Hair grows everywhere on the human skin except on the palms of our hands and the soles of our feet, but many hairs are so fine they're virtually invisible.The most oft-quoted average rate of human hair growth is 6 inches per year. Hair is made up of a protein called keratin that is produced in hair follicles in the outer layer of skin. As follicles produce new hair cells, old cells are being pushed out through the surface of the skin at the rate of about six inches a year. The hair you can see is actually a string of dead keratin cells. The average adult head has about 120,000 to 180,000 hairs and loses up to 100 of them a day; finding a few stray hairs on your hairbrush is not necessarily cause for alarm.
At any one time, about 90% of the hair on a person's scalp is growing. Each follicle has its own life cycle that can be influenced by age, disease, and a wide variety of other factors. This life cycle is divided into three phases:
- Growth(Anagen) -- active hair growth that lasts between two to six years.During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years; this period is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
- Cessation(Catagen) -- transitional hair growth that lasts two to three weeks. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.
- Rest(Telogen) -- resting phase that lasts about two to three months; at the end of the resting phase the hair is shed and a new hair replaces it and the growing cycle starts again.This hair begins to fall, causing a noticeable loss of hair. This condition is called telogen effluvium.