Terminal and Vellus Hair
Adults have two basic types of hair: terminal hair and vellus hair. Terminal Hair grows on the scalp and varies greatly among different people in color, curliness and texture. Vellus Hair is the short, fine, light-colored hair that grows on most places of the body, including the face, back and armpit.
Prior to any hair loss, the average person has about 100,000 scalp hairs. Each of these hairs grows from a follicle within the skin. These hair follicles are naturally bundled together by collagen fibers into follicular units, each containing from one to four follicles.
Phases of Hair Growth
Hairs are shed daily and replaced by new hairs as part of the normal cycle of human hair growth. At any given time, about 90% of hair follicles are in the Anagen Phase, or growing phase. Follicles may remain in the growing phase for 2 to 6 years, during which terminal hairs on the scalp usually grow about 1/2 inch per month. Next, hair follicles enter the Catagen Phase (transition phase) for 2 to 4 weeks before entering the Telogen Phase (resting phase) for 2 to 4 months. Hair shed during the resting phase — the hair you may see on your brush or in the shower — is naturally replaced by new anagen hair.
Male and Female Pattern Baldness
The majority of both men and women who lose hair beyond what is naturally shed do so because of an inherited condition called Androgenetic Alopecia. Genetic hair loss usually occurs between 20 and 45 years of age, but can appear in younger people and continue thoughout one's lifetime.