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Human Exposure Modeling - General Information
Human exposures to pollutants can result from contact with contaminated air, water, soils, and food, as well as with drugs and
consumer products. Exposures may be dominated by contact with a single medium, or concurrent contacts with multiple media may be
significant. The nature and extent of such exposures depend largely on two things: (1) human factors and (2) the concentrations of
a pollutant in the exposure media. Human factors include all behavioral, sociological, and physiological characteristics of an
individual or cohort (i.e., a group of people within a population that can be aggregated because the variation in exposure within
the group is much less than the group-to-group variation across the population) that directly or indirectly affect a person's
contact with the substance of concern. Important behavioral factors are contact rates with air, water, food, and soils. Activity
patterns, which are defined by an individual's or cohort's allocation of time spent in different activities at various locations,
are also significant because they directly affect the magnitude of exposures to substances present in different indoor and outdoor
environments. Information on activity patterns are taken from measured data collected during field and telephone surveys of
individuals' daily activities, the amount of time spent engaged in those activities, and the locations where the activities occur.
Exposure is defined as the contact between a target organism and a pollutant at the outer boundary of the organism. Exposure may be
quantified as the amount of the pollutant available at the boundary of the receptor organism per specified time period. From an
exposure modeling standpoint, the principal goal is to estimate exposure as a function of both the relevant human factors and the
measured or estimated pollutant concentrations in the contact or exposure media.
Human exposure modeling relates pollutant concentrations in the larger environmental media to pollutant concentrations in the
immediate exposure media with which a human population has direct contact. Most human exposure models simulate the movement of
either individuals or cohorts according to activity patterns through locations (called microenvironments) in a defined physical
or political region (i.e., exposure districts). The movement of individuals or cohorts coincides with pollutants at varying
concentrations. This creates the potential for contact between individuals or cohorts and pollutants, thus allowing the estimation
of exposures of various individuals or cohorts within the population to the pollutants of interest.
The models currently being used by OAQPS for estimating human exposure to criteria air pollutants and hazardous air pollutants
(e.g, APEX/TRIM.Expoinhalation, HAPEM4, HEM, and pNEM) do not include multimedia exposures. OAQPS is presently in the process of
developing the capability to model multimedia exposures as part of the overall Total Risk Integrated Methodology (TRIM) framework.
Near-term efforts include work to develop the TRIM.Expoingestion module. For further details about TRIM, go to TRIM.
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