BOX 2.2

Bioavailability Definitions

Earth Science Definitions

Bioavailability—that fraction of an element or compound in solution that can be taken up by plants or soil micro-organisms.

Biomagnification—the bioaccumulation of a substance up the food chain by transfer of residues of the substance in smaller organisms that are food for larger organisms in the chain.

Geoavailability—that portion of an element or compound’s total content in an earth material that can be liberated to the surficial or near-surface environment (or biosphere) through mechanical, chemical, or biological processes.

Health Science Definitions

Bioaccessible fraction—that fraction of a metal or a metal compound that is soluble in various body fluids (gastrointestinal, respiratory, perspiration, etc.). Solubility is dependent on individual physiology.

Bioavailable fraction—that fraction of a metal or metal compound that is absorbed by the body and transported within the body to a site of toxicological action. Absorption and transport are both dependent on individual physiology.

tions, at still higher concentrations the element may become toxic. Toxicologists express the level at which these phenomena are observed in several different ways. The first is the No Observable Adverse Effect Level (NOAEL) or No Observable Adverse Effect Concentration (NOAEC). At higher concentrations, the biological response is expressed as the Lowest Observable Adverse Effect Level (LOAEL) or Lowest Observable Adverse Effect Concentration (LOAEC). The dose-response curve lacks a deficiency zone for nonessential elements (see Figure 2.5B).

Element toxicity depends on the bioavailability of the element, its distribution in the body, the physical and chemical form of the element, and its storage and excretion parameters. In recent years, considerable interest has been focused on assessing the human health risk posed by metals, metalloids, and trace elements in the environment. It has long been recognized that large areas of the globe contain human populations characterized by having trace element excess, deficiency, or chronic poisoning (e.g., Selinus et al., 2005).

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