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KEY POINTS

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  • Persons at either end of the life span, young children or elderly people, are more susceptible to toxicity from exposure to a particular level of metal than most adults.

  • Metals that provoke immune reactions include mercury, gold, platinum, beryllium, chromium, and nickel.

  • Complexation is the formation of a metal ion complex in which the metal ion is associated with a charged or uncharged electron donor, referred to as a ligand.

  • Chelation occurs when bidentate ligands form ring structures that include the metal ion and the two ligand atoms attached to the metal.

  • Metal–protein interactions include binding to numerous enzymes, the metallothioneins, nonspecific binding to proteins such as serum albumin or hemoglobin, and specific metal carrier proteins involved in the membrane transport of metals.

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What Is a Metal?

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Metals are typically defined by physical properties of the element in the solid state. General metal properties include high reflectivity (luster), high electrical conductivity, high thermal conductivity, and mechanical ductility and strength. A toxicologically important characteristic of metals is that they may react in biological systems by losing one or more electrons to form cations.

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Many metals have been reported to produce significant toxicity in humans. These include major toxic metals (e.g., lead and cadmium), essential metals (e.g., zinc and copper), medicinal metals (e.g., platinum and bismuth), minor toxic metals including metals in emerging technology (e.g., indium and uranium), toxic metalloids (e.g., arsenic and antimony), and certain nonmetallic elemental toxicants (e.g., selenium and fluoride). An overview of metal toxicology is shown in Figure 23–1.

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Metals as Toxicants

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Metals are unique among pollutant toxicants in that they are all naturally occurring and are already ubiquitous to some level within the human environment. Regardless of how safely metals are used in industrial processes or consumer products, some level of human exposure is inevitable. Metals differ from other toxic substances because, as elements, they are neither created nor destroyed by human endeavors. Human use of metals has influenced environmental levels of metals in air, water, soil, and food. Human use of metals can also alter the chemical form or speciation of an element and thereby impact toxic potential. As elemental species, metals are nonbiodegradable. This indestructibility combined with bioaccumulation contributes to the high concern for metals as toxicants.

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Movement of Metals in the Environment

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Metals are redistributed naturally in the environment by both geological and biological cycles. Rainwater dissolves rocks and ores and transports materials, including metals, to rivers and underground water (e.g., arsenic), depositing and stripping materials from adjacent soil and eventually transporting these substances to the ocean to be precipitated as sediment or taken up into forming rainwater to be relocated elsewhere. Biological cycles moving metals include ...

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