Imperial Tobacco Canada destroyed documents related to the dangers of smoking. (Canadian Press)Imperial Tobacco Canada destroyed up to 60 early studies that linked cigarettes to addiction and carcinogens, according to a review published Wednesday in the online Canadian Medical Association Journal.

The internal studies, done from 1967 to 1984, were destroyed in 1992 on orders from head office at British American Tobacco in the United Kingdom to avoid "exposing the company to liability or embarrassment," the authors of the review say.

The studies were later uncovered in British American Tobacco files, which began to be made available through disclosure during a U.S. trial in 1998, Dr. David Hammond of the University of Waterloo said in an interview.

Hammond said he and his team undertook the review because they knew the studies existed and involved high-quality research into cigarette design and the health effects of smoking.

He said it wasn't the team's intention to judge Imperial Tobacco, but after the review of the documents it was impossible not to.

"Not only did they not reveal these studies but they destroyed them," said Hammond, whose research at the University of Waterloo focuses on cancer prevention. "It's very difficult to write this review from any other angle. What was the evidence they were so worried about being uncovered?"

Hammond's team concluded that the destruction of the research documents has implications for industry liability and litigation now underway in Canada.

Last month, Ontario launched a $50-billion lawsuit against Imperial Tobacco. British Columbia and New Brunswick have also filed lawsuits against the company.

Hammond said the uncovered studies were particularly interesting because they were funded by the cigarette company itself and showed Imperial was trying to improve the safety of its product.

"They took on the research in good faith," Hammond said. "The problem is they weren't getting the results they wanted. And the product modifications they pursued weren't working."

A lot of the studies the tobacco company ordered destroyed looked at the effects of filtered versus unfiltered cigarettes and the effects of second-hand smoke.

In many cases, the studies contradicted conventional beliefs at the time that filtered cigarettes were safer than non-filtered and that second-hand smoke was not dangerous, the review found.

In one of the older studies, a senior research scientist found that people smoking filtered cigarettes inhaled more smoke to get about the same amount of nicotine they'd get from unfiltered.

Eleven of the destroyed documents focused on original research about the effects of second-hand smoke. These studies, most of which were experiments performed on rats, indicated cellular changes from second-hand smoke.

"The scientists concluded that second-hand smoke was in fact more toxic than mainstream smoke "especially for low-delivery cigarettes," Hammond said.

Hammond said it was a huge task to find the documents, which were released piecemeal during various U.S. trials against cigarette companies.

"I'm not joking when I say there were tens of millions of pages. We located one and then tracked down the others."

Hammond said he wonders what might have changed had the cigarette companies made their findings public, adding that targeted anti-smoking campaigns might have been launched much earlier.