United States Environmental Protection Agency Office of Research and Development
National Exposure Research Laboratory
Research Abstract
Government Performance Results Act Goal: Safe Food
Significant Research Findings:
A Second-Generation Multimedia, Multipathway Exposure Model
Scientific Problem Accurately quantifying human exposures and doses of various
and Policy Issues populations to environmental pollutants is critical for the U. S.
Environmental Protection Agency (EPA) to assess and manage human
health risks. For example, the Food Quality Protection Act (FQPA) of
1996 requires the EPA to consider aggregate human exposure to a
pesticide, especially for infants and children, when setting regulatory
limits on that pesticide's usage. Pesticides and metals are several
examples of pollutants that can be present in multiple environmental
media (air, water, soil, house dust, food, surface residues) and that can
contact a human via various routes and pathways. These routes and
pathways include dietary ingestion of pesticide residues in food and
drinking water, inhalation of air containing pesticides, dermal (skin)
contact with surfaces containing pesticides (indoors and residential
lawns), and non-dietary ingestion of pesticide residues from hand- or
object-to-mouth activities. Estimation of the population's exposure
from dietary ingestion and inhalation is difficult because of the lack of
information on the patterns of human activities and food consumption,
limited knowledge of the contaminant levels in food, and challenges in
determining breathing rates for different levels of human activity.
Dermal and non-dietary exposure pathways are even more challenging
to quantify because many difficult-to-measure factors (such as the
frequency of contact with contaminated surfaces, the subsequent transfer
of the pesticide residue from those surfaces, and the frequency of putting
fingers and objects into one's mouth) influence an individual's
exposure. These issues result in both variability of exposures to
individuals within a population and uncertainty in exposure estimates.
Aggregate exposure refers to the total exposure of humans to a single
chemical through multiple environmental media and pathways. The
EPA's Office of Research and Development (ORD), National Exposure
Research Laboratory (NERL) has developed a probabilistic model that
predicts the range and distribution of aggregate personal exposures and
doses within a population as well as the uncertainty in the model
estimates. This model is called the Stochastic Human Exposure and
Dose Simulation Model, or SHEDS model.
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Research Approach The primary objective of this research is to develop a computerized
model to conduct assessments of aggregate human exposures and doses
for various populations and multimedia, multipathway pollutants. The
model framework is being developed with an initial case study for the
pesticide chlorpyrifos and the population of young children.
Chlorpyrifos has been widely used both in homes and on food crops,
and is associated with multiple media and pathways of human exposure.
The SHEDS model for pesticides (SHEDS-Pesticides) is being
developed in stages. The first generation SHEDS-Pesticides focused on
children's exposure and absorption on a single day post-pesticide
application via dermal contact with and non-dietary ingestion of surface
residues in and around the home. Distributions of exposure and mass of
3,5,6 -trichloro-2-pyridinol (TCP, a metabolite of chlorpyrifos) in
blood and eliminated in urine were modeled for different residential
uses of chlorpyrifos, different periods of time after its usage, and
different age groups of children that might be exposed. The results were
then combined with a probabilistic, multimedia, multipathway exposure
model and chlorpyrifos assessment developed as part of the National
Human Exposure Assessment Survey (NHEXAS).
The second-generation aggregate SHEDS model for pesticides extends
the first generation model by including the inhalation and dietary
ingestion routes in addition to dermal contact and non-dietary ingestion.
It also allows for characterization of uncertainty as well as variability in
predicted population estimates of exposure and dose. The algorithms
used to estimate exposure via the various routes and pathways have also
been refined. The refined 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.
Results and Initial results of the second generation SHEDS-Pesticides suggest that
Implications for those exposed, dermal contact and non-dietary ingestion routes are
the most important for shorter times after application (e.g., days to
weeks) and that food ingestion and inhalation are the most significant
routes for longer times post-application (e.g., weeks to months). High
variability in modeled estimates primarily reflects differences in
activities among individuals and differences in the concentrations
contacted by individuals during their daily activities. Uncertainty in the
model results stems from the lack of data for critical factors including
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information on pesticide usage, the pattern and frequency of touching
surfaces, hand-to-mouth and object-to-mouth activity patterns, pesticide
residue concentrations at different post-application times, pesticide
residue transfer to the skin surface upon contact, where people spend
their time in relation to where pesticide residues exist in the residential
environment, and factors related to intake and uptake into the human
body after exposure. As more exposure measurements data become
available and iterative sensitivity and uncertainty analyses are
conducted, uncertainties in model results will be reduced. By the same
token, as model development progresses, SHEDS will continue to help
refine the identification of areas of greatest uncertainty that need more
research.
