United States Environmental Protection Agency Office of Research and Development National Exposure Research Laboratory Research Abstract Government Performance Results Act (GPRA) Goal 1 Annual Performance Measure 21 Significant Research Findings: Stochastic Human Exposure and Dose Simulation Model for Air Toxics Scientific EPA provided a "road map" for its air toxics program with the July 19, 1999 Problem and Federal Register publication of the National Air Toxics Program: Integrated Urban Policy Issues Strategy. This program is designed to characterize, prioritize, and equitably address exposures to air toxics and their impact on public health and the environment. The program addresses air toxic emissions from large and small stationary sources, mobile sources, and indoor air sources as part of its strategy for reducing risks from exposure to air toxics. In addition, the program prioritizes its actions and measures progress through the use of assessments conducted at multiple geographic scales (e.g., national, regional, local). Conducting National Air Toxics Assessments (NATA) is one of 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. These models can then be used to estimate the range of potential chemical exposure across the general population as well as susceptible and highly-exposed subpopulations. EPA's National Exposure Research Laboratory (NERL) has begun developing a human exposure model to characterize population exposures to air toxics in support of risk assessment activities in the National Air Toxics Program. The model, called the Stochastic Human Exposure and Dose Simulation Model for Air Toxics (SHEDS-Air Toxics), uses a probabilistic approach to predict the distribution of inhalation exposure and dose for a specified population. The model is designed to estimate 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 the way people interact with their environment. Pollutant concentrations in the Research Approach National Exposure Research Laboratory — October 2003 ------- microenvironments where people spend their time (e.g., home, car, office, school, restaurant) are calculated based on relationships between central site outdoor and indoor or in-vehicle concentrations obtained from measurement study data. Each individual's 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. Daily-averaged exposure and dose for each individual are calculated and combined to provide a distribution of exposure and dose for the population. Statistical methods for incorporating both variability and uncertainty in the model input parameters are utilized to obtain the predicted population distribution and the uncertainty associated with the predicted distribution. The model framework has been initially developed around a benzene exposure case study. Critical exposure-microenvironments were identified based on chemical source information and analysis of human activity patterns. All relevant concentrations were compiled and evaluated from multiple sources found in the scientific literature. In addition to the chemical-specific data, human-activity-pattern data are required as inputs to NERL's and other complex human exposure models. The lack of longitudinal (long-term) exposure data is a significant limitation to estimating realistic lifetime exposures. To improve estimation of long-term exposure, data available in EPA's Consolidated Human Activity Database (CHAD) were analyzed to determine relative importance of characteristics used to define cohorts. Attributes including age, gender, ethnicity, and outdoor temperature were analyzed to characterize the relationship between cohort attributes and the time spent outdoors, indoors, and in motor vehicles. The significant characteristics were used to develop longitudinal activity profiles from single-day activity data. Results and SHEDS-Air Toxics estimates an individual's exposure to benzene in several Impact 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-Air Toxics model provides 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 model allows EPA's Office of Air and Radiation, Regional Offices, and State agencies to improve their mobile source and urban air toxics exposure assessments. Collaboration with EPA/NERL on this research effort included contributions from ManTech Environmental Technology, Inc. Examples of recent publications from this study include the following: Research Collaboration and Research Products National Exposure Research Laboratory — October 2003 ------- Graham SE and JM Burke. 2003. "Microenvironmental exposures to benzene: a critical review and probabilistic model input distribution development." EPA/600/.T-03/008. Graham SE and McCurdy. 2003. "Developing meaningful cohorts for human exposure modeling. Journal of Exposure Analysis and Environmental Epidemiology. Accepted. Future Research Future research will include model development to permit application to a wide range of individual air toxics and mixtures of compounds. Chemicals associated with mobile sources will be addressed first, followed by pollutants that have significant pathways of exposure other than inhalation. Work will also be conducted to link SHEDS-Air Toxics with dosimetry and physiologically-based pharmokinetic models to enhance the current capabilities for characterizing absorption, distribution, metabolism, and elimination. Finally, work will be conducted to incorporate or link source and/or emission models to SHEDS-Air Toxics. Initial research in this area will focus on indoor sources and associated chemicals such as various aldehydes emitted from furniture and paints. Contacts for Questions and inquiries can be directed to the principal investigator: Additional Information Stephen E. Graham, Ph.D. U.S. EPA, Office of Research and Development National Exposure Research Laboratory Mail Drop E205-02 109 T.W. Alexander Drive Research Triangle Park, NC 27711 Phone: 919-541-4344 E-mail: graham.stephen@epa. gov Federal funding for this research was administered under EPA Contract #69-D- 00-206 to ManTech Environmental Technology, Inc. National Exposure Research Laboratory — October 2003 ------- |