Over the years, many factors have been studied to see whether or not they increase the risk of breast cancer. Some of these factors have been proven to be unrelated to breast cancer and do not increase (or in some cases, decrease) risk. A full list of these factors is too long to present here. Some, however, deserve mention because they have been studied more than most or they have gotten a lot of media attention.
The factors below are listed alphabetically, not by level of risk or strength of evidence.
Research clearly shows no link between abortion (also called induced abortion) and breast cancer risk .
Since 2003, the National Cancer Institute (NCI) and the American College of Obstetricians and Gynecologists (ACOG) have agreed the scientific evidence does not support a link between abortion and breast cancer [438-439]. The NCI and ACOG routinely review the evidence on this topic (most recently in 2010 and 2013, respectively) and continue to agree there is no link between abortion and breast cancer [439-440].
Some case-control studies have suggested abortion may increase the risk of breast cancer . However, the design of case-control studies can make the accuracy of their results questionable. Case-control studies rely on the reporting of past behavior. When it comes to a sensitive topic like abortion, this can have a big impact on the information gathered. The cases in these studies (the women with breast cancer) are more likely to give information about their abortion history than the controls (the women without breast cancer). Such differences in reporting can bias study results.
Prospective cohort studies are much more likely to give accurate results on the topic of abortion. These studies gather sensitive information before women are diagnosed with breast cancer. This helps limit biased reporting. The results from cohort studies show abortion does not increase the risk of breast cancer [437,442-450].
Read our statement on abortion and breast cancer risk.
Further supporting the evidence on abortion are findings from studies on miscarriage (also called spontaneous abortion) and the risk of breast cancer. Cohort studies and a large pooled analysis have shown miscarriage does not increase the risk of breast cancer [442-442,447-449].
The NCI routinely reviews the evidence on miscarriage and breast cancer and since 2003, has concluded the evidence does not support a link between the two .
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:
A good 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 [451-458].
Sometimes a cluster (a small area, 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. Two of the best-studied breast cancer clusters are in Long Island, New York and Marin County, California (San Francisco Bay area) [455,459]. In the 1990’s, rates of new cases of breast cancer were higher than expected in these areas. Many studies looked at known risk factors and possible environmental factors (including organochlorines and electromagnetic fields) that might be related to the excess number of breast cancer cases in these areas. 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 family history of breast cancer, use of menopausal hormone therapy (postmenopausal hormones) and age at first childbirth [455,459].
Read our perspective on pesticides and breast cancer risk (November 2010).*
Read our perspective on cancer cluster studies of pesticides and breast cancer risk (November 2011).*
For a summary of research studies on environmental pollutants and breast cancer, visit the Breast Cancer Research Studies section.
*Please note, the information provided within Komen Perspectives articles is only current as of the date of posting. Therefore, some information may be out of date at this time.
Scientific evidence does not support a link between wearing an underwire bra (or any type of bra) and breast cancer risk . There is no biological reason the two would be linked. Any observed relationship is likely due to other factors.
A 1991 case-control study found that women who did not wear bras had a lower risk of breast cancer than women who did wear bras . However, the authors 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 thin, which the authors concluded might account for their decreased risk of breast cancer .
Although wearing a bra does not appear to be linked to breast cancer risk, breast size is under study as a potential risk factor.
Learn about breast size and breast cancer risk.
Neither saline nor silicone breast implants increase breast cancer risk. Findings from prospective cohort studies and a meta-analysis of 10 studies of cosmetic breast implants (including studies of both saline and silicone implants) have found no increase in breast cancer risk among women with breast implants [462-468].
A few studies have shown that women with breast implants have a lower risk of breast cancer than women without implants [466-467]. However, this is most likely due to traits of women who tend to choose breast implants (for example, these women tend to be thin). These traits likely lower the risk of breast cancer rather than the implants themselves .
The FDA is looking into a possible link between breast implants and a slight increase in the risk of anaplastic large cell lymphoma (ALCL) . ALCL is a very rare form of cancer of the cells of the immune system (it occurs in breast tissue in about three in 100 million women).
The FDA does not recommend special care for women with breast implants (nor does it recommend removing implants) . However, if you have breast implants and feel any pain around the implants or have other problems, see your health care provider.
For a summary of research studies on breast implants and breast cancer, visit the Breast Cancer Research Studies section.
Some data suggest that caffeine may affect blood hormone levels . However, the largest cohort studies to date have found no link between drinking either coffee or tea and the risk of breast cancer [471-476]. Further, a meta-analysis that combined the results of 37 studies found no link between caffeine or coffee intake and breast cancer risk .
Learn more about caffeine.
Research on deodorant/antiperspirant use and breast cancer risk 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, studies have found no link between deodorant/antiperspirant use and breast cancer risk [480-481].
Read our perspective on deodorant/antiperspirant use and breast cancer risk(April 2010).*
People are exposed to electromagnetic fields (EMF) through utility wires, electric blankets, microwave ovens and fluorescent lighting. To date, there is little evidence that regular exposure to EMF increases the risk of breast cancer.
A few case-control studies have suggested a link between EMF exposure and breast cancer risk . However, large cohort studies and a meta-analysis that combined the results of 15 studies have found no link between the two [483-488].
Read our perspective on power lines and breast cancer risk(November 2010).*
Read our perspective on cancer cluster studies of EMF and breast cancer risk(November 2011).*
For a summary of research studies on electromagnetic fields and breast cancer, visit the Breast Cancer Research Studies section.
Cohort and case-control studies have shown the use of permanent hair dyes is not related the risk of breast cancer [489-491]. A meta-analysis that combined the results of 14 studies confirmed these findings .
There also appears to be no link between the use of hair relaxers and the risk of breast cancer among African-American women and breast cancer risk .
For a summary of research studies on hair dyes and breast cancer, visit the Breast Cancer Research Studies section.
Although it is not clear why, breast cancer is five percent more likely to occur in the left breast compared to the right breast . This difference prompted a theory that being left-handed may increase breast cancer risk.
However, studies have shown that being left-handed does not increase breast cancer risk [495-496]. Further, being left-handed is not linked to reproductive factors related to breast cancer risk (such as age at first period or age at menopause) [496-497].
The data in this section come from two main types of research studies:
The goal of these studies is to give information that helps support or disprove an idea about a possible link between an exposure (like alcohol use) 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.
Learn more about different types of research studies.
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. They often look at exposures in larger doses and for shorter durations than are suitable for people. Thus, animal studies can lay the groundwork for research in people, but in order to draw conclusions for human populations, we need human studies.
All data presented within the About Breast Cancer section of this website come from human studies unless otherwise noted.
Several organizations conduct research and/or prepare summary reports of research on certain exposures shown to have a link (or no link) to breast and other types of cancer. These agencies are a good place to find detailed, up-to-date information (for example, if you have concerns over a news item on cancer).
IARC is a part of the World Health Organization. The CDC, NTP and FDA are all part of the U.S. Department of Health and Human Services.
Facts for Life: Breast Cancer Risk Factors
Is There a Connection? Breast Cancer and Abortion
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