adventitious fluid and water loss or infection. Dermal toxicity results from local tissue responses through direct contact of a substance with skin or, alternatively, may represent a manifestation of systemic toxicity following ingestion or inhalation. Allergic contact dermatitis induced by nickel is an example of a local tissue response (Centeno, 2000).

Many chemical elements occur in living tissues in such small concentrations that they are referred to as trace elements. Some trace elements are essential for human life because of their role as catalysts in cellular functions involving metabolic or biochemical processes. At present, less than one-third of the 90 naturally occurring elements obtained from the air, water, and food are known to be essential to life. The mineral elements currently considered essential for human health and metabolism include the major ions/anions sodium (Na⁺), calcium (Ca²⁺), chlorine (Cl⁻), magnesium (Mg²⁺), potassium (K⁺), silicon (Si⁴⁺), sulfate (SO₄⁻), and nitrate (NO₃⁻); trace elements such as phosphorus (P), iodine (I), and fluorine (F); and metals/metalloids such as iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), vanadium (V), selenium (Se), cobalt (Co), nickel (Ni), chromium (Cr), tin (Sn), and molybdenum (Mo) (Moynahan, 1979). Some of these species occur predominantly in silicate minerals (e.g., Mn, boron [B]), some in silicates and sulfides (e.g., Zn, Se) or as trace element impurities in phyllosilicate minerals, and some predominantly as sulfides (e.g., Cu, Mo); others (e.g., Fe) are ubiquitous (Combs, 2005). The bioavailability and bioassimilation of these essential mineral elements are dependent upon each of their unique physiochemical properties.

Various definitions have been used to describe trace element concentrations within earth science materials and humans (see Box 2.2). Interestingly, both earth scientists and public health professionals have distinguished total elemental concentrations from bioavailable concentrations. In essence, bioavailable concentration in soils refers to the concentration of an element in solution that can be taken up by plants or microorganisms. Within the human body, the term “bioavailable” refers to the amount of a particular element that can be absorbed by the body and influence human health and welfare. For earth science materials and the human body, bioavailable concentrations are always less than total elemental concentrations.

Many trace elements are metabolic requirements for humans, so too small an amount of these elements will result in deficiency (see Figure 2.5A). Although the biological response is optimal at higher concentra-