Is methylisothiazolinone safe for skin?
Preservative that is generally recommended only for use in rinse-off products such as cleansers or shampoos. Among the preservatives used in leave-on products, methylisothiazolinone is known to be sensitizing when used in leave-on products. In fact, the American Contact Dermatitis Society named methylisothiazolinone as its allergen of the year in 2013! As with most ingredients with problematic potential, much depends on the amount used, but as a general rule, if you see methylisothiaolinone in the middle of an ingredient list for a leave-on product it should be avoided, particularly if you have sensitive skin or if this preservative is used in conjunction with sunscreen actives like octinoxate or avobenzone.
Methylisothiazolinone is active against bacteria but has weak anti-fungal properties. Its use in rinse-off and leave-on cosmetic products is restricted to low amounts as a means of avoiding irritation while maintaining preservative efficacy, but even the low amount permitted (which is 0.01%) has come into question as being more sensitizing than most other preservatives (Source: www.cosmeticsandtoiletries.com/regulatory/region/europe/Preserving-with-Methylisothiazolinone-premium-246545731.html?c=n).
Along with methylchloroisothiazolinone, this preservative is even more of a frequent allergen in leave-on products, particularly hair care, baby care, and feminine hygiene (Sources: Contact Dermatitis, December 2012, pages 334-341 and November 2011, pages 276-285). Just to be clear, neither of these preservatives are NOT considered problematic when used in rinse-off products such as cleansers or body washes; it's the leave-on products, especially those meant for use around the eyes or those that contain sunscreen actives (which some people find sensitizing on their own) that avoidance is advised.