Methylchloroisothiazolinone

This totally unpronounceable ingredient (let’s take a shortcut: MCI) is a super effective preservative with both antibacterial and antifungal effects that works at very low concentrations. 

The bad news with it though is that it can sensitize the skin easily (it’s a so-called non-fragrance allergen). It’s not permitted to be used alone, but only with its BFF, methylisothiazolinone (MI), another icky preservative. Together they have a broad spectrum antimicrobial effect but are permitted to be used only in rinse-off products (under 0.0015%).