The dangers of secondhand smoke are well known.
Whether it’s walking in the wake of a cigarette, driving with a smoker or even kissing a baby after lighting up, secondhand smoke contains more than 4,000 chemicals, according to NHS Smokefree.
And now research is increasingly showing nicotine and other nasties can lurk in carpets, curtains and clothes for years or even decades.
Known as thirdhand smoke, the dangers of smoking may continue long after a cigarette has been put out.
What is thirdhand smoke?
Thirdhand smoke describes exposure to residual nicotine and other chemicals on indoor surfaces.
This is thought to react with other “indoor pollutants” to create a “toxic mix of cancer-causing compounds”, according to the Mayo Clinic.
Indoor pollutants may include volatile organic compounds (VOCs), found in everything from paint and moth repellent to cleaning products and make-up.
The US Environmental Protection Agency links VOCs to eye, nose and throat irritation, as well as headaches, nausea, and even damage to the kidneys, liver and central nervous system.
Some VOCs have been shown to cause tumours in animals, while others are “suspected or known to cause cancer in humans”.
The Mayo Clinic warns thirdhand smoke can build up over time.
Breathing the chemicals in, or even touching a contaminated surface, could be dangerous.
Crucially, thirdhand smoke cannot be eliminated via open windows or air conditioning.
The only way to avoid it seems to be to live smoke-free.
How could thirdhand smoke harm our health?
Thirdhand smoke is a relatively new area of research.
Scientists from Drexel University in Philadelphia were some of the first to look into whether nicotine residue really could linger in our homes.
After analysing an old classroom, they found a “reduced nitrogen component” in 29% of the airborne particles.
The team put this down to “thirdhand smoke components” from “interior surfaces”.
“In an empty classroom, where smoking has not been allowed in some time, we found 29% of the entire [air particles] contained thirdhand smoke”, study author Dr Anita Avery said.
In a second part of the experiment, the team filled a sealed container with cigarette smoke, which they then “flushed” with outdoor air, the Independent reported.
The following day, they pumped filtered air into the container to mimic indoor ventilation.
Despite this, the container still had 13% more harmful substances than the control boxes.
The scientists worry humid, circulating air - like that in offices - may “lift” chemicals fixed to surfaces to become airborne.
While nicotine residue may linger all around us, whether it poses any health risks is less clear.
To learn more, scientists from the Lawrence Berkeley National Laboratory in California exposed newborn mice to thirdhand smoke.
This caused the animals to weigh significantly less than unexposed controls.
Thirdhand smoke also “threw off” the mice’s blood make-up.
The newborns had significantly more of the immune-fighting protein oesophils, suggesting their immune system had been activated.
Their platelets, which cause blood to clot when the skin is cut, also went down.
The scientists concluded “thirdhand smoke exposure may have adverse effects on human health”.
READ MORE: Thirdhand Smoke Damages Human Cells
When it comes to humans, babies and toddlers could be worst affected.
Dr Jonathan Winickoff, from the Harvard Cancer Center, told Scientific American: “The developing brain is uniquely susceptible to extremely low levels of toxins.”
The paediatrician also warned a child’s height puts them closer to smoke residues in carpets or furniture.
“They tend to touch or even mouth the contaminated surfaces,” he said.
Scientists from Massachusetts General Hospital claim children ingest up to 0.25g (0.0005lb) of dust a day, more than twice that of adults.
The same team looked into whether the public believe thirdhand smoke exists.
They asked 1,478 people, 273 of which were smokers, if they thought smoke residues harm children.
Less than half (43%) of the smokers answered “yes”, compared to 65% of the non-smokers.
With limited evidence, the NHS has said it is “plausible but unproven” that smoke residues damage health.