8-Hour Ozone NAAQS

1997, 2008, and 2015 8-Hour Ozone NAAQS

The Environmental Protection Agency (EPA) is required to set National Ambient Air Quality Standards (NAAQS) for harmful pollutants, per the Clean Air Act Amendments of 1990. Every five years the EPA is required to reevaluate the NAAQS to ensure adequate protection of human health and the environment, based on the best available science. The information below describes the historical and current information about the 8-hour ozone standard in the North Central Texas (NCT) region.

1997 Ozone NAAQS

Based on EPA's bi-decade review of the ozone standard, the Agency determined the 1-hour ozone standard did not fully protect public health, so they placed more emphasis on prolonged exposure to ozone by promulgating a standard based on an eight hour average ozone concentration, the 1997 ozone NAAQS.

The 8-hour ozone NAAQS differs from the 1-hour ozone NAAQS in several major ways. First, the averaging time was extended from one hour to eight hours to reduce prolonged exposure. Additionally, the standard threshold was strengthened, from 125 parts per billion (ppb) to 84 ppb. Under the new 8-hour ozone NAAQS, the EPA also introduced the concept of a design value to determine a region's attainment status. The design value is defined as the fourth-highest eight-hour average ozone concentration averaged over a consecutive three-year period. Using the design value reduces the influence of unusual meteorological conditions in any given year, and more accurately reflects actual recurring pollution levels. Design values are also used to determine the severity of nonattainment.

On June 15, 2004, nine counties in the NCT region were designated as moderate nonattainment for the 1997 ozone NAAQS. Those counties are Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall and Tarrant. The NCT region's deadline to demonstrate attainment for this standard was June 15, 2010. Although the region's design value and the number of  ozone exceedance days are steadily decreasing, the region failed to meet the 2010 attainment deadline, and was reclassified under the 1997 ozone NAAQS as serious nonattainment in December 2010, with an attainment date of June 15, 2013. The NCT region failed to meet the 2013 attainment deadline; however, the region did meet the 1997 ozone NAAQS in 2014 with a design value of 81 ppb.

The EPA published the revocation of the 1997 8-hour ozone NAAQS on March 6, 2015, effective April 6, 2015. The nine counties were not re-classified under the 1997 standard because EPA cannot designate or reclassify an area for a revoked standard. The EPA did, however, publish a final rule on September 1, 2015, providing a Clean Data Determination for the nine NCT nonattainment counties, effective October 1, 2015.

1997 Ozone NAAQS Nonattainment Boundary
1997 Ozone NAAQS Exceedance Days
1997 Ozone NAAQS Design Value Trendline
Reclassification to Serious Nonattainment [EPA]
Additional Information about this Standard [EPA]

2008 Ozone NAAQS

On March 12, 2008, EPA revised the 8-hour ozone NAAQS from 84 ppb to 75 ppb; however, the EPA did not finalize the designations under this new standard until 2012. On the May 21, 2012, EPA published the final rule classifying the NCT region as moderate nonattainment for the 2008 ozone NAAQS, Wise County was added as the tenth nonattainment county, EPA revoked the 1997 ozone NAAQS for transportation conformity, and established December 31st of each calendar year as the attainment date for all nonattainment area classification categories. On December 23, 2014 the DC Court of Appeals published a consent decree reversing a portion of EPA's final rule for the 2008 ozone NAAQS for, including the revocation of the 1997 ozone NAAQS for purposes of transportation conformity and the December 31st attainment date deadline. As a result of this consent decree, the ten nonattainment counties must reach attainment by July 20, 2018.

2008 Ozone NAAQS Nonattainment Boundary
2008 Ozone NAAQS Exceedance Days
2008 Ozone NAAQS Design Value Trendline
2008 Ozone NAAQS Designations and Classifications [EPA]
Additional Information about this Standard [EPA]

2015 Ozone NAAQS

On October 26, 2015, EPA revised the 8-hour ozone NAAQS from 75 ppb to 70 ppb, effective December 28, 2015. The final rule also extends the ozone season for the NCT region by one month to November 30th.  EPA signed the final rule classifying nine counties (Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Tarrant and Wise as marginal nonattainment. Once the final rule is published in the Federal Register , the effective date will be 60 days from publication.  A marginal nonattainment classifiation gives the region three years to acheive attainment. 

2015 Ozone NAAQS Nonattainment Boundary
2015 Ozone NAAQS Monitor Exceedance Data
2015 Ozone NAAQS Exceedance Days
2015 Ozone NAAQS Design Value Trendline
2015 Ozone NAAQS Designations and Classifications [EPA]
Additional Information about this Standard [EPA]

Additional Information

Dallas-Fort Worth Nonattainment Area Milestones for the 8-hour Ozone NAAQS
1-Hour Ozone NAAQS Information [NCTCOG]
1-Hour Ozone NAAQS Information [EPA]
Historical Ozone NAAQS Table [EPA]