Anatomy and Behavior of Hair

Hair is manufactured by tiny organ called the follicle that lies deeply in the skin.  Hair follicles are everywhere on the body except the palms and soles.  The hair shaft itself is covered with tiny shingle-like plates called the cuticle. These overlapping plates protect the hair from chemical and physical damage. If chemicals are used to color or perm the hair, the tiny plates will raise up and make the hair look dull and easily tangle.

The internal portion of the hair is composed of a compound called keratin.  Keratin is the same material that composes finger and toe nails. . If someone has a Biotin deficiency, their keratin is weaker, with resulting breakage of hair and nails.


There are about 100,000 follicles on the human scalp, which equals 180 to 280 follicles/cm2.  That’s an average scalp density of 230 hairs/cm2.  Blondes are born with more hair and are at the upper range, redheads are at the lower end of the range.  As a person gets older and older, the density slowly becomes less and less – he or she has fewer hairs per cm2.  This progressive loss of hair should not be confused with balding, a condition in which the hair diameter gets progressively smaller with eventual disappearance of the follicle and hair shaft.  The total hair “mass” is determined by the length, the density, and the caliber of the hair.


The hair is oval when cut and viewed in cross section. Africans have very oval cross-sections with the larger diameter about twice the size as the smaller diameter. The African hair shaft has diameter irregularities along its length and resembles a twisted oval rod. Caucasian hair is slightly oval.  Asians have perfectly round, straight hair

Asian hair

Caucasian hair

African hair

Hair caliber may be fine, average, or coarse. The caliber of the hair is determined at conception. Some folks have hair that is very fine or ultrafine… like baby hair.  During the hair loss evaluation, your hair diameter will be measured with a digital micrometer.

The average hair is 70 microns in diameter, coarse hair is 80 microns, and fine hair is 60 microns.  Very fine hair is 50 microns. Ultrafine is 40 microns.  The natural caliber of your hair is important — because if very fine, it has a tendency to break.  The hair’s diameter determines its cross sectional area or mass.  Because the cross sectional area is calculated as 3.14 x r2, an 80 micron hair will have twice the mass as a 60 micron hair.  In other words, if two people both have 100,000 hairs on their head, the one with coarse hair (80 microns) will have twice the hair mass as the one with fine hair (60 microns).

Hair texture may be curly, wavy, or straight. A very oval cross section equals very curly hair. Round cross section equals straight hair. Asians who have round hair have typically straight hair. Texture may be changed by chemical alteration as in perms, straighteners, relaxers, etc. When hair is permed or straightened it becomes weakened and repeated chemical treatments may contribute to fragility and surface dullness.  A chemically damaged hair can absorb up to 50% of its weight in water. That’s why processed hair will frizz in humid weather. Abnormalities like irregular length, breakage, split ends are due to chemical and physical damage and can dramatically influence the way your hair performs, the way it looks, and its ability to resist breakage and further chemical assault.

Hair may be blonde, brown, black, or gray and color is determined by ethnicity and age.  Mediterranean and Africans typically have black hair. Scandinavians and Australians have blonde.  Many women color their hair artificially.  The natural gray root will re-appear at the rate of one half inch per month.  Natural blondes have more hairs per square inch of scalp, and redheads have less.

Scalp hair grows at the rate of ½ inch per month. At any given time 90% of hairs are growing and 10% are resting. Hair grows for about 4 years (called the anagen or growing phase) and reaches to about waist length.  It then goes into a resting period (called telogen or resting phase) for 4 months. The hair is then shed, and a new hair of the same diameter emerges from the same follicle. If 100,000 follicles are shedding once every four years, the daily average hair shed is 50-100 hairs per day. If your hair is short, the shedding is less conspicuous.  If a longhaired individual shampoos infrequently, the shedding is more conspicuous when washed.

In some families there is a genetic mutation that results in continuous anagen phase… with hair growing to floor length or longer.   The longest recorded hair length is 16 feet.

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