The short answer
Radon levels change because the gas is constantly seeping up from the soil, and the amount that enters your home depends on conditions that never hold still. The stack effect pulls more soil gas in when warm indoor air rises in cold weather, falling barometric pressure ahead of storms lets more radon up, and a closed, heated house in winter traps it instead of diluting it. HVAC operation, open windows, and even rain or frozen ground all shift the reading. That is why closed-house conditions and a longer-term test give a far more reliable picture than one short snapshot, and why a result near the 4.0 pCi/L action level should be confirmed.
Table of contents
- 1. Why radon is never a fixed number
- 2. The stack effect: why winter pulls radon up
- 3. Barometric pressure and passing weather
- 4. Seasonal swings: winter versus summer
- 5. How your HVAC system changes the reading
- 6. Open versus closed windows
- 7. Rain and frozen ground
- 8. Why this is why testing matters
- 9. Frequently asked questions
1. Why radon is never a fixed number
Radon is a radioactive gas produced as uranium in soil and rock breaks down. It is always being made underground, and it is always trying to move toward lower pressure and open air. Your home sits on top of that supply, so the real question is never whether radon is present in the soil but how much of it finds its way inside on a given day. That amount is set by a tug-of-war between the soil and the house, and the forces on both sides keep changing.
Because of that, two honest tests of the same home can land on different numbers. A reading is a measurement of conditions during the test window, not a permanent label stamped on the building. Understanding what pushes the number up or down makes the difference between trusting one snapshot and reading the fuller picture.
2. The stack effect: why winter pulls radon up
The single biggest driver of indoor radon swings is the stack effect. Warm air rises. In cold weather your heated indoor air is much warmer than the air outside, so it floats upward and leaks out through the upper parts of the house, around attic hatches, recessed lights, and the top of the structure. As that warm air escapes high up, it leaves a slight vacuum lower down, near the foundation.
That lower-level vacuum acts like a straw. The house is now at slightly lower pressure than the soil beneath it, so soil gas, radon included, gets actively pulled in through cracks in the slab, gaps around pipes, sump openings, and the joint where the wall meets the floor. The bigger the temperature difference between inside and outside, the stronger the pull, which is why the coldest months often produce the highest readings.
Think of the house as a chimney in winter. Warm air going out the top means something has to come in at the bottom, and part of what comes in at the bottom is soil gas carrying radon. The same building can read low in mild weather and high in a cold snap.
3. Barometric pressure and passing weather
Weather changes the pressure balance between the soil and your home from day to day. When outdoor barometric pressure drops, which often happens as a storm front moves in, the relatively higher pressure trapped in the soil pushes more gas upward toward the lower-pressure air inside and above the ground. The result is a temporary climb in indoor radon during and just before low-pressure weather.
Wind plays a role too. Strong wind can pressurize one side of a house and depressurize another, nudging soil gas in or out depending on the direction and the home. None of these effects are dramatic on their own, but stacked together they explain why a reading taken on a calm, high-pressure day can differ from one taken as the barometer falls. It is also why a single 48-hour test happening to land during a passing front may not represent a typical week.
4. Seasonal swings: winter versus summer
Put the stack effect and household habits together and you get a clear seasonal pattern in many homes: radon tends to read higher in the heating season and lower in mild weather. In winter the temperature difference driving the stack effect is large, the house is sealed up against the cold, and the heat is running. All three push the same direction, toward more radon coming in and less of it being diluted or vented out.
Summer is more mixed. Windows may be open in mild stretches, lowering readings, but a home that runs central air conditioning all day can behave more like a closed winter house in some respects. The takeaway is not that summer is always safe, it is that the season you happen to test in can move your number meaningfully. We cover this contrast in depth in our guide to radon levels in winter versus summer.
5. How your HVAC system changes the reading
Heating and cooling equipment moves a lot of air, and anything that moves air can change how much radon enters and where it goes. Forced-air systems with leaky return ducts in a basement or crawl space can depressurize the lower level slightly, which pulls in more soil gas, the same mechanism as the stack effect. Exhaust appliances like clothes dryers, range hoods, and bathroom fans push air out of the house and can deepen that vacuum if make-up air is coming up through the foundation.
On the other hand, systems that bring in or circulate outdoor air can dilute radon and lower a reading while they run. The practical point for testing is that the heating and cooling system should be operating normally during a measurement, set the way the household actually lives, rather than turned off or run in some special mode just for the test.
Running fans, cracking windows, or shutting off the furnace during a test does not make a home safer. It just produces a number that does not match how the home is actually lived in. The goal of a test is an honest, representative reading, not the lowest possible one.
6. Open versus closed windows
Open windows let outdoor air in, and outdoor air has very little radon in it. So while the windows are open, indoor radon gets diluted and the reading falls. That sounds like good news until you remember how little of the year most homes spend with the windows open. The moment the house is closed up for heating or cooling, the dilution stops and levels climb back.
This is exactly why testing is done under closed-house conditions: windows and exterior doors kept shut except for normal entry and exit, starting at least 12 hours before the test and continuing through it. Closing the house removes the temporary dilution and captures the higher-exposure condition the home actually experiences most of the year. A test run with windows open looks reassuringly low and tells you almost nothing about your real exposure.
7. Rain and frozen ground
What happens at the surface of the soil matters because it changes how easily radon can escape outdoors versus get pushed into the house. Heavy rain can saturate the top layer of soil and snow or frost can seal it, both of which act like a cap. With the easy escape route to open air partly blocked, more of the radon already in the soil tends to follow the path of least resistance, which can be the openings in your foundation. Rain also usually arrives with the falling barometric pressure discussed earlier, so the two effects often push the same direction.
Frozen ground in winter does something similar on a longer timescale, capping the soil and reinforcing the season's tendency toward higher indoor readings. We go deeper into the wet-weather side of this in our article on whether rain affects radon levels.
8. Why this is why testing matters
Once you accept that radon moves with weather, season, and how the house breathes, the rules around testing stop feeling arbitrary. Closed-house conditions exist to remove the temporary dilution that would make a reading look better than the home's real year-round exposure. They standardize the test so results are comparable and lean toward the conditions the home actually sees during heating and cooling season.
Test length matters for the same reason. A short-term test of a few days is a fast, useful screen, but it is a snapshot that can be caught on an unusually high or low stretch of weather. A long-term test running more than 90 days averages across the swings and gives the best estimate of true exposure. A sensible approach is a short-term test first, then a long-term test to confirm if the short result lands anywhere near the 4.0 pCi/L action level. The distinction is laid out in our guide to short-term versus long-term radon testing.
A single low reading taken in mild weather with the windows open is not proof a home is fine, and a single high reading during a winter storm is not the whole story either. Fluctuation is the reason the standards push toward closed-house conditions and longer measurement windows.
9. Frequently asked questions
This article is general information for homeowners, not engineering or medical advice. Radon behavior varies from house to house, and the only way to know a specific building's level is to test it under proper conditions. For decisions about testing or mitigation, work with a qualified radon professional.
