Picture of the hair

Hair is simple in structure, but it has important functions in social functioning. Hair is made of a tough protein called keratin. A hair follicle anchors each hair into the skin. The hair bulb forms the base of the hair follicle. In the hair bulb, living cells divide and grow to build the hair shaft. Blood vessels nourish the cells in the hair bulb, and deliver hormones that modify hair growth and structure at different times of life.

Hair growth occurs in cycles consisting of three phases.

• Anagen (growth phase). Most hair is growing at any given time. Each hair spends several years in this phase.
• Catagen (transitional phase). Over a few weeks hair growth slows and the hair follicle shrinks.
• Telogen (resting phase). Over months hair growth stops and the old hair detaches from the hair follicle. A new hair begins the growth phase, pushing the old hair out.

Hair grows at different rates in different people; the average rate is around 1.25cm (0.5in) per month. Hair colour is created by pigment cells producing melanin in the hair follicle. With ageing, pigment cells die and hair turns grey.

Hair conditions

• Alopecia areata. Round patches of total hair loss, usually from the scalp. The cause of alopecia is unknown; the hair usually grows back.
• Male pattern baldness. The most common type of hair loss in men. Male pattern baldness usually includes either a receding hairline, hair loss at the crown or both.
• Female pattern baldness. In women, hair loss usually includes uniform thinning across the scalp, with a preserved hairline. The crown may be affected, but hair loss rarely proceeds to baldness as in men.
• Dandruff (seborrhoeic dermatitis). Ongoing mild inflammation of the scalp, resulting in scaly skin that may be itchy and flake off. Seborrheoic dermatitis may also affect the ears and face.
• Tinea capitis (ringworm). A fungal infection of the scalp, creating round patches of hair loss. Although the patches can appear in a ring shape, no worm is involved in tinea capitis.
• Trichotillomania. A mental health disorder that includes the irresistible urge to pull out one's hair. The hair pulling results in patches of noticeable hair loss; its cause is unknown.
• Head lice. Tiny insects that live on the scalp and feed on blood. Preschool and primary school-aged children and adults who live with children are most susceptible to catching head lice, which are only spread through close contact. 
• Telogen effluvium. A month or two after a personal shock (such as surgery, childbirth, severe stress), hair can abruptly fall out in large patches. Typically, new hair starts regrowing right away. Postpartum alopecia - hair loss after delivering a baby - is a form of telogen effluvium and usually resolves without treatment.
• Folliculitis. Inflammation of hair follicles, usually due to an infection. Staphylococcus aureus is a bacteria that frequently causes folliculitis. Acne is a form of folliculitis that is caused by inflammation. This inflammation can sometimes be worsened by the bacteria Propionibacterium acnes.
• Piedra (trichomycosis nodularis). Fungal infection of the hair shaft. Hard nodules made of fungus cling to hair fibres, sometimes causing hair loss.
• Hirsutism. A condition in which women develop male-pattern hair (such as facial hair). An excess of testosterone due to a medical condition is usually responsible.