High radon levels in a home come from uranium and radium decaying naturally in the soil and rock beneath the foundation. Every home sits on ground that contains some amount of these radioactive elements, but the combination of local geology, your foundation’s condition, and how your house manages air pressure determines whether radon accumulates to dangerous concentrations indoors. About one in every 15 homes in the U.S. has radon at or above the EPA’s action level of 4 pCi/L.
It Starts With the Geology Under Your Home
Radon is a radioactive gas produced when uranium in soil and rock breaks down through a chain of decay steps, eventually becoming radium, which then produces radon-222. This gas is invisible, odorless, and present in nearly all rocks and soils to some degree. What varies dramatically from one property to the next is concentration.
Certain types of bedrock produce far more radon than others. Granite, shale, and phosphate-rich soils tend to contain higher concentrations of uranium. But geology alone doesn’t tell the whole story. The type of soil matters just as much as what’s in it. Radon exists in three states within soil: floating in the air between soil particles, dissolved in moisture between those particles, or clinging to the particles themselves. Loose, gravelly, or sandy soils with large pore spaces allow radon to travel farther before it decays, giving it a better chance of reaching your foundation. Dense, wet clay soils can slow that migration, though they can also trap radon close to the surface in certain conditions.
This is why two neighboring homes can have very different radon readings. Even small differences in the underlying rock formation, soil composition, or groundwater level can shift how much radon reaches each foundation.
Your Home Acts Like a Vacuum
The air pressure inside your home is usually lower than the pressure in the soil surrounding your foundation. This pressure difference is the engine that pulls radon indoors. Warm air rises through your house and escapes through upper floors, attics, and ventilation openings. As that air leaves, it creates a slight vacuum at the lowest level of the home, pulling replacement air from the path of least resistance: the soil beneath and around the foundation. This process is called the stack effect.
The greater the temperature difference between indoors and outdoors, the stronger this vacuum becomes. In winter, when your heating system warms the interior well above outdoor temperatures, the stack effect intensifies. That’s one reason radon levels often peak during cold months. But anything that exhausts air from the house without replacing it contributes to the same problem. Bathroom exhaust fans, kitchen range hoods, clothes dryers, and fireplaces all push air out of the home, increasing the negative pressure at the foundation level and drawing more soil gas in.
Forced-air HVAC systems can also play a role. If ductwork is leaky or the system is unbalanced, with more return air pulled from certain zones than supply air delivered, it can depressurize the lowest level of the home and accelerate radon entry.
How Radon Gets Through the Foundation
Radon doesn’t need large openings. It enters through any gap where the foundation contacts soil. The most common entry points include:
- Cracks in concrete slabs or basement floors, even hairline cracks that develop as concrete cures and settles
- The joint where the floor meets the foundation wall, a natural seam that rarely stays perfectly sealed
- Gaps around pipes and utility lines that penetrate the foundation
- Sump pump pits, which provide a direct opening to the soil below
- Floor drains that connect to the soil or to drain tiles beneath the slab
- Hollow block foundation walls, which can fill with radon that seeps through the blocks and enters the home through openings at the top
Foundation type matters significantly. Homes with basements have the most surface area in contact with soil and the most potential entry points. Slab-on-grade homes (where the concrete floor sits directly on the ground) have fewer joints and penetrations but are still vulnerable through cracks and utility gaps. Crawlspace homes, where exposed dirt sits directly beneath the floor, can accumulate radon in that enclosed space, which then migrates into the living area above through gaps in the flooring, ductwork, or plumbing penetrations.
Well Water Can Release Radon Indoors
If your home uses a private well, the water itself can be a secondary source. Groundwater that passes through uranium-bearing rock dissolves radon gas. When that water reaches your faucets, the radon escapes into the air, particularly during activities that agitate the water like showering, running a dishwasher, or doing laundry.
The contribution is real but typically modest compared to soil gas. As a general rule, 10,000 pCi/L of radon in water adds about 1 pCi/L to the air in your home. Most of the health risk from waterborne radon, roughly 89% of associated deaths, comes from breathing in the gas after it escapes from the water, not from drinking it. Municipal water systems rarely pose this risk because the treatment and storage process allows most radon to dissipate before it reaches your tap.
Weather Causes Daily and Seasonal Swings
If you’ve tested your home more than once and gotten different numbers, weather is a likely explanation. Radon levels inside a home are not constant. They fluctuate with barometric pressure, wind, temperature, and rainfall.
Drops in barometric pressure cause soil gas to flow upward more readily. U.S. Geological Survey research found that even short-term pressure fluctuations lasting minutes, not hours, can measurably change radon concentrations in the top layers of soil. When pressure drops, radon-bearing gas from deeper soil layers migrates upward, eventually increasing concentrations near the surface and at your foundation. When pressure rises, outside air pushes down into the soil and can temporarily displace radon away from the surface.
Rain saturates the top layer of soil, which can act like a cap. Radon that would normally diffuse upward into the open air gets trapped and redirected laterally, toward your foundation. Heavy snow cover has a similar sealing effect. Wind speed and direction also influence radon entry in less predictable ways, potentially creating pressure differences on different sides of your foundation.
These fluctuations are why short-term radon tests (the 2-to-7-day kind) give you a snapshot, not a definitive reading. Long-term tests of 90 days or more capture seasonal variation and give a more reliable annual average.
Home Features That Raise or Lower Levels
Beyond geology and weather, specific features of your home can amplify or reduce radon accumulation. Tightly sealed, energy-efficient homes reduce air exchange with the outside, which means any radon that enters has fewer chances to dilute and escape. Older, draftier homes may have lower radon levels simply because they leak enough outdoor air to keep concentrations down, though this is an unreliable and energy-wasteful form of ventilation.
Homes with large basement footprints in contact with soil have more surface area for radon to enter. Finished basements where people spend significant time are a particular concern, because the living space sits at the level where radon concentrations are highest. Upper floors typically have lower levels, since radon dilutes as it mixes with indoor air and escapes through natural ventilation higher in the building.
How High Radon Levels Are Reduced
The most common and reliable fix is active subslab depressurization. A contractor drills a small hole through the basement or slab floor, inserts a pipe into the crushed rock or soil beneath, and attaches a fan that runs continuously. This reverses the pressure relationship: instead of your home pulling soil gas in, the system pulls radon from beneath the foundation and vents it safely above the roofline, where it disperses harmlessly into outdoor air.
Variations on this approach work for different foundation types. Homes with drain tiles or perforated pipes around the foundation can have suction applied to those existing channels. Sump pump pits can be sealed with an airtight cover and used as the suction point, still allowing water drainage while capturing radon. Homes with hollow concrete block walls can have suction applied directly to the block cavities.
Crawlspace homes use a different strategy. A heavy plastic sheet is laid over the exposed dirt floor, sealed at the edges, and connected to a vent pipe and fan. This submembrane suction pulls radon from under the plastic and vents it outside, and it is the most effective approach for this foundation type. These systems typically reduce radon levels by 80% to 99%, and they run quietly in the background with modest electricity costs, similar to a light bulb left on continuously.