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. 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.
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 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 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.