The study followed 46,700 American women who were already enrolled in the National Institutes of Health’s “Sister Study,” which tracks breast-cancer-free women whose biological sisters were diagnosed with cancer. The women, age 35 to 74, answered lifestyle questions (for example, what hair products they used) and updated researchers over an average of eight years. In the end, the study found that women who’d used the dyes and chemical straighteners had a greater chance of being diagnosed with breast cancer. This was especially true if those women identified as Black.
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