Radon is created by the natural radioactive decay of uranium in rock, soil, and water. Specifically, uranium-238 slowly breaks down through a chain of intermediate elements, eventually forming radium-226, which then decays directly into radon-222, the isotope responsible for nearly all radon-related health concerns. This process has been happening since the Earth formed and will continue for billions of years.
The Decay Chain That Produces Radon
Uranium-238 doesn’t turn into radon in a single step. It passes through a series of transformations, each producing a different radioactive element. The sequence moves from uranium-238 to uranium-234, then to thorium-230, then to radium-226. When radium-226 decays, it releases an alpha particle (a tiny bundle of protons and neutrons) and becomes radon-222, a colorless, odorless gas. The entire chain eventually ends at lead-206, which is stable and no longer radioactive.
What makes radon unusual in this chain is that it’s the only product that exists as a gas. Every element before and after it in the sequence is a solid metal that stays locked in rock or soil. Radon, by contrast, can escape into the air. It has a half-life of about 3.8 days, meaning half of any given amount decays within that window. That’s long enough for the gas to migrate out of the ground and into buildings, but short enough that it doesn’t travel far from its source before breaking down into other radioactive particles.
Where the Uranium Comes From
Uranium exists in virtually all rock and soil, but concentrations vary enormously depending on geology. Ultramafic rocks (dense rocks from deep in the Earth’s mantle) may contain less than one part per million of uranium, while phosphorite deposits along the U.S. Coastal Plain can hold up to 1,350 parts per million. Granites and black shales are the most commonly encountered high-uranium rock types. Devonian-Mississippian black shales in the Appalachian Plateau region, for example, contain around 70 parts per million, and some Mississippian sandstones reach 140 parts per million.
This is why radon levels vary so much from one neighborhood to the next, or even between adjacent houses. The geology directly beneath a building determines how much uranium and radium are present, and therefore how much radon is being generated in the soil at any given moment.
How Radon Gets Into Buildings
The primary way radon enters a home is through soil gas being pulled in from the ground beneath the foundation. Warm air rising inside a house creates a slight pressure difference between the indoor air and the surrounding soil, sometimes called the stack effect. This small but persistent vacuum draws soil gas, including radon, upward through cracks in the foundation, gaps around pipes, sump pits, and even porous concrete or cinder block walls.
Radon can also dissolve in groundwater. If your home uses well water, the gas comes out of solution during everyday activities like showering, running the dishwasher, or cooking. For most homes, though, soil gas entry through the foundation is the dominant source. Building materials like granite countertops or concrete do contain trace amounts of radioactive material and can contribute small amounts of radon, but the CDC notes these contributions are minor compared to what seeps in from the ground below.
Why Radon Matters for Health
Radon itself is a gas you breathe in and out without it doing much harm directly. The real danger comes from what radon decays into: solid radioactive particles (called radon progeny) that stick to lung tissue. These particles emit alpha radiation, which damages the DNA in lung cells. Unlike other forms of radiation that can pass through the body, alpha particles deposit all their energy in a very small area, making them especially destructive at close range.
Research has revealed that the damage extends beyond direct hits. Alpha particles can generate reactive factors in surrounding tissue that cause DNA alterations in nearby cells that were never directly struck by radiation. In lab studies using human lung cells, these indirect effects produced DNA damage equivalent to what direct irradiation would cause. This means the biological target for alpha particle damage is effectively larger than a single cell nucleus, which helps explain why even relatively low doses of radon exposure carry real risk over time.
Radon is the second leading cause of lung cancer in the United States, responsible for an estimated 21,000 lung cancer deaths each year. The EPA recommends fixing your home if indoor radon levels reach 4 pCi/L (picocuries per liter) or higher, and suggests considering mitigation even at levels between 2 and 4 pCi/L, since there is no known safe threshold for radon exposure.
Testing and Reducing Radon Levels
Because radon is invisible and odorless, testing is the only way to know your home’s levels. Short-term test kits measure radon over a few days, while long-term detectors give a more accurate picture of average exposure over months. You can pick up a test kit at most hardware stores or order one from your state radon program.
If levels come back high, the most common fix is a sub-slab depressurization system: a pipe and fan installed beneath your foundation that draws radon-laden soil gas out before it enters the house and vents it above the roofline. These systems typically reduce indoor radon by 80% to 99% and cost between a few hundred and a couple thousand dollars to install. For homes with radon in well water, aeration systems or activated carbon filters can remove the gas before it reaches your taps and showerheads.