Limitations in the 1996 National-Scale Air Toxics Assessment

These three topics will help you understand the limitations of the assessment so that you will use the results to answer only those questions for which the assessment is suitable:

• Assessment limitations: Are there any features of the assessment that limit the kinds of conclusions that might be drawn?
• Variability: Do these results apply to an individual such as myself?
• Uncertainty: With what confidence can these results be accepted?

Limitations The results provide answers to questions about emissions, ambient air concentrations, exposures and risks across broad geographic areas (such as counties, states and the Nation) at a moment in time. As such, they help the EPA identify specific air toxics compounds, and specific source sectors such as stationary sources or mobile sources, which generally produce the highest exposures and risks in the country. But they also are based on assumptions and methods that limit the range of questions that can be answered reliably. They cannot be used to identify exposures and risks for specific individuals, or even to identify exposures and risks in small geographic regions such as a specific census tract. These limitations, or caveats, must always be kept in mind when interpreting the results, and the results should be used only to address questions for which the assessment methods are suited.

Variability Emissions, air concentrations, exposures and risks are not the same throughout the U.S., and are not the same for everyone. Some geographic areas have higher concentrations than others; there are some periods of time when the concentration is higher at a given location than at other times. Some individuals have an exposure and/or risk below the national average, while others have an exposure and/or risk above the national average. It is necessary, therefore, to have some idea of how the ambient air concentration, exposure and risk from air toxics varies throughout the U.S. Such a process is called a variability analysis.

Uncertainty The EPA seeks to protect health with reasonable confidence. Scientific estimates of air concentrations, exposures and risks, however, always involve assumptions that simplify the real situation but make the assessment possible given available information and resources. These assumptions introduce uncertainties into the results, since there is never complete confidence that the assumptions are entirely correct. It is necessary to understand the size of these uncertainties, the level of confidence that can be placed in any statement you might find related to the assessment, and how this confidence affects the ability to make reasoned decisions. Such a process is called an uncertainty analysis.