Yes. Radon is the leading cause of lung cancer in people who never smoked and the second leading cause overall, linked to about 21,000 U.S. deaths a year. About 22 percent of Hamilton County tests reported to the Ohio Department of Health exceed EPA's 4.0 pCi/L action level.
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The danger comes from years of exposure, not from a day spent in a basement. Radon breaks down into radioactive particles that lodge in lung tissue, and EPA's risk estimates scale with both the level and the time spent breathing it. At 4.0 pCi/L over a lifetime, EPA estimates about 7 of every 1,000 people who never smoked could get lung cancer from radon, rising to about 15 per 1,000 at 8 pCi/L. For smokers the same exposure carries far more risk, about 62 per 1,000 at 4.0 pCi/L, because tobacco smoke and radon compound each other; EPA attributes about 2,900 of the roughly 21,000 annual U.S. radon deaths to people who never smoked. Locally the risk is uneven rather than uniform. EPA's 1993 zone map puts Hamilton, Butler, and Warren counties in Zone 1 and Clermont, Boone, Kenton, and Campbell counties in Zone 2, but measured results cut across those lines. Ohio Department of Health data from licensed testers since 2010 shows about 22 percent of Hamilton County tests above 4.0 and roughly 36 to 37 percent in Butler, Warren, and Clermont counties, and within Hamilton County the share runs from single digits in parts of the urban core to nearly half of tests around Harrison. You can look up your area in our radon levels by ZIP code table, but neighbors often test far apart, so a test is the only way to know your own exposure; the radon testing page covers kits and professional options. Households with a smoker, or with someone sleeping in a finished basement, have the most to gain from acting on a high result, and our answer on how long it takes radon to cause harm explains why a few weeks between a result and a fix does not change the picture.