Issue Date: February 8, 2010
After The Smoke Clears
Nonsmokers may have a new worry—thirdhand smoke. Nicotine residues on indoor surfaces can react with nitrous acid in the air to form carcinogenic nitrosamines not present in fresh tobacco smoke, chemists at Lawrence Berkeley National Laboratory have demonstrated.
"The residual smoke on surfaces appears to become even more toxic through reactions with other atmospheric chemicals," says K. Michael Cummings, a secondhand smoke expert at Roswell Park Cancer Institute in Buffalo. "The assumption was with time the material would become less, not more, harmful."
More than 30 years ago, Stephen S. Hecht and coworkers of the University of Minnesota first showed that nicotine reacts with nitrous acid in aqueous solution. A new study now shows that such reactions can also happen with nicotine left behind from tobacco smoke and nitrous acid in the air. "We are describing a system in which these reactions can take place on indoor surfaces," says Hugo Destaillats, a chemist at LBNL (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0912820107).
Destaillats, Lara A. Gundel, Mohamed Sleiman, and coworkers measured nitrosamines formed by the reaction of nicotine adsorbed on surfaces with nitrous acid in the air. They took samples from a nicotine-coated model cellulose surface and from surfaces in the cab of a smoker’s truck. Three tobacco-specific nitrosamines were the main products of the reaction.
The researchers propose the following mechanism for the reaction: NO+ removes an electron from the pyrrolidine nitrogen of nicotine to form an unstable cation. Then, a second NO+ abstracts H from one of the three α-carbons to form an iminium ion. Subsequent reactions with adsorbed water and nitrous acid vapors yield the nitrosamines.
In addition, they found secondary products resulting from decomposition of the nitrosamines. One of these compounds, a stable pyrazole formed from the decomposition of one of the nitrosamines that has not been observed in fresh tobacco smoke, could serve as a tracer for thirdhand smoke, the authors suggest.
“Since nicotine readily adsorbs to surfaces and nitrogen oxides are ubiquitous, their findings may have some relevance to contamination by thirdhand smoke,” Hecht says. “Research would be necessary to demonstrate whether, for example, infants in homes that permitted smoking were receiving significant exposures by this route. I personally feel that exposure by this route would be minimal, but the studies need to be carried out.”
Destaillats emphasizes that this study addresses only the chemistry. “Our study should incite some of our colleagues to determine whether exposure to these reactive residues and by-products of these reactive residues can be harmful,” he says.
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