# Spreading of Dangerous Skin-Lightening Products as a Result of Colourism: A Review
## 3 Toxic Ingredients in Skin Lightening Cosmetics
The following reaction shows the dissociation of calomel into mercuric chloride and elemental mercury, which evaporates into the air:  
\[\text{Hg}_{2}\text{Cl}_{2}\rightarrow\text{HgCl}_{2}+\text{Hg}^{0}\]  
In vapour form, elemental mercury is lipid-soluble and highly diffusible through cell membranes; it is easily absorbed in the lungs, but also through the nose by the olfactory pathway [24], enters the bloodstream and rapidly spreads throughout the body, crossing the blood-brain barrier and accumulating in the central nervous system. Elemental mercury is also oxidised to mercuric form in tissue cells [24]. This reaction shows how the prolonged use of creams containing mercury salts can spread elemental mercury within the household via contaminated items (clothes, towels) or surfaces. [PERSON] et al. [27] reported several cases of widespread household contamination in Mexico and California. Mercury vapour levels detected in bedrooms ranged from undetectable to 8 ug/m\({}^{3}\); these cosme may therefore contaminate people living in the same house, notably children and the elderly. It is worth mentioning that, according to health-based risk levels for elemental mercury in air set by regulatory agencies (e.g., ATSDR, Agency for Toxic Substances and Disease Register), 1 ug/m\({}^{3}\) is the indoor air level below which staying in an environment is considered safe and no remediation is required, while 10 ug/m\({}^{3}\) is the level above which evacuation of residents is recommended; at levels ranging from 1 to 10 ug/m\({}^{3}\), evacuation is not required but remediation is recommended [27]. The toxic effects of topically applied mercury-containing products have been documented since the beginning of the 20 th century. Acute or chronic exposure can result in dermal, gastrointestinal, neurological and renal toxicity. Organic and metallic mercury, which is more lipophilic, is more typically associated with neurological damage,while inorganic mercury more often causes kidney damage. Dermatological effects include allergic contact dermatitis, redness, erythroderma, nail discolouration, purpura and paradoxical hyperpigmentation [34]. Gastrointestinal symptoms include a metallic taste in the mouth, gingivostomatitis, nausea and hypersalvation. Neuropyschiatric signs and symptoms are the most significant indicators of mercury poisoning due to the use of skin brighteners; the most frequent are tremor, muscle weakness, peripheral neuropathy, depression, psychosis, anxiety, dizziness, headache and vision loss [35]. Regarding renal damage, acute exposure to mercury usually causes tubular injury (acute tubular necrosis), while chronic exposure is more frequently responsible for glomerular injury [34]. A common clinical sign in exposed infants and children is hypertension [27]. Since mercury crosses the placenta, the use of skin lighteners containing mercury during pregnancy may lead to pre- and post-natal intoxication, leading to serious harmful effects on babies [35; 36]. Several studies have evaluated mercury levels in popular skin-whitening products, demonstrating that these cosme are an important exposure pathway to Hg and a threat to human health. For example, [PERSON] et al. [37] analysed 16 locally produced Mexican skin-whitening creams using cold vapour atomic absorption spectrometry (CV-AAS). Mercury was detected in six of the 16 samples, in amounts ranging from 878 to 36,000 ppm. In a 2014 investigation, [PERSON] et al. [29] investigated 549 US skin-lightening cosme, both online and in shops, using X-ray fluorescence spectrometry. Of the products tested, 6% contained mercury above 1000 ppm and 45% contained mercury above 10,000 ppm. [PERSON] et al. [27] reported that mercury levels of artisanal lightening creams from Mexico containing calomel ranged from 28,000 to 210,000 ppm. More recently, [PERSON] et al. [22] used X-ray fluorescence and cold vapour atomic absorption spectroscopy to assess mercury concentrations in 60 popular lightening cosme on the Jamaican market.