You've heard of secondhand smoke, but thirdhand smoke needs to be on your radar, too. "Just because you're in a nonsmoking environment, it doesn't mean you aren't exposed to tobacco," says Peter DeCarlo of Drexel University, whose study was published Wednesday in Science Advances, per the Washington Post. It found 29% of air particles in a vacant college classroom in a non-smoking building could be traced to residue from cigarette smoke. Researchers say it likely originated from a nearby balcony known for "illicit smoking activity" and from an office occupied by smokers, both connected to the classroom via air ducts, reports the Los Angeles Times. And Drexel warns "that Uber car you jump into" or "the hotel room you stay in" could be just as problematic.
"I don't think we have a clue how big the problem is," the president of the Campaign for Tobacco-Free Kids tells the Post. Effects on humans aren't well understood, but thirdhand smoke has been shown to increase the risk of lung cancer, liver damage, and diabetes in mice. And unlike secondhand smoke—which lingers only a short while but causes 7,300 lung cancer deaths a year—residual tobacco chemicals cling to walls, upholstery, and other surfaces for years, and can repeatedly become airborne, researchers say. In an experiment, air flowing through a Pyrex container that had residual cigarette smoke on its glass sides carried the chemicals, reports NBC News. Researchers note they're easily spread through ventilation systems, even those with filters. (See how thirdhand smoke touches homes.)