SHEDS-HT:
An Integrated Probabilistic
Exposure Model for Prioritizing Exposures
to Chemicals with Near-Field and Dietary Sources
- Publication date
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Abstract
United
States Environmental Protection Agency (USEPA) researchers
are developing a strategy for high-throughput (HT) exposure-based
prioritization of chemicals under the ExpoCast program. These novel
modeling approaches for evaluating chemicals based on their potential
for biologically relevant human exposures will inform toxicity testing
and prioritization for chemical risk assessment. Based on probabilistic
methods and algorithms developed for The Stochastic Human Exposure
and Dose Simulation Model for Multimedia, Multipathway Chemicals (SHEDS-MM),
a new mechanistic modeling approach has been developed to accommodate
high-throughput (HT) assessment of exposure potential. In this SHEDS-HT
model, the residential and dietary modules of SHEDS-MM have been operationally
modified to reduce the user burden, input data demands, and run times
of the higher-tier model, while maintaining critical features and
inputs that influence exposure. The model has been implemented in
R; the modeling framework links chemicals to consumer product categories
or food groups (and thus exposure scenarios) to predict HT exposures
and intake doses. Initially, SHEDS-HT has been applied to 2507 organic
chemicals associated with consumer products and agricultural pesticides.
These evaluations employ data from recent USEPA efforts to characterize
usage (prevalence, frequency, and magnitude), chemical composition,
and exposure scenarios for a wide range of consumer products. In modeling
indirect exposures from near-field sources, SHEDS-HT employs a fugacity-based
module to estimate concentrations in indoor environmental media. The
concentration estimates, along with relevant exposure factors and
human activity data, are then used by the model to rapidly generate
probabilistic population distributions of near-field indirect exposures
via dermal, nondietary ingestion, and inhalation pathways. Pathway-specific
estimates of near-field direct exposures from consumer products are
also modeled. Population dietary exposures for a variety of chemicals
found in foods are combined with the corresponding chemical-specific
near-field exposure predictions to produce aggregate population exposure
estimates. The estimated intake dose rates (mg/kg/day) for the 2507
chemical case-study spanned 13 orders of magnitude. SHEDS-HT successfully
reproduced the pathway-specific exposure results of the higher-tier
SHEDS-MM for a case-study pesticide and produced median intake doses
significantly correlated (<i>p</i> < 0.0001, <i>R</i><sup>2</sup> = 0.39) with medians inferred using biomonitoring
data for 39 chemicals from the National Health and Nutrition Examination
Survey (NHANES). Based on the favorable performance of SHEDS-HT with
respect to these initial evaluations, we believe this new tool will
be useful for HT prediction of chemical exposure potential