Questions about whether chemicals used to color or straighten hair can cause cancer have swirled for years.
The answers have been inconsistent and inconclusive, but a large new study released on Tuesday had some sobering findings: Women who used permanent hair dye or straighteners, or applied straighteners to others, had a higher risk of developing breast cancer than women who didn’t use those products.
The link was particularly evident in black women — their use of permanent dye was associated with 45 percent higher breast cancer risk, while white women faced a 7 percent higher risk. Straightener use was associated with 18 percent higher breast cancer risk.
The results suggest using hair dye and straightener “could play a role in breast carcinogenesis,” the study noted.
Still, women shouldn’t be overly alarmed by the findings, said lead author Alexandra White, an investigator at the Epidemiology Branch of the National Institute of Environmental Health Sciences.
“We know that a lot of different factors influence a woman’s risk of developing breast cancer, and these risks we see here, they are meaningful but they are small,” White told NBC News.
“Women should take that into context with everything else in their life, including their physical activity and diet. These are all factors we have to consider when we’re thinking about our long-term health risks.”
Other experts also urged women to remember that correlation does not mean causation.
“While these results are intriguing, they do not provide good evidence that hair dyes or chemical straighteners are associated with a meaningful increase in risk of breast cancer or that any increased risk association is causal,” said Paul Pharoah, a professor of cancer epidemiology at the University of Cambridge, in a statement.
“Women who have used such products in the past should not be concerned about their risks.”
The U.S. Food and Drug Administration does not have “reliable evidence” showing a link between cancer and hair dyes available on the market today. The International Agency for Research on Cancer has concluded personal use of hair dyes is “not classifiable as to its carcinogenicity to humans.”
Many of the associations in the study are not statistically significant, said Linda Loretz, chief toxicologist for the Personal Care Products Council, a trade group that represents the U.S. cosmetics industry.
"Hair dyes are one of the most thoroughly studied consumer products on the market," Loretz said in a statement.
"As with all cosmetics and personal care products, companies are required to substantiate the safety of hair dyes and straighteners and individual ingredients before marketing to consumers, and the labeling of those products must be truthful and not misleading."
What the study found:
The findings, published in the International Journal of Cancer, are based on data from 46,709 women enrolled in the Sister Study, named for the fact that all had a sister diagnosed with breast cancer but were cancer-free themselves.
The participants had a higher underlying risk of developing breast cancer, but the findings likely still apply to the general population, White noted. “We’re not studying women who all have a breast cancer gene,” added co-author Dale Sandler, chief of the Epidemiology Branch at the National Institute of Environmental Health Sciences.
At the start of the study, all women filled out questionnaires about the hair products they used in the past year. More than half of all the women, 55 percent reported using permanent dye and 75 percent of black women said they used chemical straighteners.
As researchers followed the women for an average of eight years, they found 2,794 cases of breast cancer.
Overall, they saw a 9 percent higher breast cancer risk among women who used permanent dye compared to those who skipped such products. Permanent hair dyes are both sold in drug stores and used in salons. There was little or no associated risk for semi-permanent or temporary dye use.
But in black women, any permanent dye use in the past year was linked with a 45 percent higher breast cancer risk. It went up to 60 percent if they colored their hair more frequently — at least every five to eight weeks.
Using dark or light dyes didn’t have a meaningful impact on risk, White said.
Straightener use was associated with 18 percent higher breast cancer risk, and went up to 31 percent when women used it at least every five to eight weeks. There was also a higher risk when women applied straighteners to other people, perhaps because they were inhaling fumes from the chemicals, Sandler speculated.
Why there’s concern:
Hair products contain more than 5,000 chemicals, including those that may damage DNA or interfere with the body’s endocrine system, the study noted. The chemicals can go into the blood stream and circulate through the body, including breast tissue, White said.
Previous research has found products designed for black women may contain higher concentrations of estrogens and more endocrine disrupting chemicals. Some straighteners contain formaldehyde, a known carcinogen. All of it may be a factor in the increased risk in black women, White speculated.
The authors didn’t have information on the exact dyes and straighteners used by women in the study.
What women should know:
It’s challenging to figure out what’s in a hair dye or straightener because those ingredients aren’t always reported on the box. There’s also no specific red flag to look for, Sandler said.
“In terms of a specific chemical, we haven’t actually identified the culprit and the formulas change. It’s extremely difficult to pin it down,” she noted.
Women could try switching to a semi-permanent hair dye, if it works for them: “If you’re able to cover your hair with a product that has a different chemical mix, you might consider that,” Sandler said.
Hair dyes and other cosmetics with “organic” ingredients aren’t necessarily safer, the FDA noted. Other than vegetable dye and henna, hair dyes rely on chemicals to work.
The American Cancer Society has these reminders for using hair color:
- Always wear gloves when applying hair dye.
- Don’t leave the dye on your head any longer than the directions say you should.
- Rinse your scalp thoroughly with water after use.
- Never mix different hair dye products because this can hurt your hair and scalp.
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