Women who use permanent hair dye and chemical straighteners may get more than just shinier, brighter hair, according to a new study.

Public health experts who studied the hair care habits of more than 46,000 women across the US from 2003 to 2009 have found that women who permanently dye their hair tend to also be roughly 9% more likely than other women to develop breast cancer.

In the study, conducted in women who had at least one sister with breast cancer, 55% of women surveyed said they’d used permanent hair dye in the past 12 months.

Corresponding study author Alexandra White told Insider that her study findings should be treated with some caution.

“Relative risk sounds very high, but these are not huge increases in risk,” she said. “These point estimates are small in magnitude, and need to be put in context of everything else we know that influences breast cancer risk.”

Black women may be the most in danger of all. Those who dyed their hair at least once in a year had a 45% greater chance of developing breast cancer, compared to other black women who did not dye their hair. Those who dyed their hair most frequently (every five to eight weeks or more) were 60% more likely to contract breast cancer.

Chemical straighteners appear to increase risk, too; women who used those every month or two were about 30% more likely to develop breast cancer than others.

“It’s kind of this additional burden that [black women] are experiencing by their use of chemical straighteners that, really, white women aren’t using,” White said. “74% of black women in our study are using chemical straighteners and only 3% of white women [are].”

Other studies have shown how potentially cancer-causing and endocrine-disrupting chemicals in hair dye can enter the bloodstream, circulate through the body, and reach breast tissue. Hair dressers, who deal with dyes and other hair chemicals day in and day out, are already known to have increased cancer risk, but this study took salon workers out of the results and still found a link between dyed hair and more breast cancer cases. Since it focused only on women who already had a sister with breast cancer, it helped control for any genetic factors, like the presence of a BRCA breast cancer gene.

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It’s possible that there are some material differences in the hair products marketed to white and black women that influence cancer risk in different ways. Older black women dying hair to cover their greys may be using products that include coal tar, a known cancer causer.

Glossy Brazilian keratin straightening treatments may create another exposure issue. They became extremely popular in the early 2000s, but have fallen out of favor after it was discovered that they release formaldehyde, another known cancer-contributor, when heated.

“Just because something’s on the shelf doesn’t mean it’s safe,” Tamarra James-Todd, a public health researcher at Harvard who was not involved in this study but who has been studying chemical exposures and health across different ethnic groups for over a decade, told Insider.

She said one issue is how many different products black women sometimes use on their hair to fit Euro-centric beauty ideals.

“People are altering their appearance, and layering chemicals on top of chemical on top of chemicals on themselves,” James-Todd said.

The solution, she said, is policy change in an industry that is barely regulated.

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While the risk of developing breast cancer remains low overall among hair dye and straightener users, James-Todd said that in women who are already genetically predisposed to breast cancer, it’s possible that using more hair dye could be like a cancer “tipping point.”

White, meanwhile, uses non-permanent highlights in her hair (there was no significant link between semi-permanent hair dyes or temporary colors and cancer risk in the study). Her best advice for avoiding breast cancer has nothing to do with hair care.

“I don’t know if these findings in particular would put me off of hair dye,” she said.

Instead, she’d prioritize exercise, which is inversely associated with breast cancer, and has lots of other health benefits, too.

https://www.insider.com/permanent-hair-dye-and-straighteners-may-increase-breast-cancer-risk-2019-12