There's no evidence that the major ingredients in hair relaxers - such as lye and calcium hydroxide - promote cancer. However, manufacturers need not list all of the ingredients they use in cosmetics (as some are considered trade secrets), and it's not clear whether some of these substances might be harmful.
Because many African-American women regularly use hair relaxers, often throughout their lives, it's important to know whether the products carry any long-term health risks, researchers say.
The new study found that among more than 48,000 African-American women, those who used hair relaxers most frequently were no more likely to develop breast cancer than those who rarely used the products.
''Our study provides reassurance that hair relaxers in general are not increasing the incidence of breast cancer,'' Dr.
Lynn Rosenberg, the study's lead author, told Reuters Health.
That said, ''nothing is 100-per cent certain,'' noted Rosenberg, who is associate director of the Slone Epidemiology Center at Boston University, Massachusetts.
The study did not look at individual products, she explained, and it's still possible that some contain harmful substances. For now, however, there's ''very little evidence'' that women who use hair relaxers should worry that they're raising their odds of breast cancer, according to Rosenberg.
She and her colleagues report their findings in the journal Cancer Epidemiology, Biomarkers&Prevention.
The researchers based their results on data from the Black Women's Health Study, which began following a large group of African American women from across the US in 1995. Study participants completed questionnaires on their health and lifestyle habits every two years.
Of 48,167 women who reported on their hair relaxer use, 574 had developed breast cancer by 2003. Women who used hair relaxers most frequently - seven or more times per year - were no more likely to develop breast cancer than their peers who rarely or never used the products.
The same was true when the researchers compared non-users to women who'd used hair relaxers for 20 years or more.
Compared with white women, African-American women are more likely to develop breast cancer before the age of 45, for reasons that aren't completely clear. In this study, Rosenberg pointed out, there was no evidence that hair relaxers increased breast cancer risk in older or younger women.
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