Background The common contact allergens may change over time as the environmental exposure changes. Objectives To identify the prevalence rates and changing trends of contact allergens in Taiwan over a 40‐year period. Materials & Methods The patch testing results of a referral centre from 1978 to 2018 were retrospectively reviewed. The study population was divided into four groups according to 10‐year intervals. The prevalence rates of contact sensitization to each agent and the clinical relevance were analysed. For patients with positive reactions to relevant allergens, the occupations and sites of dermatitis were analysed. Results From 1978 to 2018, a total of 4005 patients underwent patch testing. Successively increasing trends of positive reactions to cobalt, fragrance mix I, and para‐phenylenediamine (PPD) were found. Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI) was an emerging contact allergen. Hairdressers, cosmetologists, and aromatherapists became the occupations most commonly having positive reactions in the most recent 10 years. In the first two decades, the face and neck were the most commonly affected areas. Later, hands became the most commonly affected sites. Conclusions The prevalence rates of positive reactions to cobalt, fragrance mix I, and PPD increased successively. MCI/MI was an emerging contact allergen of special concern.

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