Methylchloroisothiazolinone/methylisothiazolinone contact sensitization: diverging trends in subgroups of IVDK patients in a period of 19 years.

Background: The preservative methylchloroisothiazolinone (MCI)/methylisothiazo- linone (MI) is an important allergen, and has thus been subject to regulation in some areas of exposure, mainly in cosmetics.

Objectives: To examine whether a decline in sensitization prevalence and intensity of elicitation (by the patch test) can be observed, either overall, or in certain subgroups defined by causative exposures.

Methods: On the basis of long-term contact allergy surveillance data collected by the Information Network of Departments of Dermatology (www.ivkd.org), 1992-2010, in the course of 171 883 consultations, the time trend of (different grades of) positive patch test reactions to MCI/MI (0.01% aqua) was examined in an age-stratified analysis.

Results: The overall frequency of positive patch test reactions was 2.33% (95% confidence interval 2.26-2.4%). No significant overall trend of positive reactions or of the distribution of +, ++ and + + + reactions (p > 0.05) was observed, similarly to what was found for patients in whom cosmetics were recorded as causative exposure (p = 0.55). In contrast, in patients with occupational contact dermatitis who were < 40 years of age, a significant increase in prevalence, by 0.87 percentage points per decade, was identified (p = 0.018).

Conclusions: Both applications of MCI/MI, cosmetic and industrial, should be the target of further preventive efforts.