Radon in homes comes from uranium, a naturally occurring radioactive element found in nearly all soil and rock beneath and around your foundation. As uranium breaks down over millions of years, it eventually produces radon gas, which seeps upward through the ground and into buildings through cracks, gaps, and other openings. The average indoor radon level in the United States is about 1.3 pCi/L, more than three times the average outdoor level of 0.4 pCi/L, because homes trap and concentrate the gas.
How Uranium in Soil Creates Radon
Uranium-238, the most common form of uranium in the Earth’s crust, decays through a long chain of radioactive steps. Each step produces a different element, and partway through that chain, the element radium forms. When radium decays, it releases radon-222, a colorless, odorless gas with a half-life of about 3.8 days. That short lifespan matters: radon doesn’t stick around long, but the supply is constantly replenished as more radium decays underground.
Unlike the solid elements before and after it in the decay chain, radon is a gas. That means it can migrate through tiny pore spaces in soil and rock, working its way upward toward the surface. Outdoors, it disperses harmlessly into the atmosphere. But when a building sits on top of that soil, the gas gets funneled indoors and accumulates.
Which Soils and Rocks Produce More Radon
Not all ground produces the same amount of radon. The concentration depends on how much uranium the underlying rock contains. Granite and sheared rocks are generally high-radon sources because they tend to be enriched in uranium. Volcanic regions also show elevated indoor radon levels for the same reason. Sedimentary formations like limestone and sandstone typically produce lower concentrations, though exceptions exist depending on local geology.
A study of different rock types in Nigeria found that radon concentrations in mica schist were roughly eight times higher than in granite gneiss, illustrating how dramatically levels can vary even within the same region. This is why two homes a few miles apart can have very different radon readings. Your neighbor’s result tells you almost nothing about your own.
How Radon Gets Inside
The main force pulling radon into your home is a pressure difference. Your house typically has slightly lower air pressure than the soil beneath it, especially in colder months. Warm air rises inside the building and escapes through upper floors and the attic, creating a mild vacuum at the lower levels. This “stack effect” draws soil gas, including radon, upward through the foundation. Wind pressing against the house can intensify the effect.
Radon exploits every available opening. Common entry points include:
- Cracks in concrete slab floors
- Floor-to-wall joints and construction joints
- Gaps around plumbing pipes and other service penetrations
- Sump pump openings
- Cavities inside basement walls
- Gaps in suspended floors
Even a home with no visible cracks can have elevated radon. Concrete is porous enough to allow some gas through, and the joints where walls meet floors are rarely perfectly sealed.
Building Materials as a Minor Source
The soil beneath your foundation is the dominant source, but building materials themselves can contribute a small amount. Concrete, brick, natural stone, granite countertops, sandstone, and gypsum all contain trace levels of naturally occurring uranium, radium, and thorium. As these elements decay, they release small quantities of radon into indoor air.
In practice, the contribution from building materials is very low. The CDC notes that these materials are highly unlikely to raise radiation exposure above normal background levels. If your home has elevated radon, the soil is almost certainly the cause, not your walls or countertops.
Radon From Well Water
If your home uses a private well, the water itself can be a secondary radon source. Groundwater that flows through uranium-rich rock dissolves radon gas along the way. When that water enters your home and gets agitated or heated, the radon escapes into the air. Showering, running the dishwasher, and doing laundry all release dissolved radon, briefly spiking indoor levels.
Homes on municipal water supplies rarely have this issue because the water is treated and stored in open reservoirs, allowing radon to dissipate before it reaches your tap. Well water is the concern here, particularly wells drilled into granite or other uranium-bearing rock.
Why Radon Is a Health Concern
Once radon is in your lungs, it continues decaying into solid radioactive particles (called progeny) that lodge in lung tissue. These particles emit alpha radiation, which damages cells at close range. At the concentrations found in most homes, the damage works indirectly: an alpha particle hitting one cell sends chemical stress signals to neighboring cells, causing chromosome damage in cells that were never directly hit. This “bystander effect” is the primary mechanism behind radon-related lung cancer risk at typical residential exposure levels.
Radon is the second leading cause of lung cancer after smoking. There is no known safe level of exposure, which is why even modest indoor concentrations warrant attention.
Testing and Fixing the Problem
The EPA recommends taking action if your home’s radon level reaches 4 pCi/L or higher, and suggests considering mitigation even between 2 and 4 pCi/L. Short-term test kits are inexpensive and widely available at hardware stores. You place one in the lowest livable level of your home for a few days, then send it to a lab. Long-term tests (90 days or more) give a more accurate picture of your year-round average.
The most common fix is called active soil depressurization. A contractor installs a pipe through your foundation slab and connects it to a small fan that runs continuously. The fan draws radon-laden soil gas from beneath the house and vents it above the roofline, where it disperses harmlessly. The system also reverses the pressure difference that was pulling radon in, so it addresses the root cause rather than just filtering air. Installation typically takes a day, and the systems are quiet and inexpensive to operate.
Because radon production in the soil never stops, mitigation is an ongoing process rather than a one-time fix. The fan needs to keep running, and periodic retesting (every two to five years, or after major renovations) confirms the system is still working effectively.