# Spreading of Dangerous Skin-Lightening Products as a Result of Colourism: A Review
## 1 Introduction  
Melanins are polymorphous and multifunctional biopolymers that include eumelanin and pheomelanin; their synthesis in the skin, known as melanogenesis, occurs in melanocytes at the dermal-epidermal junction within intracellular organelles called melanosomes. Melanosomes are transferred via dendrites to the cytoplasm of surrounding keratinocytes, where they play a critical role in photoproduction. Melanogenesis is a complex process that comprises both enzymatic and chemical reactions and involves several enzymes: phenylalanine hydroxylase, tyrosinase (a glycosylated polyphenol oxidase containing copper), and tyrosine-related protein-1 (TRP-1) and TRP-2 (or dopamine tautomerase) [1]. Tyrosine catalyses two distinct reactions in melanin biosynthesis: the hydroxylation of L-tyrosine to L-DOPA (L-3,4-dihydroxy-phenylalanine) and then the oxidation of L-DOPA to dopaquinone. Dopaquinone is highly reactive and, in the absence of thiol compounds, undergoes intramolecular cyclization, eventually leading to eumelanin; in the presence of thiols, such as cysteine or glutathione, it is converted to 5-S-cysteineyl DOPA or glutathionely DOPA, eventually leading, through a complex series of reactions, to pheomelanin [1, 2]. The pigments eumelanin (brown/black) and pheomelanin (red/yellow) both protect the skin from UV damage, although pheomelanin can also function as a powerful UV photosensitizer [3]. After UVB exposure, melanogenesis is induced by a variety of factors (cyclobutane pyrimidine dimers, alpha-melanocyte stimulating factor, stem cell factor, nitric oxide, adrenocortic: hormone or ACTH, endothelin-1) through different signaling pathways [1]; DNA repair also stimulates melanin production [4]. Melanin has a photoprotective effect by forming supranuclear caps in the cells of the human epidermis and thus reducing the formation of DNA photoproducts due to exposure to ultraviolet radiation; in addition, this pigment can chelate metal cations and exhibits antioxidant and radical-scavenging properties. Skin lightening is defined as 'the practice of using chemicals or any other product with depigmenting potential in an attempt to lighten skin tone or improve the complexion; these goals are achieved by decreasing the concentration of melanin to achieve a reduction in the physiological pigmentation of the skin' [5]. Products used to achieve this purpose are known as depigmenting, skin-lightening, skin-bleaching, skin-brightening or skin-evening agents [6]. Several skin-lightening compounds have been developed and are now available for pharmaceutical and cosmetic purposes. Therapeutic indications for skin-lightening agents are generally aimed at the management of pigmentation disorders such as discolouration due to hormonal changes, melasma, age spots, senile/solar lentigo, post-inflammatory hyperpigmentation, and pigmented acne scars, as they reduce the hyperpigmentation of specific areas of the body and provide a more uniform skin colour. However, skin lightening is primarily a cosmetic procedure whose function is not only to lighten dark areas of the skin but also to achieve a generally lighter tone, particularly in countries where darker skin tones are prevalent and voluntary depigmentation meets the aesthetic criterion of a lighter skin [7]. Skin-lightening agents can operate through different mechanisms: the inhibition of tyrosinase transcription (e.g., treinoin, retinol), the inhibition of tyrosinase (e.g., hydroquinone, azelaic acid, resveratrol), the acceleration of epidermal turnover (lactic acid, glycolic acid), the inhibition of melanosome transfer from melanocytes to surrounding keratinocytes, anti-inflammatory action and scavenging of free radicals, all of which have tyrosinase inhibition as their common goal [8; 9]. Many effective depigmenting compounds available today can only be used as drugs (e.g., hydroquinone, retinoic acid) and should be restricted to use under dermatological supervision, while others are permitted in cosmetic products (e.g., alpha-hydroxy acids, arbutin, retinol, ascorbic acid).