Hair growth disorders in children

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The diameter of a human hair ranges from about 18 µm to 180 µm. In people of European descent, blond hair and black hair are at the thinner end of the scale, while red hair is the thickest. The hair of people of African descent is typically thicker than the hair of other groups.

Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. The difference between body hair and scalp hair (and, in males, chin and moustache hair) is that head hair for practical purposes grows continuously, whereas body hair alternates regular periods of growth and dormancy. During the growth portion of the cycle, body hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After a few weeks, body hair growth stops. The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows to great length, whereas body hair does not. Anthropologists speculate that the functional significance of long head hair is almost certainly adornment, a by-product of natural selection.

Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.