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MODELING EXPOSURES TO PESTICIDES APPROACHES AND MODELING NEEDS
Citation:
Ozkaynak, A H., V Zartarian, J. Xue, E J. Furtaw Jr., AND M L. Rigas. MODELING EXPOSURES TO PESTICIDES APPROACHES AND MODELING NEEDS. Presented at Workshop on Exposure of Children to Pesticides, Berlin, Germany, September 27-29, 2001.
Impact/Purpose:
The primary objective of this research is to produce a documented version of the aggregate SHEDS-Pesticides model for conducting reliable probabilistic population assessments of human exposure and dose to environmental pollutants. SHEDS is being developed to help answer the following questions:
(1) What is the population distribution of exposure for a given cohort for existing scenarios or for proposed exposure reduction scenarios?
(2) What is the intensity, duration, frequency, and timing of exposures from different routes?
(3) What are the most critical media, routes, pathways, and factors contributing to exposures?
(4) What is the uncertainty associated with predictions of exposure for a population?
(5) How do modeled estimates compare to real-world data?
(6) What additional human exposure measurements are needed to reduce uncertainty in population estimates?
Description:
Estimation of exposures of children to pesticides requires careful consideration of sources and concentrations of pesticides that may be present in different environmental media and in foods and beverages consumed by children, as well as the different routes and pathways of exposures specific to daily activities of children of different ages. In recent years a number of (aggregate) exposure models has been developed by various researchers to account for exposures to a single chemical from different routes and pathways. Cumulative exposure models, dealing with aggregate exposures to more than one chemical are, however, still mostly in the developmental stage. The EPA's Office of Research and Development (ORD), National Exposure Research Laboratory (NERL) has developed a probabilistic model (Stochastic Human Exposure and Dose Simulation Model, or SHEDS) that predicts the range and distribution of aggregate personal exposures and doses within a population as well as the uncertainty in the model estimates. The model framework is being developed with an initial case study for the pesticide chlorpyrifos and the population of young children. At the present, the SHEDS model includes the inhalation and dietary ingestion routes in addition to dermal contact and non-dietary ingestion. The model can simulate an individual's exposure up to a year time frame, accounting for multiple pesticide applications in the residential environment, in addition to single day estimates for different post-application time periods. In addition, a user-friendly interface has been developed for the aggregate SHEDS-Pesticides model with both exposure researchers and regulators in mind as potential users. Future versions of the SHEDS model will include more complete characterization of pesticide dose and metabolite concentrations in the body by coupling SHEDS to NERL's Exposure Related Dose Estimating Model (ERDEM). SHEDS and other aggregate or cumulative pesticide exposure models need rigorous evaluation and independent verification against carefully designed field studies. All of the models suffer from limitations of available input information on critical exposure factors for infants and young children, especially dermal and non-dietary transfer efficiencies or coefficients by activity type, location, surface and contact characteristics. In general, models need to demonstrate, by sensitivity analysis, which inputs or parameters are of special concern for future revisions. This information will in turn assist the design of future field exposure and biomonitoring studies that will then generate the critical data necessary for refining the existing pesticide exposure models. In order to develop more robust models with more complete input data, repeated or longitudinal pesticide concentration measurements, time-activity data, and frequency of pesticide usage information in homes, day care centers and schools are also needed. Finally, the form of model outputs that are most useful to regulatory and scientific agencies and the public also needs to be identified.
This work has been funded wholly by the United States Environmental Protection Agency. It has been subjected to Agency review and approved for publication.