The hair shaft consists of three layers with keratin in its composition: medulla, cortex and cuticle.1–3 Keratins are proteins rich in cysteine ​​residues.1,2 Adjacent keratin chains join through tightly bonded disulfide bridges and other bonds such as Van der Waals, hydrogen bonds, and salt bonds.1

The hair shaft can be changed in shape by progressive or permanent chemical straightening. The first, in general, results from the use of acidic straighteners, have variable durability, and can present a cumulative effect with consecutive applications. The definitive ones, on the other hand, are permanent in the area of treatment and, as a rule, make use of alkaline substances.

Several compounds are used by the population to obtain chemical straightening, regardless of their approval by legislation. In Brazil, the technique called “progressive brushing” has spread with the use of acidic straighteners, such as formaldehyde and glutaraldehyde. The procedure has been disseminated worldwide as a Brazilian keratin treatment, among other names. Formaldehyde is classified as a colorless, highly toxic substance that, at room temperature, is a flammable gas.4 The terms methanal, methyl aldehyde, methylene oxide, formalin, oxymethylene, formic aldehyde, methylene glycol, paraformaldehyde, and oxymethane are listed as synonyms.5,6 According to the National Health Surveillance Agency (ANVISA, Agência Nacional de Vigilância Sanitária), formaldehyde and glutaraldehyde are not listed as straighteners.7 Recently, substances that release formaldehyde only during the heating process have been used, such as methylene glycol, glyoxylic acid, timonacic acid.5,8 The so-called “safe keratin treatments” use glyoxylic acid associated with carbocysteine ​​or amino acids, silicone derivatives and fatty acids.9

The ANVISA Normative Instruction -NI - number 64 of 07/27/2020 established, under the terms of the Resolution of the Collegiate Board (RDC, Resolução de Diretoria Colegiada) N. 409, its “List of allowed active principles in cosmetic products to straighten or curl hair”, which includes: thioglycolic acid and its salts, thioglycolic acid esters, sodium or potassium hydroxide, lithium hydroxide, calcium hydroxide associated with guanidine salt, sulfites, and inorganic bisulfites.10 These are the traditional hair straighteners, commonly referred to as relaxers. Although there are divergences, in general, this name is used to designate alkaline straighteners. They can be classified as lye (sodium hydroxide based), no-lye, ammonium thioglycolate, and bisulfites.3 Hydroxides are divided into lye and no-lye. Some authors categorize thioglycolate in the no-lye group.11

This article aims to identify the alterations in the hair shaft and scalp caused by the use of chemical straighteners.

A search was carried out in the Pubmed, Lilacs and Scielo databases using the terms hair straightening, Brazilian hair treatment, Brazilian blowout, hair relaxer, formaldehyde hair, glyoxylic acid hair, thioglycolic hair, sodium hydroxide hair, calcium hydroxide hair, guanidine hair, potassium hydroxide hair, lithium hydroxide hair, alisamento cabelo, escova progressiva, relaxamento cabelo, formaldeído cabelo, ácido glioxílico cabelo, tioglicolato cabelo, hidróxido de sódio cabelo, hidróxido de cálcio cabelo, guanidina cabelo, hidróxido de potássio cabelo, hidróxido de lítio cabelo. Publications between January 2000 and May 2020 were included. The search period for the research was from 03/16/2020 to 05/20/2020. A total of 703 results were obtained.

The criteria for article inclusion was the topic of hair straighteners and their effects on the scalp and hair shaft. All results unrelated to hair straighteners were excluded by reading the article titles. Review articles, letters to the editor and doctoral theses were excluded after reading the abstracts of these articles, (but articles resulting from doctoral theses published in scientific journals were included). Articles on the term “permanent” were also excluded because, although the substances overlap with chemical straighteners, the technique consists of different steps. Articles related to occupational exposure to chemical substances in straighteners and publications that analyzed the formulations without investigating their effects on hair shafts or scalp were excluded.

Studies related to other hair techniques were not excluded when the information was mixed with that of straighteners (for example, the association of techniques) or when straightening was one of the studied subjects (in this case, the review only included information about straighteners). When the article jointly addressed the researched effects and systemic side effects, the latter were not detailed in the review.

Therefore, after applying the inclusion and exclusion criteria, 649 articles were excluded. The search resulted in 33 articles, after removing the duplicates, which were selected for reading and review. The methods are depicted in Fig. 1.

Figure 1.

Article selection methodology.

(0.2MB).

Straighteners based on formaldehyde or derivatives consist, in general, of aldehyde and proteins such as hydrolyzed keratin: after exposure to heat, the aldehyde would promote a cross-link between the keratins in the product and those in the hair.9 Supposedly, straightening occurs by diffusing keratin into the cortex, filling in the defects, and restructuring the hairs; however, there is no evidence of this effect.9 The use of formaldehyde and glutaraldehyde as hair straighteners is not legal in Brazil.7

Products with glyoxylic acid are mentioned on the ANVISA website as glyoxyloyl-carbocysteine ​​and glyoxyloyl keratin amino acids in the section “recommendations on straighteners - what should I find on the label of these products”, with the consideration that they must warn the consumer about the risk of hair loss or change in hair color.7 However, the substance does not appear in the NI n. 64 of the organization.10 Carbocisteine ​​is not a straightener when used alone, but it is usually combined with glyoxylic acid.8

