The study, an analysis of data from the Nurse's Health Study (NHS), found that women who smoked were at a higher risk for death overall, and that that risk increases with the number of cigarettes smoked per day and the earlier a woman started smoking.

The Nurses' Health Study began in 1976 when 121,700 US registered nurses aged 30 to 55 first filled out a questionnaire detailing their medical history and health-related behavior, and it has since been updated and expanded every 2 years. For this study, lead author Stacy A. Kenfield, Sc.D., of the Harvard School of Public Health, and colleagues looked specifically at the relationship between smoking and mortality among the majority of the never smokers, current smokers, and former smokers who took part in NHS.

Overall, current smokers had almost 3 times the risk of death compared with women who had never smoked, and that risk grew with the number of cigarettes smoked per day. Approximately 64% of all deaths among current smokers and 28% of deaths among former smokers were attributed to cigarette smoking.

Women who still smoked were more than 20 times more likely to die of lung cancer than those who didn’t smoke, while former smokers were about 5 times more likely to die from it. Current smokers also had a 63% increased risk of dying from colorectal cancer compared with never smokers. Former smokers had a 23% increased risk. No increased risk of death from ovarian cancer was found in the study.

Among smokers, the study found clear increased risks of death from causes other than cancer as well, including heart disease, stroke, and COPD (bronchitis and emphysema).

Mortality risk among current smokers was higher among those who started smoking at age 17 or younger compared to those who started at or after 26 years of age. The researchers suggest this data should help sound the alarm to direct more attention towards developing school tobacco prevention programs and enforcing laws to keep cigarettes out the hands of kids. For more information on this topic, see Child and Teen Tobacco Use.

On the other hand, quitting smoking was found to have significant benefits over time. Women who quit smoking saw a 13% reduction in their risk of death within the first 5 years of quitting, compared to women who continued to smoke. Even better, after 20 years of not smoking, that risk dropped to a level on par with that of a never-smoker.

Much of the risk of dying from heart disease and stroke also disappeared within 5 years of quitting, and researchers found a 13% reduction in the risk of death from COPD within 5 to 10 years. Former smokers saw a 21% reduction in lung cancer risk within the first 5 years of kicking the habit, compared with women who still smoked. However, according to the researchers, it may take as long as 30 years for excess risk for lung cancer to approach that of a never smoker.

The results from the Nurses’ Health Study present one of the clearest pictures yet of both the risks of smoking and the benefits of quitting. "Effectively communicating the risks to smokers and helping them quit successfully should be an integral part of public health programs," the researchers conclude.

Need help breaking the habit? See our Guide to Quitting Smoking.