The One Hour vs. The Eight Hour Ozone Standard
The One Hour Ozone Standard
Under the one-hour standard, ozone concentrations of 0.125 ppm (125 parts per billion) or above are considered to exceed the standard. The standard is not to be exceeded in an area more than three times in three consecutive years at the same monitoring site. If the standard is exceeded four times in three years at one site, then the area is in violation of the standard and no longer in "attainment."
The Eight Hour Ozone Standard
In 1997, the U.S. Environmental Protection Agency (EPA) announced new national ambient air quality standards (NAAQS) for ground-level ozone, the primary constituent of smog. The new standard was challenged in the federal courts. The standard was upheld by the courts in March 2002, but the EPA is required to revise their implementation plan.
The eight-hour standard of 0.08 parts per million (85 parts per billion to exceed the standard) is determined by the fourth highest eight-hour daily maximum at any single monitor in an area, averaged over a three-year period.
The Eight Hour Ozone Standard Revised in 2008
In the spring of 2008, the EPA greatly strengthened its NAAQS for ground-level ozone, the primary component of smog. An area will meet the revised standards if the three-year average of the annual fourth-highest daily maximum 8-hour average at every ozone monitor is less than or equal to 75 ppb. EPA based the decision to lower the standard on scientific evidence that demonstrated public welfare was not being protected adequately.
These changes will improve both public health protection and the protection of sensitive trees and plants. EPA estimates that the revised standards will yield health benefits valued between $2 billion and $17 billion. Those benefits include preventing cases of bronchitis, aggravated asthma, hospital and emergency room visits, nonfatal heart attacks and premature death, among others. For more information regarding the schedule of implementing the 2008 revised standard visit the EPA's webpage located at: http://www.epa.gov/groundlevelozone/actions.html#stand.