Description:
The Stochastic Human Exposure and Dose Simulation model for pesticides (SHEDS-Pesticides) supports the efforts of EPA to better understand human exposures and doses to multimedia, multipathway pollutants. It is a physically-based, probabilistic computer model that predicts, for user-specified cohorts, exposures and doses incurred via eating contaminated foods or drinking water, inhaling contaminated air, touching contaminated surface residues, and ingesting residues from hand- or object-to-mouth activities. To do this, it combines information on pesticide usage, human activity data (from time/activity diary surveys and videography studies), environmental residues and concentrations, exposure and dose factors using 1-stage or 2-stage Monte Carlo probabilistic sampling methods. Under this task SHEDS is being developed to address specific applications of interest to the Agency.
The first generation SHEDS-Pesticides model was a 1-stage Monte Carlo model (variability only) focusing on dermal and non-dietary ingestion routes. The next generation is intended to characterize aggregate human exposure and dose to a variety of environmental pollutants, including various pesticides, metals, and Persistent Bioaccumulant Toxins (PBTs). The model is being developed through case studies. The first focused on children's aggregate exposures to the organophosphate pesticide chlorpyrifos via indoor crack and crevice, lawn, and garden treatments. The second focused on children's aggregate exposures to arsenic and chromium from chromated copper arsenate (CCA)-treated playsets and decks, using a scenario-specific version of SHEDS-Pesticides called SHEDS-Wood. The third case study will focus on children's aggregate exposures to pyrethroid pesticides from indoor crack and crevice, lawn, garden, broadcast, fogger, and pet treatments, and will include algorithms for co-occurrence of a single pesticide and multiple application scenarios in space and time.
While SHEDS-Pesticides has been coded as a source-to-dose model, the concentration-to-exposure module has been the primary focus of development, and relatively simple modules are currently incorporated for source-to-concentration and exposure-to-dose estimation. For example, SHEDS currently uses a lumped pharmacokinetic (PK) model to estimate dose rather than a more complete physically-based pharmacokinetic model to estimate dose. Under this and other tasks, the SHEDS concentration-to-exposure module will be interfaced with more sophisticated source-to-concentration (i.e., fate and transport) models and exposure-to-dose models (e.g., ERDEM).
Model inputs and assumptions have been based on available measurements data, and will continue to be refined as new data become available. In conjunction with other tasks, SHEDS will also be evaluated against data from new field measurement studies (e.g., CTEPP, Jacksonville, CHAMACOS) and also against estimates from other aggregate exposure and dose models. The collection of measurements and model refinement and evaluation will be carried out in an iterative manner: each iteration of SHEDS will use the best available data to identify critical pathways of human exposure and dose and the major uncertainties in those pathways, and the model results will be used to guide future measurement studies. The outcome of this research will be a modeling tool for assisting risk assessors and risk managers in regulatory decision-making when human exposure estimates are required beyond the screening level.
Keywords:
MODELING, MULTI-MEDIA, AGGREGATE EXPOSURE AND DOSE,
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Project Information:
Progress
:The SHEDS-Pesticides model is being developed through case studies. Specific progress on the case studies is detailed below.
Chlorpyrifos Case Study
The first case study used to develop and evaluate SHEDS-Pesticides algorithms focused on children's exposures to the organophosphate pesticide chlorpyrifos. This chemical was selected because of the relative abundance of data compared to other chemicals. A paper on version 1 of SHEDS-Pesticides (1-stage Monte Carlo; dermal and non-dietary ingestion routes; indoor crack and crevice, lawn, and garden scenarios; "micro-activity" dermal approach; cross-sectional time frame) was published in 2000.
Initial SHEDS-Pesticides evaluation using data from a 10-home field study was conducted by EOHSI. Research has also been conducted to further evaluate version 2 of SHEDS-Pesticides (aggregate over routes and pathways for 1 chemical at a time; 2-stage Monte Carlo; inhalation, dietary, dermal, non-dietary ingestion routes; indoor crack and crevice, lawn, and garden scenarios; "macro-activity" dermal approach; longitudinal 1-year time frame) using the NHEXAS/Minnesota Children's Study chlorpyrifos data.
