Radon levels don’t stay constant because the conditions that control how it moves from the ground into your home are constantly changing. Think of it less like a steady leak and more like something that “breathes” with the environment. Here are the main reasons it fluctuates over time:
1. Air pressure differences (the biggest driver)
Radon is pulled into buildings when indoor air pressure is lower than the soil beneath the house. Changes in weather—like passing storms or shifts in barometric pressure—can increase or decrease that suction. Lower outdoor pressure often means more radon entering.
2. Temperature and seasonal effects
In colder months, homes are sealed up and warm indoor air rises and escapes (the “stack effect”), pulling soil gases—including radon—inside. That’s why levels are often higher in winter and lower in summer.
3. Ventilation and HVAC use
Opening windows, running exhaust fans, or using heating/cooling systems changes how air flows through the house. More ventilation usually dilutes radon; less ventilation lets it build up.
4. Soil moisture and rainfall
After rain, water fills gaps in the soil and can either:
5. Time of day patterns
At night and early morning, homes tend to be more closed up and air is more stable, which can let radon accumulate. Daytime activity and ventilation often lower it.
6. Entry pathways changing slightly
Tiny cracks in foundations, sump pumps, crawlspaces, and pipe openings are radon entry points. Small shifts (like humidity causing materials to expand/contract) can subtly change how much radon gets in.
What this means in practice:
If you’re seeing big swings, that’s normal—but if the average is elevated, mitigation (like a sub-slab depressurization system) is the right fix.
We often get asked why radon levels fluctuate, often dramatically, during the 48-hour test period and whether there is cause for concern.
There are many reasons for this and the fluctuations are perfectly normal, but this emphasizes the reason that a long-term average gives the best indication of whether radon is truly elevated in the home
The most common reasons that radon levels fluctuate can be broken down into three basic categories:
- Air pressure differentials in the home;
- Use of exhaust devices that vent indoor air to the exterior;
- Weather;
Air Pressure Differentials - Induced Soil Suction & Stack Effect from Heating
Buildings can create vacuums that pull in soil gases/radon from the soil under the building or from the crawl space. These vacuums (referred to as Air Pressure Differentials APD’s) may be very small. But even small APD’s between the house air pressure and the soil gas pressure can greatly affect radon readings inside a home.
When indoor air is warmer than outside air, it rises up and exits through the upper portion of the house. This air is replaced by soil gases which contain radon. Warmer inside temperatures and colder outside temperatures create a strong stack (chimney) effect drawing radon into the home.
APD’s and the stack effect are the greatest factors causing radon levels to rise or fall inside a home and the stack effects are usually higher at night and lower during the day. This results in higher radon levels at night and lower radon levels during the day and creates diurnal radon fluctuations.
Use of Home Air Exhaust Devices
When exhaust devices push air out of the home, negative pressure is created which causes radon to be pulled in from the soil under the slab or from the crawl space. Common exhaust devices that can increase the negative pressure inside the home leading to higher radon levels include:
- Gas burning or wood fireplaces;
- Central vacuum cleaner;
- Clothes dryer;
- Bathroom fan;
- Gas combustion appliances (furnaces, space heaters, ranges, water heaters, etc.);
- Kitchen range exhaust fan.
Weather Effects
- Light rain and gentle winds have little effect on indoor radon concentrations.
- Severe storms and high winds can have a great effect on indoor radon levels.
- Rain can block soil pathways and either raise or reduce indoor radon concentrations.
Indoor radon concentrations are usually higher during heavy rain events and winter seasons. High winds can raise or lower indoor radon. When wind blows on the side of house with most door(s)/ window(s), it creates more positive indoor pressure which pushes radon out. When it blows on side with least door(s)/window(s), it creates more negative indoor pressure, which sucks more radon in.
Barometric pressure changes can change soil gas pressures and amount of radon entering a home. Indoor radon levels are usually higher in the wintertime because: 1) the heating system is pulling air up and out of the home (stack effect); and 2) frozen ground/soil saturation caps/blocks soil gas/ radon from normal exiting into the atmosphere from outside soil - causing soil gas/radon pressures to rise.
Daily radon variation (sometimes 2-3 fold) is usually greater in the summer than in the winter because of more variations in soil surface temperatures during the summer than in the winter.
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