Radon is invisible, odorless, and common, and the EPA ranks it the second leading cause of lung cancer and the leading cause among never-smokers, at about 21,000 US deaths a year.
Testing is the only way to know, and it is cheap: a short-term DIY kit on the lowest lived-in floor, then a confirmatory test if levels are high. Mitigation is typically a vent pipe and fan, a contractor job, not a rebuild. Test when buying a home, after foundation or ventilation changes, and especially if anyone smokes indoors, since radon and smoking multiply each other's risk.
What to do
Common questions
Should I test my home for radon?
Yes, because it is the rare home hazard that is both serious and trivially easy to check. Radon is a radioactive gas that seeps into homes from the soil, and you cannot see, smell, or taste it, so testing is the only way to know your level. The EPA says radon is the second leading cause of lung cancer overall and the number one cause among people who have never smoked, responsible for about 21,000 lung cancer deaths a year in the US. Testing is cheap: a do-it-yourself short-term test kit costs little and sits in the lowest lived-in level of your home for a few days, with lab analysis included, and the EPA's guidance is that if levels come back at or above the action level, you retest or run a long-term test and then fix it. Fixing is usually a vent pipe and fan system installed by a contractor, not a rebuild. Test when you buy a home, after renovations that change the foundation or ventilation, and if your neighbors found high levels. The risk math gets sharper for smokers: radon and smoking multiply each other, so a smoking household with high radon has by far the most to gain from both testing and quitting.