A December 2019 study published in the International Journal of Cancer found that use of hair dyes and chemical hair relaxers/straighteners may be associated with an increased risk of breast cancer.

Researchers with the National Institute of Environmental Health Sciences (NIEHS) used data from the large, ongoing Sister Study, which follows 46,709 women aged 35-74. Each woman has a sister with breast cancer, and the cohort study looked to see if they developed breast cancer, as well as potential causes.

The NIEHS researchers selected data regarding use of hair dyes and chemical hair straighteners and used an average follow up of eight years to see if participants developed breast cancer. They identified 2794 cases of breast cancer, and of these cases, 55% of women reported use of permanent hair dyes at the time of enrollment.

White women who used permanent hair dyes had a 7% increased risk of breast cancer, while black women had a 45% increased risk. Nonprofessional application of semi-permanent hair dyes was associated with a 27% increased risk. Personal hair straightener use was associated with an 18% increased risk, with that risk increasing with more straightener use. Nonprofessional application of hair straightener was associated with a 28% increased risk.

Concerns were noted over chemicals in some dyes and straighteners that mimic the hormone estrogen, which if kept at a sustained excess level in the body can contribute to the development of breast cancer.  

The study has several limitations. Only 9% of the study’s participants were black and most were only asked about product use once. Furthermore, concerns over real cancer risks are usually not raised until an environmental factor reaches at least a hazard ratio of 2.0 or a 100% increase in risk. The risks identified in this study do not reach that, and the study should not be interpreted as these products guaranteeing breast cancer. Instead, the researchers want women using these products to take these small risks into consideration for their daily lives.