Radon is extremely common. It’s a naturally occurring radioactive gas found in virtually every home, school, and building. The real question is how often it reaches dangerous levels, and the answer is surprisingly frequent: nearly 1 in 15 U.S. homes has radon at or above the level where action is recommended. That translates to roughly 8 million homes with elevated concentrations.
How Many Homes Have Dangerous Levels
The EPA sets its action level at 4 pCi/L (picocuries per liter), the concentration at which homeowners should take steps to reduce radon. About 1 in 15 homes exceeds this threshold. But there is no known safe level of radon exposure, which is why the EPA also recommends considering mitigation for homes between 2 and 4 pCi/L. If you include that range, the number of affected homes is significantly larger.
Indoor radon concentrations vary enormously, even between neighboring houses. Globally, levels inside buildings range from 10 to more than 10,000 becquerels per cubic meter. A home sitting on one type of soil can have ten times the radon of a house across the street built on different ground. This variability is why testing your specific home is the only way to know your exposure.
Why Some Areas Have More Radon
Radon forms from the natural decay of uranium in soil, rock, and water. Certain geological features concentrate uranium and produce more radon gas. Black shales, for instance, can contain 10 to 40 parts per million of uranium, while phosphate-rich nodules within those shales can reach 350 ppm. Silica-cemented layers in some sedimentary formations have been measured at 125 ppm in the most intensely ceite areas. Even volcanic ash deposits, common across the central and western United States, contain measurable uranium at 4 to 9 ppm, and as that ash weathers into surrounding sediment, it releases radon into the soil.
The gas seeps upward through the ground and enters buildings through cracks in foundations, gaps around pipes, and other openings. Homes with basements tend to have higher levels because more of the structure sits in contact with soil. Tight, energy-efficient construction can also trap radon indoors, allowing it to accumulate. Geography matters, but construction style, foundation type, and local soil conditions all play a role, which is why two homes on the same block can have very different readings.
The Health Risk in Numbers
Radon is responsible for about 21,000 lung cancer deaths every year in the United States, making it the second leading cause of lung cancer after smoking. The combination of radon exposure and smoking multiplies the risk dramatically, but radon is dangerous for nonsmokers too.
For people who have never smoked, lifetime exposure at 4 pCi/L (the EPA action level) leads to about 7 lung cancers per 1,000 people. At 20 pCi/L, that number jumps to roughly 36 per 1,000. For context, 7 out of 1,000 is comparable to the risk of dying in a car accident over a lifetime. Globally, radon is estimated to cause between 3% and 14% of all lung cancers in a given country, depending on national radon levels and smoking rates.
How to Test Your Home
Short-term radon test kits are inexpensive and widely available at hardware stores and online. You place the kit in the lowest lived-in level of your home for two to seven days, then mail it to a lab for analysis. Long-term tests, which remain in place for 90 days or more, give a more accurate picture of your year-round average.
Retesting is worth doing periodically, even if your first result came back low. Radon levels can shift over time as soil conditions change, your home settles, or you renovate. The EPA specifically recommends retesting if you start using a lower level of your home (like finishing a basement), if the last test is more than two years old, or if you’ve made structural changes. If you’re buying a home, any test older than two years or done on a different floor than where you plan to live is worth redoing.
What Mitigation Looks Like
If your test comes back at 4 pCi/L or above, the most common fix is a system called subslab depressurization. A contractor installs a pipe through the foundation slab and connects it to a fan that draws radon gas from beneath the house and vents it outside before it can enter your living space. This approach typically reduces radon levels by 50% to 99%, and most homes can be brought well below the action level.
Installation usually takes less than a day. The system runs continuously and uses about as much electricity as a light bulb. After installation, you should retest to confirm levels have dropped, and then check periodically to make sure the system is still performing. The fan will eventually wear out, usually after several years, but replacement is straightforward. For homes in the 2 to 4 pCi/L range, sealing cracks and improving ventilation can sometimes bring levels down without a full mitigation system, though active depressurization remains the most reliable option.