Hair is produced by hair follicles--indentations of the epidermis (outer skin layer) that contain the hair root, the muscle attached to it, and sebaceous, or oil, glands. Hair is made up of dead cells filled with proteins, most of which are known as keratins. The cells are woven together like a rope to form the hair fiber. The hair fiber, in turn, has three layers: the outer cuticle with its fish-scale-like structure; the cortex, which contains the bulk of the fiber; and the center, or medulla. Hair color is determined by melanocytes, cells that produce pigment. When these cells stop producing pigment, hair turns gray.
Although it seems as if the hair on your head is always growing, hair actually has active and rest phases. The growth phase, known as anagen, lasts for two to six years. At any given time, about 90 percent of scalp hair is in the growth stage. The remainder is in the rest phase, known as telogen; this lasts from two to three months.
Once the rest phase is over, the hair strand falls out and a new one begins to grow. As a result, it's considered normal to lose from 20 to 100 hairs a day, says Diana Bihova, M.D., a dermatologist in private practice in New York City. Only a change in your regular pattern of loss is considered abnormal--but many things, including genetic factors, diet, stress, and medications, can change that pattern.
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