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Limitations in the 1996 National-Scale Air Toxics Assessment: Uncertainty

Information provided for informational purposes onlyNote: EPA no longer updates this information, but it may be useful as a reference or resource.

What are the Components of Uncertainty

The uncertainty analysis performed for the national-scale assessment helps in understanding the confidence with which specific statements (such as the lifetime excess risk of cancer due to inhalation of benzene in North Carolina) can be made based on the information available for the assessment. No scientific statement (in risk assessment or other areas of science) can be made with complete confidence. Uncertainties always exist in these statements due to issues that will be discussed below. Transparency and openness, therefore, require an assessment of these uncertainties and the ways in which they raise or lower confidence. The national-scale assessment produced statements about variability in ambient air concentrations, exposures and risks across geographic regions for typical individuals, as described in the section on Limitations, and so the uncertainty analysis was designed to characterize the confidence with which these statements may be made. It is important to note that uncertainty does not prevent a statement of risk from being made; nor does it prevent reasonable actions from being taken. It does, however, require that the magnitude of the uncertainty, and the implications for decisions, be understood so the degree of support for the statement is not misinterpreted.

Uncertainty can arise from a variety of sources. Some of these sources can be described quantitatively, while others are best described qualitatively. A decision must be made as to how to combine these quantitative and qualitative aspects of uncertainty into an overall measure of the confidence that may be placed in a specific statement (such as the statement that the lifetime risk of cancer from air toxics compounds in a specific census tract in North Carolina is 1 in a million). The goal of an uncertainty analysis is to characterize uncertainty and confidence as rigorously as possible, trying to avoid purely subjective judgments, while recognizing the inherently qualitative nature of some aspects of uncertainty and the need for human judgment at times.

What are the components of uncertainty? To understand this issue, consider the process by which a study such as the national-scale assessment is performed:

This section on Uncertainty considers both model uncertainty and parameter uncertainty, and explains how these were developed and combined for the national-scale assessment.

More Details About the "Overall Confidence" Rankings
Which components of uncertainty did the national-scale assessment include?
How was the uncertainty analysis conducted?

Return to the main Uncertainty Page

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