The number of hairs on a person’s head varies between 85,000 for redheads and 150,000 in the case of blond people. That equates to between 300 and 900 hairs per square centimetre. An average hair is 0.08mm thick and grows between 0.1mm and 0.2mm every day. Hairs are horny filaments produced by specialised skin cells. They consist primarily of the fibrous protein keratin and are coated in a lipid layer, which gives them elasticity and enables them to repel moisture. They also contain a melanin, the specific type of which gives them their natural colour.

Thick hairs have an innermost layer called the medulla, which is not present in thin hairs. The next layer, the cortex, is covered by the outermost layer, the cuticle. The cuticle is a hard layer of scales, which are formed from dead skin cells and overlap like roof shingles.
The growth of a hair is initiated by its root. The root is embedded in the dermis and supplied with nutrients by a dense network of tiny blood vessels in the dermal papilla. From the specialised skin cells in the hair matrix, it is constantly producing new material which forms the hair shaft. The root is surrounded by two sheaths, an internal sheath and an external sheath. In the upper part of the dermis, the structure known as the follicle is attached to a muscle called the erector pili muscle, and the sebaceous glands secrete sebum which provides the hair’s surface with an additional layer of protection.

Pigment cells called melanocytes in the hair root produce brown/black (eumelanin) or reddish-yellow pigment, which determines the hair’s colour. The amount produced depends on a person’s heredity and decreases as they get older, turning their hair grey.  Natural hair types are also decided by genes. The rounder the cross section of a person’s hair strands, the straighter their hair will be, the more elongated the cross section, the curlier the hair.

Hairs grow in a repeated cycle, which can be broken down into three phases. The growth phase, called the anagen phase, is the longest of the three. During this phase, a root produces a hair shaft that then continues to grow for two to six years. At the end of the anagen phase, the cycle enters a transitional phase known as the catagen phase. Here, the root shrinks, the hair stops growing, and eventually, it falls out. This is followed by a resting phase, the telogen phase, which lasts for three to four months. By the time telogen phase is up, a new hair root will have formed from stem cells, and the cycle starts all over again with the anagen phase.