The dangers of smoking are well known - heart disease, stroke and lung cancer. And they impact both on smokers, through directly inhaling nicotine and other chemicals, and non-smokers, who are exposed to exhaled ‘secondhand’ smoke and the burning end of cigarettes and cigars.

But what about the risks of third-hand smoke? What is it, and should we be worried?

The term "third-hand smoke" refers to the residue of smoke left on clothing, carpets, furniture, skin and hair - the invisible tobacco dust that settles in the environment and stays there even after a cigarette has been extinguished.

As the below chart explains, after they land on these surfaces, the chemicals undergo an ageing process, which changes the structure of the smoke pollutants.

Smokers’ homes

The nicotine in third-hand smoke residue reacts with common indoor air pollutants, such as nitrous acid and ozone and goes on to form carcinogens, or compounds that may cause cancer, according to researchers from the Massachusetts General Hospital for Children. The chemicals are then continuously re-emitted back into the air in a process known as “off-gassing”.

Efforts to diffuse the smoke, such as opening windows or using a fan, don’t prevent third-hand smoke from forming or keep it from being inhaled. And this residue may give off harmful chemicals for years or even decades.

Although a relatively new term, third-hand smoke has been a research topic for decades. In 1953, a scientist from Washington University School of Medicine in St. Louis showed that condensate made from the residue of cigarette smoke caused cancer.

In a 1991 study, researchers found nicotine in the dust of smokers’ homes. A further study, conducted in 2004, found that nicotine was still present in homes where smokers had tried to limit exposure, such as smoking outdoors.

Babies and children are at a greater risk of being exposed to third-hand smoke because they breathe or eat the toxins when they cuddle adults, sit on car seats or crawl on floors where toxins may have built up over time. Their developing immune systems and organs make them particularly vulnerable.

To reduce this risk, the American Academy of Pediatrics issued recommendations to help limit children’s exposure.

What can be done?

The World Health Organization is calling on countries to increase action to protect people from exposure to tobacco. Bans on tobacco advertising, promotion and sponsorship can help reduce tobacco consumption, it says.

Raising taxes, warning about the dangers of smoking and offering assistance to people who want to quit are all initiatives that have helped to reduce the impact of tobacco on the health of people worldwide.