The cuticle of this hair has been significantly damaged by repeated and excessive perming

waving, by its nature, disrupts the structure of the hair: indeed, it has to do so for the perm to be successful. In order to change the shape of the hair, permanent waving agents first break the disulphide bonds that give the hair shaft its structure. The hair is then put into its new shape and 'neutralised'. Neutralisation is the name given to the re-forming of the chemical bonds in their new positions, a process that fixes the hair permanently into its new shape. The secrets of satisfactory perming lie in the manufacturer's formulation of the product and the stylist's expertise in applying the neutralising lotion after just the right length of time, so that the perm is fixed but the hair is damaged as little as possible. Permed hair should always look beautiful in spite of this deliberate 'damage'. (We shall discuss perms in more detail in the next chapter.)
    Bleaching and dyeing change hair structure too, because the dyes and the bleaches used have to penetrate the cuticle and get into the cortex where they have their effect. Some degree of chemical damage is unavoidable.
    Cosmetic procedures do not damage the hair follicle within the scalp, and so do not cause hair loss. Only a serious chemical burn to the skin of the scalp that destroys the follicle cells can do so. Burns like this can follow indiscriminate over-use of permanent waving or relaxing solutions, and therefore these solutions must be handled carefully at all times.

Damage from the sun

The ultraviolet light in direct sunlight affects the cuticle in a similar way to a bleach, and eventually the keratin protein of the hair breaks down. The result is than the hair is gradually weakened and becomes drier. The effect shows up as light streaks in the hair (sun bleaching). The reason is that sunlight breaks up some of the chemical links within the amino acid groups, in particular those between carbon atoms and sulphur atoms. It does not affect disulphide linkages or hydrogen bonds.

Mechanical damage

Though hair is so robust, it can still be damaged by over-enthusiastic brushing and combing, especially when it is wet and if there is some degree of tangling. Metal combs are particularly hard on the hair. Backbrushing and backcombing are extremely harmful, since they tug against the scales of the cuticle, which all lie pointing towards the tip of the shaft like tiles

Two views of damage to cuticles, both due to backcombing