Hair, as with skin, ages over time. As time progresses, one’s hair whitens, thins, waves, and reduces in lustre. In addition to the natural mechanisms of ageing, hair also ages as a result of damage caused by reactive oxygen species (ROS) due to repeated exposure to ultraviolet light or metallic ions and chlorine present in tap water.
Furthermore, artificial treatments such as hair perming, colouring, and drying can cause further accumulation of damage to human hair. While an essential component of perming and colouring, hydrogen peroxide still remains in the hair after extensive washing and can then react with metal ions to produce ROS – for this reason, chelating agents such as EDTA have been used.
The effects of ROS on skin have been studied extensively, and as a result, raw ingredients meant to protect against ROS are readily available for retail sale. ROS can also damage hair, for example, through the well-understood mechanism of cystine disulfide bond breakage. Upon exposure to ROS, cystine disulfide bonds become irreversibly sulphonated, producing cysteic acids and precluding further cystine bonding. The abundance of cystine bonds has been suggested to be well correlated with the degree of hair strength, and as such, the destruction of these bonds have been thought to reduce hair strength. In addition, ROS are thought to denature proteins in hair and fade hair colouring (Fig 1).
Unlike skin, hair is not composed of live cells and lacks endogenous self-repair mechanisms, and thus, the prevention of damage from ROS is of utmost importance.
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