coldest weeks of the year often produce the highest readings.

Closed-House Conditions

In summer, windows open, doors swing, and outdoor air dilutes whatever radon enters. In winter, the house is sealed up for months to keep the heat in. Less fresh-air exchange means radon that enters has nowhere to go and accumulates instead of venting. This is the same reason radon tests require closed-house conditions: you are trying to measure the home at its least-ventilated, which winter delivers naturally.

Frozen and Snow-Capped Soil

Radon is produced continuously in the ground beneath every home. In milder weather it disperses harmlessly out the top of the soil into the open air. When the ground freezes or a snow blanket caps it, that escape route closes, and the gas takes the path of least resistance instead. Often that path is the warm, depressurized space under your foundation, where the stack effect is already pulling. Frozen soil does not create radon, but it routes more of it toward your home.

Why This Makes Winter the Best Time to Test

Here is the counterintuitive payoff: the fact that winter readings run high is a reason to test in winter, not a reason to distrust the result.

Radon mitigation is about protecting you at your worst case, because a system that holds the home under control during the high season holds it all year. If you test in summer, get a comfortable 2.5 pCi/L, and skip mitigation, you may be sitting at 6 or 7 pCi/L every January without knowing it. If you test in winter and get the 6, you have caught the home near its peak and can size a solution to match. When you can only run one short-term test, run it during heating season under closed-house conditions. A long-term alpha-track test of 90 days or more that spans the winter is more accurate still, because it averages the real range you live in. Our radon testing guide covers placement and timing, and the DIY versus professional testing guide explains which kits meet measurement standards.

How Much Do Levels Actually Swing?

There is no universal multiplier, because the swing depends on your climate, your foundation, and your home’s air-tightness. A loosely built home in a mild winter might barely move. A tight, modern home in a Minnesota or Colorado winter, with a deep freeze and a strong stack effect, can read well over double its summer baseline.

What this means in practice is simple. Do not average a high winter reading against an imagined low summer one in your head and decide you are fine. You do not get to skip winter, so winter exposure counts fully. If a winter short-term test lands at or above the action level, treat it as real and move to confirmation, exactly as our guide on what to do when a test comes back high describes.

Should You Mitigate Off a Winter Reading?

Yes. A high winter reading is the right number to act on.

Some homeowners are tempted to wait for spring, retest, see a lower number, and talk themselves out of mitigation. That is backwards. The winter figure is closer to your home’s true high, and a mitigation system designed against the high case keeps you safe in every season. Sizing a system off a flattering summer reading risks an install that cannot keep up when the stack effect turns back on in November. If anything, the winter reading is the friend of a good install.

After Mitigation, Do You Still See a Winter Bump?

A properly working system holds the home well under the action level year-round, but the physics that drive winter radon up do not disappear once a fan is installed. The soil-gas pressure your system fights is simply higher in winter, so a mitigated home may show a slightly higher retest in January than in July even though both are safely low. That is normal and expected.

What is not normal is a mitigated home reading at or above 4 pCi/L on a winter retest. If that happens, do not write it off as seasonal drift. It points to a system that was undersized for winter conditions or has developed a fault, and it warrants a contractor callback. Retesting every two years, ideally during heating season, is the discipline that catches a weakening system before it leaves you exposed through a whole winter. Our maintenance guide covers the year-by-year routine.

Winter does not make radon more dangerous on any given day. It makes it more visible, by concentrating the gas indoors right when you can measure it. Use that. Test during heating season, trust the high number, and size your solution to the worst case so the cold months take care of themselves.