Mercury

AbstractMercury exists in three oxidation states: Hg0(metallic), Hg+(mercurous), and Hg2+(mercuric) mercury. In organometallic derivatives, mercuric mercury is covalently bound to one or two carbon atoms, and the organic part of the molecule is often an alkyl group or an alkoxialkyl group. The alkyl compounds are more toxic, because they are more readily absorbed, and especially the methylmercury ion is more slowly dealkylated. In its elemental form, mercury is a dense, silvery‐white, shiny metal, which is liquid at room temperature and boils at 357°C. At 20°C, the vapor pressure of the metal is 0.17 Pa (0.0013 mmHg), and a saturated atmosphere at this temperature contains a mercury concentration of 14 mg Hg/m3, which is more than 100 times the occupational exposure limit. Mercury compounds differ greatly in their solubility, with metallic mercury being very sparingly soluble, and several salts, such as mercuric chloride, being easily soluble in water. Of relevance for its environmental fate, methylmercuric salts are also soluble in water; given the affinity of the methylmercury ion to organic compounds, it bioaccumulates in aquatic and marine species.Natural sources of mercury include primarily deposits of insoluble mercuric sulfide (HgS, cinnabar). Most of the world's production of mercury comes from mines in Algeria, China, Spain, and Kyrgyzstan. Background concentrations of mercury in the environment reflect weathering and outgassing from the earth's crust and the result of volcanic activity. Human activities contribute more mercury to the environment than do natural sources. The use of mercury by humans has been traced back to thousands of years, and the high morbidity observed in mercury miners was recognized already in Roman days. Among major anthropogenic contributions to current mercury pollution are the combustion of coal, the amalgam process of extraction and purification of noble metals, losses incurred during use of mercury and its compounds in industry, and leaks from waste disposal and incineration.Direct human exposure to mercury also occurs from their application for cosmetic and therapeutic purposes. Some mercury compounds are used for topical treatment in skin whiteners, while past uses include antiseptics and infants' teething powder. Mercury is also present in dental amalgams, which remain a significant and continuing source of human exposure to elemental mercury. In addition, in some ethnic groups, metallic mercury is used for magical purposes that may cause substantial exposure to mercury vapor.Although various mercury species in the environment and in the body may be metabolized and therefore represent a continuum of mercury species, exposures at work and from the diet differ in regard to the toxic effects and the applicable exposure biomarkers. Identification of the species is therefore crucial in regard to interventions. This chapter, therefore, discusses the toxicology of mercury and mercury compounds under three main headings: elemental mercury, inorganic mercury compounds, and organic mercury compounds (mainly methylmercury). Due to the extensive literature available on mercury toxicology, this chapter will provide an overview only. Additional detail on mercury and mercurials may be obtained from recent publications, including the Global Mercury Assessment from the UN Environment Programme, the toxicological profile for mercury from the Agency for Toxic Substances and Disease Registry, the report on Toxicological Effects of Methylmercury from the National Research Council, and the public health encyclopedia.