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Model Development - Exposure Models
Abstract:
Humans are exposed to mixtures of chemicals from multiple pathways and routes. These exposures may result from a single event or may accumulate over time if multiple exposure events occur. The traditional approach of focusing on a single chemical and a single route of exposure may not provide a realistic description of real-world exposures to mixtures of chemicals and the resulting risks. Risk assessments within EPA are now evolving toward “cumulative assessments” as mandated by legislation like the Food Quality Protection Act (FQPA) and the Safe Drinking Water Act (SDWA). However, there are considerable uncertainties associated with assessing aggregate (single-chemical, multi-route/pathway) and cumulative (multi-chemical, multi-route/pathway) risks. Implementation of FQPA necessitated developing new methodologies to assess residential exposures as well as refined dietary estimates. In addition, probabilistic models have been recommended by the National Academy of Sciences and the EPA’s Council for Regulatory Environmental Models because they allow quantification at different percentiles of a population of interest, as well as the uncertainty associated with those percentiles.
In response to these needs, EPA’s Office of Research and Development (ORD), National Exposure Research Laboratory (NERL) has developed the Stochastic Human Exposure and Dose Simulation model for multimedia, multipathway pollutants (SHEDS-Multimedia). SHEDS-Multimedia is the EPA’s state-of-the-science probabilistic model for simulating human exposures to a variety of multimedia, multipathway environmental chemicals such as pesticides, metals, and persistent bioaccumulative toxins. The purpose of this model is to improve the understanding of aggregate and cumulative exposures over space and time for enhanced human health risk assessments. SHEDS-Multimedia can be used for various purposes, including estimating population distributions of exposure; understanding intensity, duration, frequency, and timing of exposures; identifying critical media, pathways, and factors; considering how to identify and address greatest uncertainties; and comparing modeled estimates against real-world data.
The general approach of SHEDS-Multimedia consists of simulating an individual using sequential time-location-activity data, probabilistically assigning that individual’s contact with environmental concentrations, estimating the individual’s exposure time profile for multiple pathways, and applying Monte Carlo sampling to simulate the population exposures. The model includes a number of unique features such as preserving within-day exposure variability; correlation of input variables; a state-of-the-science longitudinal activity simulation algorithm; innovative sensitivity and uncertainty analysis methods; and enhanced dermal and hand-to-mouth exposure algorithms.
To date, SHEDS-Multimedia has been used in EPA, academia, government, and industry for a variety of regulatory and research purposes. Initial model evaluation has been conducted with model-to-model comparisons and available biomonitoring data. In conjunction with other tasks, SHEDS-Multimedia and its modules (e.g., dietary, fugacity) will be further evaluated against data from field measurement studies (e.g., Jacksonville,* CTEPP) and against estimates from other models. The primary outcome of this research will be a state-of-the-science probabilistic modeling tool for assisting risk assessors and risk managers in regulatory decision-making when human exposure estimates are required beyond the screening level.
*Tulve, NS, Egeghy, PP, Fortmann, RC, Whitaker, DA, Nishioka, MG, Naeher, LP, Hilliard, A. 2008. Multimedia measurements and activity patterns in an observational pilot study of nine young children. Journal of Exposure Science and Environmental Epidemiology. 18(1):31-44.
Objective:
The overall goal of this task is to evaluate, refine, and disseminate a peer-reviewed state-of-the science probabilistic model for improving estimates of human exposure and dose to multimedia, multipathway pollutants. The primary objectives of this research are the following:
- Complete and peer review the SHEDS-Multimedia version 3 (aggregate residential) model (completed)
- Make publicly available the SHEDS-Multimedia version 3 (aggregate residential) model (completed)
- Evaluate and refine the dietary (food and drinking water) exposure module and incorporate it into the SHEDS-Multimedia version 4 model (underway)
- Evaluate and refine other SHEDS-Multimedia components including longitudinal activity pattern simulation, a residential fugacity-based source-to-concentration module, sensitivity analysis capabilities (ongoing)
- Complete, peer review, and make publicly available SHEDS-Multimedia version 4 (cumulative multi-chemical version combining residential and dietary modules) (underway/planned)
- Evaluate SHEDS-Multimedia and its components with available data (ongoing)
- P ublish research on SHEDS-Multimedia model, model components, and applications (ongoing)
Relevance/Significance/Impact:
This task will produce a documented, user-friendly, reliable state-of-the-science aggregate exposure model that can be used within EPA or by external customers to improve estimates of human exposure and provide a better understanding of aggregate exposures and cumulative risks. This research improves the scientific basis of the risk assessment/risk management process by estimating distributions of aggregate exposure for a variety of multimedia, multipathway pollutants. SHEDS-Multimedia is scientifically-grounded in describing the routes, magnitude, variability, and uncertainty of human exposures and doses, and in characterizing the way people interact with their environment. It allows users to identify important exposure factors and significant uncertainties. The results can be used to enhance regulatory exposure assessments, guide future measurement studies, to answer research questions about human exposure and dose to multimedia pollutants, and to improve the Agency’s and the public’s ability to protect the environment and human health.