Radon is a radioactive gas that forms naturally underground when uranium breaks down in rocks and soil. It’s colorless, odorless, and constantly seeping out of the ground beneath your feet. The reason people care about it: radon is the second leading cause of lung cancer, and it can accumulate inside buildings to concentrations well above what’s considered safe.
How Uranium Becomes Radon
Radon doesn’t appear on its own. It’s the product of a long chain of radioactive decay that starts with uranium-238, one of the most common radioactive elements in Earth’s crust. Over billions of years, uranium-238 slowly decays through a series of intermediate elements: first into uranium-234, then thorium-230, then radium-226. When radium-226 decays, it produces radon-222, the isotope responsible for nearly all indoor radon concerns. The entire chain eventually ends at stable lead-206.
What makes radon unique in this chain is that it’s a gas. Every other element in the sequence is a solid that stays locked in rock or soil. But when radium atoms decay into radon, the resulting gas can escape from mineral grains and migrate through tiny pore spaces in the ground. From there, it rises toward the surface and, if nothing stops it, dissipates harmlessly into outdoor air. The problem starts when a building sits on top of that soil.
There’s also a less common isotope called thoron (radon-220), which comes from a separate decay chain starting with thorium-232 rather than uranium. Thoron has a much shorter half-life, so it typically decays before traveling far from its source. Radon-222 is the primary concern for indoor air quality.
Which Rocks and Soils Produce the Most Radon
Since radon ultimately comes from uranium, the geology under your home matters enormously. Uranium concentrations in rock vary from less than one part per million in certain deep-earth rocks to extremely high levels in others. Granites and black shales are among the most common rock types with elevated uranium content. In the Appalachian Plateau, for instance, Devonian-Mississippian black shales contain roughly 70 parts per million uranium, and nearby sandstones reach up to 140 ppm.
Phosphate-rich sediments can be far higher. Tertiary phosphatic sediments along parts of the Coastal Plain locally contain up to 1,350 ppm uranium. Certain mineral sands carry several thousand parts per million. This is why radon risk maps vary so dramatically from one region to another, and why two houses a mile apart can have very different radon levels. The specific rock and soil composition directly beneath and around a building’s foundation determines how much radon is available to enter.
How Radon Gets Inside Buildings
Radon in outdoor air is rarely a concern because it dilutes quickly. Indoors is a different story. The primary way radon enters a home is through the soil beneath and around the foundation. Cracks in concrete slabs, gaps around pipes, sump pump openings, and porous foundation walls all provide pathways.
The driving force behind this entry is pressure. Your home typically has slightly lower air pressure than the surrounding soil, especially in the lower levels. This happens for several reasons. Warm air rising inside the house (sometimes called the stack effect) creates a mild vacuum near the foundation. Wind blowing against the exterior also creates pressure differences. Even running exhaust fans, dryers, or furnaces pulls air out of the house, which draws soil gas in to replace it. Research has identified convective entry driven by these pressure differences as the primary radon entry mechanism in single-family homes.
Wind pressure on the building’s exterior can also play a surprisingly large role in pushing soil gases into indoor air. The combined effect is that your home essentially acts as a low-grade vacuum, gently pulling radon-laden soil gas up through any available opening in the foundation.
Well Water
Soil isn’t the only pathway. Radon dissolves easily in groundwater, so homes with private wells can have a secondary source. When you shower, run the dishwasher, or cook with water that contains dissolved radon, the gas escapes into indoor air. This contribution is generally smaller than what enters through the foundation, but it adds to the total. Municipal water systems that draw from surface reservoirs are typically not a concern, since radon releases from open water before it reaches your tap.
Building Materials
Certain building materials, including concrete, brick, natural stone, granite, gypsum, and sandstone, contain trace amounts of uranium, radium, and thorium. These can emit low levels of radon. According to the CDC, however, building materials are highly unlikely to raise radiation exposure above normal background levels. The soil beneath the foundation remains the dominant source by a wide margin.
Why Radon Levels Change With the Seasons
If you’ve tested your home for radon more than once, you may have noticed the numbers aren’t constant. That’s because environmental conditions directly affect how much radon the soil produces and how readily it migrates.
Soil moisture is the single biggest factor. When soil gets wetter, radon that would otherwise dissolve in pore water gets pushed into the remaining air spaces, concentrating it. Research on Finnish homes found that soil air radon concentration doubles when the water saturation of soil increases from 30% to 70%. This means autumn rains and spring snowmelt can boost soil radon concentrations by 10 to 20%. In contrast, February and March, when soils in cold climates are frozen and dry, tend to produce the lowest soil gas radon levels.
Temperature plays a secondary role. Warmer soil in summer increases radon emanation from mineral grains by about 10 to 15% compared to winter values. Interestingly, atmospheric pressure has less influence than once thought. Studies have concluded that soil moisture and temperature, not barometric pressure, are the primary controllers of soil gas radon concentration.
Indoor radon levels also rise in winter for a separate reason: you keep your windows closed and run your heating system, which strengthens the stack effect and reduces ventilation. So while the soil may produce less radon in midwinter, your home may trap more of what does enter.
What Levels Are Considered Dangerous
The EPA recommends taking action to reduce radon if your home tests at 4 pCi/L (picocuries per liter) or higher. Because there is no known safe level of exposure, the EPA also recommends considering mitigation for levels between 2 and 4 pCi/L. Most countries worldwide have adopted similar action levels near the 4 pCi/L threshold.
Testing is the only way to know your home’s radon level. Short-term test kits are inexpensive and widely available at hardware stores. If a short-term test comes back elevated, a follow-up long-term test (90 days or more) gives a more accurate picture of your average exposure, since levels fluctuate with weather and season. If mitigation is needed, the most common fix is a sub-slab depressurization system: a pipe and fan installed through the foundation that pulls radon from beneath the house and vents it outside before it can enter. These systems typically reduce indoor radon by 80 to 99%.