This project directly supports ORD's research to improve the scientific
foundation of human health risk assessment under the Government
Performance and Results Act (GPRA) Goal 8.2, annual performance
measure (APM) 216 ("Develop a second generation aggregate exposure
model (SHEDS) for pesticides that characterizes uncertainty and
variability in model inputs and outputs"). It will help the program
offices improve the risk assessment and risk management processes by
providing more realistic exposure assessment methods than currently
used screening level methods. The SHEDS-Pesticides model will help
users test hypotheses and formulate appropriate designs for exposure
measurement studies.
This work also directly addresses GPRA Goal 3.2.4 (Safe Food,
Research to Support New Regulatory Requirements under FQPA). The
probabilistic modeling approach used by SHEDS-Pesticides can aid in
the determination of whether for a given pesticide "there is a reasonable
certainty that no harm will result from aggregate exposures to the
pesticide's chemical residue from all anticipated dietary sources as
well as all exposures from other sources for which there are reliable
information," as required by FQPA. SHEDS will help to improve the
quantification of infants and children's exposure and dose to pesticides
and provide a framework for identifying and prioritizing measurement
needs under FQPA. The revisions to the model are intended to address
the needs of EPA's Office of Pesticide Programs (OPP) regarding
FQPA. A scientist-to-scientist meeting co-sponsored by ORD NERL
and OPP in October 2001 is being held to compare and contrast the five
aggregate residential pesticide exposure models, including SHEDS, that
are being developed to address OPP's needs to address FQPA. The
results of this workshop should include future directions of both
modeling and measurements research in the area of aggregate human
exposure assessment.
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The second generation multimedia, multipathway SHEDS modeling
project was conducted primarily by a team of NERL staff scientists.
OPP has provided technical input on model development, especially on
the dietary module and planning for the model comparison meeting.
Contractor assistance in writing computer programs was provided by
ManTech Environmental Technology, Inc.
This research has been presented in several international and national
conference presentations, and in the following manuscripts:
Zartarian, V.G., Ozkaynak, H., Burke, J.M., Zufall, M.J., Rigas, M.L., and Furtaw,
E.J..Jr. "A Modeling Framework for Estimating Children's Residential
Exposure and Dose to Chlorpyrifos via Dermal Residue Contact and Non-
Dietary Ingestion." Environmental Health Perspectives, 108(6), (2000).
Buck, R.J., Ozkaynak, H., Xue, J., Zartarian, V.G., Hammerstrom, K. "Modeled
Estimates of Chlorpyrifos Exposure and Dose for the Minnesota and
Arizona NHEXAS Populations," accepted J Expos. Anal. Environ Epid.,
(2001).
Future Research The second-generation multimedia, multipathway exposure model
described here is part of a larger on-going effort to develop models that
improve our understanding of the routes, pathways, and factors that
contribute to pollutant exposure and dose, especially to infants and
children. Work is being conducted to refine the dose aspects of the
model using the dose estimating exposure model being developed in
NERL (Exposure Related Dose Estimating Model, ERDEM). That
research coupled with SHEDS-Pesticides will allow better simulation
of the body's uptake, metabolism, and elimination of pesticides.
Additional exposure pathways such as contact with residues on pets and
the track-in of pesticides into the home on shoes and clothing will also
be included in refinements to SHEDS-Pesticides, and new
measurements data will be used to refine the model inputs and evaluate
the model outputs. After the aggregate SHEDS-Pesticides model has
been evaluated, SHEDS will be extended to address cumulative
exposure and dose to multiple pollutants with similar health endpoints.
Questions and inquiries can be directed to:
Haluk Ozkaynak, Ph.D.
US EPA, Office of Research and Development
National Exposure Research Laboratory
Research Triangle Park, NC 27711
Phone: 919/541-5172
E-mail: ozkaynak.haluk@epa.gov
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