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Model Report

Stochastic Human Exposure and Dose Simulation Model for Pesticides

Last Revision Date: 07/11/2011 View as PDF
General Information Back to Top
Model Abbreviated Name:

Model Extended Name:

Stochastic Human Exposure and Dose Simulation Model for Pesticides
Model Overview/Abstract:
SHEDS-Pesticides (Stochastic Human Exposure and Dose Simulation Model for Pesticides) is a physically-based stochastic model developed to quantify exposure and dose of humans to multimedia, multipathway pollutants. Probabilistic inputs are combined in physical/mechanistic algorithms to estimate exposure and dose using a two dimensional Monte-Carlo methodology that quantifies variability and uncertainty in model inputs and outputs. To date the model has focused on simulating aggregate exposures (multimedia, single chemical) of children to pesticides (e.g., chlorpyrifos). The aggregate model is currently being expanded to address cumulative exposures (multimedia, multiple chemicals), with the first application focusing on children?s exposures to pyrethroids.
Model Technical Contact Information:
Dr. Valerie Zartarian

Dr. Haluk Ozkaynak

User Information Back to Top
Technical Requirements
Computer Hardware
PC Computer, at least 128 MB RAM, at least 500 MHz, at least 10GB hard drive, CD-ROM drive; faster processor and higher RAM recommended
Compatible Operating Systems
Windows NT, Windows 2000, Windows XP
Other Software Required to Run the Model
User friendly GUI?s
Download Information
Currently not available. The model is presently being refined and will be beta tested prior to its public release.
Using the Model
Basic Model Inputs
CHAD time-location-activity diaries, pesticide usage information (e.g., amount per application and area, application method, time, and frequency), environmental concentrations and residues (air, soil, dust, food, surface residues), exposure factors (e.g., surface-to-skin residue transfer efficiency, frequency of hand-to-mouth contact, fraction skin surface area contacted), CSFII food consumption survey data, pharmacokinetic rate constants
Basic Model Outputs
Output options include individual daily exposure and dose time profiles, population cdfs, summary statistics tables, contributions by route and pathway, sensitivity analyses, and uncertainty analyses for predicted exposure, mass metabolite in blood, and mass metabolite eliminated in urine via the inhalation, dietary ingestion, dermal contact, and non-dietary ingestion routes, and also the absorbed and eliminated dose aggregated across all routes.

Model offers user the options for generating correlations, multiple regressions and uncertainty analysis using multiple stepwise regression analysis techniques to identify the variables and parameters that are most influential and those that contribute to greatest uncertainty in the predicted estimates.

User Support
User's Guide Available?
Currently not available.

Model Science Back to Top
Problem Identification
SHEDS-Pesticides explicitly characterizes both the variability and uncertainty in the predicted human exposures and doses resulting from personal exposures to indoor and outdoor pollutants via the inhalation, ingestion, dermal, and non-dietary ingestion exposure routes.
Summary of Model Structure and Methods
Time-location-activity diaries are sampled to generate a population of simulated individuals. For each sequential location-activity combination, model inputs are sampled from probability distributions for micro-activity data, medium-specific concentrations and residues, exposure factors, and dose factors. These are combined in physically-based equations to simulate individuals' daily exposure and dose time profiles for inhalation, ingestion, dermal, and non-dietary ingestion exposure routes. A simple pharmacokinetic component is currently incorporated to predict metabolite concentrations in blood and urine. The daily dose time profiles are aggregated across all routes and pathways. Two-stage Monte-Carlo simulation is used to estimate inter-individual variability in the population and uncertainty in estimated exposure and dose distributions.

Real-time exposure and dose time profiles based on EPA CHAD diaries are computed for the various routes considered. Inhalation exposure algorithms combine microenvironmental concentrations with inhalation rates derived from energy expenditure or METS values calculated by CHAD for each of the diary entries.

More Information in "Case Studies"

Model Evaluation
Initial model evaluation has been conducted by comparing the modeled urinary metabolite concentrations against available measurement data from field studies (e.g., EOHSI/Rutgers residential chlorpyrifos study; NHEXAS Minnesota Children's Study). Although initial model evaluation has revealed that the model produces reasonable results compared to available measurements data, model evaluation will be ongoing as the model inputs and algorithms are refined and as new human exposure field measurements become available.

Conceptual, mathematical and chemical/physical verification has been performed by model developers. Peer reviewed publications on earlier version of the SHEDS-Pesticides model are available. The aggregate SHEDS-Pesticides chlorpyrifos case study was presented to and favorably reviewed by the ORD University Partnership Agreements peer consultation panel in 2002. Several iterations of sensitivity and uncertainty analyses have been conducted for the aggregate chlorpyrifos case study to identify key inputs and parameters influencing results. A scientific advisory panel specific for further evaluation and refinement of the SHEDS model is being planned.

An Aggregate Residential Exposure Model Comparison Workshop was conducted by ORD and OPP in October 2001 in which SHEDS-Pesticides was compared to other aggregate exposure models being developed (Lifeline, CARES, Calendex). The SHEDS results compared well to those of other models for the case studies considered.

Case Studies
Dermal exposure and dose from surface residues, soil, and dust are computed for each macroactivity indoor and outdoor event in the CHAD diaries by combining surface residues, soil concentrations, or dust concentrations with factors including dermal transfer coefficients, skin surface area contacted, soil/dust-skin adherence, and dermal absorption rates; removal from the skin is accounted for via hand to mouth transfer of residues, hand washing, and bathing using information on hand-to-mouth contact frequencies, hand washing and bathing frequency, and washing removal efficiencies.

Non-dietary ingestion via hand-to-mouth contact and object-to mouth activities are simulated using the CHAD-derived macroactivities and videography data based contact frequencies and saliva removal efficiencies.

The dietary ingestion algorithm uses a similar methodology employed by the US EPA's DEEM model. CSFII individual food consumption diaries are matched with available food residue files and USDA's recipe files to calculate total residue consumption by foods eaten by the US population represented by the CSFII files.

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