The Sister Study included data from 46,709 women. Researchers with the National Institute of Environmental Health Sciences found that women who regularly used permanent hair dye in the year before the study were nine percent more likely to develop breast cancer than women who didn't.
The study goes on to suggest that the number was even more dramatic among African-American women. African-American women who used permanent dyes every five to eight weeks were associated with a 60 percent increase, compared to an eight percent increase among white women.
That number was just for the permanent dye. There was little to no increase for women who used semi-permanent or temporary dye.
"Researchers have been studying the possible link between hair dye and cancer for a long time, but results have been inconsistent," said corresponding author Alexandra White, Ph.D., head of the NIEHS Environment and Cancer Epidemiology Group.
"In our study, we see a higher breast cancer risk associated with hair dye use, and the effect is stronger in African American women, particularly those who are frequent users."
As for chemical straighteners, researchers found that women who use them at least every five to eight weeks were about 30 percent more likely to develop breast cancer.
Dale Sandler, Ph.D., one of the study’s authors, said that there is some evidence from previous studies to support a link between hair straighteners and an increased risk of breast cancer, but more research needs to be done.
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