Yes, radon causes lung cancer. It is the leading cause of lung cancer among non-smokers and the second leading cause overall, behind smoking. The EPA estimates that about 2,900 never-smokers die from radon-related lung cancer each year in the United States, and the World Health Organization attributes up to 15% of all lung cancers worldwide to radon exposure.
How Radon Damages Your Lungs
Radon is a radioactive gas that forms naturally as uranium in soil and rock breaks down. It seeps into homes through cracks in foundations, gaps around pipes, and other openings. You cannot see, smell, or taste it, and it causes no immediate symptoms. There is no cough, no irritation, no warning sign of any kind. The only way to know it’s in your home is to test for it.
When you breathe in radon, its radioactive decay products settle in your airways. These particles release alpha radiation, which are heavy, high-energy particles that tear through the DNA in the cells lining your bronchial tubes. A single alpha particle crosses a cell in less than a trillionth of a second, but it deposits enough energy to snap both strands of the DNA double helix in multiple places simultaneously. The resulting breaks are complex, often shattering DNA into small fragments that get lost or reattached in the wrong position when the cell tries to repair itself.
Beyond the direct breakage, alpha particles also generate reactive oxygen molecules inside cells. These damage individual DNA building blocks and cause point mutations, where a single “letter” in the genetic code gets swapped for the wrong one. Over years of exposure, these accumulated errors in genes that control cell growth can trigger the uncontrolled division that defines cancer. The damage is cumulative: the longer you breathe radon-contaminated air, the greater the risk.
Why Smokers Face a Much Higher Risk
Radon and cigarette smoke interact in a way that is worse than simply adding their individual risks together. Research from the National Research Council’s BEIR VI report describes this as a synergistic relationship. If you smoke and live in a home with elevated radon, your already high lung cancer risk is amplified more than what you’d expect from either exposure alone. The combination is particularly dangerous when smoking precedes or overlaps with radon exposure, producing a more-than-multiplicative effect on relative risk.
Interestingly, the percentage increase in risk per unit of radon exposure is actually higher for never-smokers than for smokers. But because smokers start with a baseline lung cancer risk that is many times greater, the absolute number of additional cancers caused by radon is far larger in the smoking population. This is why radon mitigation matters for everyone, but carries special urgency if anyone in the household smokes.
The Action Level for Your Home
Radon concentrations are measured in picocuries per liter (pCi/L). The EPA sets its action level at 4 pCi/L, meaning you should take steps to reduce radon if your home tests at or above that number. The agency also recommends considering mitigation for levels between 2 and 4 pCi/L, since there is no known safe threshold for radon exposure. Any amount carries some degree of risk; 4 pCi/L is simply the point where mitigation becomes clearly cost-effective and technically achievable in most homes.
For context, outdoor air typically contains about 0.4 pCi/L of radon. The average indoor level in the U.S. is around 1.3 pCi/L. Homes sitting on certain geological formations, particularly those with uranium-rich rock like granite, shale, or phosphate-bearing limestone, can have levels many times higher. In one study of north central Kentucky, median radon levels across different rock types ranged from 2.75 to 8.10 pCi/L, with four geological categories consistently associated with elevated readings.
How to Test Your Home
Because radon is completely undetectable by your senses, testing is the only option. You can buy inexpensive test kits at hardware stores or online, and many state health departments offer free or discounted kits. There are two types to know about.
Short-term tests run for 2 to 90 days and give you a quick snapshot. They’re useful as a first screening, but radon levels fluctuate with weather, soil moisture, and how often you open windows, so a short test may not capture your true average exposure. Long-term tests run for more than 90 days and provide a much more reliable picture of your year-round radon level. The CDC notes that the longer the test, the better it reflects your actual exposure. If a short-term test comes back at or near 4 pCi/L, following up with a long-term test or a second short-term test is a reasonable next step.
Fixing a Radon Problem
If your home has elevated radon, the most common and reliable fix is a system called active subslab depressurization. A contractor installs a pipe through the foundation slab and attaches a small fan that draws radon-laden air from beneath the house and vents it safely above the roofline. According to the EPA, this method reduces indoor radon levels by 50 to 99 percent. Most installations cost between $800 and $2,500, and the fan runs continuously, using about as much electricity as a light bulb.
The system requires minimal maintenance. You’ll want to retest your home after installation to confirm the levels have dropped, and periodic retesting every few years ensures the system is still performing. For homes with crawl spaces rather than slabs, similar depressurization techniques work with slight modifications. In nearly all cases, radon can be brought well below 4 pCi/L regardless of how high the starting level is.
Who Should Be Most Concerned
Every home has some radon. The question is how much. Geography plays a major role: homes built over granite, certain shales, or other uranium-bearing rock formations tend to have higher levels. But even in areas classified as low-risk on EPA radon maps, individual homes can test surprisingly high because of localized soil conditions, construction details, or how a foundation interacts with the ground beneath it. Two houses on the same street can have very different radon levels.
People who spend the most time in lower levels of their home, where radon concentrations are typically highest, accumulate the most exposure. If your bedroom, home office, or main living space is in a basement or ground floor, testing is especially worthwhile. The risk is proportional to both the concentration and the duration of exposure, so a moderately elevated level breathed for decades carries real significance, even if it never triggers immediate symptoms.