Introduction Methylchloroisothiazolinone/Methylisothiazolinone (MCI/MI) and methylisothiazolinone (MI) are chemical preservatives found in cosmetics, industrial and household products. There is a reported epidemic of allergic reactions to these substances in several countries (e.g. increases of 4.1% per annum over 16 years in England). Workers that come into contact with the agents may develop occupational contact dermatitis. Therefore, detecting these allergens is important to better manage workers’ skin condition and exposure. The aim of this study was to determine the prevalence of contact dermatitis to MCI/MI and MI before and after changes in allergen testing was introduced.

Methods A retrospective assessment of workers referred to the NIOH Dermatology Clinic between 2006 and 2017 was conducted. Workers with work-related dermatitis were patch tested for sensitisation to MCI/MI and/or MI using the European baseline series (Chemotechnique). Frequencies of sensitisation to the allergens (MCI/MI and MI) were calculated using Microsoft Excel.

Results A total of 583 occupational referrals were seen and 413 were patch tested to determine a possible allergic aetiology for the dermatitis. Sixteen patients (3.9%) were positive to either one or both allergens (MCI/MI and MI) during the period January 2006 to June 2017. The testing concentration of the MCI/MI was doubled in 2014 and MI was introduced in 2015. More cases were identified (6/86; 7%) after changes were introduced post 2014 compared to previous allergen concentrations (10/327; 3.1%). The workers identified with sensitisation to the chemicals were from occupations such as beauticians, cleaners, a millwright, a waitress and several workers that either decanted or mixed the chemicals.

Discussion MCI/MI and MI are important occupational allergens and should be tested using the correct concentration to avoid misdiagnosing cases. The importance of keeping abreast with what is happening in the field was highlighted by the changes implemented which improved the detection of these allergens.