The largest-ever study of smoking among women in the UK has shown that female smokers lose at least 10 years of life on average. But stopping before the age of 40 – and preferably well before the age of 40 – avoids more than 90% of the increased risk of dying caused by continuing to smoke.
The Oxford University-led study, published in the medical journal The Lancet, is timed to mark the centenary of the birth of Sir Richard Doll, one of the first people to identify the link between lung cancer and smoking.
Many of the researchers who led this research worked closely with Doll, taking forward the significant work and expertise he established in Oxford in using large-scale epidemiological studies and randomised trials to answer important questions in medicine.
The study's key findings are that the hazards of smoking for women are greater than previous studies have suggested – but also that stopping smoking has bigger benefits than previously thought.
Two-thirds of all deaths of smokers in their 50s, 60s, and 70s can be put down to smoking, the results indicate, since most of the difference between smokers and non-smokers comes from smoking-related diseases. These include lung cancer, chronic lung disease, heart disease, and stroke.
But women who stopped smoking aged around 30 avoided 97% of their increased risk of premature death due to cigarettes.
Professor Sir Richard Peto of the University of Oxford, one of the study's lead authors, says: '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 Peto emphasises that, male or female, 'smokers who stop before reaching middle age will on average gain about an extra ten years of life'.
The Million Women Study recruited 1.3 million women aged between 50 and 65 in the years from 1996 to 2001. Participants completed a questionnaire about their lifestyle, medical and social factors, and then took another survey three years later. The researchers used NHS records to track any deaths of participants, and the cause of death, over an average of 12 years from the time the women first joined the study.
At the start of the study, 20% of the study participants were smokers, 28% were ex-smokers, and 52% had never smoked. The researchers found that those women who were still smokers when surveyed three years later were nearly three times as likely as non-smokers to die over the next nine years.
The researchers also showed that risks among smokers increased steeply with the amount smoked. And even for light smokers having between one and nine cigarettes a day, death rates were double those for non-smokers.
Sir Richard Doll, who died in 2005 aged 92, was Regius Professor of Medicine at Oxford University from 1969–1979 and founding Warden of Green College 1979–1983. He continued research work right up until his final months.
Professor Peto, who worked with Doll for 30 years, has said that Doll's work has helped prevent millions of premature deaths worldwide.
In 1950 Richard Doll and Bradford Hill showed that smoking was 'a cause, and an important cause' of the rapidly increasing epidemic of lung cancer in the UK.
Although best known for his work on tobacco, Doll carried out an extraordinary range of other studies. He also studied stomach ulcers, radon in houses, leukaemia, occupational exposure to asbestos, and the various minor hazards of the contraceptive pill. Apart from tobacco, his main interests later on were in the causes of breast cancer and leukaemia, and in trials of treatments for breast cancer and vascular disease.
Sir Richard Doll's centenary is being marked by a scientific meeting held in Oxford next week. Leading researchers from across the globe will gather to discuss the latest findings from large-scale studies carried out worldwide, from Japan, Australia, China and India to USA, Russia, Cuba and Mexico.
Doll wrote in 1994: 'Death in old age is inevitable but death before old age is not.' He continued: 'In previous centuries 70 years used to be regarded as humanity's allotted span of life and only about one in five lived to such an age. Nowadays, however, for non-smokers in Western countries, the situation is reversed; only about one in five will die before 70 and the non-smoker death rates are still decreasing, offering the promise, at least in developed countries, of a world where death before 70 is uncommon.'
Today's results in the Lancet reinforce Doll's conviction that: 'For this promise to be properly realised, ways must be found to limit the vast damage now being done by tobacco and to bring home, to not only the many millions of people in developed countries but also the far larger populations elsewhere, the extent to which those who continue to smoke are shortening their expectation of life by so doing.'
The study was conducted by the Million Women Study Collaborators led by Oxford University researchers in the Cancer Epidemiology Unit and the Clinical Trial Service Unit. The Million Women Study is funded by Cancer Research UK, the Medical Research Council, and the Health and Safety Executive.