Hair dye

A preparation that changes hair color by depositing pigment on or in the hair fiber, or by forming colored molecules within it. Products range from temporary coatings to permanent chemical colorants.

Oxidative hair coloring: A hair-coloring process in which small dye precursors oxidize inside the hair to form larger colored molecules. This chemistry produces most permanent hair dyes by forming color within the fiber.
Hair cuticle: The overlapping outer layer of flattened cells surrounding the hair shaft. Its condition affects how readily dye ingredients enter and leave the fiber.
Permanent hair color: Hair color produced by oxidative reactions that form relatively durable pigment within the hair shaft. It can alter natural color and cover gray hair for many weeks.
Allergic contact dermatitis: An inflammatory skin reaction caused by contact with a substance that triggers delayed hypersensitivity. Some hair-dye ingredients can sensitize skin and provoke this reaction.
Direct dye: A colorant that deposits preformed colored molecules onto or into a material without oxidative coupling. Direct dyes color hair without the precursor reactions used in permanent formulations.
Hair cortex: The main inner region of the hair shaft, containing keratin fibers and most natural pigment. Oxidative dyes form color primarily within this region.
Semi-permanent hair color: Hair color made with direct dyes that gradually fade through washing. It offers color without the same oxidative process as permanent dye.
Para-phenylenediamine: An aromatic amine used in many permanent hair dyes and a well-known contact allergen. It is a frequent cause of allergy associated with oxidative hair coloring.
Hydrogen peroxide: A reactive compound used as an oxidizer and bleaching agent in many hair-coloring products. It lightens natural pigment and helps develop oxidative dye molecules.
Keratin: A family of structural proteins that forms hair, nails, and the outer layer of skin. Hair dyes act on fibers built largely from keratin.

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