Background: Methylisothiazolinone (MI) used alone is a new preservative causing a high prevalence of contact allergy. The eliciting threshold of MI is unknown. The combination of MI and phenoxyethanol enhances the antimicrobial efficacy of MI.
Objectives: The eliciting doses of MI contact allergy in a patch test and a repeated open application test (ROAT) were investigated. In the patch test, it was determined whether phenoxyethanol influenced the reactivity to MI.
Methods: Eleven MI-allergic individuals were patch tested with two dilution series of 12 doses of MI and the same 12 doses with phenoxyethanol. The ROAT mimicked the use of a cream preserved with 100, 50 and 5 ppm MI (corresponding to 0.21, 0.105 and 0.0105 µg MI/cm(2)).
Results: Phenoxyethanol had no influence on the reactions to MI. The lowest eliciting dose in the patch test was 1.47 µg MI/cm(2). In the ROAT, 7 patients (64%) reacted to 0.21 and 0.105 µg MI/cm(2) and 2 patients (18%) reacted to 0.0105 µg MI/cm(2), corresponding to a cream preserved with 5 ppm MI.
Conclusions: A maximum of 100 ppm MI is permitted in cosmetic products. Eighteen per cent of MI-allergic patients reacted to a concentration 20 times lower in a ROAT. The amounts used in cosmetics should be reduced, and the development of MI contact allergy should be monitored closely.
© 2011 John Wiley & Sons A/S.