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You know smoking is bad for you. You know inhaling someone else's smoke is bad for you. Now a US study says third-hand smoke -- tobacco residue clinging to surfaces -- is also bad for you.

When a cigarette burns, nicotine is released in the form of a vapor that collects and condenses on indoor surfaces such as walls, carpeting, drapes and furniture, where it can linger for months, said the study, which was published in the Proceedings of the National Academy of Sciences (PNAS).

"Our study shows that when this residual nicotine reacts with ambient nitrous acid it forms carcinogenic tobacco-specific nitrosamines, or TSNAs," said Hugo Destaillats, a corresponding author of the study.

"TSNAs are among the most broadly acting and potent carcinogens present in unburned tobacco and tobacco smoke," he said.

The most likely human exposure to TSNAs is through either inhalation of dust or the contact of skin with carpet or clothes -- making third-hand smoke particularly dangerous to infants and toddlers.

Opening a window or turning on a fan to air out a room while a cigarette burns does not eliminate the hazard of third-hand smoke. Smoking outdoors doesn't help much either.

"Smoking outside is better than smoking indoors but nicotine residues will stick to a smoker's skin and clothing," said Lara Gundel, a co-author of the study.

"Those residues follow a smoker back inside and get spread everywhere. The biggest risk is to young children," she said.

"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."

Substantial levels of TSNAs were also found in the truck of a heavy smoker, the study says, adding that most vehicle engines emit some nitrous acid that can infiltrate the passenger compartment of a vehicle.

Researchers from the Lawrence Berkeley National Laboratory led the study, which they say is the first to quantify the reactions of third-hand smoke with nitrous acid.

AFP