Information for the One-Hour Ozone Standard
The Environmental Protection Agency (EPA) implemented the one-hour standard in 1990. Under this standard, the ozone
threshold value was 125 parts per billion (ppb), measured as one-hour average
concentration. An area met this ozone National Ambient Air Quality Standard 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 one-hour ozone standard in 1991.
Those counties were Collin, Dallas, Denton, and Tarrant. On June 15, 2005, the
one-hour ozone standard was revoked and replaced by the eight-hour ozone
standard. (see 1997 Eight-Hour Ozone Standard).
On October 31, 2006, the region demonstrated attainment of the one-hour ozone
standard, althought it continues to be monitored due to EPA's anti-backsliding requirements. On October 16, 2008, the EPA issued a determination of attainment for the four one-hour nonattainment counties in NCT.
Information for the Eight-Hour Ozone Standard
Information for the One-Hour Ozone Standard [EPA]
Major Air Pollutants
National Ambient Air Quality Standards Table [EPA]