Differences between hairdressers and consumers in skin exposure to hair cosmetic products: A review

Hairdressers are at high risk of developing occupational hand eczema. Opinions on the health and safety concerns of nonfood consumer products, such as cosmetics and their ingredients, consider the exposure of a “common consumer,” which may not account for occupational exposure of hairdressers. As a result, there is a parlous scenario in which serious safety concerns about occupational exposures are present. The purpose of this review is to compare the frequency of exposure to various types of hair cosmetic products among hairdressers and consumers. Database searches for this review yielded a total of 229 articles; 7 publications were ultimately included. The analysis showed that—dependent on the task—hairdressers were exposed 4 to 78 times more than consumers to a wide spectrum of hair cosmetic products used in their daily working life, ranging from shampoos, conditioners, oxidative and nonoxidative hair colors, to bleaching agents. The highest frequency was found for coloring hair with oxidative hair color. Consumer use frequency does not appear to be appropriate for representing hairdresser exposure. The current standards do not effectively address the occupational risks associated with hairdressers' use of cosmetics. The findings of this study should cause current risk‐assessment procedures to be reconsidered.


| INTRODUCTION
Hairdressers are subjected to a high degree of occupational skin strain within their professional surroundings, mainly arising from frequent wet work and skin contact to detergents and hairdressing chemicals. [1][2][3] This occupational skin exposure may lead to adverse skin reactions of an irritant and/or allergic nature. Thus, prevalence of occupational dermatoses, mostly hand eczema, is very high in hairdressers, causing personal suffering due to illness as well as high financial burden due to health expenditures for society. 4,5 In the European Union (EU), utilization of carcinogenic, mutagenic, or substances toxic for reproduction (CMR) in cosmetic products is restricted by the so-called Cosmetics Regulation, which, however, allows exceptions to this basic principle, making it possible to use such substances in cosmetic products after adequate safety assessment and appropriate risk management (mostly, maximum permissible concentrations). 6 The Scientific Committee on Consumer Safety (SCCS) elaborates opinions on health and safety risks of nonfood consumer products, including cosmetic products and their ingredients, whereby the exposure to such products by a "common consumer" is taken into account. Such typical exposure is outlined in the SCCS "Notes of Guidance" (NoG), 7 which, in turn, are based on empirical data on product use in consumers. 8,9 The exposure of hairdressers-who are in contact with such substances throughout their daily working life-is, however, not considered in the process of the evaluation of the SCCS, leading to opinions that may not adequately address occupational exposure of hairdressers. This leads to the precarious situation that grave safety concerns with regard to occupational exposures remain.
Against this background, the objective of the present article is to examine the frequency of exposureas reported in literatureto important types of hair cosmetic products as well as of wet work in hairdressers compared to non-hairdressers, termed "consumers" (representing "clients") or "self-users." 2 | METHODS

| Hairdressers' exposure
Literature searches were performed in February 2021. The time period from 1990 to 2020 was taken as the eligible publication period.
We systematically searched for ((Hairdressers* OR Hairdressing apprentice*) AND (skin) AND (exposure*)) in PubMed/Medline and Web of Science-Core Collection (WoS). Only English-language search terms were used. Search results were exported from Medline and WoS in a suitable format and imported into Zotero libraries, documenting the number of references contributed by each export/import set. In the Zotero library, bibliographical duplicates were identified and the entry including less information (e.g., no abstract) was discarded. The remaining unified library was exported in Research Information System Format (RIS) and imported into a new Rayyan (Rayyan QCRI) project by a reviewer for eligibility based on title, keywords, and abstract. The final set of references eligible for full-text screening was exported from Rayyan in Bibtex format for import into the Zotero cloud-based reference database, after the initial set of references was archived.

| Consumers' exposure
Typical use frequencies of hair cosmetics by consumers (when normally applied by professionals in a hairdressing salon) and self-users (when applying hair cosmetics at home) have been considered together. Some products are usually applied only in a hair salon (e.g., waving agents or hair relaxers), others frequently at home (e.g., shampoos and oxidative or nonoxidative hair colors). Data on consumers' exposure was obtained primarily from the NoG, as far as available, which refers back to Hall et al. (2007) as well as Hall et al. (2011),, 8,9 who suggest a framework for conducting population exposure assessments regarding European consumer exposure to cosmetic products.

