Methylisothiazolinone + methylchloroisothiazolinone are preservatives with antibacterial and antifungal properties. They belong to a class of chemicals called isothiazolinones. They are usually used as a mixture called Kathon CG.
Methylisothiazolinone + methylchloroisothiazolinone are known allergens. They are skin and membrane irritants and cause chemical burns. They can cause lesions and dermatitis, which may be further aggravated by sun exposure. These ingredients are generally found in cream + liquid beauty products and personal care products.
Methylisothiazolinone + methylchloroisothiazolinone are restricted in the EU and Canada to rinse-off products. They are banned from products that stay on the skin.
[1] Government of Canada. “Hotlist” of ingredients banned or prohibited in cosmetics and personal care products. (2019). https://www.canada.ca/en/health-canada/services/consumer-product-safety/cosmetics/cosmetic-ingredient-hotlist-prohibited-restricted-ingredients/hotlist.html#t2m
[2] Chemical and Engineering News. Preservative in leave-on cosmetics banned in the EU. (2016). https://cen.acs.org/articles/94/i16/Preservative-leavecosmetics-banned-EU.html
[3] NIH. NCBI. Methylisothiazolinone contact allergy: a review. (2011). https://www.ncbi.nlm.nih.gov/pubmed/21777214
[4] NIH. NCBI. Methylisothiazolinone (MI) is the American Contact Dermatitis Society Contact Allergen of the Year for 2013. (2013). https://www.ncbi.nlm.nih.gov/pubmed/23340392
[5] The Dermatologist Journal. Methylisothiazolinone/Methylchloroisothiazolinone must be considered strong allergens. (2016). https://www.the-dermatologist.com/content/update-isothiazolinones