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Appendix C Concepts and Terminology in Exposure Science Exposure science and allied public-health disciplines have been chal- lenged for more than 2 decades by inconsistent definitions and applications of the terms exposure and dose. From a strictly observational standpoint, three de- scriptors characterize the contact between a stressor and a receptor: amount (for example, concentration, mass, and energy), duration (for example, exposure period, duration, and frequency), and location within the system (for example, inhaled air, skin, or target tissue). In addition, some description of how endoge- nous factors may affect dose is important (for example, inhalation rate is af- fected by level of physical activity). The terms external exposure and internal and target-site exposure are ele- ments of the source-to-outcome continuum (see Figure C-1) used in the envi- ronmental-health field. The core concept is that the different levels of biologic or environmental organization are separated by barriers to transport or transport processes that need to be accounted for in understanding or describing the rela- tionship between exposure measures on each side of a boundary or level of or- ganization. From Figure C-1 it is clear that the closer measures of exposure are to the target site for the outcome being examined, the greater is the utility of the data for assessing effects of specific stressors. Conversely, the closer measures of exposure are to environmental concentrations the greater the utility of the infor- mation for source emission assessment and control (Figure C-1). The choice of exposure measure is based on the goal of the study or intended use of the data but should always be selected according to what information best minimizes confounders and supports the study's goal or hypothesis. Exposures at any level can be related conceptually and mathematically to exposures at any other level or to dose. On the basis of the above discussion, Figure C-2 (modified from Fig- ure 1-2) provides a framework that more directly characterizes the theoretical and data-collection efforts of the field of exposure science. To accommodate recent advances in biologic monitoring, Figure C-2 contains the term Internal 193
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194 Exposure Science in the 21st Century: A Vision and A Strategy 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 ex- ample, 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 biol- ogy. 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 co- herence 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. 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. 1 Internal 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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Appendix C 195 FIGURE C-2 Core elements of exposure science. This figure is modified from Figure 1- 2, with Dose being replaced with Internal Exposure. The term environmental intensity is used, because some stressors, such as temperature excesses, cannot be easily measured as concentrations. REFERENCE Lioy, P.J. 1990. Assessing total human exposure to contaminants: A multidisciplinary approach. Environ. Sci. Technol. 24(7):938-945.
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