Overview of Hair Loss (Alopecia)

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Alopecia is the medical term for baldness. According to the American Medical Association, 95 percent of cases involve androgenetic alopecia, also known as common baldness or, in men, male pattern baldness. Androgenetic alopecia is an inherited condition that affects about 25 percent of men before the age of 30 and two-thirds of all men before the age of 60. The condition is less common and less extreme in women. It can develop in older adults, resulting in an overall thinning of all the scalp hair rather than complete baldness.

Alopecia areata and androgenetic alopecia are types of nonscarring alopecia, that is temporary hair loss that does not result from damaged hair follicles. Scarring alopecia is permanent hair loss that results from damaged hair follicles. There are other forms of scarring and nonscarring alopecia caused by skin diseases and underlying illness.


Hair is made up of keratin, a protein found in the nails and outer layer of the skin. Each hair is rooted in a hair follicle, a tiny indentation in the outer layer of the skin (epidermis). Hair follicles are made up of cortical, or matrix, cells that proliferate and keratinize into the hair.

Hair grows about one-half inch (10–15 mm) every month and a single hair can last as long as 10 years. There are 3 phases of the hair growth cycle:

  1. Anagen, the active growth phase, lasts about 1000 days. At any given time, 90 percent of a person's hair is in the anagen phase.
  2. Catagen, the transition phase, lasts a few weeks. During catagen, the growth of hair stops abruptly. Only about 1 percent of hair is in catagen at any given time.
  3. Telogen, the resting phase, lasts about 100 days. There is no activity during this final phase and, at the end, the hair sheds.

The average adult loses about 50–100 hairs a day. Only in certain circumstances (e.g., pregnancy, telogen effluvium) are all hairs in the same phase of the growth cycle.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 02 Feb 2001

Last Modified: 17 Sep 2015