A hair is a strand of dead protein cells that grows from a small tunnel in your skin called a follicle. It’s made almost entirely of a tough protein called keratin, the same material that forms your fingernails. Despite being technically dead once it emerges from the skin, each strand has a surprisingly complex structure, follows a precise growth cycle, and serves real biological purposes ranging from sun protection to sensory awareness.
What Hair Is Made Of
Between 65% and 95% of a hair strand is fibrous protein, with the rest being small amounts of fat and water. That protein is keratin, arranged in tightly coiled structures that give hair its strength and flexibility. This is why hair can stretch slightly without snapping and why it resists breakdown from water and mild chemicals.
If you could slice a single strand crosswise and look at it under a microscope, you’d see three distinct layers. The outermost layer, the cuticle, is made of flat, overlapping dead cells arranged like shingles on a roof. Each of these scale-like cells is only about 0.5 micrometers thick. When cuticle cells lie flat, hair looks shiny and smooth. When they’re roughed up by heat or chemical damage, hair feels dry and frizzy. Beneath the cuticle sits the cortex, which makes up the bulk of the strand and contains the structural proteins that give hair its strength. At the very center of thicker hairs is the medulla, a loosely packed core that not all hairs have.
How a Follicle Builds a Hair
The follicle is where all the action happens. It originates in the outer layer of your skin and extends down into the deeper layers. At its base sits a bulb-shaped structure containing the dermal papilla, a cluster of cells fed by tiny blood vessels. The dermal papilla signals the surrounding cells to divide, and those cells have one of the highest division rates of any tissue in your body. As new cells form, they push older ones upward, and those older cells harden into keratin and die. By the time the strand pokes through your skin, it’s no longer alive.
As the strand moves upward, it passes through two sleeves. The inner root sheath molds the hair into its final shape. The outer root sheath surrounds the entire structure and anchors it in place. Attached to each follicle is a tiny oil gland that releases sebum, a natural lubricant, into the follicle canal. Sebum coats the emerging hair strand, preventing it from becoming brittle and keeping the surrounding skin moisturized. There’s also a small muscle connected to the follicle. When you’re cold or startled, this muscle contracts and pulls the hair upright, creating goosebumps.
The Three-Phase Growth Cycle
Hair doesn’t grow continuously. Each follicle cycles independently through three phases, which is why you shed some hairs daily while others keep growing.
The first and longest phase is active growth, lasting two to eight years for scalp hair. During this time, cells in the follicle divide rapidly and the strand gets longer at a rate of about 0.35 millimeters per day, roughly half an inch per month, or about six inches per year. The length of this phase is why scalp hair can grow so long, while eyebrow hairs, which only stay in the growth phase for two to three months, stay short even if you never trim them.
Next comes a brief transition phase lasting about two weeks. The follicle shrinks and detaches from the dermal papilla, cutting off the blood supply that fuels growth. The hair strand stops getting longer.
Finally, the follicle enters a resting phase that lasts two to three months. The old hair sits in place, no longer growing, while a new hair begins forming at the base of the follicle. Eventually the new strand pushes the old one out. At any given time, about 9% of your scalp hairs are in this resting phase, which is why losing 50 to 100 hairs a day is completely normal.
Two Types of Hair on Your Body
Not all hair is the same. Your body produces two fundamentally different types. Vellus hair is the fine, pale peach fuzz that covers most of your skin. It’s short and nearly invisible, but it serves a sensory function, helping you feel light touch, air movement, and that goosebump sensation across your skin.
Terminal hair is the thicker, darker, clearly visible hair on your scalp, eyebrows, and eyelashes. During puberty, sex hormones called androgens trigger vellus hairs in certain areas to convert into terminal hairs. This is why coarser, darker hair appears on the armpits, face, chest, and groin during adolescence. The reverse process also occurs: in pattern hair loss, terminal hairs gradually miniaturize back into vellus hairs, which is why thinning hair often looks wispy before it disappears entirely.
What Determines Hair Color
Hair color comes from two types of pigment produced by specialized cells in the follicle. One pigment is dark brown to black, and the other is reddish-brown. The color you see depends on the ratio between them.
Black hair contains the highest concentration of the darker pigment. As that concentration drops, you move through dark brown, brown, and light brown to blonde, each with only trace amounts of the reddish pigment. Red hair is the exception: it contains roughly equal amounts of both pigments, which is why it sits in its own color category. A gene that regulates pigment production controls this ratio, and having one versus two copies of a particular variant creates a dosage effect, meaning your exact shade can depend on how many copies you inherited.
Gray and white hair appear when pigment-producing cells slow down or stop working, typically with age. The hair strand itself is colorless, and without pigment, it looks white or silvery.
Why Humans Have Hair
Scalp hair is primarily a thermoregulatory adaptation. Research confirms that it significantly reduces the amount of solar heat reaching the scalp. Hair acts as a buffer zone, absorbing and reflecting sunlight before it hits the skin. Tightly curled hair is especially effective because it doesn’t lie flat, creating an air gap between the hair surface and the scalp that provides extra insulation.
This protection works in both directions. In cold environments, hair traps a layer of warm air near the skin. In hot environments, it shields the scalp from direct sun exposure and reduces the amount of sweat needed to keep cool. The human scalp, with roughly 100,000 follicles packed into a relatively small area, is one of the most hair-dense regions on the body for exactly this reason.
Beyond temperature control, hair physically protects the skin from UV damage, friction, and minor injuries. Eyelashes shield the eyes from debris, nasal hairs filter particles from inhaled air, and body hair enhances sensory perception by amplifying the signal when something brushes against your skin. Even the fine vellus hairs covering most of your body contribute to this early-warning system, registering touch and air currents before they reach the skin itself.