Do Sidestream and Mainstream Smoke Raise Breast Cancer Risk?

We know that “passive smoking” – a term used to describe exposure to secondhand smoke – can raise the risk of lung cancer. It’s thought that exposure to secondhand smoke – also called environmental tobacco smoke (ETS) is responsible for around 3,000 cases of lung cancer in the U.S. each year. More frightening yet, is that one study suggested that children who grow up with smoking parents were 50% more likely to develop lung cancer in their lifetime.


But the concern now goes far beyond lung cancer risk. It’s thought that around 46,000 heart-related deaths are due to secondhand smoke exposure each year , and now, with breast cancer awareness month just around the corner, we’re hearing that environmental tobacco smoke may raise breast cancer risk as well .

Before entering this area of controversy, a few terms are worth defining. Two of these are “sidestream smoke” and “mainstream smoke.” Sidestream smoke is the smoke that is released from the end of a burning cigarette, and mainstream smoke is the smoke that is inhaled by the smoker, and then exhaled into the environment.

For non-smokers hanging out with people who smoke, about 85% of the smoke they inhale is of the sidestream smoke variety. Does that matter? Considering that some of the toxic chemicals in sidestream smoke are more concentrated (due to incomplete combustion) and that the particle size of sidestream smoke tends to be smaller (and thus able to be absorbed and penetrate better than mainstream smoke) the likely answer is yes. Plus – sidestream smoke hangs around longer, and persists even when mainstream smoke (extinguishing a cigarette for instance) stops.

Understanding these differences may help to make sense of some recent studies. While people who actively smoke don’t seem to have a much greater likelihood of breast cancer based on studies to date (with the exception of those who are heavy smokers and begin smoking at any early age,) those who are exposed to secondhand smoke may.

The California Environmental Protection Agency reported in 2005 that secondhand smoke exposure is consistent with an increased risk of breast cancer in younger women. The U.S. Surgeon General didn’t go quite so far, and stated there is suggestive but insufficient evidence to date to know for certain. But when the numbers are broken down, there is clear cause for concern.

In 2005 researchers took a look at all studies to date that looked at the possible association of breast cancer with both active smoking and with secondhand smoke exposure. There are many numbers (you can check these out in the article below on secondhand smoke and breast cancer) but one number jumped out at me: Premenopausal women who had never smoked, but had regular exposure to secondhand smoke, had twice the risk of developing breast cancer.

Yes, this number is personal, as I am a never smoker who developed premenopausal breast cancer. Could exposure to my parents smoking as a child have contributed to this diagnosis? Certainly it doesn’t help to go there. Blaming won’t improve family relationships or do anything positive for any of us. But sharing this question could make a difference for some – for example, mothers and fathers who continue to smoke around their children.

Why would secondhand smoke increase breast cancer risk, and why, especially, does secondhand smoke seem to be even more of a concern than active smoking?

We know that at least 20 chemicals present in secondhand smoke can cause breast cancer in mice. Since some chemicals are found in higher concentrations in sidestream smoke than in mainstream smoke, that could be a possible reason.

Other reasons have been discussed in the past. Being overweight increases the risk of breast cancer in post-menopausal women, and smokers are less likely to be overweight (note: this is not a reason to start smoking.) But the increased risk noted above was primarily in premenopausal women.

The risk may also be influenced by genetics factors, with some women (and men) having a greater likelihood of secondhand smoke contributing to breast cancer based on their particular genetic profile.

It seems pretty certain that we’ll be hearing more about secondhand smoke, active smoking, sidestream smoke, and mainstream smoke in regards to possible increased risk of breast cancer. Until then, the best solution is to quit if you smoke, and avoid secondhand smoke as much as possible.



American Cancer Society. Secondhand Smoke. Updated 01/17/13.

Conlon, M. et al. Smoking (active and passive), N-acetyltransferase 2, and risk of breast cancer. Cancer Epidemiology. 2012. 34(2):142-9.

Johnson, K. et al. Active smoking and secondhand smoke increase breast cancer risk: the report of the Canadian Expert Panel on Tobacco Smoke and Breast Cancer Risk (2009). Tobacco Control. 20(1):e2.

Reynolds. Smoking and breast cancer. journal of Mammary Gland Biology and Neoplasia. 2013. 18(1):15-23.

Reynolds, P. et al. Passive smoking and risk of breast cancer in the California teachers study. Cancer Epidemiology Biomarkers and Prevention. 2009. 18(12):3389-98.

Schick, S, and S. Glantz. Philip Morris toxicological experiments with fresh sidestream smoke: more toxic than mainstream smoke. Tobacco control. 2005. 14(6):396-404.

Tang, L. et al. Effects of passive smoking on breast cancer risk in pre/post-menopausal women as modified by polymorphisms of PARP 1 and ESR1. Gene. 2013. 524(2):84-9.

Xue, F. et al. Cigarette smoking and the incidence of breast cancer. Archives Internal Medicine. 2011. 171(2):125-33.


via cancer


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