Radon is most common in areas where the underlying rock and soil are naturally rich in uranium, particularly regions with granite bedrock, dark shale formations, and phosphate-bearing sedimentary rock. In the United States, the highest-risk zones stretch across the northern Great Plains, the Appalachian Mountains, parts of the Midwest, and the Rocky Mountain states. But radon can show up at dangerous levels in any state, and the only way to know your home’s level is to test it.
Radon is a radioactive gas that forms when uranium in rock and soil breaks down. It seeps up through the ground, enters buildings through cracks in foundations, and accumulates indoors. It’s colorless and odorless, responsible for an estimated 21,000 lung cancer deaths in the U.S. every year, making it the second leading cause of lung cancer after smoking.
The Geology That Drives High Radon
Certain rock types contain far more uranium than others, and that’s the single biggest factor determining where radon levels run high. Light-colored volcanic rocks, granites, dark shales, phosphate-rich sedimentary rocks, and metamorphic rocks derived from any of these can contain up to 100 parts per million of uranium. As that uranium decays, it produces radon gas that migrates upward through soil and into the air above.
Granite is the most widespread culprit. The Sierra Nevada range in California, the Rocky Mountains, the Black Hills of South Dakota, and large stretches of the Appalachian Mountains all sit on granitic or metamorphic bedrock with elevated uranium. Dark, uranium-bearing shales create distinct high-radon zones in Ohio, Kentucky, and Indiana. In west-central Ohio, glaciers scooped up uranium-rich black shale and spread it across a wide area, creating a radon hotspot that doesn’t follow the usual narrow outcrop pattern. In Florida, phosphate-rich rocks carry elevated uranium and produce some of the highest radon readings in the Southeast.
By contrast, areas underlain by sandstone, dolomite, or limestone tend to have lower radon levels. The difference can be dramatic even over short distances. A home sitting on schist or gneiss may have radon concentrations several times higher than a home a few miles away on carbonate rock.
Highest-Risk Regions in the U.S.
The EPA divides the country into three radon zones. Zone 1, the highest risk, covers much of the northern tier of states from the Dakotas through Minnesota, Wisconsin, Iowa, Nebraska, and into Pennsylvania and New York. Parts of Colorado, Montana, and Idaho also fall into this category. These areas have predicted average indoor radon levels above 4 picocuries per liter (pCi/L), which is the EPA’s action level for recommending a fix.
The Appalachian region deserves special attention. Granites with elevated uranium run through the mountain chain, and fault zones concentrate radioactive minerals even further. Black shales and soils sitting above limestone add to the problem. Southeastern Pennsylvania’s Piedmont region, built on schists, gneisses, and quartzites, consistently produces some of the highest radon readings in the eastern U.S.
That said, the EPA is clear that every state has homes with elevated radon. Even in Zone 3 (lowest predicted risk), individual homes can test well above 4 pCi/L depending on local soil conditions, foundation type, and how tightly sealed the building is. The national average indoor radon level is about 1.3 pCi/L, compared to roughly 0.4 pCi/L outdoors.
Radon in Well Water
If your home uses a private well, the bedrock your water flows through matters. Groundwater picks up radon as it moves through uranium-rich rock, and that radon can enter your home when you run the tap, shower, or use a dishwasher. The highest groundwater radon concentrations in the eastern U.S. come from wells drilled into schists, gneisses, and quartzites. In southeastern Pennsylvania’s Piedmont province, the median groundwater radon level is 3,100 pCi/L, with individual wells reaching as high as 38,000 pCi/L. One formation, the Peters Creek Schist, has a median groundwater radon concentration of 4,300 pCi/L.
Wells drilled into granite, schist, and gneiss consistently produce higher radon levels than wells in dolomite, sandstone, or other sedimentary rock. Ninety percent of water samples from crystalline-rock aquifers in the Piedmont and Blue Ridge regions exceeded 300 pCi/L, a threshold the EPA has considered for regulation. Municipal water systems that draw from surface water (reservoirs and rivers) generally have very low radon because the gas escapes into the air before it reaches your faucet.
Building Materials as a Secondary Source
The ground beneath a home is the primary source of indoor radon, but building materials can add to the total. Concrete, brick, natural stone, granite countertops, gypsum, and sandstone all contain trace amounts of uranium, radium, and thorium. As these elements decay, they release small amounts of radon. The contribution from building materials is typically minor compared to soil gas entry, but in a tightly sealed home with limited ventilation, even small additions matter.
Why Winter Levels Are Higher
Radon concentrations inside a home are not static. They fluctuate with the seasons, and winter consistently produces the highest readings. Several forces work together to make this happen.
When you heat your home, warm air rises and escapes through the roof, vents, and upper-floor openings. This creates a pressure difference, sometimes called the stack effect, that pulls replacement air in from below, including soil gas loaded with radon. The colder it is outside, the stronger this effect becomes. At the same time, snow and ice act as a cap on the soil around your foundation, sealing radon’s usual escape routes to the outdoor air. The gas follows the path of least resistance, which becomes your foundation cracks and utility openings.
Homes are also sealed more tightly in winter. Windows stay closed, fresh air exchange drops, and radon accumulates rather than being diluted. The difference can be striking: some buildings that test between 1.8 and 2.2 pCi/L in summer have measured between 28 and 32 pCi/L in winter. If you test during warm months and get a borderline result, retesting in winter gives a more complete picture.
What the Numbers Mean for Your Home
The EPA recommends taking action if your home tests at 4 pCi/L or above. Because there is no known safe level of radon exposure, the agency also suggests considering a fix for levels between 2 and 4 pCi/L. Short-term test kits are inexpensive and available at most hardware stores. You place one in the lowest livable level of your home, typically the basement or ground floor, for two to seven days.
If your result comes back high, a radon mitigation system can reduce levels by up to 99 percent. The most common approach uses a fan and piping to draw radon from beneath the foundation slab and vent it outside before it enters your living space. Installation typically takes less than a day. Living in a high-radon geological zone doesn’t mean your home necessarily has a problem, but it does mean testing is especially important. The geology sets the odds; the test tells you what’s actually happening in your house.