Defining Bioavailability

Biological availability, bioavailability, is a dynamic concept that considers physical, chemical, and biological processes of contaminant exposure and dose. In the eco-toxicology literature, bioavailability has been used nonspecifically, in part because no single definition is recognized. Bioavailability incorporates concepts in environmental chemistry and ecotoxicology integrating contaminant concentration, fate, and an organism's behavior in the environment.

As shown in Figure 1, bioavailability incorporates many complex processes and embedded in these processes are many parameters. Because bioavailability

Bioavailability

Figure 1 Bioavailability is a concept that incorporates many integrated processes. Bioavailability bridges environmental chemistry and ecotoxicology, integrating contaminant concentration, fate, and behavior, as well as the effect of contaminants within the environment.

includes numerous processes, there have been many descriptions and interpretations, each focusing on different aspects. Definitions have ranged from the restrictive, to more integrative, to conceptually splitting each process. The latter approach determining the fate of the bioavailable contaminant may divide the bioavailable definition into two primary concepts: bioaccessibility and toxicological bioavailability. Bioaccessibility incorporates physical-chemical processes in the environment, that is, 'environmental availability', and physiological uptake processes, that is, 'environmental bioavailability'. Toxicological bioavailability considers an organism's internal distribution and metabolic effect processes, also known as 'pharmacological bioavailability' (Figure 2).

Although this differentiation at a functional level is helpful for understanding the complexities of bioavail-ability, an integration of these concepts is also useful. A synthesis of these processes is presented as an integrating definition.

• Bioavailability describes many complex processes, including the mass transfer and uptake of contaminants into organisms, which are determined by substance properties, compartment properties, the biology of organisms, and climatic influences.

• The bioavailable contaminant fraction represents the relevant exposure dose for organisms.

This definition illustrates that bioavailability is a set of processes that incorporate site-specific parameters including space and time. Although a useful ecotoxicolo-gical definition has been provided, other definitions and discipline-specific definitions will continue to abound. Unfortunately, this semantic stumbling block may confound issues and impede the implementation of bioavailability in risk analysis. Therefore, defining the bioavailable processes may afford a better perspective by focusing on a mechanistic-based understanding of bioavailability.

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