Straight to the Point: What Do We Know So Far on Hair Straightening?

Hair represents a valued aspect of human individuality. The possibility of having an easy to handle hairstyle and changing it from time to time promoted an increasing search for chemical hair transformations, including hair straightening. Hair straightening is the process used to convert curly into straight hair. The desire for straight hair used to be associated with the standards of the “universal beauty.” Currently, the preference for this style is more for the ease of handling and the simpler daily care routine [1]. Straightening may be physical or chemical processes and temporary or permanent, regarding its duration.

We performed a literature search in the scientific database MEDLINE through PubMed until July 15, 2020, using the terms “straightening” AND “hair” (125 results), “straightening” AND “alopecia” (22 results), and “straightening” AND “human hair” (103 results). We limited the search to articles available in English and considered those mentioning alternatives to straighten the hair. After excluding duplicate titles, we had a total of 33 relevant articles.

Anatomically, the hair shaft has 3 layers: cuticle, cortex, and medulla [2, 3]. The outermost part is the cuticle, which is composed of keratin and consists of layers of scales overlapping 1 and other, just like tiles on a roof. The cuticle protects the underlying cortex and acts as a barrier [3-5]. The normal, undamaged cuticle has 6–8 layers according to the ethnicity, a smooth surface, allowing reflection of light and limiting friction between shafts [5]. The outer surface of the cuticle’s scale cells is coated by a thin membrane, the epicuticle, and each cuticle cell consists of 3 layers of protein: the A-layer, a resistant layer with high cystine content; the exocuticle, also rich in cystine; and the endocuticle, low in cystine content. The cortex accounts for most of the hair shaft and is responsible for the hair. The cortex is comprised of microfibrils, long filaments oriented parallel to the axis of the fiber. Each microfibril consists of keratin intermediate filaments, also known as microfibrils, and the matrix, constituted by keratin-associated proteins [4]. It is the thickest layer located around the medulla, which is the innermost part of the hair, has melanin granules which composition is related to the shades of hair color. It is also responsible for hair volume, the great tensile strength, and mechanical resistance of the shaft, as it contains the most part of keratin [3-5].

The primary component of the hair fiber is keratin. The remaining constituents are represented by other proteins, water, lipids, pigments, and trace elements. Because of its specific conformation and chemical bonds, keratin is responsible for hair stiffness, strength, and insolubility. Among the amino acids that make up keratin, cystine is one of the most important. Each cystine unit contains 2 cysteine amino acids from different portions of the peptide chains that are connected by 2 sulfur atoms, forming a strong bond named disulfide bridge [3-5]. Another important structural component of the hair shaft is the 18-methyl eicosanoic (18-MEA) acid. It forms a hydrophobic layer that retards water from wetting and penetrating and changing the hair shaft physical’s properties. Removal of the fatty acid layer decreases the brightness of the hair, making it more susceptible to static electricity and frizzing induced by humidity [4].

The spiral shape of the hair is determined by the asymmetric protein expression in the hair follicles [2]. As it is not possible yet to modify the shape of the follicle, the only way to change hair appearance is by modifying its physicochemical properties [6].

This method was developed in the late 19th century and became popular in the early 20th century by Madame C.J. Walker, who combined hot comb with pressing oil. It is a temporary straightening since it changes only weak hydrogen bonds, in a process named keratin hydrolysis. The initial technique was the application of a petrolatum ointment base in the hair, followed by straightening it using a heated metal combing device. Over time, the technique was improved. However, with the introduction of new methods, the hot comb went out of use [1, 5, 7, 8].

Physicochemical techniques combining mechanical and thermal straightening, as hairdryer and flat iron, are temporary solutions that last until the next washing. The hair needs to be wet, so hydrogen bridges break and there is the transitional opening of the helical structure of the shaft, relaxing it. The combined use of the dryer and the flat iron dehydrates the hair, keeping it straight [1].

High temperatures, between 235 and 250°C in the dry hair and 155–160°C in the wet hair, may denature hair shaft proteins [1, 9]. Usually, hairdryers are more harmful to the hair shaft than naturally drying it [9]. However, a study showed that the use of the dryer with continuous movement, at a minimum distance of 15 cm from the hair, could be less damaging than natural drying [10].

Hydroxides are potent alkalis, widely used for straightening very curly hairs [11]. The primary substances of this group and their characteristics are described in Table 1. Sodium hydroxide, also known as lye, is indicated for straightening extremely curly hair. No-lye relaxers, such as guanidine hydroxide, are indicated for straightening wavy to curly hair and for sensitive scalp. Although milder for the scalp, it leaves calcium mineral residues on the hair shaft, making it drier, brittle, and dull [4, 11-14].

Hydroxides promote permanent straightening since it breaks the keratin disulfide bonds of the cortex. Alkalis react with the cystine in the keratin through lanthionization (Fig. 1) [1, 4]. In this process, 1/3 of the cystine amino acids are replaced by lanthionine, an analogue of cystine that contains only one sulfur atom and contributes to stabilizing the smoother configuration of the hair [4, 8]. The high pH and the conversion of cystine to lanthionine weaken the hair shaft, which is why hydroxides are related to a higher risk of hair damage [1, 4, 8, 11].