| Data extraction
In a first step, we extracted data on hairdressers' exposure including the frequency of procedures using corresponding products, mean duration of procedures, the share of hairdressers who usually conduct the respective procedure, and the share of hairdressers who usually wear gloves during the respective procedure. Regarding consumers' exposure, the frequency of procedures using the corresponding products, mean duration of procedures, and the share of people who usually conduct the respective procedure were extracted. Study characteristics (type of investigation, country, study design/method, study population, age range, and sex of the study population) of the included studies were recorded using a publication record form (PRF).
For hairdressers, data on exposure were extracted from the respective studies. Data on mean duration of procedures, data on the share of hairdressers who usually conduct the respective procedure, and data on the share of hairdressers usually wearing gloves while conducting the respective procedure was taken from Uter et al.
(1998), 10 who assessed risk factors for hand dermatitis in a cohort of hairdressing apprentices.
In a second step, the aforementioned data were condensed and have undergone further refinement. For displaying the frequency of procedures using certain products we decided to use median values for data obtained in hairdressers. To provide greater clarity, we extracted the mean duration of procedures for hairdressers and consumers as well as the share of hairdressers and consumers who usually conduct the respective procedure. All procedures usually only performed by hairdressers were discarded, since these cannot be compared to consumers regarding exposure. The following procedures were removed from further consideration: cutting wet hair (without previous coloring), cutting wet hair (after coloring), highlighting the hair (mostly using bleach with 6% to 9% hydrogen peroxide), and lowlighting (mostly using oxidative hair color with 6% hydrogen peroxide) the hair using foil, highlighting the hair (mostly using bleach with 6% to 9% hydrogen peroxide) and lowlighting (mostly using oxidative hair color with 6% hydrogen peroxide) the hair using a cap, perming the hair using waving/perming lotions (acid, alkaline, and exothermic perms), and coloring eyelashes and/or eyebrows with oxidative hair color (using mostly 3% hydrogen peroxide).

| Daily exposure doses per area
From the included studies, data on frequency of use were available.
However, the most relevant dose-metric for risk assessment con-

| Exposure factor
The factor by which hairdressers are higher exposed as consumers was calculated using the following formula:

| Study selection
A flow diagram of the study selection concerning hairdressers' exposure is presented in Figure 1. Initial searches generated 229 individual study records. After elimination of bibliographic duplicates, 170 references remained and were screened. Manual searching of references retrieved another two references. After elimination of publications that were irrelevant because they did not give details on the frequency of use of product categories, we arrived at a final number of seven publications considered for this review.

| Study types
The included studies covered a prospective population-based cohort study including a questionnaire survey combined with clinical examinations (n = 1), an observational study (n = 1), an observational study combined with a questionnaire survey (n = 1), questionnaire surveys (n = 2), and questionnaire surveys combined with clinical examinations (n = 2). The study characteristics are summarized in a PRF and can be found in the Appendix (Table S1).

| Population studied
All studies reported either on the exposure of hairdressers (n = 5) or hairdressing apprentices (n = 2). Of the two studies reporting on hairdressing apprentices, one study included only the final year hairdressing apprentices, who have an almost equal exposure in comparison to hairdressers who have finished training, and one study included hairdressing apprentices from all three apprenticeship years (according to the German dual-training system) so that it can be assumed that the exposure of the apprentices may be comparable to the exposure of trained hairdressers. Thus hairdressing apprentices are counted as hairdressers in the present review.

| Exposure studied
All of the seven included studies provided details on the frequency of use of/quantification of exposures to hair cosmetic products in hairdressers.

