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 based on the best available science. Below depicts historical and current information about the eight-hour ozone standard in the North Central Texas (NCT) region.
1997 Eight-Hour Ozone Standard:
Based on EPA's bi-decade review of the ozone standard, the Agency determined the one-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 eight-hour ozone standard.
The eight-hour ozone standard differs from the one-hour standard 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 eight-hour standard, 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 eight-hour ozone standard. 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 have been steadily decreasing, the region failed to meet the 2010 attainment deadline, and was reclassified under the 1997 eight-hour ozone standard 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 eight-hour ozone standard in 2014 with a design value of 81 ppb.
The EPA published the revocation of the 1997 eight-hour ozone standard 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.
2008 Eight-Hour Ozone Standard:
On March 12, 2008, EPA revised the eight-hour ozone standard 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 eight-hour ozone standard, Wise County was added as the tenth nonattainment county, EPA revoked the 1997 eight-hour ozone standard 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 eight-hour ozone standard, including the revocation of the 1997 eight-hour standard 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.
2015 Eight-Hour Ozone Standard:
On October 26, 2015, EPA revised the eight-hour ozone standard 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. Below are links to information on the revised standard and the proposed rule. EPA anticipates final designaitons to be effective between late December 2017 and early January 2018.