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By April Summers
What Is Hair?
Chances are, at some time in your life, you’ve been envious of somebody’s hair—long, silky locks cascading across shoulders in gentle waves.
Or maybe you’ve wanted enough hair to stick up in a messy bun that looks carefree and glamorous at the same time.
If you have short hair, maybe you dream of thick, soft, finger-tempting layers your favorite person can’t resist caressing.
Your hair features your face and tells others a bit about you.
Like it or not, people make snap judgments about your personality, habits, gender, and health based on the length, style, color, and bounce of your hair.
Hair has two distinct structures:
The hair you see protruding from someone’s scalp is its dead cells pushed out by protein cells growing deep within each hair follicle (1).
The follicle is a tunnel from the epidermis (outer layer of skin) that ends with a pocket deep inside the dermis (2).
Hair cells begin their life and die in this pocket, so the part you see sticking out of your head doesn’t hurt when it’s cut.
You have about 100,000 follicles in your scalp.
The baby bulb (root) of the hair is made of protein cells at the base of the follicle that build a shaft.
Nearby capillaries nourish the bulb as cells divide (3).
Adequate circulation is needed to provide plenty of blood and nutrients to the growing cells.
Massaging your scalp promotes circulation and stimulates hair growth (4).
The blood delivers hormones that signal hair growth and structure throughout life.
Hair bulb cells divide every 23 to 72 hours, much faster than any other cell in the body (5).
Inner and outer sheaths surround the follicles to protect and form a growing hair strand.
The shape of your hair follicle determines whether your hair is straight, wavy, or curly (6).
The inner sheath ends below the opening of a sebaceous (oil) gland, and the outer sheath continues all the way to the gland.
Scent and sweat glands are also nearby (7).
Small, fan-shaped muscles called arrector (or erector) pili or pilomotor muscles attach below the base of each hair that contracts when the body surface is chilled (8).
Contraction makes the hair stand up, causing “goosebumps” to appear, and the sebaceous glands secrete oil, or sebum, to condition the hair and skin (9).
Specific cells in the hair bulb, called melanocytes, make the pigment called melanin, determined by your genes (10).
Two types of melanin, eumelanin and pheomelanin, and their amounts create the color of each strand.
Hair has three layers, the inner medulla, middle cortex, and outer cuticle.
Most of the body of a hair shaft is the cortex.
The cuticle tightly wraps around the cortex in overlapping layers.
When hair gets damaged, dry, or frizzy, these layers “catch” on each other and snag other strands, the opposite of a silky, manageable mane where strands glide smoothly across each other.
The medulla consists of a soft, thin core of transparent cells and air spaces and only extends partway down the hair shaft.
Its length helps determine the thickness of your hair (12).
If the medulla doesn’t extend very far or is absent altogether, hair strands are thinner than those with medullae continuing further down the shaft.
Hair Growth Cycle
Scalp hair typically grows about six inches per year.
We lose hair randomly, and at any moment, some of your strands will be in one of four stages of growth and shedding: anagen, catagen, telogen, and exogen.
Most of your hair is currently growing in this active phase.
Baby root cells are dividing continuously.
Each strand stays in this phase for two to seven years until a new hair pushes it out of the follicle (13).
After years of growth, a hair’s lengthening slows during the catagen phase, and the follicle shrinks.
This is a transitional time between the growth and resting phases.
Only about 3% of your locks are in this phase right now, and it lasts about two to three weeks.
A strand in the catagen phase is called a club hair and is ready to go bye-bye.
The outer root sheath attaches to the bulb and makes a little nubbin at the end (14).
If you yank out hair during the catagen phase, you’ll see the nubbin is probably lighter in color than the rest of the strand.
Club hairs are usually full-length strands of hair.
If a hair you see in the shower is shorter than most of your others, it might not be a club hair but a broken strand resulting from poor nutrition, chemicals, heat, or violent styling tactics (15).
Telogen for scalp hair lasts about 100 days.
Six to ten percent of your hair is currently in this phase. Hair follicles are at rest, and club hairs are entirely formed.
They detach from the pocket but don’t easily fall out yet (16).
During this phase, the follicles finally let go of their club hairs, often helped along by washing, brushing, and styling.
You normally lose fifty to 100 strands per day during the exogen phase, and they can end up everywhere and anywhere, even on your pillow (17).
As new baby bulb cells divide and multiply, old hairs fall away during the exogen phase, which can last two to five months.
Your hair’s density decreases as you grow from a baby to a grown-up because your body gets bigger, and there’s more distance between follicles.
With puberty, your body makes more sebum, but then as you age, the production of oil decreases.
And women typically have far less sebum than men at any age (18).
If you have trouble growing your hair past a certain length, it’s probably partly due to a shorter anagen (growing) phase.
It’s a wide range, and you might have a two-year anagen versus another’s seven-year phase.
Damage can also limit the overall length you can expect.
Hair on the rest of your body doesn’t grow very long because eyebrows, eyelashes, armpit, and leg hair all have short active growth phases, between only thirty to forty days (19).
As you age, some of your follicles will stop growing hair altogether, causing thinning and bald or patchy areas.
This is called alopecia (20).
More than 95% of men will experience some amount of male pattern baldness on the front and top of their heads, but 25% of men start noticing a receding hairline before they turn twenty-one (21).
Women usually get to keep their hairline but experience female pattern baldness, including thinning over the whole scalp.
Some women notice a little patchiness at the front, but typically not like men.
Along with a genetic predisposition for hair loss, similar to men, hormones appear to play a part in women’s hair loss since they don’t usually notice thinning until after age forty (22).
Ringworm, or tinea capitis, isn’t caused by a worm but a fungus and creates patches of baldness that aren’t necessarily round.
It makes the scalp dry, itchy, and flaky and mainly occurs in children, not due to aging (23).
If you find yourself tearing your hair out by the roots in clumps, you may have a mental disorder called trichotillomania, an irresistible urge to pull one’s hair.
It might happen more during stress when you find yourself inspecting your roots, twirling, chewing, or eating your hair.
There are medications and therapy that can help (24).
Stress can also cause telogen effluvium.
Shock or distress from an event like childbirth or surgery pushes more hair from anagen into the telogen phase.
About 30% of hair stops growing and goes into the resting phase before falling out in amounts of 300 strands a day instead of 100 (25).
Typically, when the stressful event subsides, hair grows back, staying in anagen phase longer.
Hair loss after having a baby is a form of telogen effluvium called postpartum alopecia (26).
It doesn’t happen to all women and not during pregnancy itself.
You might notice more hair in the drain, on your pillow, or all over your clothes between a month and six months postpartum.
Sometimes, follicles can become infected and blocked by bacteria, a virus, fungus, or yeast.
Folliculitis is inflammation that causes small, red, itchy, tender bumps that may open, ooze, and scab.
Other causes of folliculitis include:
Another condition called Piedra (trichomycosis nodularis) is caused by a fungus infection of the hair shaft and occurs typically with long hair that isn’t washed for long periods.
This creates hard nodules that cling to hair fibers and can cause hair loss (28).
Not everyone regrets losing hair. It’s a normal part of life, and sometimes it’s super sexy.
After hair loss, it may grow curlier or a lighter color.
If losing hair is uncomfortable for you in any way, consult your doctor.
Disclaimer: The information contained within this site is not intended as a substitute for professional medical advice. If you have, expect to have, or suspect you may have any medical condition, you are urged to consult with a health care provider. These statements have not been evaluated by the Food and Drug Administration.
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