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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