"This was a surprising finding, and not something that we were even looking for when we started making the measurements," DeCarlo recounted in a video describing his research, which was published Wednesday in the journal Science Advances. "It made us dig deeper and try to understand how a nonsmoking classroom could have so much influence from tobacco smoke." 

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 / Updated By Maggie Fox 
 
 People all over could be breathing in the lingering particles from years-old smoke breaks, researchers reported Wednesday. 
 
 They found evidence that lingering smoke particles — called thirdhand smoke — can be picked up and spread all around buildings by forced air HVAC systems. 
 
 That could mean that people need to be aware not only of secondhand smoke from cigarettes and other tobacco products, but might need to worry about people taking smoke breaks outside, or even the residue from years ago. 
 
 “For people who do not smoke and avoid areas where smoking occurs, this is an additional involuntary exposure route for environmental tobacco smoke species,” Peter DeCarlo and colleagues at Drexel University in Philadelphia wrote. 
 
 Secondhand tobacco smoke has killed at least 2.5 million non-smokers since 1964, the Centers for Disease Control and Prevention estimates. CDC estimates that secondhand smoke causes 7,300 lung cancer deaths a year and 34,000 heart disease deaths. 
 
 The effects of thirdhand tobacco smoke are less well-studied, although several groups have warned about potential dangers. 
 
 “Over the last decade the concept of thirdhand smoke has emerged as a distinct entity that poses health risks because hazardous compounds in thirdhand smoke include many that are toxic or cancer-causing agents,” the California Consortium for Thirdhand Smoke says on its website. 
 
 Any kind of smoke can stick to surfaces and linger deep in fabrics. And the many chemicals in tobacco smoke can react with other chemicals to form new compounds. 
 
 “Of special concern relative to thirdhand smoke is that nicotine and other post-combustion tobacco constituents can interact with other environmental chemicals to form new toxicants and carcinogens,” Thomas Northrup of the University of Texas Health Science Center at Houston and colleagues wrote in the journal Public Health Reports. 
 
 The Food and Drug Administration warns about thirdhand smoke. 
 
 “Like children, dogs and cats spend a lot of time on or near the floor, where tobacco smoke residue concentrates in house dust, carpets and rugs. Then, it gets on their fur,” FDA veterinarian Carmela Stamper said in astatement. 
 
 “Dogs, cats and children not only breathe these harmful substances in, but pets can also ingest them by licking their owner’s hair, skin, and clothes.” 
 
 DeCarlo’s team ran a test in a single classroom that had been a non-smoking room for years. To their surprise, they found many components of tobacco smoke in the air. 
 
 In fact, 29 percent of the particles in the air appeared to have been affected by tobacco smoke residue. 
 
 They’re not sure how those particles got there. 
 
 “However, the classroom where measurements were performed is 20 meters (yards) down the hall from an outdoor balcony where illicit smoking activity occurs,” they wrote. 
 
 Or particles could have been picked up and spread by the building’s HVAC system. 
 
 They ran a controlled experiment and found cigarette smoke stuck well to the sides of a Pyrex glass container, and then got picked up again when air was passed through. 
 
 Forced air might do that in a building. “HVAC systems recirculate and disperse air throughout the multiple rooms of the zone served by the system, meaning that what happens in one room affects all the other rooms in the zone,” they wrote. 
 
 “For this reason, a room located near a smoking area with smoke penetration or a room occupied by a smoker can effectively expose the other occupants served by the same HVAC system to thirdhand smoke, even if they do not share space directly.”
– 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.)