On Tuesday, the U.S. Court of Appeals for the District of Columbia Circuit reversed EPA’s 2006 revision to the National Ambient Air Quality Standard (“NAAQS”) for fine particulate matter on the ground that the standard may not be sufficiently stringent. Commonly referred to as PM-2.5 (particulate matter that is less than or equal to 2.5 microns in diameter), PM-2.5 results from the emission of nitrogen oxides and sulfur dioxide, which are chemically transformed in the atmosphere into microscopic nitrate and sulfate particles. Electric generating stations utilizing coal and oil as fuel are the nation’s dominant source of sulfur dioxide emissions and a significant source of nitrogen oxide emissions. Many other types of facilities across the economy also emit nitrogen oxides. States are required to adopt plans ensuring that their air quality meets the NAAQS; the court’s decision thus could mean more stringent requirements on the use of coal and oil in electric generation.

In 1997, the Environmental Protection Agency (“EPA”) had set the NAAQS for PM-2.5 at 15 microns per square meter (ug/m3) measured on an annual average basis and at 65 ug/m3 measured on a daily basis. In 2006, EPA reviewed these standards and decided to lower the annual standard to 35 ug/m3 but to maintain the annual standard at 15 ug/m3. The decision not to change the annual standard was very controversial, as both EPA staff and its Clean Air Science Advisory Committee had recommended a tighter standard.

EPA’s decision not to tighten the annual standard was challenged by environmental organizations and a number of states, with industry groups intervening to support EPA. Based on its review of the administrative record, the court held that EPA had failed to fully explain its decision and had failed to take into account various scientific studies in determining whether the standard would be protective of vulnerable subpopulations such as children and the elderly. Recognizing that the standard nevertheless provides significant health benefits, the court left the standard in place but remanded it to EPA to further consider whether it should be tightened.

The court also ruled on industry and environmental organization challenges to EPA’s coarse particle, or PM-10, standard. The coarse particle standard is primarily of concern to agriculture, mining and earth moving operations. EPA’s 2006 revisions to its PM NAAQS maintained its pre-existing standard of 150 ug/m3 measured on a daily basis and eliminated its pre-existing annual standard altogether. The agriculture and mining industries challenged the decision to maintain the pre-existing daily standard on the ground that EPA should have made a distinction between rural and coarse PM based on the differing health effects of the two types of particles. Environmental organizations challenged the elimination of the annual standard. The court rejected both challenges.

Given the change in Administrations, it is unknown whether EPA will attempt to secure a reversal of the court’s decision, either by seeking rehearing or asking the Supreme Court to take review of the case. In any event, NAAQS are required to be reviewed every five years, and EPA has already begun work on the next review. Given strong support in the environmental community, it seems certain that an effort will be made to tighten the annual PM-2.5 standard.