Radon went from an invisible, unknown household threat to the second leading cause of lung cancer in the United States, responsible for an estimated 21,000 deaths every year. The story of how that happened begins with a construction worker, a nuclear power plant, and a radiation alarm that wouldn’t stop going off. Since then, radon has been the subject of federal regulations, home testing campaigns, and mitigation technology that can remove nearly all of it from indoor air. Yet despite decades of public health warnings, very few people actually test their homes.
The Incident That Started It All
In December 1984, a construction worker named Stanley Watras set off radiation detectors while entering the Limerick Nuclear Generating Station in Pennsylvania. The plant wasn’t even operational yet, so the contamination couldn’t have come from the facility. Investigators traced the source back to Watras’s own home, where they measured radon levels at 2,700 picocuries per liter (pCi/L). That’s roughly 700 times the level now considered safe for long-term exposure.
Watras and his family evacuated. The EPA and Pennsylvania state officials turned the house into a research lab, using it to study radon behavior and test ways to reduce it. The case made national headlines and forced a reckoning: if one family could be living in radiation levels that extreme without knowing it, how many others were too? Within a few years, the U.S. Surgeon General and the EPA began recommending that every home in the country be tested. By 1993, the National Association of Realtors joined the EPA in urging universal home testing.
How Radon Gets Into Your Lungs
Radon is a radioactive gas that forms naturally as uranium in soil and rock breaks down. It seeps through cracks in foundations, gaps around pipes, and any opening where a house meets the ground. Because it’s colorless and odorless, there’s no way to detect it without a test.
The gas itself isn’t the main problem. As radon decays, it produces tiny radioactive particles that attach to dust and aerosols in the air. When you breathe these in, they lodge in your lung tissue and emit alpha radiation, a form of energy that tears through DNA at close range. Research has shown that even a relatively low dose of alpha particles can trigger genetic damage not just in the cells they hit directly, but in neighboring cells as well. Irradiated cells release chemical signals that cause DNA alterations in unexposed cells nearby, effectively amplifying the damage beyond the initial point of contact. Over years or decades of exposure, this accumulated damage can lead to lung cancer.
About 2,900 of the 21,000 annual radon-related lung cancer deaths in the U.S. occur in people who have never smoked. For smokers, the combination of radon exposure and tobacco dramatically increases risk.
Where the Safety Threshold Stands
The EPA recommends taking action to reduce radon when indoor levels reach 4 pCi/L or higher. Most countries worldwide have adopted similar thresholds. But the EPA is also clear that no level of radon exposure is known to be completely safe, and it encourages homeowners to consider mitigation even at levels between 2 and 4 pCi/L.
The 4 pCi/L threshold is a practical guideline, not a line between safe and dangerous. It reflects what mitigation technology can reliably achieve balanced against cost. Living at that level for a lifetime still carries measurable lung cancer risk, which is why the recommendation trends toward fixing homes at lower concentrations when feasible.
How Homes Are Fixed
The standard fix is a system called sub-slab depressurization. A contractor drills a small hole through the basement floor or slab, typically going about three feet down, and installs a pipe connected to a fan. The fan creates negative pressure beneath the foundation, pulling radon-laden air from the soil and venting it safely above the roofline before it can enter the house. The system runs continuously and uses a small blower, usually well under one horsepower, so energy costs are minimal.
These systems are remarkably effective. They can reduce indoor radon concentrations by up to 99.5%, a level of performance that sealing cracks alone or improving ventilation can’t match. Installation typically costs between $800 and $2,500 depending on the home’s design and local labor rates. For a problem that causes thousands of cancer deaths annually, it’s one of the more straightforward fixes in environmental health.
Which Areas Have the Highest Risk
The EPA maintains a Map of Radon Zones that classifies every U.S. county into one of three risk categories. The map draws on indoor radon measurements, local geology, aerial radioactivity surveys, soil characteristics, and the types of foundations common in each area. Zone 1 counties, the highest risk tier, are concentrated across the northern Midwest, parts of the Appalachian region, and the northern Plains states. But elevated radon has been found in homes in all 50 states, including areas classified as low risk. Local geology can vary block by block, which is why testing individual homes matters more than relying on zone maps alone.
Why So Few People Actually Test
Despite everything known about radon, testing rates remain remarkably low. A Pennsylvania program that offered radon information and free testing kits to parents of newborns found that fewer than 1% of eligible families even requested a kit between 2002 and 2023. Among those who did request one, only about half actually used it and sent it back to the lab. A separate study found that even when people received vouchers for free testing through their doctor’s office, only about 15% followed through within a year.
The barriers are consistent across studies: people don’t know much about radon, they perceive low personal risk because it’s a naturally occurring substance, they worry mitigation will be expensive, and there’s limited legal pressure to act. Some states, like Illinois, require sellers to disclose known radon problems during a home sale, but many states have no such requirements. Without a triggering event like a real estate transaction, most homeowners never think about it.
This gap between knowledge and action is the central tension of the radon story. The science is settled, the technology works, and testing costs as little as $15 for a do-it-yourself kit. Yet radon continues to kill more Americans each year than drunk driving, and the vast majority of those deaths are preventable with equipment that fits in a basement corner.