Statistics Canada researchers measured the association between daily smoking and the risk of heart disease for more than 10,000 men and women using 16 years of data up to 2011, says an article in Wednesday's issue of the federal agency's journal Health Reports.
"Smoking cessation reduced the risk of heart disease, but the full benefits were realized only after 20 years of sustained cessation," Didier Garriguet, senior analyst, and his co-authors concluded.
"Among individuals who continue to smoke, cutting down the number of cigarettes smoked also reduced risk."
Current daily smokers were 60 per cent more likely to have been diagnosed with, or to have died from, heart disease than were people who had never smoked daily.
The rate of heart disease decreased for former smokers who abstained the longest.
Garriguet said the finding of 20 years of continuous cessation to return to the same risk levels of those who never smoked is in line with the U.S. surgeon general's finding of 15 years, although Canadians tended to smoke fewer cigarettes than in other studies.
The earlier you quit, the better
When the study began in 1994, men were more likely than women to be current daily smokers (31 per cent versus 25 per cent) or former daily smokers (39 per cent versus 28 per cent).
Among former smokers, about one-quarter of both men and women said they had quit 20 or more years earlier.
Female former smokers reported lower intensity levels — such as number of cigarettes — than did current smokers, regardless of the number of years since quitting.
Among those who said they smoked when the study began, more than half quit during followup.
A related study released by the agency found that the sooner people quit smoking, the better the chances of eventually gaining the same health quality of life as non-smokers.
People with a high health-related quality of life are free of impairments in vision, hearing, speech, ambulation, dexterity, emotion, cognition and pain.
It took about 20 years for former smokers' quality of life to return to that of those who had never smoked daily, a similarity to the first study that Garriguet said was not simply a coincidence.
Last month, Canadian, American and British researchers concluded that people who quit smoking by age 30 are close to never-smoker death rates, after considering their risks of heart attack, stroke and lung and other cancers.