Hair (Head)

Head hair is the dense, specialised hair that grows on the scalp. It protects the head from sunlight, cushions minor impacts, helps regulate heat loss, and carries strong social and personal meanings (identity, culture, fashion).

Structure of a hair fibre

The visible hair fibre is made of keratinised protein and typically has three layers:

Cuticle: overlapping scales that protect the shaft; damage here increases roughness and frizz.
Cortex: the main body containing keratin fibrils and pigment granules; it determines strength, elasticity, and colour.
Medulla: a central core that can be discontinuous or absent.

More detail:

Cuticle: made of flattened, overlapping scale cells that form the protective outer layer. Intact cuticles reflect light (shine) and reduce friction; lifting, abrasion or chemical swelling increases porosity and frizz.
Cortex: contains long keratin filaments cross‑linked by disulphide (S–S) bonds and salt/hydrogen bonds; the cortex determines tensile strength, elasticity and holds melanin granules that produce visible colour.
Medulla: often patchy or absent in fine hair; its functional role is limited but it can affect light scattering in thick fibres.

Disulphide bonds between cystine residues in keratin stabilise curl and strength; they can be broken and reformed by chemical (permanent waving, straightening), thermal (flat irons) and oxidative (bleach) processes. Repeated bond disruption increases porosity, weakens the fibre and raises breakage risk.

See also: skin for barrier and UV context.

Follicle and pilosebaceous unit

Each hair grows from a follicle embedded in the scalp skin. Key parts:

Bulb and dermal papilla: growth centre where matrix cells divide and keratinise.
Inner and outer root sheaths: guide and protect the developing fibre.
Arrector pili muscle: contracts to produce “goose bumps.”
Sebaceous gland: releases sebum (triglycerides, wax esters, squalene, free fatty acids) to lubricate hair and support the scalp’s acid mantle.

Quantitative notes:

Typical scalp hair counts vary by natural colour (approximate averages): blond ~150,000; brown ~110,000; black ~100,000; red ~90,000. Individual counts vary widely with genetics and ethnicity.
Scalp growth rate commonly quoted as ~1–1.5 cm per month; measured ranges in studies are approximately 0.6–3.36 cm/month depending on age, sex, ancestry and hair thickness.

Growth cycle (overview)

Scalp hair cycles through phases:

Anagen (active growth): typically lasts years; most scalp hairs are here at any time.
Catagen (transition): a brief remodelling stage.
Telogen (rest): a months‑long pause before shedding.
Exogen: the release of the old fibre; a kenogen gap may occur before a new anagen begins.

Average scalp growth is roughly 1–1.5 cm per month but varies with age, genetics, health, and hormonal state. See: Hair Growth Cycle.

Texture and curl

Straight, wavy, curly, and coily patterns reflect follicle shape, the hair’s cross‑section, and keratin arrangement within the cortex. Tighter curls can experience more dryness and breakage from bends in the fibre; gentle detangling, adequate conditioning, and protective handling reduce damage.

Porosity and texture:

Hair porosity describes how readily the shaft absorbs and releases moisture and products. High porosity (raised or damaged cuticle) soaks up water and dye more quickly but also loses moisture and pigment faster.

Texture (fine/medium/coarse) and cross‑section shape (round → straight; oval/ribbon → wavy/curly) determine styling behaviours, susceptibility to mechanical damage and suitable product choice.

Colour

Natural hair colour arises from eumelanin (brown/black) and pheomelanin (red/yellow) within the cortex. Greying occurs as pigment production declines with age. See: Hair Colour.

Additional Context on Hair Colour

Hair colour is a defining feature of head hair, influenced by the types and ratios of melanin pigments. For detailed information, see Hair Colour.

Black Hair: High eumelanin content, common in equatorial regions.
Brown Hair: Medium eumelanin levels, with variations like chestnut and auburn.
Blond Hair: Low melanin levels, often found in Northern Europeans.
Red Hair: High pheomelanin content, rare globally.
Grey/White Hair: Caused by reduced melanin production, often due to aging or stress.

Care basics

Wash frequency depends on sebum output and style; over‑washing can increase dryness, under‑washing can increase scalp scale and odour.
Common damage sources: heat tools, UV, oxidative dyes/bleach, harsh brushing when wet.

Damage mechanisms & prevention:

Thermal damage: styling irons at high temperatures (often >180°C) can denature keratin and lift cuticle scales; use heat protectants and lower temperatures for fragile or bleached hair.
Oxidative colour and bleaching: hydrogen peroxide and alkaline agents (ammonia/amine substitutes) open the cuticle and remove melanin; this is the single biggest cause of long‑term fibre weakening when repeated without reconstruction treatments.

Additional bleaching notes: salon bleaching and high‑lift processes use measured hydrogen peroxide strengths and persulfate boosters to remove melanin progressively. Bleaching increases porosity and fragility — staged lifts, strand tests and professional toning reduce unwanted brassiness and structural loss. See: Hair Bleaching.

Writing tip: For craft prompts and short examples on depicting bleaching scenes, see the Writing Tips section in Hair Bleaching.

Mechanical damage: wet hair is weaker; aggressive combing or tight traction styles can cause breakage or traction alopecia over time.
UV and environmental: long UV exposure breaks down melanin and keratin, causing fading and brittleness—use UV protection when possible.

Protective practices: regular conditioning, protein–moisture balance treatments, occasional bonding treatments (e.g. thiol‑based bond rebuilders), avoiding excessive heat/bleach, and varying protective styles to reduce continuous traction.

Supportive habits: conditioner after cleansing, heat protection, minimising tight traction, sun protection.

See: Hair Care.

Styling & common hairstyles

For common styles, techniques, protective styling and cultural context see: Hairstyles.

Scalp and common conditions

The scalp consists of layered soft tissues over the skull and hosts dense follicles and sebaceous glands. Frequent issues include flaking (dandruff/seborrhoeic dermatitis), itch, and head lice. See: Scalp.

Hair loss in women (high‑level)

Female pattern hair loss: gradual thinning over the crown with preservation of the frontal hairline in many cases.
Telogen effluvium: diffuse shedding after a physiological or psychological stressor.
Traction alopecia: hair loss from sustained tension (tight styles, headgear).

Learn more: Androgenetic Alopecia, Telogen Effluvium, Traction Alopecia.

Writing Tips

Anchor descriptions in tangible details (weight, movement, texture, scent) and context (style, culture, care choices).
Avoid stereotyping hair types; emphasise individual variety and care needs.

Example

"She gathered her curls into a loose knot; a few springy strands escaped, brushing her cheeks as if with a soft, static kiss."