Update on allergic contact dermatitis due to methylchloroisothiazolinone/methylisothiazolinone and methylisothiazolinone.

The combination of methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) is widely used as a preservative in cosmetics, household, and industrial products. Furthermore, MI at a concentration of 100 ppm has been permitted in cosmetic products since 2005. Recently, a considerable increase in cases of contact dermatitis to both MCI and MI have been noted, and this warrants closer monitoring by relevant authorities and, probably, stricter legislation. In fact, MI at a test concentration of 2000 ppm was recently included in the European baseline patch test series. The clinical manifestations of allergy to MCI/MI and MI are highly variable and diagnosis is often missed. In the standard patch test series of the Spanish Contact Dermatitis and Skin Allergy Research Group (GEIDAC), MCI/MI is tested at 100 ppm, but at this concentration, up to 50% of cases might go undetected. Furthermore, our data indicate that MCI/MI at 200 ppm would make it possible to diagnose more cases of contact allergy to MI. To improve the diagnosis of contact allergy to MCI/MI and MI, we believe that the test concentration of MCI/MI should be increased to 200 ppm in the GEIDAC standard series and that MI should be added in the GEIDAC standard series.