1-Hour Ozone NAAQS

The Environmental Protection Agency (EPA) implemented the 1-hour National Ambient Air Quality Standards (NAAQS) for ozone in 1990. Under this standard, the ozone threshold value was 125 parts per billion (ppb), measured as 1-hour average concentration. An area met this ozone NAAQS if there were no more than three exceedances at any one monitor in the region in a three-year period.

Four counties in the North Central Texas (NCT) region were designated as nonattainment for the 1-hour ozone NAAQS in 1991. Those counties were Collin, Dallas, Denton, and Tarrant. On June 15, 2005, the 1-hour ozone NAAQS was revoked and replaced by the 1997 8-hour ozone NAAQS.

On October 31, 2006, the region demonstrated attainment of the 1-hour ozone NAAQS, receiving a determination of attainment from the EPA on October 16, 2008. Although the region was declared in attainment by the EPA, the four counties were still held to EPA's anti-backsliding requirements. The Texas Commission on Environmental Quality submitted a Redesignation Substitute State Implementation Plan to remove these requirements.  The EPA published a final rule approving the Redesignation Substitute and making a finding of attainment for the 1-hour ozone nonattainment area, effective December 8, 2016.

Additional Resources

For more information, please contact Jenny Narvaez.