According to Hirsutism.com, humans have an estimated five million hair follicles, one million of which reside on the scalp. The only areas of the body that do not have hair follicles are the lips, palms of the hands and soles of the feet. There are three cycles of hair growth and shedding.
Growth/Shedding Cycle
Most of the hair on your head (90 percent) is in a growing phase at any single moment, according to the American Academy of Family Physicians (AAFP). This period dictates the length of your hair. The remaining 10 percent is in a stage of rest. The hairs in the no-growth period remain stagnant for three to four months before falling out.
Types of Hair
The hair that grows on what appears to be the hairless parts of the body is called vellus hair. This soft, short hair is permanent and does not grow further beyond the predetermined rate set during the anagen phase. The longer and coarse hair that appears on the eyelashes, eyebrows, pubic area and scalp are defined as terminal hair.
Varying Rates of Growth
The hormone testosterone is the primary distributor of androgen (steroid) that determines the rate of hair growth. Certain areas are less sensitive to the effects of androgen, such as the skin of the eyebrows and eyelashes. As a result there is generally minimal hair growth there.
Monthly Scalp Hair Growth
The average person grows approximately a half inch of new scalp hair per month, according to the National Institutes of Health (NIH). Most hairs grow for a period ranging from two to six years before they fall out and are replaced by new hairs.
Hirsutism & Alopecia
There are exceptions to the average growth and type of hair. The NIH says when a woman grows coarse rather than fine hairs on her chin, lips chest, stomach or back it is referred to as hirsutism. This condition occurs when the body produces an excess amount of testosterone.
On the other hand, hair loss may occur at an accelerated rate in some people. Partial or total loss of hair is a condition known as alopecia. The NIH says aging, heredity and testosterone can all contribute to baldness.
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