Over the years, many factors have been thought to increase the risk of breast cancer. While some of these have been linked to breast cancer, others have not. A full list of factors proven to be unrelated to breast cancer risk is too long to list. However, some factors deserve mention—either because they have had media attention or because they have been studied more than most.
Hair dyes
Both cohort and case-control studies have shown that the use of permanent hair dyes is not related the risk of breast cancer [362-364]. A meta-analysis that combined the results of 14 studies confirmed these findings [365].
Abortion
Although there has been some debate in the past about the link between abortion and breast cancer risk, research now strongly supports no link between the two [366]. This conclusion was further confirmed in a 2003 National Cancer Institute report [367].
While many case-control studies have suggested that abortion may increase the risk of breast cancer [368], the nature of these studies can affect the accuracy of their results [366]. Case-control studies rely on the reporting of past behavior. When it comes to a topic like abortion, this can impact the information gathered. The cases in these studies (the women with breast cancer) may be much more likely to give full information about their abortion history than the controls (the women without breast cancer). Such differences in reporting can reduce the accuracy of the study results.
Studies called cohort studies, on the other hand, are much more likely to give accurate results on the topic of abortion because they gather sensitive information before women are diagnosed with breast cancer. The results from cohort studies clearly show that abortion does not increase the risk of breast cancer [366,369-379].
Electromagnetic fields (EMF)
Electromagnetic fields (EMF) are one of many proposed environmental risk factors for breast cancer. However, there is little evidence that regular exposure to things like power lines and electric blankets increases the risk of breast cancer. While a few case-control studies have suggested a link between the two [380], most research, including larger prospective cohort studies, has not [381-387]. Overall, studies do not support a link between EMF and breast cancer.
Blood organochlorine levels (exposure to certain types of pesticides and industrial chemicals)
Environmental pollutants have been suggested as potential causes of breast cancer because many of these compounds have estrogen-like traits. Some of the most common and well-studied environmental pollutants are organochlorines. These include the pesticide DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) and industrial chemicals, such as PCB's (polychlorinated biphenyls). One way to measure exposure to these chemicals is by looking at their levels in a person's blood. The results of most studies looking at blood organochlorine levels and risk of breast cancer, including the Long Island Breast Cancer Study, have found no link between the two [388-393].
Sometimes a cluster (small areas, such as a town or county, where rates of breast cancer are higher than the U.S. average) is found and this raises interest in possible environmental causes of breast cancer. Examples of such clusters include Long Island, New York and Marin County, California [390,394]. To date, no environmental toxins related to breast cancer have been found. This does not mean that there are no environmental pollutants that can increase risk of breast cancer. However, most clusters have been explained by differences in risk factors such as ages at menarche and menopause and number of children.
Breast implants
Although breast implants have been proposed a risk factor for breast cancer, there is no evidence that either saline or silicone implants increase risk [395-399]. A few studies have shown a lower risk of breast cancer among women with implants [400-402]. However, this appears most likely due to traits of women who tend to choose breast implants, such as a lower body mass index. These factors lower risk for breast cancer rather than the implants themselves [400].
Factors not likely related to risk
Although not as well-studied as the factors described above, based on the research to date, the factors below are not likely related to breast cancer risk.
Bras/underwire bras
The idea that wearing an underwire bra, or any type of bra, might increase the risk for breast cancer, is not supported by the scientific research. There is no biological reason the two would be linked, and any observed relationship is likely due to other factors. A 1991 case-control study found that premenopausal women who did not wear bras had a lower risk of breast cancer than women who did wear bras [403]. However, the researchers stated this link was likely due to factors related to wearing a bra rather than the bra itself. The women in the study who did not wear a bra were more likely to be lean and have small breasts, which the authors concluded might account for the link [403]. Because weight increases risk of postmenopausal breast cancer, breast size is under study as a potential risk factor for postmenopausal women.
Deodorant/antiperspirant use
There have only been a few studies looking at deodorant/antiperspirant use and breast cancer risk. Research in this area was driven by concerns that chemicals found in these products might enter the skin in the underarm and cause changes in breast cells that could lead to cancer. However, the research to date doesn't support a link between the two [404,405]. One study concluded that the reason slightly more breast cancers are found in the area of the breast closest to the underarm is due to this area having more tissue than other parts of the breast [406]. Although a link between deodorant and breast cancer appears unlikely, there are too few studies in this area to say for sure.
In utero exposures and breast cancer risk among offspring
Exposures during pregnancy (in utero) have been suggested as risk factors for breast cancer among offspring. Diethylstilbestrol (DES) was a drug with estrogen-like traits that was given to women to prevent pregnancy problems until about 1970. DES was taken off the market because it was shown to increase the risk of vaginal cancers in daughters of women who took the drug. There is some concern that DES might also increase the risk of breast cancer among daughters of women who took the drug during pregnancy. Studies to date are limited and their findings are mixed. At this time, there is not enough evidence to draw solid conclusions about a potential link between in utero DES exposure and breast cancer risk [407-409].
Plastics
Links between plastics and cancer are often reported by the media and in e-mail hoaxes (one e-mail hoax falsely claims to be a study from Johns Hopkins University). However, there is no scientific research to support a link between using plastic items, such as drinking water from a plastic bottle, and the risk of breast cancer.
Trauma to the breast
To date, there is no evidence to support a link between trauma or injury to the breast and risk of breast cancer.
Where do the data come from?
The data in this chapter come from two main types of research studies: observational studies (prospective cohort or case-control) and randomized controlled trials. The goal of these studies is to give information that helps support or disprove an idea about the link between a factor (like hair dyes) and an outcome (like breast cancer) in people. Although they have the same goal, observational studies and randomized controlled trials differ in the way they are conducted and in the strength of the conclusions they reach. For more on research studies, visit the Breast Cancer Research section.
Animal studies add to our understanding of how and why some factors cause cancer in people. However, there are many differences between animals and people that make it hard to translate findings directly from one to the other. Animal studies are also designed differently than human studies and often look at exposures in larger doses and for shorter times than are suitable for humans. Thus, animal studies can lay the groundwork for human research, but in order to draw conclusions for people, we need human studies. All data presented on this Web site come from human studies unless otherwise noted.
Some organizations do research and/or prepare summary reports of research on certain exposures that have been linked to breast and other types of cancer. If you have concerns over a news item on cancer, these agencies are a good place to find detailed, up-to-date information. The International Agency for Research on Cancer (IARC) is a part of the World Health Organization. The National Toxicology Program is part of the U.S. Department of Health and Human Services and the U.S. Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC) are other federal agencies.
Updated 11/12/09