| Exposure to hair cosmetic products in hairdressers compared to consumers
Within the seven reviewed studies, data on the 12 procedures alongside with concomitantly used product types were available. Most of the identified tasks are performed exclusively by hairdressers (Table 1). Data on hairdressers' exposure including frequency of procedures using corresponding products, mean duration of procedures, the share of hairdressers who usually conduct the respective procedure, and the share of hairdressers usually wearing gloves while conducting the respective procedure, as well as consumers' exposure including frequency of procedures using corresponding products, mean duration of procedures, and the share of people who usually conduct the respective procedure were extracted as summarized in Table S2.
While shampooing/washing the hair, consumers and/or self-users are exposed to shampoo one time per day, 7 whereas hairdressers are exposed to shampoo up to 12 times per day, with a mean duration of 7.5 minutes for each washing procedure. 10 regularly by only 23.9% of the hairdressers. 10 During deep conditioning the hair with hair conditioner, consumers and/or self-users are exposed to hair-conditioning products 0.28 times per day, 20 in contrast to hairdressers who are exposed to hair conditioners up to 5 times per day, with a mean duration of 10 minutes plus possibly additional 12.5 minutes each due to potentially following use of conditioning products with head massage. 10,17 Deep conditioning is done by 90.5% of hairdressers, and 11.5% of hairdressers reported wearing gloves while doing this task; head massages are conducted by 91.7% of hairdressers and 8.4% of hairdressers wear gloves massaging the head. 10 Cutting wet hair is done either without previous coloring or previous coloring. The former strategy avoids exposure of hairdressers to residual dye monomers, as it is known that colored hair exposes hairdressers to substances released from the hair color. Consumers as well as self-users are not exposed to wet hair during the cutting of hair. Hairdresser are exposed to wet, uncolored hair for up to 9.9 times per day and to colored hair for $2 times per day, with a mean duration of a haircut being 25.0 minutes each. 10,15,[17][18][19]21 This task is conducted by 93.5% of the hairdressers, and 1.3% of them use gloves for it. 10 Regarding coloring of the full head of hair with permanent/ oxidative hair color using 6% to 12% hydrogen peroxide, consumers and/or self-users are exposed one time per month in contrast to hairdressers who are exposed to permanent/oxidative hair color for up to four times per day. 10,[15][16][17][18][19]21 In terms of coloring a full head of hair with semi-permanent oxidative hair color using 2% to 3% hydrogen peroxide or nonoxidative hair color, consumers or self-users are exposed one time per week regarding semi-permanent, nonoxidative hair color and one time per month regarding semi-permanent oxidative hair color, whereas hairdressers are exposed to such coloring agents about three times per week. 16,17 While coloring roots/regrowth only using oxidative/nonoxidative hair colors according to previous treatment, consumers and/or self-users are exposed a maximum of one time per week regarding semi-permanent, nonoxidative hair color and one time per month regarding semi-permanent oxidative hair color, in contrast to hairdressers who are exposed to the agents approximately three times per week. 17 The mean duration of each of the coloring services mentioned is 15.0 minutes, and the services are conducted by 99.3% of the hairdressers, of whom 95.2% wear gloves during this task. 10 There is no scientific data on consumers' exposure in highlighting the hair (mostly using bleach with 6% to 9% hydrogen peroxide) and lowlighting (mostly using oxidative hair color with 6% hydrogen peroxide) the hair using (aluminum) foil and using a cap. Presumably, self-users do usually not conduct the aforementioned procedures.
Hairdressers, however, are reportedly exposed to bleach or oxidative hair color while using foils for bleaching/coloring 1.2 times per week and while using a cap for bleaching/coloring 0.84 times per day. 17,18 In addition, scientific data on mean durations as well as the share of hairdressers conducting these tasks and wearing gloves while doing so are not available. For using bleach-mostly with 6% to 9% hydrogen peroxide-for bleaching hair on the full head, data for consumers and/or self-users are not available. It nevertheless needs to be considered that for consumers/self-users the label full head only applies for first-time bleaching; after that full head refers to root/regrowth only, since already bleached hair cannot be bleached as often as desired in contrast to coloring the hair with oxidative or nonoxidative hair colors.
In contrast, hairdressers are exposed to bleach-mostly using 6% to 9% hydrogen peroxide-while bleaching the hair of the full head or also the root/regrowth 2.5 times per day, with a mean duration of 15 minutes for each procedure, which is done by 96.9% of hairdressers, of which 77.9% wear gloves. 10,16,21 For perming the hair, data for consumers' exposure is lacking and self-use is highly unlikely. Hairdressers, in contrast to consumers, are exposed to different types of perming lotions (acid, alkaline, and exothermic perms) and apply perming solutions two (acid perm) to three (alkaline perm) times a day with a mean duration of 5 minutes per procedure (acid and alkaline perm) conducted by 44.3% (alkaline perm) and 97.5% (acid perm) of hairdressers, whereas 29.2% (alkaline perm) and 34.7% (acid perm) wear gloves in each perm solution application process. 10,15,17 Exposure data on coloring eyelashes and/or eyebrows has been laid out in only one study, revealing that hairdressers are exposed to oxidative hair color using mostly 3% hydrogen peroxide, conducting this task 0.8 times per day with no further specification on mean duration, share of hairdressers conducting this task, and share of hairdressers wearing gloves while carrying out the task. 17 For consumers, there are no data available for this exposure; self-use, however, is very rare.

