EPA is making ongoing efforts to ensure that its National-Scale Air Toxics Assessment utilizes the best-available scientific methods and information. Several of the components of the assessment have been subjected to the scrutiny of leading scientists throughout the country in a process called "scientific peer review."
SAB REVIEW OF NATA
EPA submitted a comprehensive report on NATA for review to the EPA's Science Advisory Board in January 2001. The SAB review concluded that NATA represents "an important step toward characterizing the relationship between sources and risk of hazardous air pollutants." The SAB commended NATA for substantial improvements in the underlying emissions inventory used to model ambient concentrations. The SAB emphasizes that there is "an overarching need for continued, improved data collection in support of NATA," including enhanced collection of ambient concentration data and emissions data. The SAB recommends that EPA adopt a consistent national HAP emissions inventory data collection and reporting rule, with proper incentives for industry to participate and comply.
OTHER SCIENTIFIC REVIEWS OF NATA COMPONENTS
1) The National Toxics Inventory (NTI) contains the 1996 emissions data EPA utilized to estimate 1996 exposure data. EPA worked extensively with states and local agencies to develop and refine emissions data in the NTI, and efforts to improve this dataset are ongoing. EPA engaged state and local regulators and industry in three rounds of emissions inventory review: 47 of the fifty states participated in the development of the NTI either by providing emissions data or reviewing the inventory. Methods for measuring and estimating emission rates which are reported in NTI have also been peer reviewed by scientists outside the EPA.
2) The Assessment System for Population Exposure Nationwide (ASPEN) dispersion model is used by EPA to estimate ambient concentrations of hazardous air pollutants at the census tract-level. ASPEN was peer-reviewed by the EPA's Science Advisory Board in 1996 in the context of its use in the Cumulative Exposure Project (CEP). Overall, the SAB concluded that the conceptual framework and underlying scientific foundation for the modeling approach for generating hazardous air pollutant concentration estimates is sound. The board stressed the importance of comparing ASPEN predictions with measured ambient concentrations. ASPEN-based exposure estimates and the underlying modeling methodology have also been peer-reviewed in scientific papers generated by EPA's Cumulative Exposure Project. A study in the Journal of the Air and Waste Management Association evaluated the performance of the model in predicting hazardous air pollutant concentrations by comparing modeled estimates with available monitoring data for a small subset of pollutants. Overall, the model performance evaluation suggests a tendency toward under-prediction largely due to the model formulation and uncertainties in the emissions inventory that was used as a model input.