Cigarette smoke transforms healthy saliva into a "deadly cocktail" which can fuel the development of mouth cancer, scientists have found.
Smoking changes the make-up of saliva
Saliva normally defends the body from illness, as it contains enzymes which can neutralise harmful substances.
But the British Journal of Cancer study by Israeli researchers says tobacco smoke destroys these protective agents.
This leaves a "corrosive mix" of tobacco chemicals and saliva that can eventually turn mouth cells cancerous.
There are almost 8,000 cases of cancers of the nose, mouth, lips, tongue, gums, tonsils, pharynx and larynx cancer in the UK each year, and 3,000 people die of the disease.
The main causes are smoking cigarettes, cigars and pipes, chewing tobacco or betel quid with tobacco and regularly drinking more than safe levels of alcohol.
Saliva contains anti-oxidants, molecules that can help protect the body against cancer.
It should act to protect the cells lining the mouth.
But cigarette smoke destroys these cells, leaving highly damaging free radicals acting in conjunction with carcinogens from the tobacco smoke.
The team from the Technion-Israel Institute of Technology, Haifa, Israel, recreated the effects of cigarette smoke on cancerous cells of the mouth.
Cancerous cells were used so it would become obvious very quickly what the effect would be on tumour development.
Half of the cell samples were exposed to cigarette smoke and the other half to the saliva and cigarette smoke mixture.
It was found that the longer the mouth cells were exposed to the contaminated saliva, the more the cells were damaged.
'Body turns against itself'
Dr Rafi Nagler, one of the lead authors of the study, said: "Most people will find it very shocking that the mixture of saliva and smoke is actually more lethal to cells in the mouth than cigarette smoke alone.
"Our study shows that once exposed to cigarette smoke, our normally healthy saliva not only loses its beneficial qualities but it turns traitor and actually aids in destroying the cells of the mouth and oral cavity.
"Cigarette smoke is not only damaging on its own, it can turn the body against itself."
Dr Lesley Walker, director of cancer information at Cancer Research UK, told BBC News Online: "This is a fairly vivid warning for smokers that the damage starts as soon as you start inhaling tobacco.
"For researchers, the value of this study is that it offers information about how we might prevent this damage occurring."