Mice exposed to household fabrics contaminated with third-hand tobacco smoke showed changes in biological markers of health after only one month, a recent study found. After six months, the mice showed evidence of liver damage and insulin resistance, symptoms which usually precede the development of type 2 diabetes.

Each year about 600,000 people die from exposure to second-hand tobacco smoke (inhaling other people’s cigarette smoke). Once the smoke clears, after a cigarette has been extinguished, nicotine and other harmful chemicals left behind can stick to surfaces and fabrics. This residue is known as third-hand smoke.

The idea of third-hand smoke has been around for a few decades, but came to prominence in 2009 after a study by Jonathan Winickoff, an assistant professor of paediatrics at Harvard Medical School, identified a link between parents’ belief that third-hand smoke may cause harm and the likelihood they would prohibit smoking within their home.

There is growing evidence that third-hand smoke contamination is extensive and can linger for extended periods. Non-smokers can be exposed to third-hand smoke from breathing residual gases, touching surfaces and swallowing dust. Chemical reactions of nicotine stuck to surfaces can lead to an increase in the amount of carcinogenic chemicals over time.

A landmark study in 2011 by Georg Matt of San Diego State University showed that nicotine levels were still elevated in house dust in non-smokers’ homes two months after the previous smoking tenants vacated. Even infants in a neonatal intensive-care unit in the US, with a strict no-smoking policy, had chemical markers of tobacco exposure in their urine after a visit from a parent who smoked.

The link between smoking and ill health, including cancer, is now well established, but what about the impact of third-hand smoke on non-smokers? There has been considerable effort in recent years to determine whether or not third hand smoke is toxic to humans.

The Californian Consortium on Thirdhand Smoke recently reviewed the evidence on third-hand smoke, showing a range of harmful effects. Recent studies have shown that exposure to third-hand smoke can damage DNA and cells, and cause metabolism and behavioural changes.

What the new study adds

The new mouse model study investigated the effects of third-hand smoke exposure over time on animal health (the first study to do so). The researchers, from the University of California, Riverside, used a smoking machine to create third-hand smoke contaminated household fabrics in mice cages, including curtain material, upholstery and carpet. Once the fabrics showed levels likely to be found in smokers homes, the mice were placed in the cage and monitored over a period of six months.

After just one month, the mice showed changes in markers of health in the blood serum, liver and brain tissues. The range and severity of the changes on the health of mice got progressively worse the longer they were exposed.

After four months, the mice showed increases in factors related to oxidative stress and liver damage. Fasting glucose and insulin levels increased with third-hand smoke exposure and, after four months, the mice already had a increased risk of type 2 diabetes.

The speed at which third-hand smoke residues cause measurable health effects in the mice is surprising. How the health effects observed in mice translate to humans, though, remains an open question.

Greater risk for children

The authors suggest that since humans mature slower than mice the exposure times may need to be longer before biological changes can be observed. Unlike the idealised mouse experiment, where they spent all of their life with the third-hand-smoke materials, children and adults will be exposed to different third-hand smoke levels throughout the day.

In the mouse experiments, inhalation or absorption of third-hand smoke residues through the skin were the main exposure methods. But children could also ingest third-hand smoke from house dust – something the mice weren’t exposed to in the study.

Children, particularly toddlers, are at greater risk from contaminated dust because they spend more time close to the ground and are more likely to put materials in their mouths.

Using measurements of third-hand smoke constituents in house dust from 80 Spanish homes, we found that for children aged one to six years old, the cancer risks from exposure exceeded the limit recommended by the US Environmental Protection Agency (EPA) in three-quarters of smokers’ homes and two-thirds of non-smokers’ homes.

We can usually smell third-hand smoke on the clothing of smokers, or when we enter a room where a cigarette has been smoked. But it is clear that low levels of tobacco residues can contaminate homes without our knowledge. This study adds to growing evidence that third-hand smoke can have serious long-term health consequences for non-smokers, particularly children.