Human Exposure Modeling (HEM) - General

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, 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.