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 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 Pinal County Air Quality determines that data may have been affected by an "exceptional event", the data and circumstances are investigated. The county may then flag the data in the Air Quality System(AQS) as being affected by an "exceptional event". The information is submitted to EPA annually for concurrence. If EPA does concur with the county's determination, the data flag is then finalized. The documentation is made available for review to the public throughout the year within the Exceptional Event's page. Comments are accepted during the Public Comment period (30 days) that will be posted within the Public Notice page of the Exceptional Events.

An "exceptional event" is defined by EPA as an event that:

1. Affects air quality

2. Is not reasonably controllable or preventable

3. Is an event caused by human activity that is unlikely to recur at a particular location or a natural event; and

4. Is determined by EPA through the process established in the regulations, 40 CFR Part 50.14.

Some examples of natural events causing an exceptional event include:

1. High winds

2. Volcanic eruptions

3. Fires including industrial source fires

4. Seismic activity

Some examples of exceptional events caused by human activity include:

1. Structural fires

2. Prescribed burning

3. Construction/demolition

4. Unusual traffic congestion

5. Agricultural tilling

6. Clean up after a major natural disaster

If you have any questions, please e-mail scott.dibiase@pinalcountyaz.gov or call (520) 866-6929.