Hair and nails are modified (hardened) types of skin and contain more sulphur than skin does. The process of hardening of the hair and nails is called keratinization. Human hair consists of the hair shaft, which projects from the skin's surface, and the root, a soft thickened bulb at the base of the hair embedded in the skin. The root ends in the hair bulb. The hair bulb sits in a sac-like pit in the skin called the follicle, from which the hair grows.

At the bottom of the follicle is the papilla, where hair growth actually takes place. The papilla contains an artery that nourishes the root of the hair. As cells multiply and produce keratin to harden the structure, they are pushed up the follicle and through the skin's surface as a shaft of hair. Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair; and the cuticle, the hard outer layer that protects the shaft.

Hair grows by forming new cells at the base of the root. These cells multiply to form a rod of tissue in the skin. The rods of cells move upward through the skin as new cells form beneath them. As they move up, they are cut off from their supply of nourishment and start to form a hard protein called keratin. As this process occurs, the hair cells die. The dead cells and keratin form the shaft of the hair.Each hair grows about 1/2 inch (about 12 millimeters) every month and keeps on growing for up to 6 years. The hair then falls out and another grows in its place. The length of a person's hair depends on the length of the growing phase (anagen) of the follicle. Follicles are active for 2 to 6 years; they rest (telogen) for about 3 months after that. After the resting phase the hair sheds. Excessive shedding can lead to hair loss.