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Ozone Standard


EPA is required under the Clean Air Act to set National Ambient Air Quality Standards (NAAQS) for ozone, as one of six criteria pollutants. In the 1970’s, EPA established a 1-hour ozone standard. Originally set at 0.08 parts per million (ppm) in 1971, the 1-hour ozone standard was revised in 1979 to 0.12 ppm. After a thorough review of the available science relating to the adverse health effects of ozone pollution, it was determined that the 1-hour ozone standard of 0.12 parts per million (125 parts per billion with rounding) did not adequately protect the public from adverse health effects.

1997 Ozone Standard: 0.08 ppm

In July 1997, the agency replaced the 1-hour standard with an 8-hour standard of 0.08 parts per million (rounded to 84 parts per billion).

EPA also changed the form of the standard from number of exceedances to one that takes concentration into account. An area will be in attainment of the 8-hour standard when the 3-year average of the annual 4th-highest daily maximum 8-hour concentration is less than 85 ppb. The form more directly relates to ozone concentrations associated with health effects; it avoids exceedances of the standard, regardless of magnitude, from being counted equally in the attainment tests.

The 8-hour standard was challenged in court. In February 2001, the U.S. Supreme Court upheld the standard, unanimously rejecting arguments from a group of industries and Midwest and Southern states that EPA must balance compliance costs against the health benefits when setting ambient air standards. (Whitman v. American Trucking Associations, Inc., 531 U.S. 457, 471 (2001)) The justices also ruled against industry arguments that EPA exceeded its lawmaking authority when it set the revised standards in 1997. However, the Supreme Court ruled that EPA's implementation policy for the revised ozone standard was unlawful and remanded it to EPA. While the court did not question the underlying health science or the level of the standard, it ruled that EPA must develop a reasonable approach to implementing the standard. In March 2002, the U.S. Court of Appeals upheld the remaining issues. These court cases established the right of EPA to set air quality standards based only on the impact to public health.

With the resolution of these issues in court, EPA began developing strategies for implementation of the 8-hour standard. Designations for areas not meeting the 1997 standard were made by EPA on April 14, 2004. See DEP's Attainment Website for the background and status of these designations.

2008 Ozone Standard: 0.075 ppm

In March 2008 EPA finalized a revision of the 8-hour ozone standard. The primary (public health) and secondary (public welfare) standards were set identically at 0.075 ppm (75 parts per billion). The methodology for calculating exceedances of the 2008 ozone standard did not change from the 1997 ozone standard. Beginning in May 2008, a number of parties filed lawsuits in the US Court of Appeals for the District of Columbia Circuit including environmental and health organizations, states, cities and industry groups. These cases have been consolidated by the court as Mississippi v. EPA.

In early 2009, with a change in administration, EPA advised that it intended to reconsider the 2008 standard. The court held this litigation in abeyance pending a final regulation incorporating the reconsideration. In January 2010, EPA proposed to set the primary standard between 0.060 and 0.070 parts per million averaged over eight hours, compared to the existing standard of 0.08. EPA also proposed to set a separate “secondary” seasonal ground-level ozone standard ranging from 7-15 parts per million-hours to protect plants and trees from damage occurring from repeated ozone exposure, which can reduce tree growth, damage leaves, and increase susceptibility to disease. In September 2011, EPA decided not to promulgate a revision of the 2008 standard at this time. EPA also advised that it would move forward with implementation of the 2008 standard, starting with designating areas in the first half of 2012. The litigation held in abeyance is now moving forward again. DEP made recommendations for designations for the 2008 standard based on 2006-2008 monitoring in 2009. EPA will most likely base its designations on 2008-2010 monitoring.

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