Hydroxides are not chemically compatible with thioglycolate, and therefore, concomitant use can lead to hair breakage. Straighteners with thioglycolate in its formula can be used in previously dyed or chemically wavy hair (Table 2) [7].

This group comprises ammonium thioglycolate (AT), aminomethyl propanol thioglycolate, and ethanolamine thioglycolate. They have less power and a higher cost than hydroxide agents. Their main application is to straighten curly and wavy Caucasian hair. AT is the most popular, which solution ranges from 7.5 to 11% concentration, and pH between 9 and 9.3. More alkaline solutions, with a higher amount of ammonia, allow the use of a lower concentration of thioglycolate with the same efficacy. As a disadvantage, they have an unpleasant odor and are more skin irritating [11].

Thiols penetrate the cortex and straighten the hair by breaking the sulfide bonds of the cystine amino acids, forming 2 cysteines for each cystine (Fig. 2). Like hydroxides, thiols also swell the hair shaft, but with no lanthionization reaction. The process is completed with the neutralization, done through the use of an oxidizing agent, usually hydrogen peroxide. When the same technique does not involve using the flat iron, it is named relaxation [11].

Straightenings with AT may cause the color to fade, particularly brown and black hair, due to the oxidation reaction. Moreover, straighteners are usually not recommended in pregnancy, although a study has shown no association between the use of these products by pregnant women and complications [15].

This technique was introduced in 2003 in Brazil. Later, other countries adopted it, naming it Brazilian keratin treatment (BKT) [16, 17]. BKT promises hair relaxation and electrostatic reduction, possibly making hair more manageable and enhancing its color and shine. Unlike hydroxides and thiols, it can be applied on hair previously straightened or dyed.

Formaldehyde is a soluble aldehyde widely used for its chemical properties [18]. It works as a preservative in the composition of various products for daily use [19]. Some synonyms of formaldehyde are methanal, formalin, formic aldehyde, oxymethylene, methyl aldehyde, and oxymethane. Several countries banned its use as an active ingredient in straightening products [20]. Formaldehyde releasers are chemical compounds that, in high temperatures, have formaldehyde as 1 of its final products. Examples of formaldehyde releasers are methylene oxide and methylene glycol [19, 21].

The synthetic hydrolyzed liquid keratin, usually extracted from sheep’s wool, due to its small dimensions, penetrates quickly to the cortex and reacts with the keratin of human hair. The heat of the dryer volatilizes the formaldehyde increasing the fixation capacity of the hydrolyzed keratin (HK) solution in the natural keratin of the hair [13]. Protein cross-linking produces a hardening film covering the hair, waterproofing it, and keeping it smooth and shiny. This film is not so protective since the shaft remains fragile and susceptible to fractures after minimal trauma [11].

Formaldehyde and the other aldehydes are not primarily straighteners. The washing water breaks the weak hydrogen bonds, and the new straight configuration is obtained and maintained. It occurs because of the cross-linking promoted by formalin on the fiber keratin and the HK previously applied. A hypothesis was raised that the additional HK could fill defects in the cortex, restructuring, and strengthening the hair. However, this theory has never been proven [16]. BKT has variable durability and can be reapplied after 6–20 weeks [16].

Formaldehyde restriction occurs due to its carcinogenic effect, especially if subjected to high temperatures. Formaldehyde is classified as a human carcinogen due to a malignant potential for respiratory and hematological systems, being associated with sinonasal cancer and myeloid leukemia. It represents a risk for the user, and the performing professional as its chronic inhalation is related to an increased incidence of malignant processes [22, 23]. Acute exposure is also harmful and causes skin lesions ranging from mild eczema to burns and skin necrosis; lymphadenopathy, headaches, allergic conjunctivitis; respiratory lesions, that may be fatal. There are also reports about tubular necrosis leading to acute renal failure [24, 25]. One study showed skin and scalp reactions suggesting a drug reaction phenomenon and not only eczemas of irritant or allergic origin [17].

Several international regulatory agencies consider formaldehyde carcinogenic and teratogenic. Unfortunately, many products applied by salon professionals contain formaldehyde or its derivatives and are marketed as safe. A study performed in Brazil showed that all the 23 brands assessed exceeded the 0.2% formaldehyde limit established by the National Health Surveillance Agency. The concentrations obtained ranged from 3 to 11%, representing 18–54 times the permitted levels [23]. Another study from Egypt showed that the occupational use of hair straightening exposes hairstylists to formaldehyde concentrations that exceeded the standard limits [21].

Uninformed stylists may not be protecting themselves, and their customers, from the formaldehyde exposure. Customers must always ask the hairstylist about the product that is being applied and be aware of the label information provided by the manufacturers. Because this information may not be completely accurate, ingredients should be thoroughly researched to bring to light the safety of these products and their effect on different hair types [16].

Glutaraldehyde is a saturated dialdehyde, marketed as a 0.2% cosmetic preservative and as a sterilizer and hospital disinfectant in concentrations up to 2%. It is potent in producing stable cross-links with DNA proteins and more toxic than formaldehyde, especially to the respiratory system. As it is potentially a carcinogen, its use as a straightener is prohibited in any concentration [11].

