Scientific Concepts and Hair Care
Hair and scalp anatomy
This domain tests the science behind services: hair structure, growth, properties, and scalp disorders. The hair shaft has three layers. The cuticle is the outer layer of overlapping scales; when it lies flat, hair looks smooth and reflects light. The cortex is the middle layer and most of the strand; it holds keratin bundles and melanin and gives strength, elasticity, and color. The medulla is the central core and may be absent, especially in fine hair. The follicle contains the bulb, papilla, sebaceous gland, and arrector pili muscle.
Growth and properties
Hair cycles through anagen (active growth, years), catagen (transition, weeks), and telogen (rest and shedding, months). Losing 50–100 hairs daily is normal. Texture, density, porosity, and elasticity are separate: texture is strand diameter; density is hairs per square inch; porosity is the cuticle’s ability to absorb moisture; elasticity is stretch and return. Hair and scalp are acidic (about pH 4.5–5.5). Alkaline products swell and open the cuticle; acidic products close it. Shampoo surfactants lift oil and debris; conditioner lowers pH and smooths the cuticle.
Disorders and referral
Noncontagious conditions include canities (gray hair), trichoptilosis (split ends), fragilitas crinium (brittle hair), monilethrix (beaded hair), and alopecia areata (patchy loss; refer). Contagious conditions include tinea capitis (ringworm) and pediculosis (lice): do not service, refer to a physician, and disinfect. Dandruff (pityriasis) is not contagious, but an inflamed or weeping scalp needs medical referral.
Worked example. A client’s wet hair snaps when stretched and feels rough. The stylist notes low elasticity and high porosity. Reasoning: rough feel points to cuticle damage; snapping when wet confirms poor elasticity. The correct action is to recommend a conditioning/protein treatment, avoid overlapping chemical services, and not diagnose a disease. The service plan changes; the cause is structural, not a scalp infection.
Traps that cost marks. Do not call the medulla essential; it is often absent. Do not confuse the cortex with the cuticle-the cortex gives strength. Do not treat dandruff as contagious or service a client with suspected tinea or lice. Hair is not living tissue; the follicle is. A float test alone is not a definitive porosity diagnosis.