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 here because they have been studied more than most or they have received quite a bit of media attention. The factors below are listed alphabetically, not by level or risk or strength of evidence.
Although there was some debate in the past about the link between abortion (also called induced abortion) and breast cancer risk, research clearly shows no link between the two .
Since 2003, the National Cancer Institute (NCI) and the American College of Obstetricians and Gynecologists have agreed that the scientific evidence does not support a link between abortion and breast cancer [459-460]. The NCI routinely reviews the evidence on this topic (most recently in 2010) and continues to agree the evidence does not support a link between the two .
The importance of study design for research on abortion and breast cancer risk
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 [458,463-471].
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 [463-465,468-470]. 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 .
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)
- Industrial chemicals, such as PCB's (polychlorinated biphenyls)
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 [472-478].
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) [476,479]. 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 EMF) 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 [476,479].
Scientific evidence does not support a link between wearing an underwire bra (or any type of bra) and an increased risk of breast cancer. 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 . 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 lean, which the authors concluded might account for the lower risk .
Because weight increases risk of postmenopausal breast cancer, breast size is under study as a potential risk factor for breast cancer.
Learn about body weight and breast cancer risk.
Learn about breast size and breast cancer risk.
Although there was some debate in the past, there is no scientific evidence to show that either saline or 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 [481-486].
A few studies have shown a lower risk of breast cancer among women with implants [485-486]. However, this is most likely due to traits of women who tend to choose breast implants (for example, these women tend to be lean). These factors lower risk for breast cancer rather than the implants themselves .
Breast implants and risk of anaplastic large cell lymphoma (ALCL)
Although breast implants do not increase the risk of breast cancer, the FDA is looking into a possible link between 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.
Read our statement on breast implants and the possible link to ALCL.
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 [490-495]. 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.
Studies have found no increase in the risk of breast cancer or other types of cancer from cell phone use [497-498].
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 [499-500].
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.
A few case-control studies have suggested a link between EMF exposure and breast cancer risk . However, large prospective cohort studies and a meta-analysis that combined the results of 15 studies have found no link between the two [502-507].
Hair dyes and hair relaxers
Cohort and case-control studies have shown the use of permanent hair dyes is not related the risk of breast cancer [508-510]. 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 .
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 [514-515]. There are also no links between being left-handed and reproductive factors related to breast cancer risk (such as age at first period or age at menopause) [515-516].
Trauma to the breast
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 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 Understanding Breast Cancer section of this website come from human studies unless otherwise noted.
Finding information on risk factors for breast cancer
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.
*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.