“Methylisothiazolinone is a powerful synthetic biocide and preservative within the group of isothiazolinones. Methylisothiazolinone has the chemical formula C4H5NOS.” – Wikipedia
This ingredient belongs to the groups:
According to the European Commission’s Scientific Committee on Consumer Safety, methylisothiazolinone is a contact sensitizing agent. Allergies, swelling, eczema, red flammable, scaly skin, and pruritus of skin have been reported after using products containing the substance. The release of the allergy may be delayed and you may have used products with the substance for a long time before reacting.
In Canada, this substance is prohibited in leave-on cosmetic products. The maximum allowed concentration in rinse-off products is restricted to 0.0015% due to the risk of sensitization. The mixture of methylisothiazolinone and methylchloroisothiazolinone in the same product is also restricted, the total concentration cannot exceed 0.0015%.
The European Chemicals Agency (ECHA) classifies methylisothiazolinone as “fatal in contact with skin, fatal if inhaled, toxic if swallowed, causes severe skin burns, is very toxic to aquatic life with long lasting effects, causes serious eye damage, may cause damage to organs through prolonged or repeated exposure, may cause an allergic skin reaction and may cause respiratory irritation.”
The Campaign for Safe Cosmetics, a project of Breast Cancer Prevention Partners (previously the Breast Cancer Fund) list it as a substance that causes organ system toxicity, allergies, and possible neurotoxicity.
The Swedish Society for Nature Conservation (SSNC) list methylisothiazolinone as a substance causing allergies.
Methylisothiazolinone has multiple commercial and industrial applications. In cosmetic products⋆, the substance is most commonly used as a preservative in products such as sunscreens, shampoos, sanitary wipes, makeup and lotions. The substance’s industrial use includes energy production, metal working, and paint manufacturing.
We use the European Commissions definition of Cosmetics:
“Cosmetics range from everyday hygiene products such as soap, shampoo, deodorant, and toothpaste to luxury beauty items including perfumes and makeup”.
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