Methylchloroisothiazolinone

Methylchloroisothiazolinone, or MCI, is a synthetic preservative used to protect against the growth of bacteria, yeasts, and fungi in water-based products.

MCI is commonly combined with methylisothiazolinone (MI) in a 3:1 ratio.

This ingredient is safe in cosmetics when formulated to be non-sensitizing and concentrations must not exceed 7.5 ppm in leave-on products / 15 ppm in rinse-off products.

Unfortunately, this ingredient has a well-documented sensitization story. Studies spanning several decades have shown this ingredient + MI to cause skin irritation and allergies.

The use of this ingredient varies around the world:

• The EU allows this ingredient in rinse-off products up to 15 parts per million (ppm)., but banned in leave-on cosmetics.
• The US allows this ingredient in concentrations of up to 15 ppm in both rinse-off and 7.5 ppm leave-on products.
• Canada allows this ingredient to be used with MI in rinse-off products. The combination must not exceed 0.0015%.

This just means this ingredient is most likely non-sensitizing in rinse-off products unless you've been diagnosed with a contact allergy to isothiazolinones.