Radon is a naturally occurring radioactive gas that forms underground and seeps into the air you breathe. It’s colorless, odorless, and the heaviest naturally occurring gas, which means it tends to collect in low-lying enclosed spaces like basements and ground floors. In the United States alone, radon exposure is responsible for an estimated 21,000 lung cancer deaths every year, making it the second leading cause of lung cancer after smoking.
How Radon Forms in the Ground
Radon doesn’t come from a factory or a spill. It’s produced by a chain of radioactive decay that starts with uranium-238, an element found naturally in rocks and soil worldwide. Over billions of years, uranium-238 slowly breaks down into a series of other elements. One of those is radium-226, and when radium-226 decays, it produces radon-222, the form of radon that matters most for your health.
Because radon is chemically inert (it doesn’t bond with other elements), it moves freely through soil and rock rather than getting trapped in mineral deposits. Once it forms, it migrates upward through gaps in the earth, eventually reaching the surface and dispersing into outdoor air at harmless concentrations. The problem starts when something sits on top of the soil and traps it: your house.
Why Some Areas Have More Radon Than Others
The amount of radon that reaches the surface depends heavily on local geology. Rocks that contain more uranium produce more radon, and certain rock types are significantly worse than others. A study analyzing indoor radon levels across Georgia found that homes built on gneiss, schist, quartzite, and granite had the highest indoor concentrations, followed by limestone-dolomite-shale and limestone-sandstone-shale formations. The lowest levels were found over unconsolidated marine sediments, though even those weren’t risk-free.
This is why radon maps exist but can’t tell you whether your specific house is safe. Two homes on the same street can have very different radon levels depending on the exact soil composition, foundation type, and ventilation. The only way to know your level is to test.
How Radon Gets Into Your Home
Radon enters buildings primarily through advective flow, meaning soil gas containing radon gets physically pulled or pushed indoors. This happens because of small pressure differences between the air inside your home and the soil beneath it. Your house is slightly depressurized relative to the ground for several reasons: warm air rising inside creates a suction effect at the foundation, wind against exterior walls changes pressure dynamics, and HVAC systems can pull air upward and out. These pressure differences are tiny (often less than 5 pascals) but sustained enough to draw radon-laden soil gas through any available opening.
The entry points are predictable. Cracks in concrete slabs, gaps around plumbing and utility penetrations through the foundation, sump pits, and joints between the foundation wall and floor all provide pathways. Even intact concrete allows some radon to diffuse through, though cracks and gaps are the dominant routes. Changes in atmospheric pressure also create oscillating gas flow beneath the foundation, pumping soil gas in and out of the building independent of any indoor heating or ventilation effects.
Once inside, radon accumulates. Basements and ground-floor rooms with poor ventilation tend to have the highest concentrations because the gas is dense and enters from below.
What Radon Does to Your Lungs
Radon itself is an inert gas, so if you breathe it in, most of it gets exhaled right back out. The danger comes from what happens when radon decays inside your lungs. It breaks down rapidly into solid radioactive particles that lodge in the lining of your airways. These particles release alpha radiation, which is a heavy, high-energy form of radiation that damages DNA in nearby cells. Over years of exposure, this accumulated DNA damage can trigger uncontrolled cell growth: lung cancer.
Smoking dramatically amplifies the risk. The combination of radon exposure and tobacco smoke is far more dangerous than either one alone, because irritated and damaged lung tissue from smoking is more vulnerable to radiation-induced mutations. Most of the 21,000 annual radon-related lung cancer deaths in the U.S. occur in people who smoke or formerly smoked, though radon causes lung cancer in people who have never smoked as well.
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. At that level, you should fix the problem as soon as possible. Between 2 and 4 pCi/L, the EPA still recommends considering mitigation, because there is no known safe level of radon exposure. Any amount carries some degree of risk; 4 pCi/L is simply the threshold where the agency says the cost of fixing the problem is clearly justified by the health benefit.
Testing is straightforward. Short-term test kits sit in your lowest livable floor for 2 to 7 days and cost under $20 at most hardware stores. Long-term tests (90 days or more) give a more accurate picture of your average exposure, since radon levels fluctuate with weather, season, and ventilation patterns.
How Radon Mitigation Works
The most common fix is called active soil depressurization. A contractor installs a pipe through or alongside your foundation slab that connects to a small fan. The fan runs continuously, pulling soil gas from beneath the foundation and venting it above your roofline, where it disperses harmlessly into outdoor air. This reverses the pressure dynamic that draws radon indoors in the first place.
The system is effective. A field study by Health Canada across 52 homes found an average radon reduction of 90.7%, with a median reduction of 93.5%. The best-performing installations cut radon by 99.6%. Even the least effective system in the study still reduced levels by 47%. Installation typically takes a day, and the ongoing cost is limited to electricity for the fan, which is roughly comparable to running a light bulb continuously. Sealing visible cracks and gaps in the foundation complements the system but on its own is rarely enough to solve a radon problem.