Why Do Radon Levels Change?
We know that seeing two completely different radon test results can feel frustrating. This variation actually makes perfect scientific sense.
Our team at Iowa Radon Systems fields questions about shifting numbers almost every day. A buyer might see a low summer result on an old report and then get a high reading during a winter inspection.
We understand that this does not mean either test was wrong. Knowing exactly why these shifts happen helps you understand radon levels winter vs summer, whether you are booking radon testing or reading an old report.
We see these massive seasonal swings because radon gas naturally pulls from the soil under your home.
The exact amount entering your living space depends entirely on the pressure difference between the dirt and your house. Our state sits entirely in EPA Radon Zone 1, which means all 99 counties face a high risk.
Recent 2026 data shows the average indoor radon level in Iowa is 8.5 pCi/L, far above the national average of 1.3 pCi/L. We use this statistic to remind homeowners that local weather changes will heavily influence those already high baseline numbers.
The Stack Effect in an Iowa Winter
We call this upward movement the thermal stack effect, and stack effect radon intrusion dominates cold weather months. Many homeowners notice radon higher in winter due to these specific pressure changes. Warm air naturally rises. In winter, the air inside your home is significantly warmer than the frigid air outside.
Our heating systems cause that warm indoor air to rise and leak out through the attic, upper windows, and roof gaps. Replacement air has to come from somewhere to balance the pressure.
We find that several specific factors force this gas into your living space:
- Foundation vulnerabilities: The house pulls a huge volume of air in right at the bottom through cracks in the slab, around pipes, and through the sump pit.
- Temperature extremes: Our Iowa winters bring average January temperatures down around 14 degrees Fahrenheit in the northwest part of the state.
- Pressure vacuums: This extreme temperature gap creates a strong pressure differential, often measured in Pascals, acting like a giant vacuum on the soil beneath your foundation.
- Appliance interference: We consistently observe that exhaust fans, clothes dryers, and fuel-burning appliances add even more suction to the pull.
Sealed-up homes
We spend winter with the windows tightly shut to keep the heat inside. This lack of fresh air circulation means less outdoor air comes in to dilute the trapped radon gas.
Our technicians note that whatever radon gets inside simply builds up within the living space. Homes without a mitigation system can quickly exceed the Environmental Protection Agency action level of 4.0 pCi/L during these months. See what radon level is safe for how to read your number.
Frozen ground
We also have to account for changes in the soil itself. When the top layer of dirt freezes solid, it creates an impermeable ice cap over the yard.
Our field observations confirm that soil gas cannot escape into the open air as it normally would. The radon gas moves sideways and finds the path of least resistance. The easiest path is often the warm, unfrozen soil pressing directly against your basement walls and floor.
What this means
A winter test is often closer to your absolute worst-case scenario. We warn homeowners that a summer test might show a lower number than the home’s true annual average.
Summer Readings
Summer weather completely changes the pressure dynamics inside a building. We see weaker stack effects in July because indoor and outdoor temperatures stay much closer together.
People open their windows more often, and the ground thaws out completely. Our testing data shows that many homes do read much lower during the warm summer months. That rule is definitely not universal, though.
We know that specific summer conditions can actually spike your readings:
- Central AC usage: Central air conditioning systems can create minor negative pressure that pulls in soil gas. Our technicians often spot this during intense heat waves.
- Heavy thunderstorms: Heavy summer thunderstorms saturate the topsoil with rainwater. The surface water blocks the gas from naturally escaping the yard.
- Forced concentration: We find that this forces concentrated radon directly into the basement.
Basement vs. Upper Floors
Radon concentration usually hits its peak on the lowest level of the house. We always test the lowest livable level because it sits closest to the radioactive source in the dirt.
The winter stack effect can carry some of that gas upward to the bedrooms. Upper floors typically still read lower than the basement. You should test the lowest floor you currently use or could potentially finish as living space.
We provided a quick breakdown of how these seasonal patterns generally affect your lowest level.
| Season | Typical pattern | The science behind the shift |
|---|---|---|
| Winter | Often much higher | Strong stack effect, closed windows, frozen topsoil cap |
| Spring | Highly variable | Ground thaw and heavy rain change the soil moisture levels |
| Summer | Often noticeably lower | Weaker stack effect, open windows, more natural ventilation |
| Fall | Steadily rising | Heating season begins, people close their windows |
How to Read a Seasonal Result
A professional test provides a snapshot of your air quality at a specific moment in time. We help clients interpret these numbers based on the time of year they ran the equipment. You need to contextualize your results to understand your true risk exposure.
- High winter result: We advise taking this number very seriously. It reflects the real, concentrated conditions your family lives with for several months each year.
- Low summer result: A summer reading near 2.0 to 4.0 pCi/L often spikes higher during a January freeze. Our experts suggest you do not treat this low number as the final word.
- Two results that disagree: The long-term average matters most. You can compare exact test types in our comprehensive guide to short-term vs. long-term tests.
- Real estate test: Our standard protocol requires strict closed-house conditions since you only get one short window to test. Following those rules ensures the 48-hour continuous monitor captures accurate data.
When Should You Test?
We recommend testing any time of year as long as you follow the required closed-house conditions. You must keep all windows and outside doors shut for 12 hours before the equipment starts and for the entire duration of the test.
- Short-term testing: Our professional continuous radon monitors can give you a highly accurate, quick answer in just 48 hours. This provides immediate clarity for real estate transactions.
- Long-term testing: We often deploy alpha track tests for 90 days or more to capture the full seasonal swing. This approach gives you the most complete picture of your annual exposure.
Homeowners frequently wonder about the best time to test for radon. We consider fall a highly strategic time to test an Iowa property, as it bridges the extreme radon levels winter vs summer.
The heating season is just starting, so the numbers represent realistic winter conditions.
Our installation teams have plenty of time to put in a mitigation system before the coldest, highest-risk months arrive. A mitigation system runs all year, and its active suction easily overpowers the winter stack effect.
We want to help you protect your home from a gas that causes roughly 400 lung cancer deaths in Iowa annually. Contact a state-certified professional today to schedule your continuous monitor test and secure your peace of mind.