The human body is covered with approximately five million hair follicles, each independently cycling through the three phases of the hair-growth cycle. Lips, soles of the feet and palms of the hands are among the limited places on the human body where hair is not found. Unlike many animals whose hair growth patterns are controlled by the change of season and the resulting variation in length of day, human hair growth occurs randomly. Although hair is dead material (mainly the protein keratin), it and the follicles that produce it are part of the body's epidural or skin structure.
Approximately 80 percent to 90 percent of a healthy individual's hair is in the anagen phase, a time when the hair is actively growing. In a New England Journal of Medicine article, "The Biology of Hair Follicles," Dr. Ralf Paus and Dr. George Cotsarelis point out that on average, an individual human scalp hair remains in the anagen phase for about 1000 days or almost three years. However, this phase can last as long as eight years. The amount of time a hair spends in the anagen phase controls how long the hair will grow. Although scalp hairs spend years in anagen before moving on to the next phase, an eybrow follicle only spends months in the anagen phase.
The catagen phase is a short period of transition between the anagen and telogen phases. This phase lasts only one to three weeks. The catagen phase involves a period of major cell death, and only a remnant of the hair follicle remains at the completion of this phase. However, near the end of the catagen stage, movement of the dermal papilla occurs, setting the stage for re-growth.
The solving of mysteries in many detective shows and stories, as well as real-life crimes, often hinge on the forensic analysis of hairs left behind by the perpetrator. These hairs are generally shed during the telogen phase. In fact, humans lose between 50 and 150 scalp hairs each day. During this phase, hairs are only anchored by friction between the club-shaped root and the follicle. According to the FBI Hair, Fiber, Crime and Evidence Report, these hairs shed easily and when examined have undamaged roots. It takes force to remove hair during the anagen phase; these hairs exhibit stretching and may have tissue attached to the root. The telogen phase lasts approximately two or three months, a resting phase before the growth phase returns.