Some of the world's biggest tobacco firms researched the lethal radioactive substance polonium – present in cigarettes – over a 40-year period but never published the results, according to a new scientific article.
Experts have examined more than 1,500 internal documents from tobacco companies.
Polonium 210 is known to cause lung cancers in animals and studies suggest it is responsible for 1 per cent of all lung cancers – equivalent to 11,700 deaths globally – each year in the US.
It is also the substance that poisoned the Russian dissident Alexander Litvinenko in London in 2006.
Yet tobacco companies, while attempting but failing to remove the substance from their products, have kept quiet about their research, experts say.
One of the documents – all of which were made public through legal actions – said publication would be "waking a sleeping giant". The authors of the article, published in the September edition of American Journal of Public Health, also say tobacco companies feared possible litigation.
The quoted studies show polonium is present on the tobacco leaf and inside it as part of its chemical make-up. Tobacco company scientists spent years trying to remove the substance by washing the leaf, achieving only partial success. Attempts at genetic modification and creating filters to remove it also failed.
The research article, led by Monique Muggli, from the Mayo Clinic in Minnesota, says: "Philip Morris documents show the majority of the company's internal reports regarding PO-210 [Polonium 210] were not published. One manuscript believed by some Philip Morris scientists to be favourable to the tobacco industry was withheld from publication for fear of heightening public awareness of PO-210."
It then quotes an internal document that says publication of that research, from 1978, "has the potential to wake a sleeping giant".
Ms Muggli added that, while tobacco companies tried to obscure other health controversies, their line on polonium seemed to be simply not to raise the issue.
"Unlike other smoking and health issues, where the industry line was to create doubt, in relation to polonium 210 and the radioactivity of cigarettes, the companies wanted to hide from that issue publicly. They continue to minimise the recognition of radioactivity in their products in smoking and health litigation," she said.
A spokeswoman for British American Tobacco said it was not known which constituents of cigarette smoke caused cancer and argued that polonium 210 is also present in food.
"It's fairly common knowledge polonium 210 is in cigarette smoke because it's present in all such plant types, including strawberries," she said. "There was a 1977 study that found, of the daily intake of the polonium 210 in a smoker, 77.3 per cent came from food and 17 per cent from tobacco. The World Health Organisation is trying to determine which constituents of tobacco smoke are most important in diseases including lung cancer, but as yet have not concluded polonium 210 is a priority constituent."
A spokesman for Philip Morris said many reports into polonium and cigarette smoke had been published over the last 30 years and links were available on their website. He added that Philip Morris had published some of its research but no company would publish all its internal findings.