Hair care products used primarily by black women in the U.S. contain a variety of chemicals that have been linked to asthma, hormone disruptions and even cancer, a recent study found.

An analysis of 18 commonly used hair cosmetics such as relaxers (which chemically straighten hair), root stimulators and anti-frizz products detected 66 chemicals with potentially toxic effects. The majority of such compounds were not mentioned on the products’ ingredient labels, researchers say.

Eight in 10 of the products studied contained parabens and phthalates, which are known endocrine disruptors—substances that disturb the body’s hormone balance. Regular exposure to phthalates can cause early puberty and preterm births. The researchers also detected nonylphenol, a compound associated with obesity and a higher risk of breast cancer, in 30 percent of the hair treatments. “We found dozens of these chemicals in the products, with multiple chemicals in each product,” says Jessica Helm, a research fellow at the Silent Spring Institute and lead author of the study, published in April in Environmental Research. “Some contained as many as 30, and that is important because these chemicals can have additive effects in combination,” she adds.

Helm and her colleagues selected the hair care products based on a 2005 survey of their use among women of all ethnicities in New York City. Each product chosen for testing was used regularly by at least 5 percent of respondents, primarily black women.

The researchers detected several chemicals—including benzophenone, diethanolamine and nonylphenol—that are banned in the European Union and strictly regulated in California under the state’s Proposition 65 list of harmful chemicals. These substances are mostly used as preservatives or plasticizers to create the products’ creamy consistency. Five of the compounds studied had the highest concentrations in products aimed at children, and the largest variety of chemicals was often present in products marketed for everyday use.

Chemicals in hair cosmetics can be absorbed by the body via the skin or inhalation. The researchers say regular use of hair care products containing these substances could compound health problems that are already more common among black women, such as early or delayed puberty, preterm birth, asthma and obesity.

One problem the researchers identified was incorrect labeling. Some of the substances were not mentioned on the product packaging at all whereas others were referred to by vague terms such as “fragrances.” In the U.S. companies are required to list intentionally added chemicals on product packaging—but there is no definition of “intentional,” Helm says, which gives companies some leeway in how they label their goods.

Linda Loretz, chief toxicologist at the Personal Care Products Council, a lobbying group for U.S. cosmetics manufacturers, said in a statement that the mention of unlabeled chemicals in the study was “incredibly misleading,” and noted these are likely trace levels of ingredients that were not intentionally added. “Nothing is more important to cosmetic and personal care products companies than ensuring the safety of their products that are trusted by millions of families every day,” she added.

Black women are more likely than white or Hispanic women to suffer from hormonal diseases that are exacerbated by substances that disrupt hormonal balance. A 2016 study showed black women in the U.S. have higher concentrations of such chemicals in their bodies than do women from other ethnicities.

The researchers did not compare the hair products they tested to products aimed at white or Hispanic women, but a 2012 study found 50 percent of hair products used by black women contained endocrine disruptors, compared with just 7 percent of those used by white women. The new study also did not test the products’ health effects directly; it merely observed the presence of these chemicals.

Nevertheless, “hair product use is likely an important culprit of environmental health disparities in African-American women compared to white women,” says Tamarra James-Todd, the lead author of the 2012 study and a professor of environmental reproductive and perinatal epidemiology at Harvard University. She says more must be done to educate African-American women and men about the effects of these substances. “Currently, most women and men likely do not know that these harmful chemicals are in the products, especially since companies are not always required to put all ingredients on labels,” she says.

Helm and her team were particularly concerned about the high concentration of chemicals in products aimed at children. They point out that two relaxer kits they tested were marketed as “no lye,” possibly suggesting to buyers they are safer than other products. But one of the kits contained a strong concentration of bis(2-ethylhexyl) phthalate, a plasticizer associated with obesity, heart problems and low fertility. In a mouse study childhood exposure to this compound was also linked to delayed puberty.

The researchers point out that women are more susceptible to these chemicals during certain stages of their lives such as puberty or pregnancy. But they caution against blaming hair care products alone for the larger health burdens experienced by black women, a view shared by Ami Zota, an assistant professor of environmental and occupational health at The George Washington University who did not take part in the studies. “The reality is that African-Americans in the United States face a wide range of environmental and social stressors, like racism, food instability and housing instability,” she says. “The study is helping shine a light on how different beauty product usage may play a role in this, but we want to be careful not to overgeneralize the results.”

The International Association of Color Manufacturers, which represents companies that produce hair colorants and other cosmetics, did not respond to a request for comment.

Helm says she hopes manufacturers will improve the testing and labeling of their products to make sure the full range of chemicals they contain is known before they go on the shelves. But Zota thinks tougher regulation and more consumer awareness are called for. “The reality is that all of us use multiple products a day, and that these products should be undergoing a lot of scrutiny for health and safety concerns,” she says.