What Causes High Radon Levels In A Home? Why It Happens

Radon is a radioactive gas that forms naturally when uranium, thorium, or radium break down in soil and rock. It rises from the ground into the air and can seep into a home through any opening that touches the soil. A home’s radon level depends on the geology beneath it, how the house was built, and how the two interact — not on whether the house is old, new, clean, or well-kept.

What Causes High Radon Levels In A Home?

The cause is almost always the ground the house sits on. Radon forms when naturally occurring uranium and radium decay in soil, rock, and sometimes groundwater. As a gas, it moves upward through the soil and enters a home through small gaps and cracks that are in contact with the ground.

Because radon comes from the soil, the single strongest predictor of a home’s radon level is local geology. Areas with granite, shale, phosphate-rich rock, or certain glacial deposits tend to produce more radon. But high levels have been found in every U.S. state, including areas with no obvious geological warning signs. The EPA estimates that about 1 in 15 homes in the United States has elevated radon levels, and radon is considered the second leading cause of lung cancer after smoking.

This is where a common misunderstanding matters. Two houses on the same street, built the same year, can have very different radon levels. The difference is often not the neighborhood — it is how each house interacts with the soil under it.

How Does Radon Get Inside a House?

Radon enters through the path of least resistance. A house acts like a weak vacuum relative to the soil around it, and that pressure difference pulls soil gas indoors.

Several mechanisms drive this:

• Stack effect: Warm indoor air rises and escapes through the upper floors and roof. That loss creates slight negative pressure at the lower levels, which pulls soil gas in through the foundation.
• Pressure differences: Appliances like furnaces, water heaters, and exhaust fans can lower indoor air pressure, increasing the pull on soil gas.
• Soil and weather: Radon moves more readily through loose, permeable soil. Changes in temperature, wind, and barometric pressure can shift how much gas flows toward the house.

The gas then enters through any gap that connects the home to the soil. Common entry points include:

• Cracks in concrete floors and walls
• Construction joints and floor-wall seams
• Gaps around pipes, wires, and sump pumps
• Crawl spaces and dirt floors
• Drain tiles and basement floor drains
• Porous hollow-block foundation walls
• Well water, though this is a smaller contributor in most homes

Once inside, radon can build up — especially in basements and lower levels, where the gas is closest to its source and ventilation is often poorer.

Why Do Some Homes Have More Radon Than Others?

Geology sets the baseline, but the house itself decides how much of that baseline ends up indoors. Two homes over the same soil can differ substantially because of how they are built and how they are used.

Factors that tend to raise indoor radon levels include:

• Foundation type: Basements and slab-on-grade foundations have more direct soil contact than homes on piers or with well-ventilated crawl spaces.
• Soil permeability: Loose, gravelly, or highly fractured soil lets gas move more freely toward the foundation.
• Home tightness: A well-sealed, energy-efficient home can trap radon that enters, allowing it to accumulate. This is one reason newer, tightly built homes are not automatically lower-risk.
• Air pressure balance: Strong exhaust systems or leaky ductwork can depressurize a home and increase soil gas draw.
• Lower-level living space: Bedrooms, offices, or play areas in basements mean more time spent where radon concentrations are typically highest.

Construction quality matters too, but not always in the direction people expect. A house with many small foundation cracks is not necessarily worse than a seemingly sealed one, because the dominant driver is pressure and soil gas movement, not just the number of visible openings.

Does the Type of Foundation Matter?

Yes. Foundation type changes how much soil surface area is exposed to the home and how easily gas can migrate inside.

Homes with basements and slab-on-grade foundations generally have the highest potential for elevated radon, because a large portion of the living space sits directly against soil. Crawl-space homes can be lower-risk when the crawl space is well-ventilated and separated from living areas, but poorly sealed or unventilated crawl spaces can still allow radon to enter. Homes on piers or pilings, with open air underneath, typically have the least direct soil contact.

This is a general pattern, not a rule. A well-built basement home in a low-radon area can test low, while a crawl-space home in a high-radon area can test high. The only way to know a specific home’s level is to test it.

Does Radon Come From Anything Inside the House?

Radon comes from the ground, not from building materials, furniture, or household products in any meaningful way. This is a frequent point of confusion.

Some building materials — such as certain concrete made with alum shale, or some granite countertops — can emit very small amounts of radon or its parent elements. In the vast majority of homes, these contributions are minor compared to what enters from the soil. A handful of studies have looked at granite countertops specifically, and the general finding is that they are not a significant radon source in typical homes.

Well water is a more real, if still secondary, source. Radon dissolves in groundwater and can be released into the air when water is used for showering, washing, or cooking. This matters most in homes served by private wells drawing from radon-rich bedrock. For most homes on public water supplies, water is not a major contributor.

Why Do Radon Levels Change Over Time?

A single test result is a snapshot, not a permanent label. Radon levels in the same home can vary from day to day and season to season.

Factors that cause fluctuation include:

• Weather: Storms, changes in barometric pressure, and temperature swings can change how much soil gas moves toward the house.
• Season: Levels often run higher in winter, when windows stay closed and the stack effect is stronger. But this is not universal — some homes test higher in other seasons.
• Home operation: Opening windows, running exhaust fans, or changing heating and cooling patterns can shift indoor concentrations.
• Renovations: Adding a basement bedroom, sealing a crawl space, or changing ventilation can alter how radon moves and accumulates.

Because of this variability, short-term tests can miss a home’s true average. For a decision that affects long-term health, a long-term test — typically placed for several months under normal living conditions — gives a more reliable picture of average exposure.

Does a Newer or Older Home Make a Difference?

Age alone does not determine radon risk. A brand-new home can test high, and a century-old home can test low.

What matters more is construction style and soil contact. Newer homes are often built tighter for energy efficiency, which can trap radon that enters. Some newer homes are also built with radon-resistant features — a gas-permeable layer under the slab and a vent pipe — which can lower indoor levels when installed and operating correctly. Older homes may have more gaps and cracks, but also more natural air exchange that dilutes indoor gas.

The takeaway is simple: you cannot predict a home’s radon level from its age. You have to test.

What Actually Reduces Radon Levels?

The most established method for lowering indoor radon is a soil depressurization system, often called a radon mitigation system. These systems use a pipe and fan to draw soil gas from beneath the foundation and vent it above the roofline, so it never enters the home. When properly designed and installed, these systems reliably reduce indoor radon concentrations.

Other measures — sealing cracks, improving ventilation, or installing a radon-resistant foundation during construction — can help, but sealing alone is generally not considered sufficient. The pressure-driven flow of soil gas can move through paths that sealants do not fully close.

Because radon is odorless and invisible, testing is the only way to know whether a home has a problem and whether any fix is working. The U.S. Environmental Protection Agency recommends testing all homes below the third floor, and testing again after any major renovation or mitigation work. If a test shows elevated levels, a qualified radon professional can recommend the right approach for that specific home.