Long-term exposure to passive smoking (second-hand smoke) may increase premenopausal breast cancer risk

Jun 2 2005

Breast cancer is one of the most commonly diagnosed cancers among women in Western countries. In North America, for example, there are an estimated 233,000 new cases every year and 45,000 deaths. Despite the well-characterized role of genetics and reproductive risk factors, as well as the suggested roles of physical activity and alcohol, more than half of breast cancer risk remains unexplained.

A new study, published online May 31, 2005 in the International Journal of Cancer, the official journal of the International Union Against Cancer (UICC), suggests that long-term exposure to passive smoking (second-hand smoke) may increase premenopausal breast cancer risk. The study is available via Wiley InterScience.

Researcher Kenneth C. Johnson, PhD. of the Public Health Agency of Canada--which is part of the Government of Canada--analyzed data from the 20 published studies which had examined the relationship of passive smoking to breast cancer. Most but not all of the 20 studies reported increased risk, leading the author to explore what might explain the differences in observed risks among the studies. Factors he evaluated included the impact of study design (case-control or cohort); when the study was published (before or after the year 2000); where the study was conducted (Asia, Europe, North America); what outcome measure was used (breast cancer death or diagnosis); and the quality of the second-hand smoke exposure measure.

The completeness of the second-hand smoke measures varied widely. Studies that collected quantitative long-term information on the three major potential lifetime sources of passive smoking exposure (childhood exposure from parents, adult residential exposure and adult occupational exposure) were considered to have the most complete exposure measures.

For all studies combined, long-term second-hand smoke exposure was associated with a 27 percent increase in breast cancer risk among women who were lifetime non-smokers. The quality of the second-hand smoke exposure history best differentiated those studies that suggest--from those that do not suggest--increased risk. In the subset of 5 studies with the best exposure assessment, the breast cancer risk was increased by 90 percent, but was only elevated 8 percent for the remaining studies with poorer second-hand smoke measures.

Overall, premenopausal breast cancer risk was elevated 68 percent with passive smoking among life-long non-smokers. The premenopausal risk was up 119 percent for the 5 studies with more complete second-hand smoke measures.

Cigarette smoke mutagens have been reported in the breast fluid of non-lactating women, and nicotine has been found in greater concentrations in the breast fluid of smokers than in the plasma. Animal studies demonstrate that mammary tumours in rats, mice and/or hamsters can be induced by a number of tobacco smoke carcinogens.

For women who had smoked, the breast cancer risk estimate was increased by 46 percent when compared to women with neither active nor regular passive smoke exposure. Active smoking was associated with a 108 percent increase in breast cancer risk, in those studies with better second-hand smoke measures. Studies exploring biological mechanisms are needed to explain the unexpectedly high passive and active smoking risks, as well as their similarity.

Dr. Johnson concludes, "this analysis of breast cancer risk finds that studies with comprehensive passive exposure assessment suggest that both passive and active smoking are risk factors for premenopausal breast cancer."

http://www.interscience.wiley.com/journal/ijc