African American women are at the highest risk, and especially if they use hair dye every five to eight weeks. The average increased risk among all women is around 9 percent.
Chemical straighteners increase breast cancer risk by around 30 percent for women from any ethnicity, although African American women are more likely to use the products.
Semi-permanent and temporary dyes don't seem to have the same cancer risk, say researchers from the Sister Study, which tracks the health of 46,709 women. They assessed the breast cancer risk for women who used the products for a year.
The hair products are yet another source of chemicals that increase the breast cancer risk. "We are exposed to many things that could potentially contribute to breast cancer, and it is unlikely that any single factor explains a woman's risk. While it is too early to make a firm recommendation, avoiding these chemicals might be one more thing that women can do to reduce their risk," said researcher Dale Sandler.