(HealthScoutNews) -- Even if you've smoked for half a century, a new study suggests you can still add a few years to your life by kicking the habit.

However, it's still much smarter to dump cigarettes at a much younger age, say researchers at Duke University. According to their calculations, a female smoker who quits at age 35 may boost her life span by seven years, compared to less than two years for one who stops at age 65.

"Smoking cessation is clearly one of the best things you can do to increase your life expectancy," says study co-author Donald H. Taylor Jr. "We know that smoking shortens life, but this puts numbers on it."

Duke researchers set out to understand how smoking affects longevity. They turned to an ongoing American Cancer Society study of 1.2 million Americans that began in 1982.

The researchers examined a subgroup of 877,243 people, of whom about 20 percent had died by the late 1990s. The researchers then estimated life spans for three groups -- smokers, those who never smoked, and those who smoked and then quit.

The findings appear today in the June issue of the American Journal of Public Health. The Duke researchers acknowledge the results apply mostly to the white, middle-class, educated people who made up a large chunk of the people studied by the American Cancer Society.

According to the Duke study, male smokers who quit by age 35 increase their life span by 6.9 to 8.5 years, and female smokers by 6.1 to 7.7 years.

On average, compared to men who smoked until they died, male smokers who quit at 35 would live to be 76 instead of 69. Women who quit would live to be 80 instead of 74.

Nonsmoking men live to an average of 78 years, while women live to be 81, according to the study.

It's not entirely clear why men get a bigger boost from quitting smoking, but Taylor points out that men smoke more cigarettes over the course of their lives. "That may be one reason why smoking is more lethal to men," he says.

Older Americans can also benefit from quitting smoking. The study suggests that male smokers who quit at age 65 increase their life span by 1.4 to 2 years, and women smokers by 2.7 to 3.7 years.

For men, that means a typical increase in life span from 69 to 71 years; for women, from 74 to 77.

The increase is significant, Taylor says. "It's not like 10 days or two weeks. This suggests the body is pretty resilient."

The study sends a "very important" message to smokers who think it's too late to quit, says Dr. Norman H. Edelman, a consultant on scientific affairs to the American Lung Association.

"There are some who say, 'I've gotten away with it so long, I must be immune to the effects' or 'It's too late to quit smoking.' This shows that [the latter] is clearly not true," Edelman says. "You'll get a benefit in terms of how long you live."

Taylor cautions that smokers shouldn't use the study as an excuse to puff away until they qualify for Social Security.

The study is "bad news if some kids think they'll start smoking and quit when they're 65, and undo all the harm," he says. "That's just not true."

What To Do

The Internet has plenty of resources to help you quit smoking. Try Kickbutt.org or the American Lung Association.

Reference Source 101

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