Breast Cancer Risk Factors

Breast Cancer Risk Factors Infographic | City of Hope Click to download infographic

Many factors can impact a woman’s chances of developing breast cancer. These can be divided into categories of demographic characteristics, genetic predisposition, reproductive factors, lifestyle factors and environmental factors. 

Demographic Characteristics

Demographic predictors of breast cancer risk are age and race or ethnicity. Like most cancers, one’s risk of breast cancer increases as one gets older. 

Further, black women have greater risk before age 40 years than white women, but after that, white women have higher risk and the highest risk overall. 

Latina white women have lower risk than non-Latina white women and black women.

Breast cancer risk among Asian women is lower than that of white women and black women, but Asian women’s risk increases the longer they have lived in the United States. In fact, the breast cancer risk of U.S.-born Japanese women is nearly as high as that of non-Latina white women.  

Genetic Predisposition

We have long known that women with a first degree family history of breast cancer have greater risk than women who have no such family history. Having a first degree family history means that a woman’s mother, sister or daughter has been diagnosed with breast cancer. In the 1990s, two genes were identified that define about 25 percent of the increase in risk associated with family history. Specific mutations in these two genes, known as BRCA1 and BRCA2, are associated with high risk of breast cancer. 

Reproductive factors

The first studies of breast cancer identified reproductive risk factors: a woman’s age at her first menstrual period (menarche), her age at her last menstrual period (menopause), whether or not she had pregnancies, her age at her first full-term pregnancy and how many full-term pregnancies she had. Breast cancer risk is lowest for women who were older when they had their first menstrual period, were younger when they had their last menstrual period, had a full-term pregnancy (or first birth) at a young age, and after births, breastfed their infants. Although the first birth is most important, each subsequent birth lowers risk some more.
 
Because the breast changes during or as a result of each of these reproductive events, the timing of other exposures in relation to reproductive events may be important. Research has focused on reproductive transitions such as during pubertal development and ending with the onset of menstrual periods, the time between a first menstrual period and a first full-tem pregnancy, and the time when a woman transitions from menstruating to being menopausal.

Lifestyle factors

Lifestyle factors that increase breast cancer risk include being overweight or obese, drinking alcohol and being inactive. Regular exercise lowers breast cancer risk, whereas time spent sitting has an independent effect and increases risk. Use of hormone therapy increases breast cancer risk, especially use of combined therapy, estrogen plus a progestin (which are synthetic forms of estrogen and progesterone).  

Environmental factors

Only one environmental factor is known to increase breast cancer risk and that factor is exposure to ionizing radiation. Exposure to environmental pollutants and toxic chemicals are possible risk factors for breast cancer.  Any increase in breast cancer risk associated with exposure to environmental pollutants will likely depend on the amount and type of exposure, the age when the exposure occurs and the properties of the pollutant.

Our study focuses on women being exposed to two such exposures during the menopausal transition and how this exposure may impact a woman’s risk of breast cancer. The two environmental pollutants, both of which are widespread in the environment today, are polybrominated diphenyl ethers and bisphenol A.