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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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