An alarming new National Institutes of Health study suggests that women — and black women in particular — who use permanent hair dye or chemical straighteners could have a greater risk of developing breast cancer than women who don’t use them. And applying chemical straightener or dye to another person’s hair at home could also raise breast cancer risk.

Researchers analyzed the health data of almost 47,000 women ages 35 to 74 participating in the Sister Study over eight years; these subjects were cancer-free at the onset, but each one had a sister who developed breast cancer. Almost 2,800 were later diagnosed with breast cancer. And the women who colored their hair with permanent dye (which grows out, as opposed to semi-permanent dye that fades out) were 9% more likely to get breast cancer.

But the risk was much higher for black women, who saw a 45% greater chance of being diagnosed with breast cancer after using permanent dye every five to eight weeks or more, compared to white women, who had a 7% higher risk. The researchers theorized that different chemical formulations of hair dyes developed for black women and white women may play a part, or that thicker hair types may absorb more dye, and therefore more chemicals. But as most of the previous studies looking for links between hair dye and breast cancer have been on white women, more research needs to be done.

As for chemical hair straighteners, women who used them every five to eight weeks were about 30% more likely to develop breast cancer, regardless of race — although more black women in the study used straightening products than white women.

There was little to no increased risk for using semi-permanent or temporary dye, with one exception. Women who applied semi-permanent dye or chemical straighteners to other women in a nonprofessional setting did see a higher risk of developing cancer, perhaps because those using at-home kits inhaled the toxic chemicals, or got some of the chemicals on their hands and forearms.

More than one-third of American women over 18 are believed to use hair dye, according to the NIH report, fueling a $61 billion hair and nail salon industry. A recent U.K. survey found that one in 20 women spends more than $65,000 coloring her hair over her life time. And breast cancer is the most prevalent cancer in the U.S., affecting almost 85 out of every 100,000 Americans.

Previous research into whether these dyes could increase cancer risk have been small or produced conflicting results, according to the National Cancer Institute, although there is evidence of hair dye correlating with an increased risk of bladder cancer in hairdressers and barbers. Most research has also looked into the risks of blood cancers (leukemias and lymphomas) and bladder cancer, not breast cancer.

Part of the difficulty in determining whether there is real danger — not to mention pinpointing a particular chemical or treatment to avoid — is that hair dyes can contain more than 5,000 different chemical compounds, and those formulas are constantly changing. The researchers in the latest NIH study called out some potential culprits, such as formaldehyde, a known carcinogen that is an active ingredient in Brazilian keratin straightening treatments. Permanent dyes also contain aromatic amines, which are colorless chemicals that have been found to bind with breast tissue DNA — possibly damaging the DNA and leading to the development of breast cancer. Permanent dyes have higher concentrations of these aromatic amines than the semi-permanent dyes do, which may be why the temporary dyes didn’t show the same increased risk of cancer.

So is it safe to dye or straighten your hair? Unfortunately, there still isn’t a clear-cut answer without further research. The International Agency for Research on Cancer has ruled that personal use of hair dyes is “not classifiable as to its carcinogenicity to humans” after reviewing the existing evidence before this report. (It does warn that workplace exposure as a hairdresser or barber is “probably carcinogenic,” however.) The U.S. Food and Drug Administration has also said there is no “reliable evidence” showing a link between cancer and hair dyes available on the market.

Dr. Dale Sandler, chief of the National Institute of Environmental Health Sciences research branch and co-author of the report, also noted in a statement that the new research shows correlation, not causation, and “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.” The women in the study had a family history of breast cancer, for example, which limits the generalization of these findings.

He stopped short of telling women to stop dyeing or straightening their hair — but he also warned them to be cautious. “While it is too early to make a firm recommendation, avoiding these chemicals might be one more thing women can do to reduce their risk of breast cancer,” he suggested. Similarly, doctors have also advised pregnant women against getting their hair dyed as a safety precaution, even though there’s no definitive evidence that it can hurt the developing baby.

The American Cancer Society also notes that minimizing other cancer risk factors — such as not smoking, eating a healthy diet, being physically active, and getting routine medical screening exams — can also protect against cancer. For those who use hair dye, the American Cancer Society also suggested using some newer formulas that are plant-based (and avoiding formaldehyde), or getting hair treated less frequently. (In the new study, cancer risk increased with the frequency of hair dye and chemical relaxer use.) You can also switch from permanent dyes to the semi-permanent and temporary dyes, which didn’t show the same heightened risk of cancer.

And if applying dye or straightening agents at home — to yourself or another person — practice the following safety tips to minimize your exposure to the chemicals:

  • Follow the directions in the package. Pay attention to all “Caution” and “Warning” statements.
  • Be sure to do a patch test for allergic reactions before putting the dye in your hair. Do a patch test before every use.
  • 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. This can hurt your hair and scalp.
  • Never use hair dye to dye your eyebrows or eyelashes. This can hurt your eyes. You might even go blind. The FDA does not allow using hair dyes on eyelashes and eyebrows.

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