Anyone walking into a smoker's abode can tell you that the traces of tobacco use don't vanish when a cigarette or cigar is extinguished. But just what happens to this "third-hand" smoke once the air has cleared—and can it still be harmful?

A team of researchers at the Lawrence Berkeley National Laboratory found that remnants of a smoke don't just inertly settle onto surfaces, they can react with a common gas (nitrous acid, which is emitted from gas appliances and vehicles, among other sources) to create carcinogenic compounds known as tobacco-specific nitrosamines (TSNAs). The group reported the findings in a study published online February 8 in the Proceedings of the National Academy of Sciences.

These "TSNAs are among the most broadly acting and potent carcinogens present in unburned tobacco and tobacco smoke," Hugo Destaillats, a chemist at the lab's Indoor Environment Department and coauthor of the paper, said in a prepared statement.

Second-hand smoke itself contains TSNAs, but the presence of nitrous acid in an environment can increase their numbers in the hours after smoking has ceased. "Whereas the sidestream smoke of one cigarette contains at least 100 nanograms equivalent total of TSNAs, our results indicate that several hundred nanograms per square meter of nitrosamines may be formed on indoor surfaces in the presence of nitrous acid," Mohamad Sleiman, lead author of the study and also of the Berkeley Lab, said in a prepared statement. And given that nicotine can linger on surfaces for weeks and months—even on hard ones, such as walls or dashboards—the finding shows that this form of exposure might be even more persistent than first- or second-hand smoke.

"Our findings indicate that third-hand smoke represents an unappreciated health hazard," Sleiman said. And the potential harm comes not just from breathing these molecules in, but also from direct skin contact and incidental ingestion, the authors noted, calling into question some purported benefits of smoking outside. "Smoking outside is better than smoking indoors, but nicotine residues will stick to a smoker's skin and clothing," Lara Gundel, a researcher at the lab and coauthor of the study, said in a prepared statement. "These residues follow a smoker back inside and get spread everywhere."

Third-hand smoke is a relatively new concept, but like the better-known bugaboo, second-hand smoke, the most vulnerable population is likely children. "Dermal uptake of the nicotine through a child's skin is likely to occur when the smoker returns, and if nitrous acid is in the air, which it usually is, then TSNAs will be formed," Gundel said. Young children are also more likely to consume more dust—and thereby any present TSNAs—than adults, they report.

To stamp out third-hand smoke, the authors recommend public places be entirely smoke-free and individuals avoid smoking indoors and in their cars. Also, in enclosed spaces that have seen plenty of puffs over the years, they suggest replacing furniture, carpet and even wallboard to cut down on the amount of TSNA exposure.

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