ACS. Harvard, and NCI researchers conclude that smoking causes breast cancer: This is important to all women

Researchers at the American Cancer Society, Harvard, and National Cancer Institute published “Smoking and Mortality – Beyond Established Causes” in the New England Journal of Medicine on February 12, 2015.  This paper attracted a lot of media attention because it added several new diseases to the list that smoking causes, including breast cancer, which added another 60,000 deaths to the annual toll.
 
The fact that researchers at the American Cancer Society and Harvard are now recognizing that smoking causes breast cancer is a big development, since they have been the main skeptics of the conclusion that secondhand smoke caused breast cancer in younger, primarily premenopausal women that the California Environmental Protection Agency reached ten years ago, in 2005.
 
The ACS/Harvard/NCI paper concluded that smoking increased the risk of breast cancer by about 30% (a relative risk of 1.3 with a 95% confidence interval extending from 1.2 to 1.5) in older women (55 and older).  This is about the same risk that Ken Johnson and I estimated based on smaller studies than in the new paper.
 
The new estimate likely underestimates the true amount of breast cancer that smoking causes for two  reasons: 
 

• The researchers compared smoking women to nonsmoking women, which includes passive smokers.  Since secondhand smoke increases the risk of breast cancer, this means that the risks will be understated compared to what would be found with a cleaner control group of nonsmoking women not exposed to secondhand smoke.
• The breast cancer risks of smoking (and secondhand smoke) are largest in young women, before they have had a baby.  (Breast cells change in a way that makes the cells less sensitive to the chemical carcinogens in cigarette smoke) when a women lactates .)  The risks estimated for older women will be lower than in younger women. 

 
These points notwithstanding, this is an important paper.  It is time to actively integrate breast cancer into tobacco control efforts, particularly for the CDC and US Surgeon General to add breast cancer to the official list of disease smoking and secondhand smoke “causes.”
 
This recognition will be particularly important in campaigns to make casinos and bars smokefree.  A lot of young women work in these environments and are getting exposed to secondhand smoke at a time when it has its strongest effects on increasing breast cancer.