Radon comes from the natural breakdown of uranium in rock, soil, and water. Uranium-238 slowly decays through a chain of radioactive elements, eventually becoming radium-226, which then decays into radon-222, a colorless, odorless gas. This process happens continuously in the earth’s crust, and the gas seeps upward through soil and into the atmosphere, or into buildings sitting on top of it.
Rock and Soil: The Primary Source
The earth itself is the biggest emitter of radon. Uranium exists in varying concentrations across nearly all rock and soil, but certain geological formations produce far more radon than others. Granite is one of the most well-known high-radon rock types because it tends to contain elevated levels of uranium. Black shales from the Carboniferous period, metamorphosed sedimentary rock, and volcanic formations are also significant sources. Soils derived from carbonate rock, particularly in areas with limestone cave systems (karst terrain), can release substantial amounts of the gas.
Glacial deposits that originated from uranium-bearing rock carry that radioactive signature with them, which is why some regions far from granite bedrock still have elevated radon levels. River, lake, and marine sediments that accumulated uranium over geological time also contribute.
How much radon actually reaches the surface depends heavily on soil permeability. Sandy, gravelly soils with large pore spaces allow the gas to migrate upward easily and exhale into the atmosphere. Clay-heavy or compacted soils with low permeability trap radon below the surface, creating pockets of concentrated gas underground. Buildings sitting above those pockets are especially vulnerable, because the gas accumulates below the foundation and is drawn inside through pressure differences between the soil and the building’s interior.
How Radon Gets Into Your Home
Radon doesn’t need much of an opening. It enters through cracks in basement floors and walls, gaps where pipes and wires penetrate the foundation, floor-to-wall joints, construction joints, control joints in concrete slabs, and sump pump openings. Homes with suspended floors can pull radon through gaps between floorboards. Even cavities inside walls can act as channels for the gas to travel upward through a building.
The driving force is simple: warm air rising inside your home creates a slight vacuum at lower levels, pulling soil gas inward through any available opening. This effect is stronger in winter when the temperature difference between indoors and outdoors is greatest, which is why radon levels in many homes peak during cold months.
Groundwater and Well Water
Radon dissolves readily into groundwater as it passes through rock and soil containing radium. Private wells drilled into granite or other uranium-rich bedrock can deliver water with significant radon concentrations directly into your home. The gas stays dissolved while the water is under pressure underground, but it releases into the air the moment it reaches atmospheric pressure at your tap. Showers, dishwashers, and washing machines all agitate the water enough to release dissolved radon into your indoor air.
Public water systems that draw from surface reservoirs tend to have very low radon levels because the gas escapes naturally before the water reaches your home. The concern is primarily with private wells, especially in areas with granitic bedrock.
Building Materials
Some construction materials emit low levels of radon on their own. Concrete, brick, natural stone, granite countertops, sandstone, and gypsum all contain trace amounts of uranium, radium, and thorium. As these elements decay, they release small quantities of radon gas into the room.
The CDC notes that for most building materials, these levels are very low. A granite countertop, for instance, adds a negligible amount of radon compared to what seeps up through the soil beneath a home. Building materials are worth knowing about, but they’re rarely the primary driver of elevated indoor levels.
Natural Gas
Natural gas picks up radon underground during extraction. The gas travels through pipelines and reaches homes and businesses still carrying trace amounts. However, the actual exposure is minimal. For typical residential gas usage, the estimated radiation dose from radon in natural gas is less than 1% of the dose you’d receive from the radon already present in an average home from ground sources. Combustion dilutes it further. Commercial users with high gas consumption receive somewhat more exposure, but even then the doses remain small relative to other radon sources.
Outdoor Air
Radon is everywhere outdoors, just at very low concentrations. The average outdoor level in the United States is about 0.4 pCi/L. At that level, the gas disperses harmlessly into the atmosphere. The problem arises when radon accumulates in enclosed spaces, particularly basements and ground-floor rooms with poor ventilation, where concentrations can build to levels dozens of times higher than outdoor air.
Why Radon Levels Matter
Radon is responsible for an estimated 21,000 lung cancer deaths per year in the United States, making it the leading cause of lung cancer among nonsmokers. The risk compounds dramatically for smokers: cigarette smokers exposed to radon face 10 to 20 times the lung cancer risk of nonsmokers with the same radon exposure. For children exposed to both radon and tobacco smoke, the risk increases at least 20-fold.
The EPA recommends taking action to reduce radon if your home tests at 4 pCi/L or higher. Because no level of radon exposure is considered completely safe, the agency also suggests considering mitigation for levels between 2 and 4 pCi/L. Testing is the only way to know your home’s radon level, since the gas is invisible and odorless. Short-term test kits are inexpensive and widely available, and professional mitigation systems (typically a fan and pipe that vent soil gas from beneath the foundation to outside) can reduce indoor levels by up to 99%.