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Background: Hair cosmetic products contain several, partly potent contact allergens, including excipients like preservatives. Hand dermatitis in hairdressers is common, scalp and face dermatitis in clients or self‐users (summarised here as 'consumers') may be severe. Objective: To compare frequencies of sensitization to hair cosmetic ingredients and other selected allergens between female patch tested patients working as hairdressers and consumers without professional background, respectively, who were tested for suspected allergic contact dermatitis to such products. Methods: Patch test and clinical data collected by the IVDK (https://www.ivdk.org) between 01/2013 and 12/2020 were descriptively analysed, focusing on age‐adjusted sensitization prevalences in the two subgroups. Results: Amongst the 920 hairdressers (median age: 28 years, 84% hand dermatitis) and 2321 consumers (median age: 49 years, 71.8% head/face dermatitis), sensitization to p‐phenylenediamine (age‐standardised prevalence: 19.7% and 31.6%, respectively) and toluene‐2,5‐diamine (20 and 30.8%) were most common. Contact allergy to other oxidative hair dye ingredients was also more commonly diagnosed in consumers, whereas ammonium persulphate (14.4% vs. 2.3%) and glyceryl thioglycolate (3.9 vs. 1.2%) as well as most notably methylisothiazolinone (10.5% vs. 3.1%) were more frequent allergens in hairdressers. Conclusions: Hair dyes were the most frequent sensitizers both in hairdressers and in consumers; however, as indication for patch testing may differ, prevalences cannot directly be compared. The importance of hair dye allergy is evident, often with marked coupled reactivity. Workplace and product safety need to be further improved. [ABSTRACT FROM AUTHOR]

Background: The burden of occupational hand eczema in hairdressers is high, and (partly strong) allergens abound in the hair cosmetic products they use. Objectives: To systematically review published evidence concerning contact allergy to an indicative list of active ingredients of hair cosmetics, namely, p‐phenylenediamine (PPD), toluene‐2,5‐diamine (PTD), persulfates, mostly ammonium persulfate (APS), glyceryl thioglycolate (GMTG), and ammonium thioglycolate (ATG), concerning the prevalence of sensitization, particularly in terms of a comparison (relative risk; RR) between hairdressers and non‐hairdressers. Methods: Following a PROSPERO‐registered and published protocol, eligible literature published from 2000 to February 2021 was identified, yielding 322 publications, and extracted in standardized publication record forms, also considering risk of bias. Results: Based on 141 publications, the contact allergy prevalence to PPD was 4.3% (95% CI: 3.8–4.9%) in consecutively patch tested patients. Other ingredients were mostly tested in an aimed fashion, yielding variable, and partly high contact allergy prevalences. Where possible, the RR was calculated, yielding an average increased sensitization risk in hairdressers of between 5.4 (PPD) and 3.4 (ATG). Additional evidence related to immediate‐type hypersensitivity, experimental results, exposures, and information from case reports was qualitatively synthesized. Conclusions: An excess risk of contact allergy is clearly evident from the pooled published evidence from the last 20 years. This should prompt an improvement in working conditions and product safety. [ABSTRACT FROM AUTHOR]