Nicotine combines with the commonly found nitrous acid to form a cancerous toxin that collects on walls and floors
Old tobacco smoke does more than simply make a room smell stale - it can leave cancer-causing toxins behind, American researchers report.
They have found that cancer-causing agents called tobacco-specific nitrosamines stick to a variety of surfaces, where they can get into dust or be picked up on the fingers. Children and infants are the most likely to pick them up, the team at Lawrence Berkeley National Laboratory in California reports.
"These findings raise concerns about exposures to the tobacco smoke residue that has been recently dubbed 'third-hand smoke'," the researchers write in the Proceedings of the National Academy of Sciences.
They suggest a good clean up could help remove these potentially harmful chemicals and say their findings suggest other airborne toxins may also be found on surfaces.
"TSNAs (tobacco-specific nitrosamines) are among the most broadly acting and potent carcinogens present in unburned tobacco and tobacco smoke," Berkeley chemist Hugo Destaillats, who worked on the study, says in a statement.
"The burning of tobacco releases nicotine in the form of a vapor that adsorbs strongly onto indoor surfaces, such as walls, floors, carpeting, drapes and furniture. Nicotine can persist on those materials for days, weeks and even months."
The nicotine combines with another common compound called nitrous acid to form tobacco-specific nitrosamines or TSNAs, Destaillats and colleagues found.
Unvented gas appliances are the main source of nitrous acid indoors, and vehicle engines emit it too.
The researchers did laboratory tests with cigarette smoke, and also tested a 45-year-old pickup truck driven by a heavy smoker. The TSNA compound formed quickly if nitrous acids were around - notably in the truck compartment but also in rooms where cigarette smoke wafted.
It would be easy to ingest this new compound, they say, calling it "an unappreciated health hazard".
"Because of their frequent contact with surfaces and dust, infants and children are particularly at risk," they write.
"Nicotine, the addictive substance in tobacco smoke, has until now been considered to be non-toxic in the strictest sense of the term," Kamlesh Asotra of the University of California's Tobacco-Related Disease Research Program, which paid for the study, says in a statement.
"What we see in this study is that the reactions of residual nicotine with nitrous acid at surface interfaces are a potential cancer hazard, and these results may be just the tip of the iceberg."
James Pankow, who also worked on the study, says it may raise questions about the safety of electronic cigarettes, or "e-cigarettes" which produce a nicotine vapor but not smoke.
The researchers say regulators who have cracked down on second-hand smoke with smoking bans may decide to consider policies on third-hand smoke.
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