For decades, scientists have debated whether hair dyes frequently used by women might contribute to cancer. The research has been mixed and inconclusive, but now government investigators have turned up a disturbing new possibility.

Black women who regularly used permanent dyes to color their hair were 60 percent more likely to develop breast cancer, compared to black women who did not report using dye, according to an analysis published this week in The International Journal of Cancer.

White women using hair dye did not see a significantly increased risk. The reasons are unclear: It may be because different products are designed for women of different ethnic and racial backgrounds, or that variations in hair texture alter the amounts of dye that are applied or absorbed through the skin.

The study also implicated hair straighteners, finding a 30 percent increase in the risk of breast cancer among women of all races who reported regular use of the products . African-American women were much more likely than white women to use hair straighteners, the researchers noted.