Glyoxylic acid (GA), also called formic acid or oxoacetic acid, is an organic compound and the simplest of the aldehydic acids. After the ban of formaldehyde as a straightener in several countries, GA became widely used in hair straightening products. According to the literature, GA would release formaldehyde when subjected to high temperatures [13, 21, 26-29].

GA straightenings have semipermanent results, maintaining the straight configuration of hair for up to 5–10 washes. That protein denaturation occurs through the reaction between the acid and the structure of the amino acids. GA penetrates through cuticles and acts on the hydrogen bonds and the amino acid tyrosine of the cortex. When GA is incorporated, tyrosine residues become exposed by the interactions of hydrogen bonds with the carboxylic portion of the acid. Although there is no break in the disulfide bridges, there is a change in the shape of these bonds. These reactions, when associated with the heat, originate a bio-polymerized structure, straightening the hair [13, 21, 26-28].

GA derivatives, like glyoxyloyl carbocysteine and glyoxyloyl keratin aminoacid oxoacetamide, are also used as straighteners [30]. Such substances yield a temporary change in the hair structure [13]. Their mechanism of action is similar to other acid straighteners: besides the already known formation of cross-links involving 2 or more polypeptide chains; aldehydes interact with hair’s aminoacid chain, forming an unusual product in the hair shaft: imine and hemiacetal. The temperature is highly associated with this process, because high temperature, increase the formation of these products [30].

The use of formaldehyde releasers (“formaldehyde-free”) is growing in popularity each day with promises of safetiness with no scientific embasement [13]. In Brazil, National Health Surveillance Agency is still discussing the possibility of using this product and its derivatives [31]. Despite the film-forming effect that GA promotes to the hair, giving it a shiny appearance, this substance disturbs the integrity of the keratin and the tensile strength of the shafts, which become more fragile.

Irritant and allergic contact dermatitis are the most common adverse events [11, 16, 23]. Formaldehyde can cause various types of reactions including contact dermatitis, urticaria, and mucous membrane irritation, especially of the conjunctiva and respiratory tract [20]. Erythematous-desquamative and papulopustular lesions may happen in the scalp or other areas of the body, in a pattern that suggests drug-induced skin reactions [17].

Telogen effluvium may occur, usually because of excessive or improper use of straightening agents [11]. Traction alopecia, especially along the hairline, is related to the pulling force applied to the hair during the process. It may be acute or chronic, and it is a particular concern in children and Afro-descendants. Scarring alopecia is rare and may occur because of repeated traction or in case of burns by straighteners or by heated petroleum jelly during the process [32]. In a recent study, exposure to chemical straightening was significantly more common in patients with cicatricial alopecia compared with those with no cicatricial alopecia [33].

Hair breakage is another possible adverse effect that occurs if the hair is exposed to straighteners for a long time or if there is the use of incorrect pH or substances in remarkably high concentrations [32]. For acid straighteners, there are changes in hair properties as combing, tensile strength, color, amino acid content, and internal structure. Straightener’s pH value has a strong influence on these properties. The presence of more hydrogen ions as the pH value decreases causes more damages to the shaft and affects the elasticity of the fiber. It is important to control the pH value to avoid more damage and possible breakage of shafts [30].

Hair is an essential aspect of human individuality. The possibility of having an easy to handle hairstyle and changing it from time to time generated an increasing search for straightening hair among all races around the globe. However, this trend brings some concerns about the chosen techniques, used products, methods of application, and risks involving the safety of both the user and the performing professional.

Professionals must consider product concentrations, pH, and exposure time to correctly perform the chosen technique and to reduce possible adverse effects. Patients who want to have their hair straightened should seek “ethical straightening,” made with approved substances according to the law of their countries. Use of BKT should be discouraged due to the risks of formaldehyde, possible discrepancies between the formulation shown on the label and the actual presence of formaldehyde or its releasers in hazardous concentrations, and lack of safety information regarding other substances such as GA. The characteristic odor of formaldehyde associated with irritation in the eyes and oropharynx should lead to the immediate suspension of the procedure because it indicates that there is formaldehyde in the product in very high concentrations and, therefore, dangerous to health.

Patients have to remember that permanent dyes should only be performed at least 2 weeks after the straightening, reducing the chance of excessive damage to the shaft. Temporary and semipermanent colorings can be used at the same time as straightening. Patients must also be aware of the incompatibility between different products and techniques due to the risk of hair breakage. Furthermore, the possibility of allergic reactions, scalp irritation and burning, alopecia, and long-term hair damage always must be kept in mind. Dermatologists need to understand the peculiarities of different techniques of straightening, the possible health risks involved to accurately orient patients on how to get the best outcome without risking one’s safety.

The authors have no conflicts of interest to declare.

The authors did not receive any funding.

T.M.B., D.F.M., and G.M.P conceived the study and wrote the manuscript; S.C.F. and P.S.D. collected data; and F.W analyzed and interpreted data. All the authors discussed the results and reviewed the final manuscript.