Formaldehyde-free hair-straightening products may still threaten health, concerning study finds

Formaldehyde-free hair-straightening products have gained popularity as a potentially safer option than their formaldehyde-containing counterparts — but a new series of medical cases calls the safety of these alternative products into question.

Formaldehyde — as well as liquid forms of the chemical, called formalin and methylene glycol — is added to hair-straightening and hair-smoothening products as a preservative to extend shelf life and to help lock the hair's texture in place for a long time.

However, in recent years, various countries have moved to ban the chemical from hair-straightening products due to health concerns.

When heated, as it is during these hair treatments, formaldehyde is released into the air as a gas. This can cause immediate problems, such as skin irritation, coughing, wheezing and a burning sensation in the eyes. But especially when it happens repeatedly, formaldehyde exposure can also cause long-term issues such as fertility problems, asthma and a raised risk of cancer, such as leukemia and nasopharyngeal cancers.

Given these issues, formaldehyde-free versions of these products are often touted as safer options. But that doesn't mean they're harmless, according to the new case series, published Aug. 28 in the journal Clinical Toxicology. In particular, the report highlights a potential risk of kidney injury tied to glyoxylic acid, an ingredient found in many formaldehyde-free hair-straightening products that serves as a straightening agent.

The report details the cases of 13 women and teens who were treated at the Sourasky Medical Center in Tel Aviv following exposure to formaldehyde-free hair-straightening products. The patients began feeling ill within two to 72 hours of undergoing these hair-straightening treatments at various salons.

"Even before these studies were published," the study authors wrote, "our medical toxicology service had begun to receive similar reports, prompting us to visit local hair salons, including illegal salons mentioned by affected patients, where we discovered the presence of glyoxylic acid in the hair-straightening products."

Related: Hair-straightening products contain chemicals that boost risks of cancer, kidney injuries and breathing issues

The 13 patients, who ranged from 15 to 53 years old, developed symptoms such as vomiting, abdominal pain and scalp rashes. Twelve of the patients developed acute kidney injury, as evidenced by a buildup of the waste product creatinine in their blood, a decrease in urine output and the presence of small crystals in the urine they did expel.

The one person who didn't develop kidney injury had received treatment very quickly — they were admitted to the hospital just six hours after their hair treatment, and within two hours of admission received thiamine and pyridoxine, which are B vitamins thought to help the body break down glyoxylic acid.

In addition to finding glyoxylic acid-containing products at the salons their patients visited, the medical center's toxicology team also found that some of the salons had inadequate ventilation.

The patients stayed in the hospital for between one and 10 days. None of the patients required their blood to be filtered with hemodialysis and no deaths occurred.

It’s worth noting that the case series included relatively few patients, didn't include any kidney biopsies and was in part conducted retrospectively, as the authors looked back at cases that took place between April 2021 and March 2025. However, this isn't the first time glyoxylic acid has been linked to kidney issues. Hair products containing the ingredient have been tied to kidney injuries in anecdotal reports and in one woman's case of repeated kidney damage, for example.

"These findings highlight a potentially underrecognized risk of kidney toxicity associated with glyoxylic acid-containing hair products," the authors concluded. "Prompt recognition and early treatment, particularly with thiamine and pyridoxine, may reduce the severity of kidney damage. Further studies and regulatory action are needed to better define the risk and prevent future cases."

Meanwhile, in the U.S., the Food and Drug Administration (FDA) seemed poised to ban formaldehyde in hair-straightening and hair-smoothing products back in April 2024. However, the proposed ruling was postponed several times in 2024, and the effort has not progressed in 2025. For now, the FDA simply discourages consumers from purchasing and using hair products containing formaldehyde or related ingredients.

Some experts have said that glyoxylic acid should also be banned from hair-straightening and smoothening products, but no moves have been made on that front.