Permanent hair dyeing may increase the risk of breast cancer, according to a December 2019 article in the prestigious International Journal of Cancer.

Research also shows that hair products contain more than 5,000 chemicals, including some considered hormone disruptors. These chemicals can affect how estrogen and other hormones work in the body by blocking or mimicking them, which can alter the body's hormonal balance. Some chemicals in hair dye have been found to cause mammary gland tumors in rats.

These findings on hair dyeing and hair straightening are part of a larger study conducted by scientists at the US National Institute of Environmental Health Sciences, which included 50,884 women living in the United States and Puerto Rico. Women who used permanent hair dye every 5 to 8 weeks or more in the 12 months before joining the study had an 8 percent higher chance of developing breast cancer than women who did not use hair dye.

“Researchers have been studying the possible link between hair dye and cancer for a long time, but the results have been inconsistent,” said corresponding author Alexandra White, chief of the NIEHS Environment and Cancer Epidemiology Group. “In our study, we see a higher risk of breast cancer associated with hair dye use, and the effect is stronger in African-American women.” Dale Sandler, chief of the NIEHS Epidemiology Branch, said, “We are exposed to many things that could contribute to breast cancer, and it is unlikely that any single factor explains the risk of developing the disease. Although it is too early to make a firm recommendation, avoiding these chemicals may be one more thing women can do to reduce their risk of breast cancer. The most practical and wise approach is to use what is efficient and harmless and to minimize anything known to be harmful.”