Women who stop smoking by the age of 40 dramatically cut risk of early death, enjoying up to ten more years of life, a major new study has found.
The study of 1.3 million women found that quitting smoking by the age of 30 allowed women to avoid up to 97 per cent of the extra risk of premature death.
The results, which are published in The Lancet medical journal, showed that lifelong smokers died a decade earlier than those who did not smoke at all.
Those who stopped at thirty lost an average of a month of life and if they stopped at 40 they died a year younger.
Most of the increased death rate resulted from smoking-related diseases such as lung cancer, chronic lung disease, heart disease or stroke.
The risk rose steeply with the quantity of tobacco smoked, but even light smokers who puffed fewer than 10 cigarettes a day doubled their likelihood of dying.
The authors of the Million Women Study wrote: "Smokers lose at least 10 years of lifespan. Although the hazards of smoking until age 40 years and then stopping are substantial, the hazards of continuing are 10 times greater."
Women aged 50 to 65 were enrolled into the study, designed to investigate links between health and lifestyle, from 1996 to 2001.
Participants completed a questionnaire about living habits, medical and social factors and were re-surveyed three years later. Women were monitored for a total of 12 years on average, during which there were 66,000 deaths.
Initially, 20% of the women were smokers, 28% were ex-smokers, and 52% had never smoked.
Those who still smoked at the three year re-survey were almost three times more likely than non-smokers to die over the next nine years.
Both the hazards of smoking and the benefits of quitting were greater than previous studies had suggested, said the researchers.
Professor Sir Richard Peto, one of the co-authors at Oxford University, said: "If women smoke like men, they die like men - but, whether they are men or women, smokers who stop before reaching middle age will on average gain about an extra 10 years of life."
He added: "Both in the UK and in the USA, women born around 1940 were the first generation in which many smoked substantial numbers of cigarettes throughout adult life.
"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."
Professor Rachel Huxley, from the University of Minnesota, said: "In most of Europe and the USA, the popularity of smoking among young women reached its peak in the 1960s, decades later than for men.
"Hence, previous studies have underestimated the full eventual impact of smoking on mortality in women, simply because of the lengthy time lag between smoking uptake by young women and disease onset in middle and old age."
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