In response to a court order, on Thursday, June 14, 2012, the United States Environmental Protection Agency (USEPA) proposed updated national ambient air quality standards (NAAQS) for fine particulate matter, known as PM2.5. A group of plaintiffs, including 11 states, filed suit in February 2012 after USEPA failed to meet an October 2011 deadline for revising the existing standards, as required by Section 109(d)(1) of the Clean Air Act. The United States District Court for the District of Columbia directed USEPA to propose updated standards no later than June 14, 2012. The Clean Air Act requires the NAAQS to be reviewed every five years, but USEPA has rarely complied with the five-year review provision. The proposal sets in motion a requirement by states to classify air quality regions and then establish additional requirements in regions that do not attain the new standards.

USEPA proposed revisions to the suite of primary and secondary standards for PM. The current primary PM2.5 annual standard is 15 micrograms per cubic meter (µg/m3). USEPA proposed to lower the standard to within a range of 12µg/m3 to 13µg/m3, and is taking comment on whether to set the standard as low as 11µg/m3. By proposing a range, USEPA stated it hopes to receive input from the public as to the most appropriate final standard. According to the proposal, USEPA plans to keep the existing primary 24-hour fine particle standard of 35µg/m3 set in 2006 (USEPA's last review of the PM NAAQS), as well as the current primary standard for coarse particles, the PM10 standard of 150µg/m3.

With regard to secondary standards, USEPA proposes a new, separate 24-hour fine particle standard defined in terms of a visibility index to improve visibility in urban areas, and to retain the current standards to address non-visibility welfare effects. The new standard will be set at either 30 deciviews or 28 deciviews.

The proposal will also revise its Prevention of Significant Deterioration regulations to provide grandfathering from the new requirements for permit applications for which the public comment period has already begun at the time the revised PM NAAQS take effect. USEPA expects that 99% of U.S. counties are projected to meet the proposed standard without any additional action.

The following link is to a map showing USEPA's projections regarding counties that are not expected to meet the proposed annual standards (http://www.epa.gov/airquality/particlepollution/2012/map.pdf). Sources of PM2.5 include fuel combustion from automobiles, power plants, petroleum refineries, wood burning, industrial processes, and diesel powered vehicles such as buses and trucks.

USEPA will accept public comment for 63 days after the proposed standards are published in the Federal Register and expects to issue the final standards by December 14, 2012.