How hair grows


Hair doesn’t grow in individual strands but rather emerges from
the scalp in groups of one to four (and sometimes even five or six)
strands. Hair follicles are arranged in the skin in naturally occur-
ring groups called follicular units.
At any given time, about 90 percent of a person’s terminal hairs are
actively growing. This growth phase, called anagen, can last for
two to seven years, though the average is about three. Scalp hair
grows at an average rate of about 0.44 millimeters a day, or about
1
⁄2 inch per month.
The 10 percent of scalp hairs not actively growing are in a resting
state called telogen that, in a healthy scalp, lasts about three to five
months. When a hair enters its resting phase, growth stops, 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.
Hairs grow in naturally growing groups of one to four hairs each.
These are called follicular units. The follicular unit has two types of
hair. The terminal hair and a vellus hair or two. These vellus hairs
are smaller, shorter, and finer than the terminal hairs found in the
follicular unit.
When a hair falls out on its own, a small black bulb can often be
seen at the end of the hair. If it’s pulled out while it is in its growth
(anagen) phase, it will produce pain when it is pulled out and a
white, sticky mucus swelling will often be seen at the bottom of the
hair shaft. Most people assume that this is the growth center of the
hair and that pulling it out means the hair won’t grow again.
But cells of the dermal papillae and the growth centers found along
the side of the hair shaft near the sebaceous gland will remain in
the place after the hair is pulled out. These cells will multiply and
cause a new hair to grow at the same location in a month or so. So
pulling out a single hair should not kill that hair.
Humans lose about 100 hairs per day. The presence of a much
larger number of hairs on your comb, in the sink, or in the tub can
be the first sign of excessive hair loss. We become sensitized by
balding in our family history and a thinning appearance on the
scalp. Although we may see hair in the drain often, if we are not
balding or thinning we might think nothing of it.

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