Radon is found in soil, rock, groundwater, and indoor air virtually everywhere on Earth, though concentrations vary enormously by location. It forms naturally underground as uranium in rock and soil decays, producing a colorless, odorless radioactive gas that seeps to the surface and can accumulate inside buildings. Outdoors, it dilutes to harmless background levels of 5 to 15 becquerels per cubic meter. Indoors, especially in basements and ground-floor rooms, it can build to concentrations hundreds of times higher.
How Radon Forms in the Ground
Radon-222, the most common and significant isotope, is a product of the uranium-238 decay chain. Uranium is present in nearly all soil and rock, though in widely varying amounts. As uranium atoms decay through a series of intermediate elements, they eventually produce radon, an inert gas that migrates through tiny spaces in soil and fractures in rock. Because it’s a noble gas, radon doesn’t bind chemically to anything. It simply drifts upward through the ground and escapes into the air.
Rock and Soil Types With the Highest Levels
Not all geology produces equal amounts of radon. The rocks most associated with elevated indoor radon are uranium-rich metamorphic and igneous formations. A study of Georgia’s geology found the highest indoor radon levels and the greatest lung cancer risk in homes built above gneiss, schist, and quartzite formations, followed by granite, then limestone-dolomite-shale, and limestone-sandstone-shale combinations. The state’s Blue Ridge and Piedmont regions, rich in uranium-bearing minerals, were particular hotspots.
Granite bedrock is a well-known radon source, but carbonate rocks like limestone and dolomite also matter. Homes built above carbonate bedrock tend to have higher indoor radon than those over siliciite-rich sandstone formations. The permeability of the soil layer plays a role too: loose, gravelly soils allow radon to travel more freely toward the surface than dense clay.
How Radon Gets Inside Buildings
The primary route into any building is through the foundation. Soil gas carrying radon enters through cracks in concrete slabs, gaps at floor-to-wall joints, construction joints, spaces around plumbing pipes, sump pump openings, and cavities inside walls. Even a foundation that looks solid has enough micro-cracks and penetrations to let significant amounts of radon through. The slight negative pressure inside a heated building (warm air rises and escapes, pulling replacement air from below) actively draws soil gas upward through these pathways.
Building materials themselves can emit small amounts of radon. Granite countertops, concrete, and some natural stone contain trace uranium and thorium that slowly decay into radon gas. However, the EPA considers this a minimal concern compared to soil-sourced radon. Any radiation from granite countertops drops off rapidly with distance and is extremely unlikely to raise exposure above normal background levels. The radon released from these materials also dilutes with normal room ventilation.
Radon Levels by Floor
Radon concentration drops dramatically as you move higher in a building. A Swiss study of homes in alpine areas measured average radon levels of about 1,146 becquerels per cubic meter in cellars, 176 on the ground floor, and 113 on the first floor above that. In other words, the basement had roughly six to ten times the radon of the living floors above it. The extent of this drop depends on how well the basement is sealed from the rest of the house. Barriers between the cellar and ground floor, such as a solid ceiling, sealed doors, and minimal ductwork connections, significantly reduce how much radon reaches living spaces.
Homes without basements aren’t immune. Slab-on-grade construction still sits directly on soil, and radon enters through the same types of cracks and joints. Homes on crawl spaces can also accumulate radon, particularly if the crawl space is poorly ventilated.
Radon in Water
Groundwater picks up radon as it flows through rock and soil. Private wells typically contain far higher radon concentrations than municipal supplies that draw from surface water. The average concentration in public water systems sourced from groundwater is about 20 becquerels per liter. Surface water from lakes and streams contains roughly one-tenth that amount, because radon escapes into the air at the water’s surface before it reaches a treatment plant.
When you run a faucet or shower, dissolved radon releases into your indoor air. That said, tap water is usually a smaller contributor to indoor radon than soil beneath the house. The main concern is for households relying on private wells in uranium-rich geology, where water radon levels can be substantially above the public-supply average.
Workplaces With Elevated Radon
Underground mines have long been recognized as high-radon environments. Studies of uranium miners provided some of the earliest and strongest evidence linking radon exposure to lung cancer. But mines aren’t the only workplace concern. Caves and caverns can accumulate striking radon levels. Measurements inside a tourist cavern found mean radon concentrations ranging from 970 to 2,600 becquerels per cubic meter in the main chamber, and 5,400 to 6,000 in a smaller side cave. These levels are well above what you’d find in most buildings and represent a real occupational exposure for cave guides and staff who spend hours underground daily.
Other workplaces with potential radon accumulation include utility tunnels, subway systems, underground storage facilities, and water treatment plants that process high-radon groundwater.
What Affects Indoor Levels Day to Day
Radon concentrations inside a home aren’t static. They fluctuate with weather conditions, particularly barometric pressure. When atmospheric pressure is higher, it pushes down on the soil surface and reduces the rate at which soil gas migrates upward into buildings. One study found that for every 5-point increase in barometric pressure, the odds of a home testing at or above the EPA’s action level dropped by 62%. Interestingly, the same study found no significant link between indoor radon and season, topography, or precipitation, even though many people assume winter levels are always highest.
Building age matters as well. Homes built in the last 40 years tended to have higher indoor radon than older homes, likely because modern construction is more airtight, trapping soil gas inside rather than letting it leak out through drafty walls and windows.
How to Know if Your Home Has High Radon
The EPA’s action level is 4 picocuries per liter (pCi/L), equivalent to 150 becquerels per cubic meter. If your home tests at or above that threshold, the EPA recommends installing a mitigation system, typically a vent pipe and fan that pulls radon from beneath the foundation and exhausts it above the roofline. The agency also recommends considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe level of radon exposure.
Testing is the only way to know your home’s radon level. Two houses on the same street, built on the same soil, can have very different concentrations depending on foundation condition, ventilation, and subtle differences in underground geology. Short-term test kits are inexpensive and widely available at hardware stores. For a more accurate picture, long-term tests that measure over 90 days or more account for the natural day-to-day fluctuations in radon levels.