Infants and toddlers face increase risk from 'third-hand smoke'

Nicotine that accumulates on indoor surfaces long after a person has put out their cigarette is converted into cancer-causing chemicals, US scientists have warned.

Researchers at the Lawrence Berkeley National Laboratory discovered that the nicotine in so-called 'third-hand smoke' reacts with a common indoor air pollutant called nitrous acid to form dangerous chemicals.

Lab tests using cellulose surfaces to mimic indoor materials exposed to smoke revealed that levels of carcinogenic tobacco-specific nitrosamines (TSNAs) increased by ten times after three hours of exposure to nitrous acid, which is mainly emitted by un-vented gas appliances. These included cancer-causing chemicals such as NNA, NNN and NNK.

Tests on surfaces inside the truck of a heavy smoker confirmed there were high levels of TSNAs here as well, as a result of nicotine reacting with nitrous oxide from the vehicle's engine.

Ed Yong, Cancer Research UK's head of health information, commented: "This is an interesting piece of research that adds the possibility of an extra level of harm from tobacco smoke. However, this study doesn't tell us what threat, if any, third-hand smoke could pose to our health.

"There is clear evidence about the harmful effects of secondhand smoke to children, especially in homes and cars. The most important step parents can take to protect their families from the dangers of cigarette smoke is to make their homes and cars smokefree."

Hugo Destaillats, a chemist at Berkeley Lab's Indoor Environment Department, explained that burning tobacco releases nicotine vapour which clings to surfaces such as walls, floors, carpets and furniture - sometimes for months at a time.

"Our study shows that when this residual nicotine reacts with ambient nitrous acid it forms carcinogenic tobacco-specific nitrosamines or TSNAs," he confirmed.

The scientist added that TSNAs are "among the most broadly acting and potent carcinogens present in unburned tobacco and tobacco smoke".

Lead author Mohamad Sleiman revealed that TSNAs form rapidly and warned that third-hand smoke "represents an unappreciated health hazard through dermal exposure, dust inhalation and ingestion".

The study authors, whose findings are published in the Proceedings of the National Academy of Sciences, believe that the risk may be highest for infants and toddlers, as these are most likely to inhale dust or touch contaminated carpet or clothes.