Radon is a naturally occurring radioactive gas that forms in soil and rock, then seeps into homes through the foundation. It has no color, no odor, and no taste, which makes it impossible to detect without a test. It’s also the second leading cause of lung cancer after smoking, responsible for an estimated 21,000 lung cancer deaths per year in the United States alone.
How Radon Forms in the Ground
Radon starts with uranium, a radioactive element found naturally in most soil and rock. Uranium decays slowly over billions of years through a chain of intermediate elements. One of those intermediates is radium-226, which decays directly into radon-222, the isotope responsible for nearly all indoor radon exposure. When radium atoms in soil or rock release radon, the gas migrates through tiny air pockets in the ground and eventually reaches the surface.
Because radon is a noble gas (element 86 on the periodic table), it doesn’t bond with other elements. It simply drifts. That chemical inertness is part of what makes it dangerous: it moves freely through soil, through cracks in concrete, and into the air you breathe.
Why Some Areas Have More Radon
The type of rock and soil beneath a home is the biggest factor in how much radon it produces. Certain geologies are particularly prone. Limestone, especially phosphatic limestone from the Ordovician period, along with black shale from the Devonian period, tends to concentrate uranium and generate more radon. A Kentucky-based study found that homes built on carbonate bedrock (like limestone) had indoor radon levels averaging 2.8 pCi/L higher than homes built on other types of rock.
Granite-rich regions are also commonly associated with elevated radon. But geology alone doesn’t determine your risk. Soil permeability, moisture content, seasonal temperature changes, and the construction of your home all play a role. Two houses on the same street can have very different radon levels.
How Radon Gets Inside a Home
Radon doesn’t need a large opening. It enters through any gap where your home contacts the soil: cracks in a basement floor, joints where the floor meets the wall, gaps around pipes, sump pump openings, and even porous building materials like concrete and brick. Homes with crawl spaces, slab-on-grade foundations, and basements are all vulnerable. Even manufactured homes can have elevated levels if they’re skirted and sitting on soil.
Several forces actively pull radon indoors. Warm air rising through your home creates what’s called the stack effect, drawing cooler, soil-contact air in at the lowest level. Exhaust fans, dryers, furnaces, fireplaces, and hot water heaters all remove air from inside the house, and some of the replacement air gets pulled from the soil beneath the foundation. Strong winds flowing over and around a home can create a vacuum effect that does the same thing. In short, your home acts like a gentle vacuum on the ground beneath it, and radon hitches a ride with the soil gases that get drawn in.
What Radon Does to Your Lungs
Radon itself is a gas you inhale and exhale. The real damage comes from its decay products, which are solid radioactive particles. When radon decays inside your lungs, it releases alpha particles: heavy, high-energy bits of radiation that tear through cells at close range. Each alpha particle leaves a dense track of ionization as it crosses through tissue, and when that track passes through a cell’s nucleus, it can break both strands of the DNA helix in multiple places simultaneously.
These double-strand DNA breaks are the most serious type of radiation damage. Your cells have repair systems that attempt to stitch the broken DNA back together, but the process is error-prone. Fragments can be lost, incorrectly reattached, or swapped with pieces from other breaks along the same alpha track. The result is cells that survive but carry permanent genetic alterations: deletions, rearrangements, and point mutations. Over years of exposure, these accumulated errors can push a cell toward uncontrolled growth.
Alpha particles also generate reactive oxygen molecules inside cells, which cause additional, subtler DNA damage that leads to miscoding during cell replication. The combined effect of direct DNA breakage and oxidative damage is what makes long-term radon exposure a potent carcinogen.
Risk Levels for Smokers and Non-Smokers
Radon’s danger multiplies dramatically if you smoke. At 4 pCi/L, the EPA’s action level, roughly 62 out of every 1,000 smokers exposed over a lifetime could develop lung cancer. For people who have never smoked, that number drops to about 7 out of 1,000 at the same concentration. Smoking damages the lungs’ ability to clear radioactive particles and creates cells that are already more vulnerable to further mutation, so radon and tobacco together are far worse than either one alone.
That said, radon is the leading cause of lung cancer among non-smokers. You don’t need to smoke for radon to pose a meaningful risk, particularly at higher concentrations or over many years of exposure.
Safe Levels and Action Thresholds
There is no known safe level of radon exposure. The EPA recommends fixing your home if levels reach 4 pCi/L (150 Bq/m³) or higher, and suggests considering action for levels between 2 and 4 pCi/L. The World Health Organization sets a lower reference level of 2.7 pCi/L (100 Bq/m³). The average outdoor radon concentration is about 0.4 pCi/L, so even the EPA’s action level represents a tenfold increase over what you’d breathe outside.
How to Test Your Home
Testing is the only way to know your radon level, and it’s straightforward. You have two main categories of devices: passive and active.
Passive devices don’t need electricity. The most common short-term option is a charcoal canister, which you place in the lowest lived-in level of your home for two to four days, then mail to a lab for analysis. These kits are inexpensive and widely available through the National Radon Program Services and hardware stores. For a more accurate long-term picture, alpha track detectors sit in your home for three months to a full year before being sent to a lab. Because radon levels fluctuate with weather, season, and how you use your home, long-term tests give a more reliable annual average.
Active devices are continuous monitors that require power and typically need a trained operator. They measure radon levels in real time and can show you the range of variation during the testing period. Some models are designed to detect tampering, which makes them popular during real estate transactions.
If a short-term test comes back at or above 4 pCi/L, the standard recommendation is to follow up with a second test (either another short-term or a long-term test) before deciding on mitigation.
How Radon Mitigation Works
The most common and effective fix is called sub-slab depressurization. A contractor drills a small hole through your basement floor or slab, inserts a pipe, and connects it to a fan that runs continuously. The fan creates suction beneath the foundation, pulling radon-laden soil gas out before it can enter your living space and venting it above the roofline where it disperses harmlessly.
This system typically reduces indoor radon by 80 to 99 percent. Installation usually takes a day, and the cost for most homes falls between $800 and $2,500 depending on the foundation type and local labor rates. The fan runs around the clock and uses roughly as much electricity as a single light bulb. Once installed, you should retest periodically to confirm the system is still performing, since fans can wear out over time and foundation conditions can shift.
For homes with crawl spaces, a similar approach involves sealing the crawl space with a heavy plastic membrane and depressurizing the area beneath it. Sealing cracks and openings alone, without active depressurization, rarely reduces radon enough to solve the problem on its own.