Canadian, American and British researchers analyzed smoking histories and death records for 113,752 women and 88,496 men in the U.S. over seven years.
"Those that quit by age 40 avoid about 90 per cent of the risk of continuing to smoke," said study author Dr. Prabhat Jha, head of the Centre for Global Health Research at St. Michael’s Hospital in Toronto.
"Those that quit by 30 are close to never smoker death rates."
The message is that it's never too late to quit, he said.
But the researchers cautioned it is not safe to smoke until 40 and then stop because the risk is still substantial.
"About one in six of these former smokers who dies before the age of 80 years would not have died if their death rates had been similar to those for persons who had never smoked who were similar in educational levels, adiposity and alcohol use," the study's authors wrote.
Dr. Graham Berlyne, a respirologist and chief of medicine at St. Joseph's Health Centre in Toronto, called the study important for its size and focus on the first generation of women who started smoking when they were young and continued through their adult lives.
"The years of smoking are not erased but the damage done is halted and the lungs have a huge capacity so that we can still function even having lost some capacity," Berlyne said.
Tracy Hager, 39, of Toronto, is using nicotine patches and lozenges to quit. She hasn't lit up in six weeks but her two teenagers have started.
"I was so disappointed, but I can't really blame anybody else but myself," said Hager, who is using the study's findings as further motivation.
The study was funded by U.S. National Institutes of Health, the Canadian Institutes of Health Research and the Bill and Melinda Gates Foundation.
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