Radon gas in homes comes from the natural radioactive breakdown of uranium in the soil and rock beneath your foundation. Nearly all soil contains trace amounts of uranium, which slowly decays into radium and then into radon, a colorless, odorless gas that seeps upward through the ground. When a house sits on top of that soil, it can trap and concentrate radon to levels far higher than what you’d find outdoors. The average outdoor radon concentration is about 0.4 pCi/L, while indoor levels can easily reach 4 pCi/L or more, the threshold at which the EPA recommends taking action.
The Radioactive Decay Chain in Soil
Radon-222, the isotope that causes problems in homes, is a decay product of radium-226, which itself comes from uranium-238. This decay chain has been running for billions of years in the Earth’s crust, and it happens everywhere. You don’t need to live near a uranium mine for radon to be present. Ordinary soil, rock, and groundwater all contain enough uranium to produce radon continuously. The gas is inert, meaning it doesn’t bind to soil particles, so it migrates freely through gaps and pores in the ground until it either dissipates into the open air or finds its way into a building.
How Your House Pulls Radon In
A home doesn’t just passively let radon drift inside. It actively draws the gas in through a pressure difference between indoor air and the surrounding soil. Warm air inside your house naturally rises and escapes through upper floors, windows, and attic spaces. This creates a slight vacuum at the foundation level, pulling soil gases, including radon, upward into the lowest parts of the building. This process, called the stack effect, is the primary mechanical driver of radon entry.
Any factor that lowers indoor air pressure relative to the soil intensifies the pull. Exhaust fans, clothes dryers, fireplaces, and forced-air heating systems all push air out of the house, increasing that negative pressure at the foundation. The house essentially becomes a low-pressure zone sitting on top of a reservoir of radon-containing soil gas.
Entry Points in the Foundation
Radon enters through any gap or opening where the foundation meets the ground. The most common pathways include cracks in concrete slabs, gaps around pipes and utility penetrations, construction joints where walls meet the floor, sump pump openings, and floor drains. Even poured concrete that looks solid has microscopic pores that allow some gas migration. Homes with crawl spaces are particularly vulnerable because the soil surface is often exposed directly to the air beneath the house, giving radon an unobstructed path indoors.
Block-wall foundations are especially permeable. The hollow cores of cinder blocks act as channels, allowing radon to travel upward through the wall and seep into living spaces through small openings at the top of the wall or around window frames.
Why Soil Type Matters
The type of soil beneath your home significantly affects how much radon reaches the foundation. Loose, permeable soils like gravel, sand, and coarse glacial deposits allow radon to travel long distances through the ground before it decays. Research on radon transport in permeable environments has found that these coarse-grained soils show the highest seasonal radon variation and the greatest potential for delivering large volumes of gas to a building’s footprint.
Clay soils, by contrast, are much tighter. Studies measuring radon at various depths have found that clay layers in the soil structure act as barriers, producing noticeably lower radon concentrations compared to neighboring depths. If your home sits on heavy clay, radon has a harder time reaching the foundation, though it can still accumulate to concerning levels if the uranium content in the soil is high enough or if cracks in the clay provide migration pathways.
Weather and Seasonal Fluctuations
Indoor radon levels are not constant. They fluctuate with weather, season, and how you use your home. Winter typically brings the highest readings for two reasons: the stack effect is stronger when there’s a large temperature difference between warm indoor air and cold outdoor air, and homes are sealed up with windows closed, reducing ventilation that would otherwise dilute the gas. A long-term Swiss study found that outdoor temperature was the single strongest predictor of indoor radon, with colder weather consistently producing higher concentrations.
Rain also plays a role. Heavy rainfall saturates the soil surface, sealing off the normal escape routes radon would use to dissipate into the open air. Since the soil directly beneath a house stays relatively dry and unsealed, radon migrates preferentially toward the building instead of venting harmlessly outdoors. Summer thunderstorms and prolonged rain events can cause temporary spikes even during the warmer months when levels are generally lower. Interestingly, barometric pressure and humidity appear to be less significant factors than temperature and precipitation.
Less Common Sources
While soil is the dominant source, two other contributors are worth knowing about.
Well water. Groundwater that passes through uranium-bearing rock can dissolve radon. When that water comes out of your tap during showers, dishwashing, or laundry, some radon escapes into the air. However, the EPA estimates that only about 1 to 2 percent of indoor radon comes from drinking water. Unless your well water has extremely high radon concentrations, it’s rarely the main problem.
Building materials. Concrete, brick, natural stone, granite, and gypsum all contain trace amounts of uranium, radium, and thorium, and they can release small amounts of radon. The CDC notes, however, that these materials are “highly unlikely” to raise radiation levels meaningfully above normal background exposure. If your home has elevated radon, the foundation’s contact with soil is almost certainly the cause, not the materials the house is built from.
Why Some Homes Test High and Neighbors Don’t
Radon levels can vary dramatically from one house to the next, even on the same street. This happens because the uranium content in soil is not evenly distributed. Pockets of higher-concentration rock or soil can sit beneath one home’s footprint but not the house next door. Differences in foundation type, the number and size of cracks, how airtight the home is, and ventilation habits all compound the variation. A neighbor’s low reading tells you nothing about your own home.
New construction is not immune either. Tighter building envelopes designed for energy efficiency can actually increase radon levels by reducing the natural air exchange that would dilute the gas. A well-sealed modern home may trap more radon than a drafty older house, even on the same lot.
Testing and Action Levels
Because you can’t see, smell, or taste radon, testing is the only way to know your levels. Short-term test kits measure radon over 2 to 90 days and give you a quick snapshot. Long-term kits, which run for more than 90 days, provide a more accurate picture of your home’s year-round average and are better at capturing seasonal variation.
The EPA recommends testing if your home has never been tested, if you’re buying or selling, before and after renovations, or before converting a basement into a living space. If you install a radon mitigation system, retesting a few months afterward confirms that it’s working.
The EPA’s action level is 4 pCi/L, but the agency also recommends considering mitigation for levels between 2 and 4 pCi/L, because there is no known safe level of radon exposure. Most countries worldwide have adopted similar thresholds. Mitigation typically involves a sub-slab depressurization system, which reverses the pressure dynamic by using a fan and piping to pull radon from beneath the foundation and vent it above the roofline, where it disperses harmlessly into the atmosphere.