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New threat to health over 'third' hand smoke

First came the evidence that smoking causes lung cancer, then researchers found a link between second-hand smoke and chronic illnesses. Now a study has pointed an accusing finger at "third-hand" smoke.

Scientists have found that significant quantities of cancer-causing chemicals are produced on indoor surfaces contaminated by tobacco smoke, even when a smoker has been away from the room for hours or even days.

The potentially damaging substances in "third-hand" smoke are present in sufficient amounts on chairs, tables, carpets and even skin to pose a danger to non-smokers, particularly young children, according to research.

Scientists found that nicotine can stick to indoor surfaces for days where it interacts with nitrous acid formed from the gas nitrous oxide, released by car exhausts and gas appliances.

When combined, the two chemicals form tobacco-specific nitrosamines (TSNAs) which can cause cancer, said Mohamad Sleiman of the Lawrence Berkeley National Laboratory in California.

The concept of third-hand smoking emerged from a study a year ago in Boston, which said that a cocktail of toxins can linger on soft furnishings, clothes and other surfaces for hours or even days after a cigarette is put out.

The effect is particularly acute in confined spaces such as a car or a small room.

The latest study attempted to analyse the toxins involved and to quantify the risk.

It found that some of most damaging substances are produced when nicotine, which is not considered to be one of the damaging constituents of cigarette smoke, interacts with the pollutant nitrous oxide, created by the combustion of petrol and gas.

"Our findings indicate that third-hand smoke represents an unappreciated health hazard through [skin] exposure, dust inhalation and ingestion," Dr Sleiman said.

"Smoking outside is better than smoking indoors, but nicotine residues will stick to a smoker's skin and clothing," said Lara Gundel, who collaborated on the project.