Most women know what increases their odds of developing breast cancer. Some risk factors, such as age or having a family history of breast cancer, can’t be changed, while women do have some control over other risks, such as being overweight or not getting enough physical activity.

Now: Recent research is providing more clarity on a possible risk factor that’s perplexed scientists for years due to mixed study findings. The new study, published in International Journal of Cancer, found that hair dye is associated with higher breast cancer risk—and it goes a step further by uncovering another potentially harmful type of hair product.

Study details: As part of the Sister Study, researchers analyzed data from nearly 47,000 women who filled out questionnaires that included information regarding their use of hair products during the prior 12 months. Study participants were ages 35 to 74 and did not have breast cancer themselves but did have a sister with the disease. By studying sisters who share genes, environments and experiences with a sister who developed breast cancer, the researchers hope to gain a better understanding of the risk factors associated with breast cancer.

Key findings: Overall, study participants who used permanent hair dye during the year before they enrolled in the study were 9% more likely to develop breast cancer during an eight-year follow-up period than those who didn’t dye their hair.

When researchers did more analysis, they discovered some intriguing additional associations. For example, the breast cancer link was much stronger among black women who used hair dye—they were 45% more likely to develop breast cancer than non-users of hair dye, while increased risk was 7% for white women who dyed their hair.

The study also found that chemical hair straighteners, when used at least every five to eight weeks, increased breast cancer risk by about 30% in all women studied, although these products are more commonly used by black women. Researchers theorize that the chemicals found in hair products may play a role in the development of breast cancer, and the products used by black women may have more carcinogens than those used by white women. The results could also be due to differences in application methods or hair texture.

Even though the researchers stated that the hair products included in the study were associated with increased breast cancer risk, they pointed out that the use of permanent hair dye or a hair-straightening product doesn’t mean that a woman will necessarily develop breast cancer.

“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,” said study coauthor Dale Sandler, PhD, chief of the National Institute of Environmental Health Sciences Epidemiology Branch. “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.”

More studies will be done to see if the new findings can be replicated. The research team also noted that little to no increase in breast cancer risk was found in women using semipermanent or temporary dye.

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