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A large study led by government scientists is renewing concern about whether chemicals used to dye and straighten hair raise the risk of cancer.

National Institutes of Health (NIH) researchers found an increased chance of breast cancer among women who regularly used permanent hair dye, particularly African-Americans. Black women who used dye at least every two months had a 60% higher breast cancer risk, while white women had an 8% higher risk. Straightener use was linked to an 18% increase in risk.

Decades of studies of hair dye and various cancers have produced conflicting and inconclusive results. The authors of the latest study, published in the International Journal of Cancer, said their results need to be confirmed and put in context.

"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," senior author Alexandra White, and NIH epidemiologist, told the "Today" show.

"Women should take that into context with everything else in their life, including their physical activity and diet," White said. "These are all factors we have to consider when we're thinking about our long-term health risks."

Co-author Dale Sandler, also an NIH epidemiologist, echoed that sentiment in a press release, but also suggested that women might want to err on the side of caution.

"While it is too early to make a firm recommendation," Sandler said, "avoiding these chemicals might be one more thing women can do to reduce their risk of breast cancer."

The National Cancer Institute, part of the NIH, explains on its website that early hair dyes contained chemicals that were found to cause cancer in animals. But in the 1970s, manufacturers eliminated some of these chemicals to make products safer.

"It is not known whether some of the chemicals still used in hair dyes can cause cancer," the National Cancer Institute says. "Given the widespread use of hair dye products, even a small increase in risk may have a considerable public health impact."

The new findings are based on data from 46,709 women in the Sister Study, a huge NIH study that followed women who had a sister with breast cancer.

More than half of the women reported using permanent hair dye, and 75% of black women said they used chemical straighteners.

After the women were followed for an average of eight years, 2,794 breast cancers were diagnosed. That translated to an overall 9% higher risk of breast cancer in women who used permanent dyes compared to those who did not.

Although the women's family history put them at higher risk of developing breast cancer to begin with, the researchers believe the hair dye link still can be extended to the general population.

What might explain the more elevated risk in black women? The researchers point to previous studies of potentially toxic chemicals, including the hormone estrogen, which can accelerate the growth of cancerous breast cells.

The new results are consistent "with toxicological assessments that report higher concentrations of estrogens and endocrine-disrupting compounds in hair products marketed to black women."