Exposure instead of Dose in the “Exposure” box. This modification is intended to address the importance of measuring and quantifying exposure within the organism, but at least a level of organization away from the target site, for example, a specific tissue, or cell, in an organism or a specific compartment of an ecosystem.

The use of the term internal exposure1 is potentially a major shift for the field in that it can be used both in conceptual and theoretical discussions and in experimental design to characterize the processes associated with exposure biology. The quantitative definition of internal exposure is the same as originally discussed by Lioy (1990) and others, but it was described as an internal dose. As the field moves forward, the internal-exposure values can help to establish coherence in the quantitative units that are used to describe the exposure values associated with different routes of entry to the target (for example, mg/kg/day), whether human or ecologic. Therefore internal exposure links the internal-marker measurements of exposure (for example, blood and urine) directly to traditional external measures of exposure, and these in turn can be linked to a dose that is described for toxicologic sites of action or for clinical analyses.

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FIGURE C-1 Another view of the source-to-outcome continuum for exposure science. Exposure science can be applied at any level of biologic organization: the ecologic level, the community level, or the individual level—and within the individual at the level of external exposure, internal exposure, or target-site.

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1Internal exposure is defined as the contact between an agent or a receptor one level of physical or biologic organization past the external boundary toward the target site.



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