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The NIH tracked 46,709 women. Then they looked at their beauty routines. The Sister Study found that women using permanent hair dye faced a 9 percent higher risk of developing breast cancer. For Black women dyeing their hair every five to eight weeks, that risk jumped to 60 percent. Semi-permanent and wash-out dyes showed almost zero link. The data comes with a caveat. Every woman in the study already had a sister with breast cancer, meaning these chemicals likely aggravate an existing genetic vulnerability.