A healthy person loses from twenty to 100 hairs a day. This is part of the normal growth cycle of hair. A hair typically grows for two to six years, then enters a resting stage for two to three months, after which it falls out. a new hair grows in its place. At any time, 90% of the hairs are in the growth phase.

In the United States, about 57.5 million people have some degree of baldness. Pattern baldness, in which the top of the head loses hair, affects 35 million men and 20 million women. The women tend to be past menopause, when lowered amounts of the hormone estrogen contribute to hair loss, which is more even on the scalp than in men. Pattern baldness is called androgenic alopecia, because it is associated with testosterone, an androgenic (male) hormone. About 2.5 million people have an inherited condition called alopecia areata, in which the body manufactures antibodies that attack the hair follicles, resulting in oval bald spots in mild cases, to complete loss of scalp and body hair in severe cases.

Various conditions can cause temporary hair loss. Lowered estrogen levels shortly before and after giving birth may cause a women's hair to come out in clumps. Taking birth control pills, cough medications, certain antibiotics, vitamin A derivatives, antidepressants, and many other medications can also cause temporary hair loss. A sustained high fever may prompt hair loss six weeks to three months later.

Many people losing their hair seek treatment. One successful treatment is minoxidil (Rogaine), a drug originally used to lower high blood pressure. It was approved as a treatment for baldness in men in 1988 and in women in 1991. Rogaine causes new hair to grow in 10 to 14% of cases, but in 90% of people it slows hair loss. However, when a person stops taking it, any new hair falls out.

Hair transplants move hair follicles from a hairy part of a person's body to a bald part, and they are successful. Several other approaches, however, are potentially damaging--to the wallet as well as the scalp. Suturing on hair pieces often leads to scarring and infection. In 1984, the Food and Drug Administration banned hair implants of high density artificial fibers, because they too become infected easily. Products called thinning hair supplements are conditioners, often found in ordinary shampoo, that merely make hair feel thicker. They are generally concoctions of herbs and the carbohydrate polysorbate. Labels claim the product releases hairs trapped in the scalp.

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