Modeling aggregate and cumulative chemical exposures from near and far field sources
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Aggregate and cumulative chemical exposures (individuals’ total exposure to a single chemical and multiple chemicals, respectively) are metrics used by decision makers to evaluate chemical risks. While, biomonitoring studies can characterize these metrics for certain chemicals, most chemicals will require the use of simulation models. The Human Exposure Model (HEM) is an ongoing project to characterize aggregate and cumulative exposures to a wide range of chemicals. The current focus of HEM is near-field exposure sources (consumer products, indoor air, diet, and the workplace) since these sources have been shown to provide the majority of exposure for chemicals in commerce, but when completed HEM will account for far-field sources of exposure (water and ambient air). After the user defines a population of interest, the characteristics of each simulated individual (demographics, physiology, housing, location, and family structure) are randomly selected by HEM to reflect variation in the modeled population. Based on these characteristics, HEM determines the exposures and resulting doses received from each near- and far-field source for each chemical on each day of a simulated year. The source-specific doses are summed to give the individual’s aggregate and cumulative doses. The results are distributions of aggregate and cumulative doses that reflect differences among individuals as well as random variation. Data are taken from various public sources: individuals’ characteristics from national surveys; composition of consumer products from EPA’s Chemicals and Products Database; and physical-chemical properties from EPA’s Chemistry Dashboard. Using these data streams, HEM is currently parameterized for the assessment of exposures from the use of consumer products for over 1000 chemicals. Assessments of other sources can be run if the user provides data on the presence of chemicals in the relevant media. HEM contains default information on activity patterns, breathing rates, rates of water consumption, and dietary intakes, and uses physical-chemical properties-based absorption rates to convert such media concentrations into doses. HEM is currently being used in case studies to evaluate chemicals of concern and to investigate the differences between source-specific and aggregate exposures.