People who are exposed to secondhand smoke face a higher risk of mouth cancer, a study suggests.
When a smoker lights up, most of the fumes do not enter their lungs, but circulate in the air around them.
Secondhand – or passive – smoking occurs when someone inhales these fumes, raising their risk of conditions like lung cancer, heart disease and asthma.
To better understand whether it triggers mouth cancer, scientists from King's College London analysed five studies with close to 7,000 participants between them.
Results reveal those who were exposed to secondhand smoke were 51% more likely to develop oral cancer, which mainly affects the lips, tongue, palate, gums or inner cheeks. It is unclear how this exposure was defined.
Around 8,300 people are diagnosed with mouth cancer every year in the UK, making up approximately one in every 50 forms of the disease.
Smoking, chewing tobacco and excessive alcohol intake have been linked to a heightened risk of oral cancer, which can also develop in the salivary glands, tonsils and mouth-windpipe junction.
The disease has also been linked to high-risk variants of the human papillomavirus (HPV), which can spread during unprotected oral sex.
Smoking "constitutes the largest exposure of humans" to cancer-causing chemicals, with one in five tumour-related deaths caused by tobacco, the King's scientists wrote in the journal Tobacco Control.
Secondhand smoke, most of which is invisible and odourless, is thought to contain more than 4,000 toxins.
Worldwide, a third (33%) of males, 35% of females and two in five (40%) children endure passive exposure, despite not smoking themselves, according to the scientists.
Secondhand smoke is made up of both a "mainstream" – fumes exhaled by a cigarette user – and a "side stream" – released from the tip of a cigarette between puffs.
Exposure during pregnancy has been linked to premature labour, a low birthweight and sudden infant death syndrome, also known as cot death.
Children who live with smokers may also develop breathing problems and allergies.
With the impact of secondhand smoke on oral cancer being less clear, the King's scientists analysed studies carried out in Asia, Europe, North America and Latin America.
Of the more than 6,900 participants, over 3,400 had been exposed to secondhand smoke.
The results suggest these individuals were 51% more likely to develop oral cancer.
The risk doubled among those exposed for more than 10 years.
"This supports a causal association between secondhand smoke exposure and oral cancer," concluded the scientists.
"Moreover, the analyses of exposure response, including by duration of exposure (more than 10 or 15 years) to secondhand smoke, further supports causal inference."
The scientists acknowledged just five studies were included in their analysis. In addition, not all the research adjusted for the effects of alcohol consumption.
Nevertheless, the team added: "The identification of the harmful effects of secondhand smoke exposure provides guidance to public health professionals, researchers and policymakers as they develop and deliver effective secondhand smoke exposure prevention programmes".
How to ward off secondhand smoke
The best way to protect loved ones against secondhand smoke is to quit the habit.
At the very least, ensure your home and car is smoke-free. Smoking in a vehicle that is carrying a person under 18 has been illegal in the UK since October 2015.
Smokers should always light up outside. Smoke can linger in the air for up to three hours after a cigarette has been put out, even with windows open.
Smoking in just one room also provides insufficient protection, because fumes spread throughout the house.
Watch: Secondhand smoke could spread coronavirus, medic warns