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Smokers can contaminate nonsmoking indoor spaces with thirdhand cigarette smoke.

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‘Thirdhand’ smoke can expose moviegoers to the emissions of up to 10 cigarettes

Scientists have warned about the dangers of secondhand smoke for decades. And in recent years, a variety of smoking bans have saved nonsmokers from passively sucking in harmful cigarette chemicals. But a new study finds that even those restrictions might not be enough: Compounds from tobacco smoke can infiltrate well-ventilated, nonsmoking rooms and even movie theaters by hitching a ride on peoples’ clothes, skin, and hair.

The findings are “fascinating,” says Georg Matt, a psychologist at San Diego State University who has spent 20 years studying thirdhand smoke—chemicals left behind on surfaces from tobacco fumes. He and his colleagues, who were not part of the new study, have long wondered why indoor areas with smoking bans are often contaminated with cigarette chemicals. One study, for example, found that thirdhand smoke lingers in vacant homes up to 2 months after smokers move out; another found that it remained in a casino for 6 months after a smoking ban.

But most studies look at indoor spaces that were recently smoked in. The new work focused on a movie theater in Mainz, Germany, that has had a strict smoking ban for 15 years—enough time for the preban contaminants to clear out, says Drew Gentner, an environmental engineer at Yale University and the study’s lead author. To determine the quality of air in the theater, he and his colleagues placed a mass spectrometer—a machine that measures chemicals—by one of the theater’s ventilation ducts. The device monitored the air as patrons filed in and out of the theater and watched a variety of films.

Over the course of 4 days, the researchers found sharp spikes in 35 tobacco-related chemicals, including toxic compounds such as benzene and formaldehyde, when audience members entered the theater, the team reports today in Science Advances. Given the theater’s strict ban on smoking, the only way the contaminants could have made their way in was by sticking to the clothes and bodies of audience members who had been around smoke prior to entering.

When the theater showed R-rated movies such as Resident Evil, levels of thirdhand smoke compounds were up to 200% higher than they were during viewings of the G-rated family movie Wendy. That’s because the R-rated films attracted older viewers, who were more likely to have recently encountered cigarette smoke, the researchers say. Over the course of a typical R-rated film, audience members were exposed to the equivalent emissions of one to 10 cigarettes of secondhand smoke, Gentner says.

Matt says these findings suggest cigarette smokers—or people who have been exposed to smoke—are carrying the compounds with them and depositing them as they slowly evaporate. The process, known as “off-gassing,” is the reason why smokers smell like cigarettes, explains Peter DeCarlo, an expert in air pollution at Johns Hopkins University, Baltimore, who was not involved in the study. “You’re not smelling the chemicals that are bound to the person’s clothes,” he says. “You’re smelling the ones that are coming off.”

All of this means that hazardous pollutants in tobacco smoke likely put nonsmokers at some risk—though just how much is unknown, the researchers say. (The harmful effects of secondhand smoke are firmly established.) They expect exposure to thirdhand smoke is an even bigger problem in confined and poorly ventilated spaces, such as subway cars and small rooms in people’s homes.

DeCarlo hopes studies like this will raise awareness of the toxins that could be present even in smoke-free buildings. Meanwhile, Matt says it might be unrealistic to expect smokers or those who have been around smokers to shower and wash their clothes before going to shared spaces. “The only solution … is to reduce smoking rates.”