Anyone walking into a smoker’s abode can tell you that the traces of tobacco use don’t vanish when a cigarette is extinguished. Does this so-called thirdhand smoke pose a health hazard? Researchers at Lawrence Berkeley National Laboratory found that remnants of cigarette smoke don’t just inertly settle onto surfaces. Instead the leftover nicotine can react with nitrous acid vapor, an environmentally common chemical emitted from gas appliances and vehicles, among other sources. The reaction produces carcinogenic compounds known as tobacco-specific nitrosamines (TSNAs).

Secondhand smoke itself contains TSNAs, but the presence of nitrous acid in an environment can increase their numbers by several times in the hours after smoking has ceased. And because nicotine can linger on surfaces for weeks or months, this form of exposure might be even more persistent than firsthand or secondhand smoke, with the TSNAs being inhaled, ingested and dermally absorbed into the body. Children are likely to be the population most vulnerable to thirdhand smoke, just as they are for the secondhand variety. The findings, published online February 8 by the Proceedings of the ­National Academy of Sciences USA, are preliminary, and more research remains to be done to determine if thirdhand smoke has definite human health implications.