Mercury
Authors: Foulkes Ernest 
Published: August 2005

Mercury is a heavy metal widely distributed in nature, both as the metallic element (Hg0) and as inorganic or organic compounds of its oxidation products, the mercurous (Hg+) and mercuric (Hg2+) ions. These mercury species are readily interchangeable in the environment and in the body, and evidence suggests that their toxicity in many cases reflects the actions of the mercuric ions formed by oxidation of mercury or by breakage of the mercury–carbon bond. To that extent, different toxicities of individual compounds can largely be attributed to differences in their toxicokinetic properties such as rates of absorption, distribution, degradation, or oxidation to Hg2+, and finally excretion. Monosubstituted organic compounds of mercuric mercury may also react directly with biological molecules before degradation to Hg2+.

Although all biological tissues contain traces of mercury, no essential biological function has been identified for the metal. To the contrary, because the end product of the metabolism of mercury and mercurials is usually the mercuric ion, which has high affinity for proteins and other biological molecules, many of the organic and inorganic compounds of the metal strongly inhibit biological reactions in very low concentrations. The high chemical reactivity of Hg2+also helps explain the relatively nonspecific nature of mercury toxicity in the target organs. For instance, mercuric mercury accumulates primarily in the kidney, but in that organ it inhibits a large number of different enzymatic and other functions.

Natural sources of mercury include primarily deposits of the metal itself or of insoluble mercuric sulfide (HgS, cinnabar). Most of the world's production of mercury comes from mines in Algeria, China, Spain, and Kyrgyzstan. The background concentration of mercury in the environment reflects outgassing from the earth's crust and the result of volcanic activity. Large amounts of mercury are also contributed to the environment by human activities. The use of Hg by humans has been traced back thousands of years, and the high morbidity observed in mercury miners was well recognized in Roman days. Among major anthropogenic contributions to mercury pollution are the combustion of fossil fuels, the application of inorganic fertilizers and sewage sludge to agricultural lands, the amalgam process of extraction and purification of noble metals, losses incurred during the extensive use of mercury and its compounds in industry, and leaks from waste disposal. Natural and anthropogenic releases of mercury into the atmosphere in the 1980s were roughly equivalent and reached values of more than 6000 tons per year.

Human exposure to mercury and mercurials also was, and in part still is, associated with their direct application for cosmetic and therapeutic purposes. Even in the twentieth century, such uses have included topical treatment with mercury‐containing skin whiteners, antiseptics, and infants' teething powder. Organic mercurials have also been prescribed routinely as diuretics for treating salt and water retention. These human applications have mostly been abandoned, but the presence of the element in dental amalgams remains a significant and continuing source of human exposure.

The interchangeability of various mercury species in the environment and in the body and the overlap between the toxic effects of individual mercurials and between the biomarkers available for monitoring exposure to these mercurials make it impractical to consider the different mercurials under entirely separate headings. This chapter, therefore, discusses the occupational toxicology of mercury and selected mercury compounds under three main overlapping headings: A. Elemental Mercury, B. Inorganic Mercury Compounds (primarily the chloride salts of mercuric and mercurous mercury, although other inorganic mercury salts are also cited), and C. Organic Mercury Compounds (mostly methylmercuric chloride and phenylmercuric acetate). A very extensive literature has accumulated on all of these topics. However, no attempt will be made here to provide an exhaustive survey; a detailed listing, for instance, of all occupational uses and reported health effects of each chemical species of mercury in every animal species tested and in humans, under all exposure conditions would have to cover thousands of references and is hardly necessary to emphasize the major problems potentially associated with human occupational and general exposure to mercury and its compounds. Additional details on mercury and mercurials may be obtained from a number of informative recent publications, including the EPA document on mercury, a toxicological profile placed into the Federal Register by the Agency for Toxic Substances and Disease Registry (ATSDR), the NIOSH Manual of Analytical Methods, and the authoritative review of Clarkson. The volume edited by Chang on theToxicology of Heavy Metalsprovides an extensive discussion of the toxic properties of mercury. Environmental health criteria for inorganic mercury were discussed by WHO. An earlier volume on the history of mercury contains much fascinating information on this important element.