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Ambient air quality can be affected by many circumstances. Depending on the parameter measured at a site, the values can be affected by sources (mobile and stationary), activities (human and natural), and weather, occurring either nearby or some distance from that particular location. Many activities can be controlled, altered, or regulated in some manner in order to improve air quality. Other activities affecting air quality are outside the scope of regulation.
A mechanism is set up through U.S. EPA for reviewing the data affected by either uncontrollable or non-preventable activities. Data which are in exceedance of a National Ambient Air Quality Standard (NAAQS) are analyzed to determine if they are affected by events which meet this "exceptional event" criteria. If a state determines that data may have been affected by an "exceptional event", the data and circumstances are investigated. The state may then flag the data in AQS as being affected by an "exceptional event". The documentation is made available for review by the public and submitted to U.S. EPA for concurrence. If U.S. EPA does concur with the state's determination, the data flag is then finalized.
U.S. EPA defines the term "exceptional event" to mean an event that:
Some examples of natural events causing an exceptional event include:
Some examples of exceptional events caused by human activity include: