Radon levels inside a home are driven by a combination of what’s in the ground beneath it, how the building is constructed, and what the weather is doing outside. The EPA’s action level is 4 pCi/L, but there’s no known safe exposure level, so understanding what pushes concentrations higher can help you take practical steps to reduce your risk.
The Ground Beneath Your Home
Radon starts as uranium buried in rock and soil. Uranium-238 decays through a long chain of intermediate elements until it produces radon-222, a colorless, odorless gas with a half-life of about 3.8 days. That short lifespan means radon is constantly being generated and constantly decaying, so the supply from below never stops as long as uranium is present in the ground.
The type of bedrock matters significantly. Homes sitting above carbonate bedrock (limestone and dolostone) tend to have higher indoor radon. Granite-rich soils also contain more uranium. But geology alone doesn’t determine your exposure. The soil between the bedrock and your foundation acts as either a highway or a barrier. Loose, gravelly, or sandy soils with high permeability let radon travel easily toward the surface. Clay-heavy or water-saturated soils slow it down. Particle size, porosity, and how much moisture is in the ground all influence how freely the gas migrates upward.
How Your Home Pulls Radon In
A house doesn’t just passively receive radon. It actively draws soil gas inward through a process called the stack effect. Warm air inside your home rises and escapes through upper floors and the attic, creating a slight vacuum at the lowest level. That negative pressure pulls soil gases, including radon, up through the foundation. The effect intensifies with greater temperature differences between indoors and outdoors, taller buildings, and wind pressing against the exterior.
This is why the lower levels of a home, particularly basements and ground-floor slabs, consistently show the highest radon concentrations. The depressurization is strongest there, right where the building contacts the soil. Anything that increases the pressure difference, like running exhaust fans, clothes dryers, or fireplaces without adequate makeup air, can amplify the pull.
Entry Points in the Foundation
Radon needs a physical gap to get inside. The most common pathways include the perimeter crack where the floor slab meets the foundation wall, shrinkage cracks in poured concrete, gaps around plumbing and electrical penetrations, sump pits, and exposed soil in crawl spaces. Even small, hairline cracks provide enough of an opening. The EPA’s radon-resistant construction guidelines specifically call for sealing all openings, cracks, and crevices in the foundation floor and walls to block soil gas entry.
Weather and Seasonal Swings
Radon levels are not constant. They fluctuate with the weather, sometimes dramatically. The single biggest seasonal pattern: winter concentrations routinely exceed summer levels by two to five times. Studies measuring homes across seasons found indoor radon ranging from 70 to 168 Bq/m³ in winter but only 17 to 30 Bq/m³ in summer. A Greek study of 25 homes recorded winter averages of 137.5 Bq/m³ versus 96.1 Bq/m³ in summer.
Several forces converge in cold months. The temperature gap between heated indoor air and freezing outdoor air strengthens the stack effect. Windows stay closed, cutting ventilation and letting radon accumulate. Frozen ground near the surface can also cap soil gas, forcing it to travel laterally toward the warmer soil surrounding your foundation, where it finds easier entry.
Barometric pressure is another powerful driver. When atmospheric pressure drops, radon concentrations in shallow soil rise because the reduced pressure overhead allows gas to migrate more freely toward the surface. Research has quantified this clearly: for every 5-point increase in barometric pressure, the odds of a home reading at or above 4 pCi/L dropped by 62%. Falling pressure before storms, common in winter, does the opposite. This is why radon levels can spike noticeably ahead of weather fronts.
Home Age and Energy Upgrades
Older homes aren’t automatically worse, but home age does show up as a predictor in large studies. Homes built within the last 40 years were significantly associated with different radon profiles, partly because construction standards, foundation types, and insulation practices have changed over time.
Energy-efficient retrofits deserve special attention. Adding exterior insulation makes a building’s envelope tighter, which reduces air leakage. That sounds like a good thing, but tighter envelopes can increase the depressurization effect at the foundation level, pulling more soil gas inside while simultaneously reducing the natural air exchange that would dilute it. If you’ve recently weatherized your home, added insulation, or replaced windows, your indoor radon levels may have increased even if nothing changed in the ground beneath you.
Ventilation and Air Exchange
Radon concentration is ultimately a balance between how fast the gas enters and how quickly it gets diluted or exhausted. A home with a high air exchange rate, where fresh outdoor air regularly replaces indoor air, will have lower radon even if the entry rate is significant. A tightly sealed home with minimal ventilation lets radon build up to much higher levels from the same source.
This is why basements with no windows or mechanical ventilation tend to have the worst readings. Opening windows, running a balanced ventilation system, or installing a heat recovery ventilator all increase dilution. Conversely, anything that reduces airflow, closing off a basement, sealing a crawl space without adding ventilation, or simply keeping the house buttoned up through winter, allows radon to concentrate.
Well Water as a Source
If your home uses a private well, the water itself can carry dissolved radon into the house. When you run a shower, wash dishes, or do laundry, radon escapes from the water into the air. The general rule of thumb: 10,000 pCi/L of radon in water adds roughly 1 pCi/L to the air inside your home. Most well water doesn’t carry enough radon to be the primary driver of high indoor levels, but in areas with uranium-rich bedrock, well water can be a meaningful contributor on top of what’s already coming through the foundation.
Building Materials
Concrete, brick, and natural stone all contain trace amounts of naturally occurring radioactive elements. Granite countertops have drawn occasional concern. In practice, the EPA notes that radiation levels released from building materials are typically very low, even with additives in concrete. Brick releases slightly more than wood, but neither comes close to what the soil beneath a foundation produces. For the vast majority of homes, building materials are a negligible factor compared to ground-source radon and ventilation patterns.
What Matters Most
If you’re trying to understand why your radon test came back high, or why readings vary between tests, the biggest factors are the uranium content of your local geology, the permeability of the soil, the integrity of your foundation, how tightly sealed your home is, and the season and weather conditions during testing. Winter tests during low-pressure weather will capture near-worst-case levels. Summer tests with open windows may underestimate your actual exposure during the months you spend the most time sealed indoors.
Because so many variables interact, two identical-looking houses on the same street can have very different radon levels. The only reliable way to know your situation is to test. Short-term test kits give a snapshot, but long-term detectors (90 days or more) capture the seasonal swings that short tests miss.