Acid straighteners remain a constant in straightening products. Locally, formaldehyde can cause dermatitis, including allergic eczema and psoriasiform eczema.12,13 The medical literature reports of scalp reactions may be underestimated, as irritant contact dermatitis following formaldehyde use is often considered to be a normal effect by users. In the hair shaft, formaldehyde and glyoxylic acid, despite the macroscopic appearance of hair shine, worsened cuticle irregularity on electron microscopy and decreased resistance to breakage.14 However, other studies have not shown the same damage.15,16 Although there are changes in strength, hair color, and other evidence of damage to the shaft, the Work for the combing the hair decreases with acidic straighteners, as shown in a study with glyoxylic acid.17

As for the straighteners permitted by law in Brazil, they comprise those listed in NI n. 64 of ANVISA.10 The hydroxides act due to their high pH, ​​opening the cuticles and allowing the action of the alkaline reducing agent in the cortex, where the disulfide bonds are broken. The hair is then mechanically straightened with the restructuring of the bonds between the keratins. Then, the hair is exposed to the acidic substance to terminate the process and close the cuticles (neutralization process).1,8,44 Alkalis react with cystine and produce lanthionine, a process called lanthionization. This process replaces one-third of the cystine with lanthionine.8 There is no need for heat or external traction.8 In contrast, thioglycolate does not act by lanthionization, but through a selective reduction of cystine bonds without changing the entire protein, being neutralized with hydrogen peroxide or sodium bromate, and the bonds are rearranged.8,44 Heat should be applied for the heat treatment step.8,44 Its use is commonly referred to as the “Japanese brushing”.8 It maintains 90% of the cystine content, and 10% becomes cysteic acid.8 There is also ammonium bisulfite, which is less aggressive, less effective, and more commonly used in household products.3

Relaxer-type straighteners are not free of side effects. Allergies, hair shaft frizz, dry hair, thinning and weakening of hairs, trichoptilosis, and hair color change have been reported.18–20,30 On the scalp, desquamation, pain, atrophy, even burns, and potentially severe cases of contact eczema with secondary infection have been reported.18–21

The long-term consequences of acute inflammation are not known, but they are even thought to be related to central centrifugal cicatricial alopecia.22 One hypothesis would be that cytokine changes caused by straighteners might play a role in the inflammatory process.38 The role of cytokines, including their relationship to the sensory diferences among several types of products, has been studied.39

Hair loss has also been described , including irreversible alopecia.18–20 Data regarding the association with CCCA diverge.23–25,43 There is an increased risk of traction alopecia with relaxers, especially if associated with cornrow braids, and generalized proximal trichorrhexis nodosa was associated with hair straightening.26,27

The hair shaft, with the use of relaxers, show an increase in the inter-scale distance, the opening of the scales, irregularity of their edges, fissures, severe lifting of the cuticle with exposure of the cortex, damage to the fiber, increase in particles extracted from them.28–30,35

The use of straighteners concomitantly with other hair techniques seems to increase the vulnerability to damage. Hair relaxing in addition to the use of artificial hair implants has been related to areas of permanent alopecia.32 Hair relaxers resulted in a greater propensity for protein loss, and when combined with hair dye, sodium hydroxide was even more aggressive.11 The summation of ammonium thioglycolate straightening, discoloration, and straightening iron resulted in hairs with irregular contours, cuticle detachment and deformation, and possible cortical damage.35

Regarding lye and no-lye straighteners, a difference was found in the number of amino acids of treated hair compared to virgin hair. The decrease in cystine, present in the disulfide bonds that give hair strength, is evident.37 Cysteine deterioration was observed at the molecular level in European straight hair exposed to sodium hydroxide33 and straightening with lye relaxer degraded tryptophan;34 however, the use of thioglycolate did not result in additional loss of protein or tryptophan.30 Comparing lye and no-lye straighteners, there was a reduction in cystine after the use of both, with a greater loss with no-lye straighteners.36

Hairs of different ethnicities seem to diverge regarding their susceptibility to straightening, with mixed findings in the literature.31,35 Structural damage can be minimized by using conditioning agents, some ceramides, and waxes.40–42 Despite the side effects, many women choose to straightening their hair.19,25

The study limitations were: the search was restricted to three databases only; the search was carried out in two languages only; there is a scarcity of research, resulting in low scientific rigor for article inclusion; accepting different study designs.

The review of data related to the last 20 years, despite limitations, discloses the significant occurrence of alterations in the hair shaft and scalp secondary to hair straightening. Clinical and subclinical inflammation of the scalp, damage to hair, hair loss due to breakage were evident. The long-term effects are still unknown.

Health professionals, especially dermatologists, as well as beauty salon professionals and their clients, need to know about the risks and benefits of these chemical treatments.

ANVISA’s recent norm limits the registration of straighteners; however, it does not clarify the new “acid straighteners”, such as glyoxylic acid.10 The effects of acidic straighteners are a matter for research, especially because of the Brazilian multicenter study that hypothesized the association of frontal fibrosing alopecia with formalin-straightening procedures.45

Studies carried out with scientific rigor are necessary to better understand the responses of hair structures to chemical agents, especially their relationship with the development of alopecia.

This research did not receive any specific funding from public, private or nonprofit funding agencies.

None declared.

Joane Nathache Hatsbach de Paula: Design and planning of the study; collection, analysis, and interpretation of data; critical review of the literature; drafting and editing of the manuscript.

Flávia Machado Alves Basílio: Design and planning of the study; effective participation in research orientation; critical review of the manuscript; approval of the final version of the manuscript.

Fabiane Andrade Mulinari-Brenner: Effective participation in research orientation; critical review of the manuscript; approval of the final version of the manuscript.

None declared.