LONDON (AP) -- Breathing in other people's cigarette smoke makes nonsmokers 82 percent more likely to suffer a stroke, a new study suggested Tuesday -- indicating the dangers of so-called passive smoking are much worse than originally believed.

The study by researchers at the University of Auckland in New Zealand is the largest and most rigorous to date, and gives more ammunition to those campaigning to have smoking banned in all workplaces and public areas.

Current estimates of how smoking increases the risk of various diseases are dramatically underestimated because the ill effects of secondhand smoke inhalation are not taken into account, say the researchers, whose work is published in the British medical journal Tobacco Control.

That means research into the hazards of tobacco smoke has artificially narrowed the true gap between smokers and people whose bodies really are tobacco-free, said Dr. Rodney Jackson, a professor of epidemiology at the University of Auckland and one of the authors of the study.

"We've never really had the right comparison group because everybody has been slightly poisoned," Jackson said.

Because New Zealand's anti-passive-smoking legislation is among the most progressive in the world, it is easier to separate out people who have been exposed to secondhand smoke, he said.

Two previous studies have linked stroke with secondhand smoke. Studies also show passive smoking increases the risk of heart disease, heart attack, lung and breast cancer, and breathing- related diseases.

The suggestion that studies into the dangers of smoking underestimate the real risk is ``an important point that has not been well appreciated," said Stanton A. Glantz, a secondhand smoke expert at the University of California-San Francisco who was not connected with the study.

The study examined 521 stroke patients in Auckland and compared them with 1,851 randomly selected healthy people matched by sex and age to see the effect of smoking and secondhand smoke on the chances of suffering a stroke. None of the subjects was older than 74.

"Half the people who have strokes are 75 or older, so these are premature strokes that should not be happening," said Ruth Bonita, the lead author of the study, who now runs the non- communicable disease section at the World Health Organization.

Anyone who smoked at least one cigarette a day was termed a smoker. Those who had either never smoked or hadn't smoked for at least a decade were considered nonsmokers. Other former smokers were divided according to how long it had been since they stopped.

People were classed as having been exposed to secondhand smoke if they lived with or worked in the same room as someone who regularly smoked in front of them for more than one year during the past 10 years.

The fewer cigarettes people smoked each day, and the longer ex-smokers had abstained, the better off they were, but the difference between them and nonsmokers was not as dramatic once secondhand smoke was taken into account.

Overall, smokers were four times more likely to suffer a stroke than nonsmokers. But when the nonsmokers exposed to secondhand smoke were excluded from the equation, smokers were six times more likely to have a stroke. That's a difference of about 50 percent.

Again in the basic investigation, people who had quit smoking more than two years before were no more likely to suffer a stroke than those who had never smoked. But once the passive smoking factor was removed, those people still had nearly double the chance of suffering a stroke.

Lifetime nonsmokers and those who had quit smoking more than a decade earlier were 82 percent more likely to have a stroke if they were breathing secondhand smoke.

Men fared worse, with a doubling of their stroke risk, compared with a 1.5 times increased chance for women.

Dr. Konrad Jamrozik, a smoking and heart expert at Harvard University, wrote in an editorial in the journal that the study is "a very significant step forward in our knowledge."

He said based on the research in New Zealand, where fewer than 20 percent of adults smoke, secondhand smoke is potentially implicated in one in eight of all strokes before age 74.

Further studies, especially in populations where more people smoke and strokes are more common, such as Asia, are now needed to investigate whether the findings would apply elsewhere, he said.