Hair dye mainly contains ammonia (NH3), hydrogen peroxide (H2O2) and dye molecules. First, the ammonia opens up the hair cuticle, allowing the hydrogen peroxide and colorant to penetrate into the hair cortex. Hydrogen peroxide reacts with, or oxidizes, the remaining melanin in the hair, making it colorless. The small dye molecules then penetrate the hair cortex where they deposit, increase in size and become trapped, creating a new overall hair color.
