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Exceptional Events

When air quality exceeds National Ambient Air Quality Standards (NAAQS), IDEM determines if the data are affected by an exceptional event as defined by the U.S. EPA. Exceptional events are unusual or naturally occurring events that can affect air quality but are not reasonably controllable using techniques tribal, state or local air agencies may implement in order to attain and maintain the National Ambient Air Quality Standards (NAAQS).

Types of exceptional event include natural events such as high winds/dust storms, wild fires and seismic/volcanic activity, in which human activity plays little or no direct causal role. Other examples of exceptional events include exceedances or violations caused by emissions from controlled burning.

The federal Clean Air Act recognizes that it may not be appropriate to use monitoring data influenced by exceptional events collected by the ambient air quality monitoring network when making certain regulatory determinations. When exceptional events influence monitoring data and cause exceedances or violations of the NAAQS, air agencies may request the exclusion of event-influenced data, and the U.S. EPA may agree to exclude these data from the data set.