The Clean Air Act requires the United States Environmental Protection Agency (EPA) to set limits on how much of a particular pollutant can be in the air anywhere in the United States. National Ambient Air Quality Standards (NAAQS) are the pollutant limits set by the EPA; they define the allowable concentration of pollution in the air for six different pollutants – Carbon Monoxide, Lead, Nitrogen Dioxide, Particulate Matter, Ozone, and Sulfur Dioxide.

The GCLMPO is a part of the Charlotte Metrolina Area, which historically has had issues with air quality, primarily from ground-level Ozone. In 2004, the EPA designated the Metrolina Area, including Gaston and Lincoln counties, as moderate non-attainment for ozone NAAQS under the 1997 8-hour standard. With significant improvements, in 2013 the EPA redesignated the Metrolina Area as attainment for the 8-hour ozone standards. In May 2008, the EPA revised the ozone NAAQS to provide increased protection of public health and welfare, especially for children and “at risk” populations. With the new 2008 standards, the Metrolina Area was designated by the EPA as marginal nonattainment.

The Clean Air Act requires the GCLMPO to demonstrate that the fiscally-constrained metropolitan transportation plan and the transportation improvement program conform to the State Air Quality Implementation Plan, and not cause new air quality violations, worsen existing violations, or delay timely attainment of the NAAQS.