There are two types of hair covering the human body. The first type is a very fine, short hair of a type called vellus. The second type, or Terminal hairs, are the ones that are most noticeable. They are much thicker, and grow up to about 85 cm in length, on average. Although in some cases they can be much longer.
Hairs can start growing while the baby is still in it's mothers womb, and in many cases keep growing for the rest of a persons life, even continuing to grow after death. A hair folicle psses through three distinct phases when producing a hair, and then starts again with a new hair. This gives rise to the hair growth cycle.
The first stage is called "Anagen - the growing phase." This phase starts the moment that hairs start to grow, and typically continues for a period of three to seven years. The exact length of the growing phase is dependant on many factors, such as, the age and the sex of the person, but the main determination of how long the hair grows for is genetics. During this phase, there is very active growth in the hair bulb, causing the hairs to grow at about a centimeter a month. The hair follicle introduces melanin pigmantation into the hair bulb, giving rise to the various hair colours we see. As the follicle gets older, less pigmentation is produced, so the hairs turn grey, or white, even thought the hair itself remains strong and healthy.
The second stage is called "Catagen - the resting phase." This phase follows immediately after the growing phase, and last for 2 to 4 weeks. During this time, the hair follicle stops growing more hair, and producing the melanin pigment. Also during this resting phase the hair follicle moves closer to the surface of the skin ready for the next phase.
Last phase, called "Telogen - the shedding phase." After the hair follicle has rested, a new hair starts growing, and the old hair is either pushed out, or falls out. This is a painless process, and accounts for most of the hair that comes out when the hair is brushed, or washed. This phase last anywhere between three and four months, and accounts for about 10% of bodies hair at any time.
After these telogen hairs have fallen out the hair growth cycle starts again with the anagen phase. A typicle hair follicle can through as many as 20 cycles in a natural lifetime. However, when the hair is plucked early, in either the anagen or catagen phase, then a new hair starts growing immedeately with the anagen phase.
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