It results primarily from the action of sunlight on hydrocarbons and nitrogen oxides emitted in fuel combustion. It also reacts with other materials such as rubber compounds, breaking them down. In response to a lawsuit filed by the American Lung Association, the U.S. Environmental Protection Agency in July 1997 set a new stricter ozone standard of 0.08 ppm averaged over an eight-hour period.
Compliance is based on the fourth highest reading per year averaged over three years.
The EPA is expected to designate ozone nonattainment areas for the new 8-hour standard in early 2001. However, based on 1994-96 monitoring data, the EPA has determined that a total of 270 counties in 33 states will violate the new standard. An estimated 117 million people living in those counties are at risk from the health effects of elevated ozone levels. In response to an industry lawsuit, the Court of Appeals for the D.C. Circuit in 1999 ruled that the 1997 national ozone standard could not be implemented. That decision is currently being appealed to the U.S. Supreme Court by the U.S. Department of Justice and the American Lung Association.