Tobacco smoke contains several known cancer-causing substances. But previous studies on the link between cigarette smoking and breast cancer have provided inconsistent and sometimes controversial results.
Researchers say a major issue is that lifetime smoking exposure consists of many factors, including active and secondhand or passive smoke exposure, which can be difficult to measure accurately.
In this study, researchers looked at the effects of personal smoking history as well as passive smoke exposure on breast cancer risk using data from 111,140 women who were followed from 1976 to 2006 for active smoking status and 36,017 women who provided information from 1982 to 2006 on secondhand smoke exposure.
Overall, 8,772 cases of breast cancer were reported during the follow-up period.
The results showed breast cancer risk was higher among the following groups:
Heavy current and past smokers (25 or more cigarettes daily).
Those who started to smoke before age 17.
Women who had smoked for at least 20 years.
Current and past smokers with a history of 20 or more pack-years (number of packs of cigarettes smoked per day multiplied by the number of years smoked).
“Heavy smokers who started smoking early in life, smoked for a long duration and smoked a high quantity were at the highest risk of breast cancer,” write researcher Fei Xue, MD, ScD, of Brigham and Women’s Hospital and Harvard University, and colleagues in the Archives of Internal Medicine.