The largest-ever study of a million women has found that those who smoke lose a decade of their lives, while kicking the deadly habit before the age of 40 avoids more than 90 percent of the increased risk of dying caused by continuing to smoke, while stopping before the age of 30 avoids over 97% of it.

The research has just been published in the online edition of the British journal The Lancet, to mark the100th anniversary of the birth of Sir Richard Doll, one of the first people to identify the link between lung cancer and smoking.

A total of 1.3 million women were recruited to the study between 1996 and 2001, at ages 50 to 65 years. Participants completed a questionnaire about lifestyle, medical and social factors and were resurveyed by mail three years later. The National Health Service’s central register notified the researchers when any participant died, giving the cause of that death.

Women were traced for an average of 12 years from the time they first joined; thus far, 66,000 study participants died.

Initially, a fifth of the study participants were smokers, 28% were ex-smokers and 52% had never smoked. Those who were still smokers at the three-year follow-up survey were nearly three times as likely as non-smokers to die over the next nine years, even though some reduced their risk by stopping smoking during this period.

This threefold death rate ratio means that two-thirds of all deaths of smokers in their 50s, 60s, and 70s are caused by smoking, as most of the difference between smokers and non-smokers came from smoking-related diseases such as lung cancer, chronic lung disease, heart disease, or stroke. The risks among smokers increased steeply with the amount smoked, although even for those who were light smokers (one to nine cigarettes daily) at the start of the study, mortality rates were double those for non-smokers.

The key finding is that both the hazards of smoking and, correspondingly, the benefits of stopping are bigger than previous studies have suggested; smokers who stopped around age 30 avoided 97% of their excess risk of premature death, and although serious excess hazards remained for decades among those who smoked until age 40 before stopping, the excess hazards among those who continued smoking after age 40 were 10 times bigger.

According to University of Oxford co-author Prof. Richard Peto, who was for many years a co-researcher with Doll on smoking epidemiology, “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.” Peto added: “Both in the UK and the US, 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.”

Prof. Rachel Huxley of the University of Minnesota, in a published comment, said: “That we had to wait until the 21st century to observe the full consequences in women of a habit that was already widespread in the mid- 20th century, when tobacco smoking pervaded much of the developed world, might seem paradoxical. But this is because in most of Europe and the US, 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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