Impact/Purpose:

The goal of this research to develop models and computational tools to improve understanding of the functional relationships underlying human exposure to air toxics, and to reduce uncertainty in estimates of exposure to individuals and populations of concern. This goal will be met by addressing a number of key objectives in this task and include:

Model Research and Development: develop a multiroute/pathway/media human exposure and dose model for air toxics.

Model Application: apply model to an urban area (Houston, TX) to estimate human population exposure and dose.

Model Performance Evaluation: compare and evaluate exposure and dose estimates with independent model estimates or personal exposure measurements

Description:

This project aims to improve the scientific basis for the Environmental Protection Agency's (EPA's) assessments of human exposures to air toxics by developing improved human exposure models. The research integrates the major components of the exposure paradigm, i.e., sources, transport, transformation and fate, human exposure and uptake dose, into a human exposure model for air toxics. New databases, models, modeling algorithms, and model performance evaluation techniques are being developed to enhance the current National Air Toxics Assessments (NATA) and mobile source air toxics assessment methodologies used by Office of Research and Development (ORD) laboratories and various EPA program offices. Conducting NATAs is one of the four components in EPA's risk-based National Air Toxics Program, and includes all of the exposure and risk assessment activities. NATA activities are intended to provide EPA and others with improved characterization of air toxics exposures and risks for both stationary and mobile sources, as well as relative risks from indoor air exposures. To improve the scientific basis of exposure and risk assessments conducted under the National Air Toxics Programs, models that better characterize the relationship between concentrations measured at central site monitors and residential, vehicular, and other microenvironmental exposures need to be developed.

The EPA's National Exposure Research Laboratory (NERL) is developing a human exposure and dose model called the Stochastic Human Exposure and Dose Simulation model for Air Toxics (SHEDS-AirToxics) to characterize population exposure to air toxics. Currently, SHEDS-AirToxics uses a probabilistic approach to predict the distribution of benzene exposure and internal dose for a specified population. The model estimates this distribution by simulating the time series of exposure and dose for individuals that demographically represent the population of interest. U.S. census data are used to build the simulation population, and human-activity-pattern data are assigned to each simulated individual to account for interaction with their immediate environment. Each individual?s inhalation exposure and dose profile is estimated from the time spent in each location, the concentration in that location, and the activity-specific inhalation rate while in that location. The model also can estimate exposure and dose from ingestion of food containing air toxic residues and from dermal absorption of chemicals while bathing. A daily-averaged exposure and dose for each individual can then be calculated and combined to provide a distribution of exposure and dose for the population. SHEDS-AirToxics can also account for both the inherent variability and the uncertainty in the factors connecting emissions, sources, environmental concentrations and the magnitude, duration, frequency, and time-series of human exposures. Future developments include expansion of model capabilities for additional air toxics of concern such as aldehydes, metals, and polycyclic organic material and focused exposure modeling applications will be designed to identify important deficiencies (e.g., data gaps, modeling assumptions) that lead to large uncertainties in exposure modeling estimates. In addition human exposure model research, Microscale Emission Factor (MicroFac) models for selected air pollutant emissions are being developed to provide site-specific emissions needed to support human exposure models.

This research will provide exposure model development, applications, and model performance evaluations that are critically needed to improve the present exposure modeling methods used to assess risk to air toxics. This will assist EPA, NERL, scientific collaborators and its clients, and the scientific community in identifying the sources of significant exposure and to understand the best approaches for limiting or mitigating those exposures.

Record Details:

Record Type:PROJECT

Start Date:10/01/2000

Projected Completion Date:09/01/2004

Record ID: 56208

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Project Information:

Progress :SHEDS-AirToxics Modeling

Modeling research needs for air toxics were identified and a benzene exposure modeling case study was initiated. Inputs for microenvironmental concentrations relating to inhalation exposures were compiled and documented. A prototype version of the Stochastic Human Exposure and Dose Simulation model for air toxics (SHEDS-AirToxics) has been created by integrating and modifying relevant components of the previously developed SHEDS models for particulate matter (SHED-PM) and pesticides (SHEDS-pesticides), and a user's manual and technical guide was drafted. In addition, discrete analyses on longitudinal human activity patterns was conducted.

Physical Process Modeling

MicroFac models for carbon monoxide (CO) and particulate matter (PM) emissions were developed and an emissions model for air toxics was initiated to provide site-specific emissions needed to support human exposure models. NERL's Meteorological Instrument Cluster of 3 trailers (MIC3) supporting two portable Sound Dectection and Ranging instruments (SODARs) and a 10-m Tower can be used to characterize local meteorological conditions. A pilot microenvironmental meteorolgical study using this equipment was set up for the Research Triangle Park, NC (RTP) area to be used in support of human exposure modeling of mobile source emissions. Equipment was also placed in lower Manhattan, NY as part of EPA's response to the events of September 11, 2001 and in Baltimore, MD. Research performed during the pilot microenvironmental human exposure model studies (including Computational Fluid Dynamics [CFD] simulations) and the MIC3 meteorological measurement studies was applied to modeling of the World Trade Center site in Manhattan, NY.

Relevance :EPA's Office of Air and Radiation (OAR) has been conducting National Air Toxics Assessments (NATA) of human exposure to hazardous air pollutants (HAPs). NATA's current approach is to combine a dispersion model with a simple, cohort-based exposure model. The National Exposure Research Laboratory's (NERL's) human exposure modeling program for air toxics will provide modeling tools (e.g., algorithms, modules, evaluations) to improve the scientific basis of OAR's national assessments.

SHEDS-AirToxics estimates an individual's exposure to benzene in several specific microenvironments (e.g., indoors at home, in vehicles, outdoors) using ambient benzene concentrations and exposure factors. Inhaled dose is calculated from the microenvironmental exposure concentrations (all using hourly ambient concentration data) and activity-level-specific breathing rates. The model includes exposure calculations specific to mobile source pollutants and activities. The model not only provides improved human exposure estimates to air toxics, but points to parameters where additional research is needed to reduce the uncertainty in exposure assessments. The SHEDS-AirToxics model will provide independent estimates of population exposures to air toxics in support of EPA?s research programs in human exposure, human health effects, and source emissions characterization. This human exposure model will allow EPA's Office of Air and Radiation, Regional Offices, and State agencies to improve their mobile source and urban air toxics assessments.

Additional modeling research on mobile source, roadway, and urban exposure is being utilized to develop a prototype source-to-inhalation exposure model for mobile source air toxics. The research focuses on near-field sources (e.g. mobile sources and small isolated sources of air toxic emissions) that are not well described by traditional air quality or compartmental models because of the averaging requirements in time and space. Characterization of such near-field exposures would dramatically improve the exposure estimation capabilities of the models such as SHEDS-AirToxics.

Clients :EPA Program Offices (OAQPS, OTAQ, OIA); EPA Regions; States; Research Community.

Project IDs:

ID Code :9524

Project type :OMIS