# Spreading of Dangerous Skin-Lightening Products as a Result of Colourism: A Review
## 4 Hydroquinone  
Hydroquinone (1,4-dihydroxybenzene; Figure 1) is a water-soluble phenolic compound in the form of colourless or white crystals; it is a ubiquitous molecule and is found in tea, coffee, beer, berries, propolis and some mushrooms. A natural derivative of hydroquinone is \(\alpha\)-arbutin, a glycoside in whose structure the hydroquinone is bound to a D-glucose molecule [38] (Figure 2). Arbutin exists in aqueous solutions in \(\alpha\), \(\beta\), or \(\gamma\)-anomeric form, with \(\beta\)-anomer being the dominant form. Arbutin had skin-lightening properties due to the inactivation of tyrosinase, and possesses antioxidant properties that can contribute to its depigmenting action [39]. Although there is the possibility that a small amount of hydroquinone can be produced by cutaneous microorganisms or UV radiations when arbutin is applied to the skin, this molecule has intrinsic depigmenting properties, that are not dependent on the release of hydroquinone [39]. There are many industrial uses for hydroquinone: it is used in photographic developers as a polymer inhibitor, as an antioxidant for fats and oils, and as an intermediate for rubber processing chemicals. It also serves as a depigmenting agent in concentrations ranging from 2% to 10% [40]. Interest in hydroquinone as a skin-lightening agent arose in the 1960s following the accidental discovery of its skin-whitening effect on black American workers who were exposed to it on a daily basis in the rubber industry [8]. Hydroquinone's use for depigmenting purposes became widespread in the following decades and is currently utilised in managing various hyperpigmentation disorders such as melasma, chloasma, freckles, age spots and post-inflammatory hyperpigmentation caused by acne or trauma [34]. Hydroquinone is present in numerous prescription-only preparations; for example, in the Netherlands, pharmaceutical wholesalers supplied pharmacacies with 90 kg of hydroquinone and it is estimated that 5000-10,000 formulations of 2% hydroquinone and 10,000-15,000 formulations of 5% hydroquinone were prescribed in a single year [40]. Although hydroquinone has been the dermatological gold standard for skin lightening for over 50 years, in recent times regulatory agencies in Japan, Europe and  
Figure 1: Hydroquinone structure. Figure 2: Arbutin structure. the United States have questioned its safety [41]. Considering the risks to human health associated with its application, hydroquinone has been banned from cosme in Europe since 2001 [40] and its use in skin-lightening cosmetic formulations is currently illegal (see the \"Regulations\" section for more details). Cosmetics containing hydroquinone have also been banned in several other countries, such as the United Kingdom, Australia and Japan. Several researchers have focused their attention on the mechanisms of depigmentation produced by hydroquinone. It has been shown that hydroquinone inhibits the activity of the tyrosinase enzyme by blocking the oxidation of tyrosine to DOPA and the subsequent oxidation of DOPA, causing the cessation of melanin synthesis; hydroquinone treatment decreases the number of melanised melanosomes and leads to the formation of abnormally melanised melanosomes, which eventually die [42]. Recently, [PERSON] et al. [43] demonstrated that tyrosinase is not the only cellular target of hydroquinone, since this compound severely affects certain cytoskeletal structures (microtubules, actin filaments) in cultured melanophores of Xenopus laevis in a dose-dependent manner. Depigmentation by hydroquinone is reversible; after its suspension, melanin synthesis resumes [43]. For depigmentation purposes, hydroquinone is generally applied once a day in the form of a 2-10% cream; treatment is normally limited to the face, decollete and back of the hands, but in some cases the compound is used on other parts of the body, reaching up to 5% of the body surface [40].