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 an analysis of cancer-causing agents produced by the interaction of stale cigarette smoke and other indoor pollutants.

They 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 by Jonathan Winickoff of the Harvard Cancer Centre in Boston who 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.

“Time-course measurements revealed fast TSNA formation... within the first hour. Given the rapid adsorption and persistence of high levels of nicotine on indoor surfaces, including clothing and human skin, our findings indicate that third-hand smoke represents an unappreciated health hazard through [skin] exposure, dust inhalation and ingestion,” Dr Sleiman said.

The study, published in the journal Proceedings of the National Academy of Sciences, found that levels of TSNAs rose tenfold within a few hours of exposing a nicotine-contaminated surface to “high but reasonable” concentrations of nitrous oxide, about 60 parts per billion. They found similarly high levels of TSNAs in the cab of a lorry driver who smoked heavily.

“Smoking outside is better than smoking indoors but nicotine residues will stick to a smoker’s skin and clothing. These residues follow a smoker back inside and get spread everywhere,” said Lara Gundel, who collaborated on the project.

Third Hand Smoking

“The biggest risk is to young children. Dermal uptake of the nicotine through a child’s skin is likely to occur when the smoker returns and if nitrous acid is in the air, which it usually is, then TSNAs will be formed,” Dr Gundel said.

Hugo Destaillats, a senior member of the Berkeley team, said: “We know that these residual levels of nicotine may build up over time after several smoking cycles, and we know that through the process of ageing, third-hand smoke can become more toxic over time. Our work highlights the importance of third-hand smoke reactions at indoor interfaces, particularly the production of nitrosamines with potential health impacts.”