In which hair layer do changes occur during permanent waving, chemical relaxing, and oxidation hair coloring?

• A. Cuticle
• B. Cortex
• C. Medulla
• D. Hair shaft

Answer: (B)

Changes from permanent waving, chemical relaxing, and oxidation hair coloring mainly occur in the cortex—the part of the hair that contains the bulk of keratin fibers and the structures that determine shape and color. In these processes, bonds within the cortex are altered. For waving and relaxing, reducing agents break the disulfide bonds between keratin molecules in the cortex and then allow those bonds to reform in a new arrangement when a neutralizer is applied, reshaping the hair. For oxidation hair coloring, oxidizing agents react within the cortex to form new color-bearing structures and to modify melanin, which sits in the cortex as the natural pigment. The outer cuticle layer controls how easily chemicals reach the cortex but is not where the structural changes that create curl or color occur. The medulla is a central core present in some hairs but is not the site of these chemical transformations, and the hair shaft is the whole strand, not a specific layer.