Radon is a naturally occurring radioactive gas found in soil, rock, groundwater, indoor air, and certain building materials. It forms underground when uranium and radium in the earth’s crust decay, and it seeps upward through soil into the air we breathe. Because it’s colorless and odorless, most people encounter it without ever knowing. In the United States alone, radon exposure causes an estimated 21,000 lung cancer deaths every year, making it the second leading cause of lung cancer after smoking.
Radon in Soil and Rock
The ultimate source of radon is uranium, a radioactive element naturally present in the earth’s crust. As uranium decays over time, it transforms into radium, which then decays into radon gas. This process happens continuously in virtually all soil and bedrock, but certain geological formations produce far more radon than others.
Granites, migmatites, and some clays and tills are particularly rich in uranium and radium. If your home sits on or near these rock types, the soil beneath your foundation likely generates higher concentrations of radon. The U.S. Geological Survey has mapped radon potential across the country using a composite score based on five factors: local geology, soil permeability, aerial radioactivity measurements, typical home construction styles, and indoor radon screening data. These maps group areas into three risk zones, and they’re a useful starting point for understanding your region’s baseline risk.
Soil permeability matters as much as uranium content. Loose, gravelly, or sandy soils allow radon to travel easily toward the surface. Dense clay soils slow it down. A home built on highly permeable soil over uranium-rich bedrock faces the highest natural exposure.
Radon in Your Home
Most indoor radon enters through the soil beneath a building’s foundation, pulled inside by small differences in air pressure between the house and the ground. Warm air naturally rises inside a home, creating a slight vacuum at the lowest levels. This is called the stack effect, and it works exactly like the draft in a chimney: as warm air escapes through upper floors, replacement air gets drawn in through any opening near the ground. That replacement air often carries radon-laden soil gas with it.
The specific entry points include cracks in concrete slabs and basement walls, gaps at floor-wall joints, sump holes, floor drains, openings around pipes and utility lines, pores in concrete block walls, and spaces left around plumbing traps beneath tubs and showers. Even heating ducts run beneath slabs can act as channels. Essentially, any gap or crack in the foundation that connects indoor air to the soil beneath can serve as a radon pathway.
Pressure-driven airflow from below the foundation is the dominant entry mechanism in most buildings with elevated radon. Diffusion plays a smaller role: because radon concentration in soil gas is higher than in indoor air, the gas naturally migrates through foundation materials even without pressure differences, though this contributes less overall.
Radon in Water
Radon dissolves easily in groundwater, especially in areas with uranium-rich geology. Private wells drilled into bedrock in these regions can contain significant radon concentrations. Municipal water systems that draw from surface sources like reservoirs tend to have much lower levels, partly because radon dissipates quickly when water is exposed to open air and partly because treatment processes reduce it further.
When radon-containing water is used indoors for showering, washing dishes, or running faucets, the gas escapes into the air. The health risk from this released gas is far greater than the risk from drinking the water itself. A National Research Council analysis estimated that of roughly 19,000 annual radon-related deaths in the home, about 160 resulted from inhaling radon released from household water. The risk of stomach cancer from actually drinking radon-dissolved water is extremely small, accounting for an estimated 20 deaths per year. So the concern with radon in water is primarily about what it adds to your indoor air, not what it does in your stomach.
Radon in Building Materials
Materials made from sandstone, concrete, brick, natural stone, gypsum, and granite contain trace amounts of uranium, radium, and thorium. These elements decay and release small amounts of radon gas. Granite countertops, for example, have received attention for this reason.
In practice, the contribution from building materials is minimal. The CDC notes that these materials are highly unlikely to raise radiation exposure above normal background levels. The far more significant source is soil gas entering through the foundation. If your home has elevated radon, the fix almost certainly involves addressing what’s coming up from the ground, not replacing your countertops or walls.
How Much Radon Is Too Much
The EPA recommends taking action if your home’s radon level reaches 4 picocuries per liter (pCi/L) or higher. The agency also suggests considering remediation for levels between 2 and 4 pCi/L, since there is no truly safe level of radon exposure. Most international guidelines fall in a similar range. There is no threshold below which radon is considered completely harmless; risk simply decreases as concentration drops.
These numbers only matter if you actually test. Because radon is invisible and odorless, testing is the only way to know your exposure level.
Testing for Radon
Home radon test kits come in two categories. Short-term kits, typically charcoal canisters, stay in your home for two to seven days before being mailed to a lab for analysis. They give a snapshot of radon levels during that brief window, but because radon fluctuates daily and seasonally, they’re less reliable as an estimate of your true year-round exposure.
Long-term kits use alpha-track detectors and remain in your home for more than 90 days, ideally close to a full year. The closer the measurement period is to 365 days, the more accurately it reflects your actual annual average. Both types are passive devices that require no power or special setup. You place them in the lowest livable area of your home, leave them undisturbed, and send them to a lab when the testing period ends.
A common approach is to start with a short-term test for a quick read. If results come back at or above 4 pCi/L, a follow-up test (either another short-term or a long-term test) confirms the finding before you invest in mitigation.
Reducing Radon Levels
The most effective and widely used fix is called sub-slab depressurization. A contractor installs a pipe through the foundation slab into the gravel or soil beneath, then attaches a fan that continuously draws soil gas from under the house and vents it above the roofline, where it disperses harmlessly. This reverses the pressure difference that pulls radon indoors. These systems can reduce indoor radon concentrations by up to 99%, and they run quietly and continuously once installed.
For a typical residential home, installation costs generally range from around $800 to $2,500, depending on the foundation type, local labor rates, and system complexity. The fan uses roughly as much electricity as a standard light bulb running 24 hours a day. Once installed, the system requires little maintenance beyond occasionally checking that the fan is running and the manometer (a simple gauge on the pipe) shows proper airflow.
Other approaches include sealing foundation cracks, improving ventilation in crawl spaces, and installing aeration or carbon filtration systems for well water with high radon. Sealing cracks alone is not sufficient to solve a radon problem, but it can improve the performance of a depressurization system by reducing the number of entry points that compete with the suction pipe.