Chromated Copper Arsenate (CCA) Case Study
The second case study, conducted at the request of EPA's Office of Pesticide Protection's (OPP's) Antimicrobials Division (AD), focused on children's exposure and dose to arsenic and chromium from CCA-treated wood on playsets and decks. A scenario-specific version of SHEDS-Pesticides (SHEDS-Wood) was developed in 2002 and applied to conduct a preliminary CCA assessment for children. The SHEDS-Wood version 1 methodology was presented successfully to OPP's Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) Science Advisory Panel (SAP) on August 30, 2002.
In 2003 recommendations from the 2002 SAP (SAP panel members, public, peer review comments) were incorporated into SHEDS-Wood version 2, and the model was applied to assess children's exposures and doses from CCA-treated wood, as part of the Agency's first probabilistic cancer risk assessment. ORD/NERL's SHEDS-Wood preliminary exposure assessment and OPP/AD's preliminary risk assessment (using the SHEDS-Wood results) were presented to the SAP in December 2003, and favorably received. SAP comments were incorporated into SHEDS-Wood version 3, which could be used to assess exposures of CCA alternatives. The final CCA exposure assessment report, incorporating SAP comments, will be completed by December 2004. A work assignment with Battelle will result in a recommended methodology and SAS code for Bayesian methods for addressing model uncertainty that could be applied to future versions of SHEDS-Wood and the other SHEDS models.
Pyrethroids Case Study
The third aggregate case study focuses on children's exposures to the pyrethroid pesticide permethrin. Coding of SHEDS-Pesticides version 3 is underway, expanding version 2 to account for additional scenarios (e.g., pets, fogger) and to include co-occurrence algorithms for various single chemical uses over space and time. A task order with Battelle/Harvard is developing an indoor residential fugacity model for pyrethroids that will be interfaced with SHEDS to enhance the source-to-concentration module. In FY05 SHEDS-Pesticides version 3 will be documented and applied to an aggregate permethrin exposure assessment case study.
Relevance
:This research improves the scientific basis of the risk assessment/risk management process by providing a probabilistic aggregate and cumulative human exposure model that can be applied for a variety of multimedia, multipathway pollutants. SHEDS-Pesticides 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 guide future measurement studies.
The Food Quality Protection Act of 1996 (FQPA) requires the Environmental Protection Agency (EPA) to consider both aggregate and cumulative human exposure, particularly for infants and children, when making pesticide regulatory decisions. The probabilistic modeling approach used by SHEDS-Pesticides can aid the determination of whether a "reasonable certainty of no harm" exists at different percentiles of a population, as well as the uncertainty associated with those percentile estimates. To support FQPA, SHEDS will help to improve quantification of infants and children's aggregate and cumulative exposure and dose to pesticides; to provide a framework for identifying and prioritizing measurement needs under FQPA; and to identify critical pathways of children's exposure and their major uncertainties.
SHEDS-Pesticides meets the needs of numerous NERL customers. The primary clients to date have been OPP's Health Effects Division (for the aggregate residential model development and for the pyrethroids case study) and OPP's Antimicrobials Division (to assist them in their public health risk assessment for CCA-treated wood). OPPT has expressed interest in applying SHEDS for high volume chemicals and consumer products. OSWER has expressed interest in the dermal exposure algorithms in SHEDS. OAQPS has expressed interest in incorporating the SHEDS dermal and dietary algorithms into their TRIM model. Within ORD, this research provides guidance to the NERL measurements program, assists NCEA with risk assessments for populations and pollutants of concern, and fosters collaboration with NHEERL (e.g., integration of SHEDS with exposure-to-dose and dose-to-effects modules as part of the Safe Foods Initiative for pyrethroids). It also provides a product for regional offices to conduct more realistic exposure assessments than currently used screening level methods. SHEDS is an integral part of NERL's collaboration with academia on source-to-dose modeling through the university partnership agreements.
Clients
:HEASD Human Exposure Measurements and Human Exposure Modeling Scientists; ORD (NERL, NCEA, NHEERL); OPP, OPPT; ; OAQPS (TRIM); regional offices, Human Exposure and Dose Scientific Community
Project IDs:
ID Code
:3948
Project type
:OMIS