Why does lightened and bleached hair get brassy?Updated 20 days ago

A key reason that lightened and bleached hair gets brassy is oxidation, a chemical reaction that can happen when metals and minerals in hard, unfiltered tap water build up on hair and interact with things like oxygen, sunlight, and heat. Oxidation, combined with the fact that chemically lightened hair is more porous (more open to taking in external substances) means that lightened and bleached hair is more vulnerable to brassiness. High-pH chemicals in bleach lift and damage the hair's outer cuticle layer, creating gaps and breaking down internal protein bonds (keratin), which essentially turn the hair shaft into an open sponge that absorbs more of the metals and minerals that lead to brass-causing oxidation.