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The term metal has important meanings in physics and chemistry. In environmental medicine, arsenic and selenium are often considered part of the metals group. Nutritionists often refer to trace metals as those constituting less than 1 g of the human body, an arbitrary limit which would exclude iron. Although “toxic metals” is a common term, all metals may actually exert toxic effects, and the dose and time of exposure determines whether or not toxicity ensues. Frequently, heavy metals (with a gravity of 4 g/cm3 and above) are considered most important with regard to adverse health effects. This belief stems from the observation that the toxicity of the metals tends to increase toward the right and bottom of the periodic table, where the molecular weight of elements increases. However, increased atomic number and increased gravity are of little medical significance and would not account for the toxic potential of beryllium. Instead, the relative toxicity on a molar basis would seem to be related to the affinity to various ligands and the resulting biochemical activity. On the basis of such considerations, metals may be separated into hard metals (class A), with a lower affinity toward sulfur and nitrogen than toward oxygen, and the soft metals (class B), where the opposite is the case.1 Among the metals considered in this chapter, aluminum and beryllium belong to the generally less toxic class A, while the other metals are either borderline or class B metals.
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In contrast to organic compounds, which may be broken down by detoxification processes, metals will remain metals. However, some changes may occur due to oxidation/reduction, as with mercury vapor and chromate, and most metals will be bound to organic compounds, notably proteins such as metallothionein. Some metals form rather stable organometal compounds with a covalent bond between carbon and the metal. Some organic compounds, such as tetraethyl lead and tributyltin, are dealkylated in the body. On the other hand, methylation in the liver is an important part of arsenic and selenium kinetics. These metabolic processes affect the toxicity and may vary between individuals.
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When present as airborne particles, retention in the airways is governed by physical principles related to the aerodynamic diameter of the particles. Some metal compounds are corrosive and exert their effect on the mucous membranes. Such is the case with osmium tetroxide and zinc chloride. In other situations, systemic effects, whether mediated by oral or respiratory intake, are most important and will then depend on the amount absorbed. Solubility of metal compounds is of major significance and, in the gut, some interaction between metals may occur. Thus, zinc and copper tend to mutually inhibit the absorption of the other metal. The same appears to be true for iron and cobalt, but the absorption of several divalent metals is increased in iron deficiency. In addition, phosphate and other components may decrease the absorption, due to formation of insoluble compounds. The variability ...