Prabhat Jha, a a professor in the Dalla Lana School of Public Health at Canada's University of Toronto, and colleagues, write about their findings in the 24 January online issue of the New England Journal of Medicine, NEJM.
Jha, who is also head of the Centre for Global Health Research at St Michael's Hospital in Toronto, says in a statement:
"Quitting smoking before age 40, and preferably well before 40, gives back almost all of the decade of lost life from continued smoking."
However, this is by no means a way of saying it is safe to smoke until that time and then stop, says Jha, who adds:
"Former smokers still have a greater risk of dying sooner than people who never smoked. But the risk is small compared to the huge risk for those who continue to smoke."
Youtube video: "Quitting smoking before the age of 40", St. Michael's Hospital
Unique StudyThe study confirms recent evidence from Britain, Japan and the US that smoking tends to take ten years off people's lives, wherever they live in the world.
But it is unique in that it looks at the risks of smoking and the gains of quitting in a sample that is representative of the overall American population, as opposed to groups like nurses or volunteers who tend to be healthier.
It is also one of the first investigations of smoking in the generation of women who took up the habit when they were young and continued into adulthood.
For their study, Jha and colleagues used smoking-cessation histories from over 200,000 men and women aged 25 and over who were interviewed between 1997 and 2004 as part of the US National Health Interview Survey, which covers a broad cross-section of the American public each year.
Using death records, they then related the survey data to causes of deaths that had occurred by the end of 2006, and calculated the risks for current smokers, as compared with those who had never smoked. They also took into account other factors that can influence the risks, such as obesity, educational level, age, and alcohol consumption.
Youtube video: "Women who smoke like men, die like men", St. Michael's Hospital
The ResultsThe overall results showed that for people aged 25 to 79 years, the rate of death from any cause among current smokers was about three times that among those who had never smoked.
Most of the extra deaths among smokers were due to diseases than can be caused by smoking.
"The probability of surviving from 25 to 79 years of age was about twice as great in those who had never smoked as in current smokers," write the authors, who add:
"Life expectancy was shortened by more than 10 years among the current smokers, as compared with those who had never smoked."
They also found that smokers who quit between the ages of 35 and 44 gained about 9 years of lifespan, while those who quit between 45 and 54, gained 6 years.
For women, the risks of dying from smoking-related causes are 50% greater than those suggested in studies done in the 1980s.
"Women who smoke like men, die like men," says Jha.
Huge Global ProblemMost of the world's 1.3 billion smokers live in low and middle income countries. While in many high income countries more than half of the people who ever smoked have quit, stopping smoking is still rare in poorer nations.
If current trends continue, smoking will kill 1 billion people in the 21st century. In the 20th century it killed 100 million.
Jha, who advises governments around the world on disease control, says taxation is the single most effective way to get adults to quit smoking and prevent children from starting.
Funds from the National Institutes of Health, the Canadian Institutes of Health Research and the Disease Control Priorities-3 project of the Bill and Melinda Gates Foundation helped pay for the study.
Youtube video: "Global smoking patterns", St. Michael's Hospital
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Written by Catharine Paddock PhD