Extreme heat: what are the consequences for the hair?

A human scalp hair fibre is composed of two main parts: the cuticle and the cortex. The cuticle is a thin protective outer sheath that surrounds the inner cortex. The largest mass of the hair shaft is the cortex, which contributes to the mechanical properties of the hair. The grains of melanin are located within the cortex and are responsible for the colour of the hair. The fibre is primarily composed of keratins, a group of insoluble cystine-containing helical protein complexes that represent 65 to 95% of the weight of the hair. The remaining constituents are water, lipids, pigments, and trace elements. Disulphide bonds are also found between two amino acids, contributing to the elasticity of the hair.

Sunlight exposure on human hair causes photo-degradation. This is particularly evident in the bleaching of the hair fibre due to the oxidation of melanin by free radicals, as well as an alteration of the keratin. The degradation of hair proteins is induced by wavelengths of 254–400 nm. As the cuticle protects the cortex, damage in this area usually occurs after significant damage to the hair cuticle. There is also an alteration of the hydrolipidic film that covers the cuticle, which leads to the opening of the scales and a loss of water.

Shorter UV radiations induce undesirable effects such as an increase in permeability, leading to a loss of shine and colour. UV exposure involves considerable changes in the structure of keratins, including the photo-oxidation of amino acids, sterol and fatty acids, resulting in the breaking of disulfide bonds, the decomposition of lipids, a decrease in melanin as well as numerous micro-molecular lesions.

In general, melanins are capable of providing photochemical protection to hair proteins, particularly at lower wavelengths, by absorbing and filtering incident radiation and then dissipating this energy as heat. This absorption capacity can be explained by the presence of a system of conjugated carbonyl groups and double bonds that capture a large part of the radiation and also immobilise many formed free radicals. UV exposure induces a decrease in melanins which results in a reduction of the photochemical protection of the hair. The hair appears drier and loses its colour.