The findings, published in the Journal of the National Cancer Institute, are especially relevant to black women, who make up the majority of straightening product users in the United States.
The scientists praised the study and called for further research to confirm the findings.
Women who use these products frequently, more than four times a year, are twice as likely to develop uterine cancer, primarily endometrial cancer, not to be confused with cervical cancer.
Similar associations were not found for other hair products, such as dyes, bleaches, highlights, or perms.
“We estimate that 1.64% of women who have never used a hair straightening product will have developed uterine cancer by the age of 70, but for frequent users this risk increases to 4.05%,” he estimates in a statement. Alexandra White, lead author of the study.
“The doubling of this rate is worrying,” he added.
Cancer of the uterus accounts for about 3% of new cancer cases in the United States, but it is the most common cancer of the female reproductive system. The prognosis is usually good if caught early, but treatment often involves removal of the uterus, which would make it impossible to have children.
The study is based on data from nearly 33,500 American women, followed for nearly 11 years.
Because black women use hair straightening products more often and tend to start younger, “these results could be particularly interesting for them,” said study co-author Che-Jung Chang.
Approximately 60% of women who said they had used hair straightening products in the past year identified as black.
– Brazilian straightening –
“What’s concerning is that there are chemicals in these products that essentially act like estrogen in the body,” White said, disrupting normal hormonal processes, and this could influence cancer risk.
A second possibility is that some products include carcinogens, such as formaldehyde, to break the bonds between the keratin proteins in the hair, changing its structure and straightening it.
Although the study didn’t specifically ask women what products they used, a keratin treatment known as “Brazilian straightening” was popular when women enrolled in this study, between 2003 and 2009, although its use has since declined.
The researchers did not collect information on specific goods and brands, but note that several chemicals in these types of products may contribute to increased cancer risk. In addition to formaldehyde, which is usually known as formalin, they cite parabens, bisphenol A and metals.
Compared to other goods, hair-straightening products may promote the absorption of chemicals through injuries or burns to the scalp, or through the use of straightening irons whose heat breaks down chemicals, the study says.
Other studies have already established a link between relaxers and an increased risk of breast cancer.