9.1 Introduction

Our aging and increasingly ethnically mixed societies continue to spur the usage of hair care products containing hair colorants. While the principal application continues to be coverage of grey hair, another significant factor in this growth is the increasing cosmetification of young adults, males as well as females, in western societies. Both target groups seek to either add colour to their hair (original or different), or to lighten/bleach their hair to generate a preferred lighter colour or permit subsequent addition of colour(s) shades lighter than their natural colour. The hair colour market worldwide is already massive, worth 9 billion euro per year (Source: Euromonitor), with some estimates of up to 50% of the adult Western population (men and women) using hair colorants, many to cover grey.

This chapter focuses on the chemistry of cosmetic colorants and agents that modify hair colour, though changes in hair colour can also be due to iatrogenic biochemical processes i.e. side effects of medication. ^[1] For example, the anti-malarial drug chloroquine and the spasmolytic drug mephenesin can lighten hair colour, though original hair colour returns upon discontinuation of drug treatment. Furthermore, melanin formation ( via melanogenesis) can be inhibited by chemicals such as hydroquinone and its ether derivatives. ^[2] It should be emphasised that alterations in hair colour by these biochemical interventions should not be regarded as cosmetic treatments because of their mechanism of action and potential side effects (allergy, irritation, hair loss).