The normal human scalp contains between 100,000 to 150,000 follicles that produce thick terminal hair. These hairs do not emerge individually from the scalp, but are arranged in small groups of 1 to 4 hairs each, called follicular units. There are approximately 50,000 to 65,000 follicular units on the human scalp. By comparison, the human body has approximately 5 million follicles that produce fine vellus hair.
At any given time, about 90% of terminal hairs on one’s head are actively growing. This phase, called the anagen phase, can last from 2 to 7 years, though the average is about three years. In the catagen phase, which is the shortest phase lasting about 2-3 weeks, growth stops, the middle of the follicle constricts and the lower portion expands to form the “club.” The other remaining 10% of scalp hairs are in a resting state called telogen that, in a normal scalp, lasts about 3 to 4 months.
When a hair enters its resting phase, growth stops, and the bulb detaches from the papilla, and the shaft is either pulled out (as when combing one’s hair) or pushed out when the new shaft starts to grow. When a hair is pulled out or falls out on its own, a small white swelling is found at the bottom of the hair shaft. Most people assume that this is the growth center of the hair, but it is just the clubbed, detached lower end of the hair shaft. The dermal papillae and the growth center of the hair remain in the scalp.
Scalp hair grows at a rate of about 0.44 mm/day (or 1/2 inch per month). Each hair follicle goes through the hair cycle 10-20 times in a lifetime.
Humans normally lose about 100 hairs per day. Everyone has a few hairs stuck to the comb each time they comb their hair. The presence of a large number of hairs on the comb, in the sink, or in the tub, can be a sign of hair loss caused by disease or medications. Common genetic balding, however, is not caused by excessive hair loss, but rather by the successive replacement of hair that is normally lost with smaller, finer hair – a process called “miniaturization.”