Hair straightening and acute renal injury: The latest data straight up

By Warren R. Heymann, MD, FAAD
April 3, 2024
Vol. 6, No. 14

The global hair straightener market has experienced significant growth, with a market size of US$ 587.9 million in 2022, projected to be US$ 759.9 million by 2028. (1) I expected the market to already be in the billions of dollars. Regardless, those who desire to straighten their hair will do so by physical or chemical processes. These are summarized in the excellent review by Baretto et al. and include physical or thermal straightening agents (hot comb, hair dryer, and flat iron), alkaline straighteners (hydroxides, thiols), and acid straighteners (formaldehyde, glyoxylic acid, and its variants). (2) This commentary will focus on recent reports of acute renal injury related to hair straightening products.

Although there are an increasing number of reports of acute renal injury with hair straightening, we do not know the numerator or denominator. Perhaps it is a rare phenomenon; alternatively, it could be underrecognized or present as milder chronic disease. Straight to the point — dermatologists should be aware that such cases exist, so we will be better able to inform our patients as we learn more about this topic.

In April 2024, the FDA is tentatively scheduled to unveil a proposal to ban formaldehyde in hair-straightening products. (3) According to the Office of Information and Regulatory Affairs (of the Office of Management and Budget), “This proposed rule would ban formaldehyde (FA) and other FA-releasing chemicals (e.g., methylene glycol) as an ingredient in hair smoothing or hair straightening products marketed in the United States. These chemicals are used in certain cosmetic products that are applied to human hair as part of a combination of chemical and heating tool treatments intended to smooth or straighten the hair. Use of hair smoothing products containing FA and FA-releasing chemicals is linked to short-term adverse health effects, such as sensitization reactions and breathing problems, and long-term adverse health effects, including an increased risk of certain cancers” [notably sinonasal cancer and myeloid leukemia]. (4,2)

Formaldehyde is banned in Brazil, Canada, and the European Union as an active ingredient in straightening products, leading to increased use of glycolic acid derivatives in hair-straightening products. (5) Abu-Amer states, “Many hair-straightening products are labeled as formaldehyde “free” but actually contain chemicals such as glyoxyloyl carbocysteine or methylene glycol which release formaldehyde and other toxic chemicals when heated…Glyoxylic acid contains an aldehyde functional group and glyoxylic acid behaves as an aldehyde when heating during the hair straightening process, thus releasing high levels of formaldehyde gas exceeding the capacity of exposure. On top of that, glyoxylic acid absorbed through the scalp may have converted to oxalic acid which may precipitate in the kidney tissue.” (6)

Ahmed et al. reported two cases — a 10-year-old girl and a 17-year-old woman who both developed acute tubule-interstitial nephritis approximately a week after using formaldehyde-free hair straightening products. Both patients experienced scalp inflammation following these agents’ use, which were applied to the scalp for 4 and 6 hours, respectively, presumably allowing systemic absorption of the product. Both patients successfully responded to systemic steroids. (7)

Mitler et al. reported the case of a healthy 13-year-old female who developed abdominal pain, nausea, persistent vomiting, and oliguria 3 hours after applying a commercial hair straightening product advertised as formaldehyde-free. Her kidney biopsy showed acute tubular necrosis with microcalcifications. Her renal function improved while on continuous venous–venous hemodialysis, completely normalizing within six months. (8)

Bnaya et al. retrospectively reviewed 26 Israeli patients who experienced severe acute kidney injury (AKI) who reported prior treatment with hair-straightening products in 2019-2022, with a median age of 28.5 (range, 14-58) years. The most common symptoms at presentation were nausea, vomiting, and abdominal pain. A scalp rash was noted in 10 (38%) patients. Two patients experienced a recurrent episode of AKI following a repeat hair-straightening treatment. Seven patients underwent kidney biopsies, demonstrating intratubular calcium oxalate deposition in six and microcalcification in tubular cells in one. All biopsies showed signs of acute tubular injury, and an interstitial infiltrate was noted in four cases. Three patients required temporary dialysis. The authors concluded that acute oxalate nephropathy was the dominant finding on kidney biopsies, which may be related to the absorption of glycolic acid derivatives and their metabolism to oxalate. (5)

Robert et al. reported the case of a 26-year-old Tunisian woman with three episodes of AKI, each coinciding with a hair straightening cream with 10% glyoxylic acid (but no glycolic acid). She reported a burning sensation during each procedure, followed by scalp ulcers. The authors used the product on mice. The plasma creatinine level in the exposed mice increased significantly 28 hours after cutaneous application of the hair-straightening cream. Three-dimensional CT scans of the exposed mice’s kidneys showed dense tubule-molding calcium oxalate monohydrate deposits. In contrast, no such deposits were seen in the control mice. The authors conclude, “These results provide evidence that hair-straightening cream containing glyoxylic acid is responsible for calcium oxalate–induced nephropathy after hair-straightening procedures of the type described here. Glyoxylic acid was patented and introduced recently in hair-straightening products as a seemingly safer alternative to formulations containing formaldehyde. In consideration of the potential nephrotoxicity of topical glyoxylic acid, products containing this compound should be avoided and, we would proffer, discontinued from the market.” (9)

In conclusion, patients should be very wary of any “formaldehyde-free” hair straightener and the risk for AKI, especially if they experience any scalp inflammation with the product. While the desire to straighten hair may never abate, nor should our search for safe hair straighteners.

