The ground beneath your home creates the greatest radon threat. Uranium-rich rock and soil are the primary source of indoor radon, and the gas seeps upward through cracks and gaps in your foundation. While radon can also enter through well water or building materials, these contribute far less. Only 1 to 2 percent of indoor radon comes from drinking water. The overwhelming majority rises directly from the earth below.
Why Soil Is the Dominant Source
Radon is a radioactive gas produced when uranium in rock and soil naturally decays. It’s invisible, odorless, and constantly escaping from the ground. Outdoors, it disperses harmlessly into the atmosphere. The problem starts when it migrates upward into an enclosed building, where it gets trapped and accumulates to dangerous concentrations.
Not all ground produces the same amount of radon. Certain geological formations are especially rich in uranium and radium (the elements that decay into radon). Granites, migmatites, and some types of clay and glacial till generate the highest levels. If your home sits on one of these formations, the radon potential is significantly elevated. Areas with granite bedrock also tend to have high radon levels in groundwater, which compounds the issue for homes relying on drilled wells.
How Radon Gets Inside
Your house works like a gentle vacuum. Warm air rising inside the building creates slightly lower air pressure near the foundation compared to the surrounding soil. This pressure difference literally pulls radon gas out of the ground and into your home through any available opening: floor-to-wall joints, cracks in the slab, gaps around pipes, construction joints, and drainage structures. Wind hitting one side of a building can intensify this pressure difference, pushing even more soil gas inside.
Every foundation type is vulnerable. Basements offer the most direct contact with soil and the most potential entry points, but slab-on-grade homes can have just as many openings. Crawlspaces are also affected because the vacuum inside the home extends down into the crawlspace, drawing radon upward even when vents are present. Manufactured homes with skirting around the base can trap radon underneath, creating the same problem. All conventional construction types have been found to exceed the action level of 4 pCi/L (picocuries per liter of air).
Weather Conditions That Raise Levels
Radon concentrations inside a home aren’t constant. They fluctuate with weather, and certain conditions make the threat worse. Outdoor air temperature is the single most influential factor. During cold months, the temperature difference between a heated home and the cold ground amplifies the pressure differential that pulls radon inside. At the same time, windows stay closed and sealed to conserve heat, so less fresh air dilutes the accumulated gas. This is why winter typically brings the highest indoor radon readings.
Drops in atmospheric pressure also matter. Lower barometric pressure makes it easier for radon to escape from the soil, and research from Serbia found a modest but consistent negative correlation between outdoor pressure and indoor radon levels. Storm systems and windy conditions can create additional pressure imbalances around a building’s foundation, further enhancing radon entry.
Radon From Water and Other Sources
Radon can dissolve in groundwater, and when that water is used inside a home, the gas escapes into the air during showering, washing, and other activities. However, the contribution is small. The EPA estimates that water with 4,000 pCi/L of radon adds only about 0.4 pCi/L to your indoor air. Water treated to the stricter standard of 300 pCi/L contributes just 0.03 pCi/L. For comparison, the EPA’s action level for indoor air is 4 pCi/L, so even moderately contaminated water accounts for a fraction of a typical radon problem. Homes on municipal water systems face almost no risk from this pathway because the water has been treated and aerated before delivery.
The Health Stakes
Radon is the second leading cause of lung cancer in the United States, responsible for approximately 21,000 lung cancer deaths every year. About 2,900 of those deaths occur in people who have never smoked. Globally, the World Health Organization estimates radon causes up to 15 percent of all lung cancers.
Smoking dramatically amplifies the danger. Smokers exposed to elevated radon are an estimated 25 times more at risk of developing lung cancer than non-smokers at the same exposure level. The combination of tobacco smoke and radon is far more lethal than either hazard alone, which is why radon testing is especially critical in households where anyone smokes.
Testing and Fixing the Problem
The EPA recommends that homes with radon at or above 4 pCi/L be fixed, and suggests homeowners consider mitigation even at levels between 2 and 4 pCi/L. There is no known safe level of radon exposure, so lower is always better. Short-term test kits are widely available at hardware stores and give a preliminary reading within a few days.
The most common and reliable fix is called subslab depressurization. A pipe is installed through the foundation slab into the gravel or soil beneath, and a small fan draws radon from under the house and vents it above the roofline, where it disperses safely. This system can reduce indoor radon by 50 to 99 percent, and the EPA notes some systems achieve reductions near the top of that range. Installation typically takes a day, and the fan runs continuously with modest energy costs comparable to a light bulb. Because the primary threat comes from the soil, addressing that pathway solves the problem for the vast majority of homes.