A hair fiber is the visible strand of hair that grows from your scalp (or anywhere else on your body). It’s made almost entirely of a tough protein called keratin, about 90% by weight, with small amounts of lipids, water, and minerals making up the rest. What surprises most people is that the hair fiber itself is not alive. It’s a hardened column of dead cells pushed upward from the living root beneath your skin.
How a Hair Fiber Forms
Every hair fiber begins inside a tiny pocket in your skin called a follicle. At the very bottom of the follicle sits the hair bulb, where new cells are constantly being produced. A small structure called the papilla feeds blood to this area, supplying the nutrients those cells need to divide.
As new cells form, they stick together, harden, and fill with keratin. Because fresh cells keep attaching from below, the older hardened cells get pushed upward and eventually out through the skin’s surface. That emerging column of hardened, keratinized cells is the hair fiber you see and touch. Once it leaves the follicle, no blood supply reaches it, which is why cutting hair doesn’t hurt.
The Three Layers of a Hair Fiber
If you sliced a hair fiber crosswise and looked at it under a microscope, you’d see three distinct layers, each with a different job.
The cuticle is the outermost layer. It’s made of flat, overlapping dead cells arranged like shingles on a roof. This scale-like armor protects everything inside from friction, heat, and chemicals. When these shingles lie flat, hair looks shiny and smooth. When they’re lifted or chipped away by damage, hair feels rough and loses moisture easily.
Beneath the cuticle is the cortex, which makes up the bulk of the fiber’s mass. It contains long, spindle-shaped cells packed with keratin proteins and structural lipids, all running parallel to the length of the strand. The cortex gives hair its strength, elasticity, and color.
At the center is the medulla, a loosely packed, somewhat disordered core. Not every hair has a visible medulla. Fine hairs often lack one entirely, while thicker, coarser hairs typically have a more prominent one. Its exact function isn’t fully understood, but it may play a role in insulation.
What Gives Hair Its Color
Hair color comes from a pigment called melanin, which is produced by specialized cells in the hair bulb and deposited into the hardening keratin cells before they leave the follicle. Two types of melanin do the work. Eumelanin produces brown and black shades, while pheomelanin produces red and yellow tones. The ratio between them, along with the size and density of the pigment granules, determines your exact shade.
Black hair follicles contain the largest and most densely packed pigment granules. Brown hair has somewhat smaller ones. In blonde hair, the granules are so lightly pigmented that often only the structural shell of each granule is visible under a microscope. Red hair is different altogether: its granules have an irregular, clumpy distribution of pheomelanin rather than the smooth, dense fill seen in darker shades. When melanin production slows with age, new hair fibers grow in with less pigment, eventually appearing gray or white.
Size and Shape Vary Widely
A single hair fiber is remarkably thin. Human hair diameters generally range from about 40 to 120 micrometers, though some estimates put the lower end as low as 15 micrometers. For reference, 100 micrometers is roughly the thickness of a standard sheet of paper.
Cross-sectional shape varies too, and this largely determines whether hair appears straight, wavy, or curly. Round cross-sections tend to produce straight hair, while oval or flattened cross-sections create waves and curls. Even among people of the same ethnicity, individual hair fibers can differ noticeably in both diameter and shape.
How Hair Fibers Interact With Moisture
Because keratin is a protein, hair fibers are naturally hygroscopic, meaning they absorb and release moisture from the surrounding air. On a humid day, your hair is literally heavier and swollen with water compared to a dry day. This moisture absorption is one reason curly hair tends to frizz in humidity: water enters the fiber unevenly, causing some sections to swell more than others.
How much moisture a hair fiber absorbs and retains depends on its porosity, which is essentially a measure of how intact the cuticle layer is. Hair with normal porosity can absorb about 75% of its maximum water capacity within just four minutes. Low-porosity hair has tightly sealed cuticle layers that resist water. You’ll notice it takes a long time to get fully wet, products tend to sit on the surface rather than sinking in, and chemical treatments like dye don’t always take as expected.
High-porosity hair is the opposite. The cuticle has been compromised, whether from heat styling, chemical processing, UV exposure, or simply the natural texture of the hair. Naturally curly and coily hair types tend to be more porous than straight hair. High-porosity fibers absorb water quickly but lose it just as fast, which is why this hair type often feels dry despite frequent conditioning. Sealing hydrated hair with oils helps slow that moisture loss by creating a barrier over the damaged cuticle.
Testing Your Hair’s Porosity
Two simple at-home tests can give you a rough idea of your hair’s porosity, though neither is precise. In the float test, you place a clean strand of hair in a glass of room-temperature water and wait. If it sinks quickly, the fiber is porous and absorbing water through gaps in the cuticle. If it floats for a long time, the cuticle is tightly sealed and resisting absorption. In the spray test, you mist clean, dry hair with water and watch whether the droplets bead up on the surface (low porosity) or absorb almost immediately (high porosity).
Understanding porosity helps you choose the right products. Low-porosity hair benefits from lightweight, water-based products and gentle heat to help open the cuticle during conditioning. High-porosity hair responds better to heavier creams, butters, and oils that fill gaps in the cuticle and lock moisture in.