Smoking until middle age does reduce lifespan somewhat — women in the study who smoked until age 40 were about 1.2 times more likely to die over a 12-year period, compared with those who never smoked.

However, those who smoked their whole lives were nearly three times more likely to die over that same time period, compared with those who never smoked.

In other words, women who stopped smoking by age 40 were able to avoid about 90 percent of their excess risk of dying from smoking, the researchers said. And those who stopped smoking by age 30 avoided 97 percent of this risk.

The findings of the study — which involved more than 1 million women born in the 1940s —are similar to what has already been seen in studies of men.

“Women born around 1940 were the first generation in which many smoked substantial numbers of cigarettes throughout adult life,” said study researcher Sir Richard Peto, of the University of Oxford. “Hence, only in the 21st century could we observe directly the full effects of prolonged smoking, and of prolonged cessation, on premature mortality among women,” Peto said.

Participants were enrolled in the study around age 55, and were followed from 1996 to 2011. They completed a questionnaire about their lifestyle, medical and social factors, and were resurveyed three years later. During the 12-year study, about 66,000 participants died.

At the study’s start, 20 percent of the participants were smokers, 28 percent were ex-smokers, and 52 percent had never smoked.

Those who were still smokers at three years after the study began were nearly three times more likely to die over the next nine years than those who did not smoke.

The excess mortality among smokers was mainly due to diseases that can be caused by smoking, such as lung cancer, the researchers said.

The study was published in the journal the Lancet.

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John Seffrin, chief executive of the American Cancer Society, issued this warning while speaking at a high-level forum of the world’s 100 leading cancer experts gathered in the Swiss resort of Lugano.

They said governments must do far more than they have done to control the global tobacco industry, either by raising cigarette prices dramatically, outlawing tobacco marketing or by taxing the multinational profits of the big cigarette firms.

According to scientists, smoking kills more than half of all smokers, mostly from cancer, and yet despite it being the single biggest avoidable risk of premature death, there are about 30 million new smokers a year.

They said that if the current trends continue – with cigarette companies targeting the non-smoking populations of the developing world – then hundreds of millions of people will be dying of cancer in the second half of this century.

Some of the experts attending the World Oncology Forum went further by calling for an outright ban on cigarettes and for the tobacco industry to be treated as a terrorist movement for the way it targets new markets with a product that it knows to be deadly when used as intended.

“We have a major global industry producing a product that is lethal to at least half the people who use it. It will kill, if current trends continue, a billion people this century,” the Independentquoted Dr Seffrin as saying.

“It killed 100 million in the last century and we thought that was outrageous, but this will be the biggest public health disaster in the history of the world, bar none. It all could be avoided if we could prevent the terroristic tactics of the tobacco industry in marketing its products to children.

“There is a purposeful intent to market a product that they know full well will harm their customers and over time will kill more than half of them. The industry needs to be reined in and regulated,” he said.

Worldwide, tobacco causes about 22 percent of cancer deaths each year, killing some 1.7 million people, with almost 1 million of them dying from lung cancer. Yet the numbers of new smokers among the young is rising faster than the numbers giving up.

The latest study into the health effects of smoking, which was published in The Lancet and involved 1.3 million women, showed that tobacco is even more dangerous than previously supposed but the benefits of giving up smoking are greater than expected.

Sir Richard Peto of Oxford University, a co-author of the Million Women study who worked closely with Sir Richard Doll, is also the scientist who first calculated how many people this century will die from tobacco-induced cancers.

“We have about 30 million new smokers a year in the world. On present patterns, most of them are not going to stop, and if they don’t stop, and if half of them die from it, then that means more than 10 million a year will die – that’s 100 million a decade in the second half of the century,” Professor Peto said.

“So this century we’re going to see something like a billion deaths from smoking if we carry on as we are. In Europe we have about 1.3 million premature deaths per year now, of which about 0.3 million are deaths by tobacco. There’s nothing else as big as that.

“If you put all causes together, you wouldn’t get a total that’s half of that caused by tobacco, and tobacco kills more people by cancer than other diseases. Smoking is still the most important cause of cancer… If you smoke a few cigarettes a day, it will be the most dangerous thing you do,” he added

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