Normal hair growth consists of three phases.
- Anagen: This is when a particular hair grows. The root divides rapidly, as rapidly as any cells in the body, and a hair will stay in this phase of growth for 2 to 6 years depending on the individual. If one person’s growth phase lasts 6 years, they might be able to grow long hair; another person with hair that stays in the growth phase for a mere 2 years will have a shorter maximum length. Most hairs (approximately 85%) are in this phase at any one time.
- Catagen: This phase is very short (14 to 21 days) and it here that hair stops growing. The outer sheath of hair attaches to the root of the hair and begins to weaken at its base. About 5% of hairs are in this intermediate phase.
- Telogen: This phase lasts for about 3 months and the hair cells are completely at rest. Approximately 10% of hairs are in this phase. Hairs called “club hairs” are now ready to be shed at a rate of about 25–100/day. This is normal.
Disruptions in either the anagen phase or telogen phase can cause hair loss. Anagen effluvium occurs with hair shedding during the anagen phase. It can be caused by many things, including drugs (e.g. chemotherapy), infections (e.g. fungal infections like tinea capitis or kerion), radiation, and autoimmune disease (e.g. alopecia areata).
Telogen effluvium is hair shedding that occurs during the telogen phase. This occurs after a major stressor that induces up to 70% of hairs into the telogen phase. As new hair grow they push these hairs in the anagen cycle out of the scalp and excessive hair loss occurs over a period of time, usually about 2 months after the stressor. Many forms of stress can cause this to happen, including surgery, major illness, childbirth, extreme weight loss, or even the loss of a loved one.