The research, based on results from the Million Women Study*, is published Online First in The Lancet today [Saturday, October 27] to mark the 100th anniversary of the birth of Sir Richard Doll, one of the first people to identify the link between lung cancer and smoking.
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 post three years later. The NHS central register notified the researchers when any participant died, giving the cause of that death. Women were traced for an average of twelve years from the time they first joined; thus far, 66 000 study participants died.
Initially, 20% of the study participants were smokers, 28% were ex-smokers, and 52% had never smoked. Those who were still smokers at the 3-year resurvey were nearly three (2.97) times as likely as non-smokers to die over the next 9 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 (1 cigarettes per day) 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 ten times bigger.
According to co-author Professor Sir Richard Peto, at the University of Oxford, UK, "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 ten 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."**
Writing in a linked Comment, Professor Rachel Huxley, at the University of Minnesota, USA, adds "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 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."
NOTES TO EDITORS:
* The study was conducted by the authors for the Million Women Study Collaborators. The Million Women Study is funded by Cancer Research UK, the Medical Research Council, and the Health and Safety Executive.
** Quote direct from author and cannot be found in text of Article.
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