| Exposure to wet work in hairdressers compared to consumers
In addition, information about wet work exposure in hairdressers was extracted ( Table 3). These data support the known fact that hairdressers are exposed to a considerable amount of wet work within their daily working life, which can add up to even more than 4 hours per working day for a considerable amount of hairdressers, 15,17,22 whereas wet work is likely less performed by the "common consumer." 23

| DISCUSSION
In this review we have shown that hairdressers are exposed up to 78 times more than ordinary consumers to customary hair cosmetic products. This applies for a wide spectrum of products ranging from shampoo, conditioner, oxidative and nonoxidative hair colours, to bleaching agents. Assuming comparable product composition, the daily dose per area of single-product ingredients can be compared (hairdressers vs consumers).
To the best of our knowledge, this is the first review to compare occupational exposure in hairdressers with exposure in consumers regarding relevant potentially skin-harming products. In order to assess typical use frequencies of hair cosmetics by consumers, data from the SCCS NoG for the testing of cosmetic ingredients and their safety, 11th revision, 7 were taken into account, to the extent available. For assessing hairdressers' exposure, a structured literature search was conducted, leading to the inclusion of seven studies in the present review. All included study types can be regarded as adequate, although it should be mentioned that that questionnaire surveys possibly suffer from a certain bias due to the subjective nature of the answers. The study populations cover a wide age structure so that the age of the included hairdressers may reflect the overall age structure in the hairdressing trade. Because the largest proportion of workers in the hairdressing sector of the EU is female, 20,21 the present high female share among hairdressers was to be expected and does thus not constitute a potential bias.
All of the included studies (n = 7) provided details on the frequency of use of/quantification of exposures to hair cosmetic products in professional hairdressers. Because the period from 1990 to 2020 was taken as an eligible publication period, it should be mentioned that the frequency of exposure to important types of addressed cosmetics might have undergone a change over the last years depending on current trends and changing preferences. In the future, the current frequency of exposure to important types of hair cosmetic products in hairdressers may be assessed via observational studies and in consumers via questionnaire surveys. The latter should also differentiate between consumers (subjected to exposure mainly on the scalp and only partly on the hands) and professional hairdressers (exposed mainly on the hands in the occupational setting).
Obviously, as consumers themselves, some hairdressers might also be more exposed to hair cosmetic products within their private surroundings, this being a not quantifiable add-on exposure to the occupational exposure. As a default, the same exposure pattern as laid down in the NoG was used for the self-use share of exposure in hairdressers. It was not only substantiated that hairdressers are exposed to products at a much higher rate than the ordinary consumer while conducting  [43][44][45] Hair styling and setting products (e.g., hairspray) Aerosols, resins, fragrances

