Winter in Michigan and Radon: Why Cold Months Mean Higher Risk

Michigan winters have a reputation that is well earned. From late October through March — and sometimes well into April — the state experiences extended periods of cold, snow, and conditions that drive residents indoors and keep them there. Windows stay shut. Doors stay closed. Homes are sealed against the elements in ways that simply do not happen during the warmer months. This seasonal reality has a direct and measurable effect on radon levels that every Michigan homeowner should understand, not because it changes what needs to be done about radon, but because it explains why the problem is more acute here than in more temperate parts of the country.

What Happens to Radon in a Sealed Home

During summer in Michigan, most homes breathe. Windows open on comfortable evenings. Screen doors let air move through. Natural ventilation — the casual, habitual exchange of indoor and outdoor air that happens when people live normally in a house during pleasant weather — dilutes the radon that enters through the foundation. It does not eliminate the problem, and a home with significantly elevated radon levels will still have a problem in summer. But the dilution effect of regular air exchange is real and measurable.

When temperatures drop and Michigan homes close up for winter, that dilution largely disappears. The same amount of radon entering through the same foundation pathways now has far less opportunity to escape or mix with fresh outdoor air. It accumulates. Concentrations that might register at a moderate level during a summer test can climb noticeably during the heating season as the home operates as a progressively more sealed system day after day, week after week, month after month.

The heating system itself contributes to this dynamic. Furnaces and other combustion appliances consume air from inside the home during operation, which depressurizes the interior slightly and increases the suction effect that draws soil air — and the radon it contains — upward through foundation entry points. A well-sealed, actively heated Michigan home in January is operating under conditions that are about as favorable for radon accumulation as any residential environment gets.

The Stack Effect and Michigan Winters

There is a specific physical phenomenon that amplifies radon accumulation in cold weather called the stack effect, sometimes called the chimney effect. It works like this: warm air inside a heated home is less dense than the cold air outside and naturally rises toward the upper levels of the home, where it eventually escapes through gaps around windows, electrical outlets, attic hatches, and other upper-level openings. As warm air rises and escapes from the top of the home, replacement air is drawn in from the bottom — from the soil through the foundation.

The greater the temperature difference between inside and outside, the stronger the stack effect. In a Michigan January, with outdoor temperatures well below freezing and indoor temperatures maintained at 68 or 70 degrees, the stack effect is at its most powerful. The home is actively drawing air from the ground beneath it with more force than at any other time of year. Radon comes with that air.

This is why radon levels measured in Michigan homes during winter are typically higher than levels measured in the same homes during summer. It is not that the soil is producing more radon in cold weather — the geological process is continuous and relatively constant year-round. It is that the home's behavior during winter creates optimal conditions for radon to enter and accumulate.

What This Means for Testing

The seasonal variation in radon levels raises an important practical question: does it matter what time of year you test? The answer is nuanced and worth understanding clearly.

For the most accurate picture of your home's worst-case radon exposure — the conditions your family faces during the months they spend the most time indoors — testing during the heating season produces the most representative result. A test conducted in January under proper closed-house conditions in a Michigan home reflects the radon environment that family will face for five or six months of the year.

A test conducted in July, with windows occasionally open and natural ventilation reducing accumulation, may produce a lower number that does not fully represent the year-round exposure profile. It is not an inaccurate result for the conditions under which it was conducted — it simply reflects a more favorable set of conditions than the home experiences during its most radon-prone months.

This does not mean you should wait until winter to test. Testing at any time of year gives you actionable information, and acting on that information is always better than waiting. If a summer test comes back above the action level, the winter levels are almost certainly higher — the case for mitigation is clear regardless of season. If a summer test comes back below the action level but in the range between 2 and 4 pCi/L, a follow-up test during the heating season can give you a more complete picture of your year-round exposure profile.

Long-term tests, which run for 90 days or more and capture conditions across multiple seasons, naturally account for some of this variation by averaging results over a period that includes both more and less favorable radon conditions. For homeowners who want the most comprehensive view of their annual radon exposure, a long-term test that spans part of the heating season is the most informative single measurement available.

The Michigan Winter as a Useful Frame

There is a way to think about Michigan's winters that reframes the radon conversation helpfully. Every year, Michigan homeowners spend five or six months in a home that is operating under conditions specifically favorable to radon accumulation. That is not a worst-case scenario — it is the annual reality of living in this climate. A family that has not tested their home has spent every one of those winters in an unknown radon environment, potentially at elevated levels, with no information and no protection.

Testing and mitigation do not change the winter. They change what the winter means for your family's health. A home with a functioning mitigation system maintains controlled radon levels regardless of whether it is January or July, regardless of how tightly sealed it is against the cold, and regardless of how strongly the stack effect is operating. The system accounts for the conditions that make Michigan winters a radon risk rather than leaving your family exposed to them.

Getting tested before the next heating season begins — or at any point during the year — means one less Michigan winter affecting the air your family breathes. That is a concrete and meaningful outcome, and it is available to every homeowner who chooses to act on it.

A Note on Existing Mitigation Systems

For homeowners who already have a mitigation system installed, Michigan winters are a useful reminder to check the manometer. The system should be maintaining the same pressure differential in January that it maintained in September. If the manometer reading looks different from what it normally shows, or if the liquid appears level when it should be displaced, winter is not a good time for the system to be underperforming. A quick visual check during the heating season — particularly at the start of the season when the home first closes up — is a small habit with real value.

The two-year retest we discussed in our post on what to expect after installation is also worth timing thoughtfully. Conducting your retest during the heating season gives you a result that reflects the conditions under which your system is working hardest — the most meaningful confirmation that it is performing as it should.

At All Michigan Mitigation, we serve West Michigan homeowners year-round, and we understand the specific radon dynamics that Michigan's climate creates. Whether you are testing for the first time heading into a heating season or checking on an existing system's performance, we are here to help.

Visit us at allmichiganmitigation.com to schedule your test or reach out with any questions about seasonal radon dynamics in your home.

All Michigan Mitigation is a West Michigan radon testing and mitigation company dedicated exclusively to helping homeowners understand and eliminate radon risk in their homes.