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USEPA SHEDS MODEL: METHODOLOGY FOR EXPOSURE ASSESSMENT FOR WOOD PRESERVATIVES
Ozkaynak, A H., V Zartarian, J Xue, AND W. Dang. USEPA SHEDS MODEL: METHODOLOGY FOR EXPOSURE ASSESSMENT FOR WOOD PRESERVATIVES. Presented at Annual Conference on Soils, Sediments and Water, Amherst, MA, October 20-23, 2003.
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?
A physically-based, Monte Carlo probabilistic model (SHEDS-Wood: Stochastic Human Exposure and Dose Simulation model for wood preservatives) has been applied to assess the exposure and dose of children to arsenic (As) and chromium (Cr) from contact with chromated copper arsenate (CCA)-treated playsets and residential decks. Short-term (for As and Cr), intermediate-term (for As and Cr), and lifetime (for As only) absorbed doses are estimated for: dermal contact with playset or deck residues; dermal contact with soil concentrations around treated playsets or decks; ingestion of CCA-containing soil near treated playsets or decks; and ingestion of wood residues via the hand-to-mouth pathway.
SHEDS-Wood calculates the predicted exposure and dose to As and Cr using age and gender representative time-location-activity diaries from EPA's Consolidated Human Activity Database (CHAD). Based on user-specified inputs, exposure days and exposure events within a day are simulated. The time series of exposure and dose are computed using pathway-specific exposure equations and the real-time diary activities for which a contact event is possible. Model inputs, represented as analytical distributions (e.g., lognormal, beta) include: fraction of outdoor time and days per year a child plays on/around playsets and decks; As and Cr residue and soil concentrations on or near CCA treated playsets or decks; and various exposure factors such as residue-to-skin transfer efficiencies, soil-to-skin adherence factor, saliva and bathing removal efficiency, daily incidental soil ingestion rate, fraction of hand skin surface area contacting soil, frequency of hand to mouth activity, maximum dermal loading, and dermal and GI absorption rates. Model results for the simulated population are analyzed to determine the dominant pathways influencing predicted exposures and dose. Both deterministic and statistical methods are used to assess the sensitivity of results to key input variables. Uncertainty analyses are performed using the 2nd Stage Monte-Carlo simulation results and a nonparametric bootstrap methodology.
This work has been funded wholly by the United States Environmental Protection Agency.
It has been subjected to Agency review and approved for publication.