How Hair Grows

By Dr. Josh Zeichner | Sunday, August 22, 2010

Photo Credit: ©iStockphoto.com/becon

Ever wonder why you’ve got the hair you’ve got, why the hair on your face feels different than the hair on your head, or how -- no matter how often you shave -- it just keeps growing? It’s time to get the facts:

Hair is a non-living fiber composed primarily of keratin.

Special bonds within the fiber determine the hair’s physical properties, such as curls.

Hair color is based on the type and amount of pigment (known as melanin) deposited in the hair as it grows.

Brown and black hair contain one type of melanin, while red and blond hair contain another.

Hair with more pigment results in a more intense color, while hair with low pigment levels appears gray.

Hair lacking pigment altogether is white.

There are many different types of hair on the body, but two main categories are vellus and terminal hairs. Vellus hairs are small and grow no longer than approximately a half-inch. Terminal hairs, on the other hand, are large and can grow to long lengths. Examples of terminal hairs are those on the scalp and in a man’s beard.

Hair growth is regulated by hormones, such as testosterone. During puberty, hormone levels increase and stimulate vellus hairs to transform into terminal hairs, especially in areas such as a man’s beard.

Hair grows at the rate of approximately a half-inch per month in a three-part cycle:

Stage 1: Anagen is the actively growing stage. The longer anagen is, the longer the hair will ultimately grow. The duration of anagen differs per body part. For example, in the scalp, anagen may last two to six years, while in the mustache only one to four months. Most hairs are actively growing at any given time, at a rate of approximately a half-inch per month.

Stage 2: Catagen is the hair’s degenerating phase, during which hairs thin and lighten at the base.

Stage 3: Telogen, the final phase, is when hair sheds from the body before a new anagen phase begins.

And lastly, FYI: There is no evidence that shaving causes hair to grow in thicker.

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