 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. Due to ongoing analysis of the effects of these pollutants on public health, the NAAQS for these pollutants are often revised to provide additional protection. Below depicts the history of the ozone standard in the North Central Texas region.
Every five years the EPA reevaluates the NAAQS to ensure adequate protection of human health based on the best available science.
1-Hour Ozone Standard:
The 1-hour Standard was instigated 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 are no more than three exceedances at any one monitor in the region in a three year period.
Four counties in the Dallas-Fort Worth (DFW) region were designated as nonattainment for the 1-hour ozone standard in 1991. Those counties were Collin, Dallas, Denton, and Tarrant. On June 15, 2005, the 1-hour ozone standard was revoked due to being replaced by the 8-Hour Ozone Standard in 2004 (see 1997 8-Hour Ozone Standard).
On October 31, 2006, the region demonstrated attainment of the 1-hour ozone standard and it continues to be monitored due to anti-backsliding regulations.
1997 8-Hour Ozone Standard:
Based on EPA's bi-decade review of the 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. Under the new 8-hour standard, an area averages the fourth highest 8-hour average concentration for three years to determine whether the area is in attainment. The longer averaging time and the averaging of three years’ data reduce the influence of unusual meteorological conditions in any given year, making the design value more accurately reflect recurring pollution levels.
On June 15, 2004, nine counties in the DFW region were designated as moderate nonattainment for the 8-hour ozone standard. Those counties are Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall and Tarrant.
The EPA had originally been considering the entire consolidated metropolitan statistical area for designation, but Henderson, Hood and Hunt counties were not included based upon data that the Texas Commission on Environmental Quality (TCEQ) provided to support this exclusion. The regional attainment date for this standard is June 15, 2010. For a timeline of the federal ozone standards to date, please click here.
The 8-hour ozone standard differs from the 1-hour standard in several major ways. First, the averaging time was extended from 1 hour to 8 hours to reduce prolonged exposure. Additionally, the standard threshold was strengthened, from 125 parts per billion (ppb) to 84 ppb. Attainment for the 1-hour standard was based upon the number of exceedances at a given monitor, whereas attainment for the 8-hour standard is determined by a regional design value.
Design value is defined as the fourth-highest 8-hour
average ozone concentration averaged over a consecutive 3-year period. The region will demonstrated attainment when the design value for all monitors in the region is below 85 ppb. Design values are used not only in determining attainment versus nonattainment, but also determine the severity of nonattainment.
The North Central Texas Council of Governments (NCTCOG) monitors the number of days per year 8-hour ozone concentration equal or exceed 85 ppb. Since 8-hour ozone data began to be monitored, ozone exceedance days have been steadily decreasing. This demonstrates that controls already in place as a result of the 1-hour Dallas-Fort Worth Attainment Demonstration State Implementation Plan (SIP) had a positive effect on reducing prolonged ozone episodes.
2008 8-Hour Ozone Standard:
On March 12, 2008, the EPA revised the 8-hour ozone standard from 84 ppb to 75 ppb per the most recent scientific evidence about the effects of ozone. Under this revision, the Texas Commission on Environmental Quality is recommending Hood County be added to the region's nonattainment counties. Upon EPA’s review, final designations will be made on or before March 12, 2010, and State Implementation Plans will be due to EPA by 2013.
Additional Information:
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