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Model Report

Stochastic Human Exposure and Dose Simulation for Air Toxics

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

SHEDS-AIRTOXICS
Model Extended Name:

Stochastic Human Exposure and Dose Simulation for Air Toxics
Model Overview/Abstract:
The Stochastic Human Exposure and Dose Simulation model for Air Toxics (SHEDS-AirToxics) is a multimedia, multipathway population-based exposure and dose model for air toxics developed by the US EPA's National Exposure Research Laboratory (NERL). SHEDS-AirToxics uses a probabilistic approach to estimate distributions of air toxic exposure and dose for a user-specified population based on air toxic concentrations supplied as input to the model. SHEDS-AirToxics provides estimates of the range in air toxic exposure and dose across a population (variability), and the percent of the simulated population exposed to different levels. The stochastic basis for the model also allows for the uncertainty in any given percentile of the exposure or dose distribution to be estimated.
Keywords:
Model Technical Contact Information:
Dr. Stephen Graham
US EPA ORD
National Exposure Research Laboratory
graham.stephen@epa.gov
(919)-541-4344

Dr. Janet Burke
US EPA ORD
National Exposure Research Laboratory
burke.janet@epa.gov
(919) 541-0820


User Information Back to Top
Technical Requirements
Computer Hardware
A minimum of 128 MB RAM, 300 MHz processor, 800MB free disk space for program and database files, and a CD-ROM drive; however, 512 MB RAM, 1.8 GHz processor, and 1.0GB free disk space is recommended for optimal model performance.
Compatible Operating Systems
Microsoft Windows operating system, (Windows 98, 2000, or XP)
Other Software Required to Run the Model
Graphical user interface (GUI) allows for user specification of the model run scenario and input data, as well as for analysis and display of model outputs.
Download Information
Not currently available for download.
Using the Model
Basic Model Inputs
User must supply hourly ambient outdoor air toxic concentrations for the population of interest (either monitoring data or output from an air quality dispersion model can be used).

Additional input databases provided with the model include: US Census 2000 demographic data that are used to generate individuals that demographically represent the user-specified population; human time-location-activity data from EPA's Consolidated Human Activity Database (CHAD) assigned to each simulated individual to account for the microenvironments people spend time in; US FDA's Total Dietary Study (TDS) food residue data and USDA's Continuing Survey of Food Intakes by Individuals (CSFII) food consumption survey data used for estimating dietary exposure.

Distributions for the parameters of equations used to estimate microenvironmental air toxic concentrations based on the ambient outdoor air toxic concentration (i.e. mass balance equation parameters such as penetration and air exchange rates) are input by the user through the GUI screens.

Smoking prevalence data is required to be input through the GUI if exposure to environmental tobacco smoke is included in the model run scenario. Traffic volume data and HAP usage information (e.g. refueling frequency) are also required for estimating inside vehicle and refueling exposures, respectively.

Basic Model Outputs
Model outputs include of air toxic exposure and dose distributions (daily average and annual average) for the user-specified population. Other output options include individual annual exposure and dose time profiles, summary statistics tables, and contributions of exposure and dose by route (inhalation, dietary ingestion, dermal contact).
User Support
User's Guide Available?
Not currently available.

Model Science Back to Top
Problem Identification
SHEDS-AirToxics explicitly characterizes both the variability and uncertainty in the predicted human exposures and doses resulting from exposures to indoor and outdoor pollutants via the inhalation, ingestion, and dermal exposure routes. The model uses a probabilistic approach to estimate distributions of air toxic exposure and dose for a user-specified population based on air toxic concentrations supplied as input to the model.

SHEDS-AirToxics provides estimates of the range in air toxics exposure and dose across a population (variability), and the percent of the simulated population exposed to different levels. The stochastic basis for the model also allows for the uncertainty in any given percentile of the exposure or dose distribution to be estimated.

Summary of Model Structure and Methods
SHEDS-AirToxics simulates the time series of air toxics exposure and dose for demographically representative individuals through random assignment of human time-location-activity data from CHAD to each simulated individual.

Regarding inhalation exposure and dose, air toxic concentrations in the indoor locations where people spend time (such as home, work, or school) are calculated using equations that depend on the outdoor air toxic concentration, and can also account for an indoor air toxic source (i.e., cigarette smoking). Each simulated individual's time series of exposure is then calculated from the time spent in each location (indoors, outdoors, in a vehicle) and the air toxic concentration in that location. Activity-specific inhalation rates while in each location are also calculated by the model and combined with the exposure concentrations to estimate an intake-dose profile for each individual. Daily-averaged values are calculated from the daily exposure and dose profiles, and then combined to provide distributions for the user-defined population.

The estimation of dietary exposure and dose requires several components and an array of data linkages. Simply put, residue data are combined with diet diaries based on the specific food or food type that are then linked to the simulated individual time-location-activity diary based on age, gender and day-type (weekend and weekday).

Dermal exposure and dose is estimated for two scenarios: bathing with contaminated water and while refueling, the occurrence of which is determined probabilistically.

Two-stage Monte Carlo simulation is used to estimate inter-individual variability in the population and the uncertainty in estimated exposure and dose distributions.

Model Evaluation
At this time, only conceptual, mathematical, and chemical/physical verification has been performed by model developers.


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