"The harms of smoking are reversible and can decline to the level of nonsmokers," said study author Stacey Kenfield, whose report is in the May 7 issue of the Journal of the American Medical Association. "For some conditions like chronic obstructive pulmonary disease, it can take more than 20 years, but there is a rapid reduction for others."
"It's never too early to stop, and it's never too late to stop," added Kenfield, who is a postdoctoral research fellow in the department of epidemiology at the Harvard School of Public Health in Boston.
Smoking is still the leading preventable cause of death in the United States. Not only does tobacco smoke cause lung cancer, it is also implicated in heart disease, other cancers and respiratory diseases.
According to the World Health Organization, an estimated 3 million people in industrialized countries will have died as a result of tobacco use by 2030, and an additional 7 million people in developing countries face the same fate.
This research is a continued follow-up on the Nurses' Health Study, a large trial involving more than 100,000 women. Scientists now have 22 years of data on the participants.
Current smokers had almost triple the risk of overall death compared with women who had never smoked.
Current smokers also had a 63 percent increased risk for colon cancer compared with never-smokers, while former smokers had a 23 percent increased risk. There was no significant association between smoking and ovarian cancer.
And women who started smoking earlier in life were at a higher risk for overall mortality, of dying from respiratory disease and from any smoking-related disease.
However, a smoker's overall risk of dying returned to the level of a never-smoker 20 years after quitting. The overall risk declined 13 percent within the first five years of abstaining.
Most of the excess risk of dying from coronary heart disease vanished within five years of quitting.
For chronic obstructive pulmonary disease, the return to normal took more than 20 years, although there was a 13 percent reduction in the risk of death seen within five to 10 years after quitting.
And the risk for lung cancer didn't return to normal for 30 years after quitting, although there was a 21 percent reduction in risk within the first five years compared with women who continued to smoke.
Many previous studies on tobacco use had focused on men and on lung cancer, the authors stated. They also only looked at smoking status at the beginning of the study. "We got smoking information every two years, so we feel we have a more accurate estimate of current and past smoking," Kenfield said. "We saw increased risks for current smokers [than previous studies], and we think that's because we know who the current smokers are."
"This shows the power of quitting smoking," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "We've known this for a number of years, but the beauty of this study is it is a very large and well-studied group of people. When I tell people to quit smoking, I say the effect of the heart precedes that of the lungs. If you've smoked, you need to be cognizant that you're still at an increased risk of lung cancer."
More information
Visit the American Lung Association for more on women and smoking.