PRINCIPLES OF TOXICOLOGY

14 Properties and Effects of Metals
PROPERTIES AND EFFECTS OF METALS
STEVEN G. DONKIN, DANNY L. OHLSON, and CHRISTOPHER M. TEAF
Metals are extensively used in commercial and industrial applications and, as a result, exposure can
occur from direct and indirect pathways. These exposures may be associated with such processes as
smelting, welding, grinding, soldering, printing, and many other product manufacturing operations.
This chapter discusses a number of fundamental characteristics and health effects of metals, including
• Classification of metals
• Chemical and physical properties of metals
• Absorption, distribution, metabolism, and excretion of metals
• Mechanisms of metal-induced toxicity
• Toxicologic information on selected representative metals
14.1 CLASSIFICATION OF METALS
Metals are elements which are naturally occurring, ubiquitous, and resistant to natural degradation.
The study of metal toxicity must take into consideration several characteristics unique to this group of
toxicants. While all metals are toxic at some level of exposure, many metals are essential nutrients
required at some minimum intake level for good health. Therefore, the distinction must be made
between necessary minimal exposure and toxic overexposure. Because life has evolved in the constant
presence of metals, most organisms, including humans, have various built-in mechanisms for coping
with potentially harmful levels of both essential and nonessential metals. It is when the frequency,
magnitude, or duration of exposure exceeds the capacity of these detoxifying mechanisms that metal
toxicity may become a concern.
Several general physical and chemical properties set metals apart from other elements. Among these
are strength, malleability, reflectivity, high electrical and thermal conductivity, and weakly held valence
electrons resulting in a tendency to ionize in solution. Some of these properties are of interest from a
toxicologic standpoint because they affect the absorption, distribution, metabolism, and resulting
biological effects of metals. In addition, the fact that metals are elements and do not degrade in the
environment means that they have a very high persistence, resulting in a greater potential for exposure
than other, less persistent, toxic chemicals. In addition to their uncomplexed or elemental state, metals
may exist in the environment as complexes with other substances. These complexes may differ
dramatically in their chemical and toxicological properties (e.g., elemental mercury vs. methyl
mercury).
While humans possess some fairly effective means for detoxifying and excreting metals at exposure
levels normally encountered in the environment or the workplace, exposure in some occupational
settings where metals are routinely used (e.g., smelting, plating) may be substantially higher. These
situations require a heightened level of protection usually attained by work practices, protective
equipment, or technological innovation. In addition, the increased mobilization of some metals within
Principles of Toxicology: Environmental and Industrial Applications, Second Edition, Edited by Phillip L. Williams,
Robert C. James, and Stephen M. Roberts.
ISBN 0-471-29321-0 © 2000 John Wiley & Sons, Inc.
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