Point to Remember: An increasing number of reports implicate acute kidney injury due to hair straighteners containing glyoxylic acid.

Our Expert’s Viewpoint

Temitayo A. Ogunleye, MD, FAAD
Associate Professor of Clinical Dermatology
University of Pennsylvania Perelman School of Medicine

The historical context of chemical hair straightening in the United States is long and complex, performed initially by Black populations for ease of styling and to meet cultural standards that allowed for higher status based on hair texture. The first reported chemical straightener was discovered in 1910 by Garrett A. Morgan while working in a sewing machine repair shop attempting to invent a new lubricating liquid for the machine needle. It is believed that he wiped his hands on a wool cloth and noted that the texture smoothed and subsequently tested it on an Airedale terrier and noticed straightening of the dog’s fur. He then tested it on himself, replicated these results, and patented G.A. Morgan’s Hair Refiner Cream, the first reported chemical straightener.

Over a century later, there have been many iterations of chemical straighteners that have been developed to aid in maintaining a straighter/smoother hair texture. In the last few years, there have been varied levels of concern regarding the long-term safety profile of several of these products and their possible carcinogenic and/or endocrine disrupting effects. As noted, the FDA may very shortly enact a ban on formaldehyde containing or releasing products, (e.g. formaldehyde, formalin, methylene glycol). Of note, this will not include chemical relaxers (aka alkaline straighteners) that many of our patients of color may still use. It is currently unclear whether the ban will also include glyoxylic acid containing products that are the subject of the above case reports describing calcium oxalate–induced nephropathy after their use.

After reading these reports, I wonder how many of these cases may have been missed, mainly because the effect was not known, or it did not occur acutely. Is there a genetic susceptibility in some populations that leads to an acute presentation but could be more insidious in others? Are we missing chronic presentations of oxalate toxicity in other populations? And what do we tell our patients?

Thankfully, this may not be a concern for much longer if the FDA ban takes effect and if it is inclusive of all forms of formaldehyde containing or releasing products. That said, when counseling patients who wish to continue any form of chemical straightening, it is wise to recommend decreasing the frequency of usage to at least every 12-16 weeks, if possible. The long-term effects of usage of many of these products are unclear and judicious use is advisable.

1. https://www.globenewswire.com/news-release/2023/11/01/2771332/28124/en/Global-Hair-Straightener-Market-Valued-at-587-9-Million-in-2022-Expected-to-Reach-759-9-Million-by-2028.html

2. Barreto T, Weffort F, Frattini S, Pinto G, Damasco P, Melo D. Straight to the Point: What Do We Know So Far on Hair Straightening? Skin Appendage Disord. 2021 Jun;7(4):265-271. doi: 10.1159/000514367. Epub 2021 Mar 30. PMID: 34307473; PMCID: PMC8280444.

3. Cohen R. FDA move to ban formaldehyde in hair straighteners called too little, too late. Shots – Health News from NPR, February 6, 2024. https://www.npr.org/sections/health-shots/2024/02/06/1229421257/formaldehyde-hair-straighteners-curl-relaxers-cancer-risk-fda

4. https://www.reginfo.gov/public/do/eAgendaViewRule?pubId=202304&RIN=0910-AI83

5. Bnaya A, Abu-Amer N, Beckerman P, Volkov A, Cohen-Hagai K, Greenberg M, Ben-Chetrit S, Ben Tikva Kagan K, Goldman S, Navarro HA, Sneineh MA, Rozen-Zvi B, Borovitz Y, Tobar A, Yanay NB, Biton R, Angel-Korman A, Rappoport V, Leiba A, Bathish Y, Farber E, Kaidar-Ronat M, Schreiber L, Shashar M, Kazarski R, Chernin G, Itzkowitz E, Atrash J, Iaina NL, Efrati S, Nizri E, Lurie Y, Ben Itzhak O, Assady S, Kenig-Kozlovsky Y, Shavit L. Acute Kidney Injury and Hair-Straightening Products: A Case Series. Am J Kidney Dis. 2023 Jul;82(1):43-52.e1. doi: 10.1053/j.ajkd.2022.11.016. Epub 2023 Jan 5. PMID: 36610611.

6. Abu-Amer N, Silberstein N, Kunin M, Mini S, Beckerman P. Acute Kidney Injury following Exposure to Formaldehyde-Free Hair-Straightening Products. Case Rep Nephrol Dial. 2022 Jul 11;12(2):112-116. doi: 10.1159/000525567. PMID: 36160636; PMCID: PMC9386411.

7. Ahmed HM, Rashad SH, Ismail W. Acute Kidney Injury Following Usage of Formaldehyde-Free Hair Straightening Products. Iran J Kidney Dis. 2019 Mar;13(2):129-131. PMID: 30988250

8. Mitler A, Houri S, Shriber L, Dalal I, Kaidar-Ronat M. Recent use of formaldehyde-'free' hair straightening product and severe acute kidney injury. Clin Kidney J. 2021 Jan 11;14(5):1469-1471. doi: 10.1093/ckj/sfaa272. PMID: 33959274; PMCID: PMC8087118.

9. Robert T, Tang E, Kervadec J, Zaworski J, Daudon M, Letavernier E. Kidney Injury and Hair-Straightening Products Containing Glyoxylic Acid. N Engl J Med. 2024 Mar 21;390(12):1147-1149. doi: 10.1056/NEJMc2400528. PMID: 38507759.

---