Dermal, respiratory
No Styling products (e.g., hair gels) are applied by the hairdresser without gloves, and the hands are usually not washed until the service is finished. In order not to get setting products (e.g., hairspray) into the customers' face, the hairdresser protects it using their own hand while spraying with the other, getting the hairspray on the protecting hand. Dermal and respiratory effects of hairsprays have already been reported. 46,47 Metal tools/objects (e.g., tweezers, crochet hooks) Nickel and/or cobalt Dermal No Tweezers are used to pluck eyebrows and to remove undesired facial hair. A nickel and cobalt release (and also a co-release of both) from tweezers has already been reported. 26,27 Crochet hooks are used for highlighting/ lowlighting the hair using a cap. A nickel release from tweezers in the hairdressing trade has already been reported. 27,48 many working tasks, but also that a considerable amount of identified procedures are usually performed only by hairdressers, such as perming the hair using diverse types of perming solutions (Table 1).
These procedures provide a set of hairdressing-exclusive services on their own and provide an extra exposure for hairdressers that consumers presumably do not have.
Because data on 12 procedures alongside with concomitantly used product types were available, many of the most relevant procedures exposing hairdressers to hair cosmetic products are covered in this review. However, there are other additional tasks conducted within the hairdressing trade that expose workers to additional cosmetic products for which no data are yet available. Additional sources of exposure to potentially harmful substances are listed in Table 4, among them cosmetic glues for applying eyelashes, nails, and hair extensions, hot wax and sugaring paste for hair removal, hair styling and setting products, as well as metallic work tools. Occupational exposure may be fluctuating due to changing product formulations. Regarding acrylates, two current investigations focus on the increasing use of (meth)acrylates in gel nail paints-such as daylight curing "hybrid" gel nail polish-and their role as sensitizers. 24,25 With regard to the common contact allergens nickel and cobalt, two recent studies found release of these metal allergens in allergologically relevant amounts from a broad spectrum of tools that are used on a daily basis in hairdressing. 26,27 The frequency of use of the products listed in Table 4 could be evaluated in forthcoming studies. As permanent remodeling of the hair structure-that is, curling or straightening-with various new methods (i.e., so-called Brazilian blowouts or keratin treatments for smoothing the hair structure) has gained more popularity over the last years, the utilized chemicals prospectively need particular attention regarding dermal and/or respiratory effects in hairdressers.
It was also shown that concerning all procedures that should only be conducted wearing adequate protective gloves, the share of hairdressers actually wearing gloves is disenchanting. Especially against the background of hairdressers reusing already worn gloves, 28 contamination due to wrong use or while taking off the gloves, 29 breakthrough times of gloves, 30  Data on wet work has again supported the known fact that this exposure is likely present in considerable extent in hairdressers, 15,17,22 whereas this is not the case for the "common consumer." 23 Wet work leads to an impairment of the epidermal barrier function concomitant with the genesis of a proinflammatory milieu. Thus, penetration of skin with hazardous substances, such as allergens, is facilitated and the risk of the development of allergic contact dermatitis-especially of the hands-is promoted. Recently it was shown that PPD itself, a common ingredient in hair dyes, can turn down the function of tight junctions and stratum corneum proteins, even when there are no clinical symptoms present. 32 These effects on the skin and combined exposures to irritants and allergens lead to a higher susceptibility and risk of skin sensitization, which need to be taken into account in future risk assessment, for example, by applying extra safety factors. First attempts of quantitative risk assessment have indeed indicated that for PPD and toluene-2,5-diamine (PTD) contact that sensitization may occur when skin protection is not applied adequately. 33 Moreover, aggregated exposure-that is, exposure to the same ingredient via different products-can be expected concerning auxiliary ingredients such as preservatives, fragrances, or emulsifiers. This will further increase cumulative exposure to such substances, both in consumers, and, even more so, in hairdressers. It should additionally be specified that the exposure of hairdressers should always be evaluated with the starting level being on the level of a consumer, as it was shown that hairdressers tend to apply many hairdressing procedures to themselves. 34

| CONCLUSION
The results of the present review imply that an assumed frequency of use for consumers is not suitable for representing exposure-and with it, morbidity risk-of hairdressers. Higher exposure leads to a greater risk of skin irritation and sensitization, which consequently also has ramifications regarding systemic exposure via skin. The regulations in force do not adequately address the occupational dangers linked with the use of cosmetic products in hairdressers. A rethinking of present risk assessment practices should be prompted.