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What Is a Metal?

What defines a “metal” is not always obvious and the differences between metallic and nonmetallic elements may be subtle (Vouk, 1986). Metals are typically defined by physical properties of the element in the solid state, but can vary widely with the metallic element. Current development of nano-sized metal particles revealed additional unrealized behaviors of metals and their interactions with biological systems. General metal properties include high reflectivity (luster), high electrical conductivity, high thermal conductivity, and mechanical ductility and strength. A characteristic of metals of toxicological importance is that they exhibit variable oxidation states and may react in biological systems by losing one or more electrons to form cations (Vouk, 1986). In the periodic table, within a group there is often a gradual transition from nonmetallic to metallic properties going from lighter to heavier atoms (e.g., Group IVa transitions from carbon to lead). Various names are applied to subsets of metallic elements including alkali metals (e.g., lithium and sodium), the alkaline earth metals (e.g., beryllium and magnesium), the transition (or “heavy”) metals (e.g., chromium and nickel), and the metalloids (e.g., arsenic and antimony), the latter of which show characteristics of metals and nonmetals.

Over 75% of the elements in the periodic table are metals and several more are considered metalloids. This chapter discusses metals and certain metal complexes or molecules that have been reported to produce significant toxicity and disease in humans. The discussion includes major toxic metals (e.g., lead and cadmium), essential metals (e.g., zinc and copper), medicinal metals (e.g., platinum and bismuth), and minor toxic metals including metals of technological significance (e.g., indium and uranium). This chapter will also discuss toxic metalloids (e.g., arsenic and antimony) and certain nonmetallic elemental toxicants (e.g., selenium and fluoride). An overview of toxic effects of metals is shown in Fig. 23-1.

Figure 23-1.

Overview of metal toxicology.

Metals as Toxicants

Humankind's use of metals has been critical to the progress and success of human civilization. It is difficult to imagine an advanced civilization without extensive use of metals and metal compounds. Eras of human development are defined by the development of specific metals, such as the many copper alloys of the Bronze Age and advancements in the Iron Age. However, metals are unique among pollutant toxicants in that they are all naturally occurring and, in many cases, are ubiquitous to some level within the human environment. Thus, regardless of how safely metals are used in industrial processes or consumer endpoint products, some level of human exposure is inevitable. Furthermore, life evolved in the presence of metals and organisms have been forced to utilize and manage the potential toxicities of these omnipresent elements. Evolution of complex biological systems required incorporating elemental metals into structural and enzymatic processes, such ...

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