Are there risks in using glyoxylic acid for smoothing?

Glyoxylic acid, a cosmetic ingredient under the scrutiny of ANSES.

The glyoxylic acid is an organic acid frequently found in so-called Brazilian smoothing products, a technique that has become very popular in recent years for straightening hair and maintaining its shine for several months. Initially based on formaldehyde, Brazilian smoothing products now use glyoxylic acid, which is capable of penetrating the hair fiber to break the disulfide bonds that connect keratin fibers. These bonds, responsible for the natural shape of the hair, are relaxed, allowing the hair to change texture and become smoother. After the product is applied to the scalp and throughout the hair, the hair is passed through a straightening iron, which seals the keratin and sets the new structure of the hair.

According to a report published on October 16, 2024, by the French National Agency for Health Safety (ANSES), glyoxylic acid could potentially be harmful and may cause kidney failure. This warning comes after the hospitalization of four individuals for acute kidney failure, all of whom had used this technique since the beginning of 2024. As a precautionary measure, ANSES, the General Directorate for Competition, Consumer Affairs, and Fraud Control (DGCCRF), and the General Directorate for Health (DGS) now recommend that hair salons and individuals avoid performing Brazilian hair straightening.

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A risk of renal insufficiency posed by glyoxylic acid?

Acute kidney failure is a common condition affecting approximately 8.3% of outpatient patients and up to 32% of hospitalized individuals. It presents with abdominal or lower back pain, nausea, and vomiting, and requires immediate hospitalization for dialysis. Unlike chronic kidney failure, acute kidney failure refers to a temporary and reversible kidney dysfunction, caused by hemorrhage, septicemia, drug poisoning, or urinary tract obstruction. Another factor has recently been suspected: Brazilian smoothing products containing glyoxylic acid.

These hair care products had already been implicated in a report published in 2023, which listed 26 patients treated in 14 medical centers in Israel between 2019 and 2022 for acute kidney failure. All these hospitalizations followed exposure to Brazilian smoothing products. Among these patients, seven (26%) underwent kidney biopsies, revealing intratubular deposits of calcium oxalate in six of them, suggesting a potential link with exposure to glyoxylic acid.

After glyoxylic acid enters the bloodstream, it is metabolized into oxalic acid by the enzyme lactate dehydrogenase found in the cytoplasm of liver cells (hepatocytes). At a physiological pH of 7.4, the oxalic acid then dissociates to form oxalate ions, which can precipitate as calcium oxalate crystals in the renal tissues, compounds that damage the kidneys.

The toxicity of glyoxylic acid and its ability to penetrate the skin barrier were notably demonstrated in a recent study conducted on rats. Four groups of five rats were formed, on which various creams were tested: a Brazilian smoothing product containing, among other things, 10% glyoxylic acid, a cream containing 10% glyoxylic acid, another cream containing 10% glycolic acid, an active ingredient sometimes accused of having the same level of danger as glyoxylic acid, and a control cream.

Scientists have observed that the applications of glyoxylic acid cream and hair straightening product significantly increased urinary oxalate excretion and caused a calcium oxalate nephropathy after transcutaneous absorption, unlike glycolic acid and the control. These increases are visible in the figure below in the "Urine oxalate" and "COM crystals in urine" histograms, the latter referring to the concentration of calcium oxalate crystals present in the urine. It is interesting to note that these crystals are only present in the rats of the "hair straightening product" and "10% glyoxylic acid cream" groups, which seems to confirm the theory that this active ingredient penetrates the skin before reaching the kidneys.

Impact de l'acide glyoxylique contenu dans les produits de lissage sur le bon fonctionnement des reins. — Impact of glyoxylic acid contained in smoothing products on the proper functioning of the kidneys.
Source: ROBERT T. & al. Cosmetics for hair straightening containing glyoxylic acid lead to crystalline nephropathy. Kidney International (2024).

Furthermore, an increase of approximately 80% in plasma creatinine, a chemical waste product produced by muscle metabolism, was detected for the "hair straightening product" and "10% glyoxylic acid cream" groups. This is shown by the "Plasma creatinine" histogram in the figure above. The measurement of urinary or blood creatinine is commonly used to assess kidney function and an increase in this waste product indicates that kidney function has been impaired. On the contrary, the applications of the control cream and the 10% glycolic acid cream had no impact on kidney function.

Scientists also conducted a computed tomography (CT-scan) of the kidneys. Areas highly concentrated with calcium oxalate crystals appear in red. The three CT scans performed are consistent with the results of the histograms and show a significant concentration of oxalate crystals in the kidneys of the rats in the "hair straightening product" and "10% glyoxylic acid cream" groups, unlike those in the "10% glycolic acid cream" group. The presence of oxalate crystals in the kidneys of the rats in the "10% glyoxylic acid cream" group was confirmed by Yasue staining, the small black dots visible in the figure above revealing the crystals.

Even though correlations seem to exist between the use of glyoxylic acid and an increased risk of acute kidney failure, it is important to exercise caution as several limitations remain.

Indeed, the previously mentioned study is based on groups of only five rats, a sample size too small to establish solid conclusions applicable to humans. On the other hand, the number of reported cases of individuals experiencing acute kidney failure following a Brazilian smoothing treatment is extremely low considering the popularity of this practice, used by millions of people for decades, with no similar precedents until the past five years. This relatively recent timeframe invites questioning: environmental factors, lifestyle changes, or even individual characteristics of the patients could play a role and warrant thorough investigation. Studies with larger sample sizes and rigorous follow-up are necessary to clarify the link between glyoxylic acid and the rare cases of reported acute kidney failure.

Following investigations, ANSES considers that glyoxylic acid is “probably the cause of acute renal failure”. As a precautionary measure, we recommend avoiding Brazilian hair straightening. If symptoms suggestive of acute renal failure appear after Brazilian straightening, consult a doctor as soon as possible, specifying the context of their appearance.