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Exposure to “third-hand smoke” – residue left behind on carpets, clothing and furniture – appears to increase the risk of liver damage and diabetes in mice.

Residues of cigarette smoke cause a cocktail of toxins to accumulate on surfaces and clothes. Such toxins are thought to resist removal by industrial cleaners.

To investigate the potential health risks of third-hand smoke, at the University of California, Riverside, and her team exposed curtains, upholstery and carpets to levels of smoke similar to those found by the US Environmental Protection Agency in smokers’ homes.

They then exposed caged mice to segments of these fabrics for up to six months, taking brain, liver and blood samples at various intervals.

After one month, these mice experienced around a 50 per cent increase in inflammatory molecules in their blood and liver compared with control mice who weren’t exposed to the fabrics.

Two months in, the team saw increased cell damage in the rodents’ liver and brain. At four months, cortisol levels had increased by 45 per cent compared with the controls. High cortisol levels have been linked to weight gain and a weakened immune system.

At four months, mice exposed to the smoky fabrics saw a 30 per cent rise in the levels of fasting blood glucose and insulin – both measures put the mice at increased risk of diabetes. These levels were even higher at the six-month mark.

First, second, third

As always, it is difficult to extrapolate the findings to humans. The results make sense, are biologically plausible and important, says at the Mayo Clinic in Rochester, Minnesota, but the effect on humans still needs to be investigated. “It’s important to note that a one-off or casual exposure is not going to have measurable impact on people,” he says.

Martins-Green believes third-hand smoke is just as dangerous as first and second-hand smoke. She warns parents against assuming that only smoking in their child’s absence, or only smoking outdoors, protects their kids from the smoke’s harmful effects. “Cotton shirts are a terrible sink,” she says. “A parent goes outside to smoke, but then crawls into bed and reads a book to their kid.”

Children are especially vulnerable to third-hand smoke because they breathe faster than adults and are in closer contact with household surfaces, says at Massachusetts General Hospital, whose team coined the term “third-hand smoke”. He says that their effective doses may be 20 times higher than adults’.

One thing Hays, Winickoff and Martins-Green all agree on is that third-hand smoke is difficult to banish. “In order to get rid of it in the wallboards, you have to get rid of the wallboards,” says Winickoff.

Read more: Passive smoking linked with diabetes for first time