On October 17, 2006, EPA published in the Federal Register (71 FR 61144) a final rule making revisions to the PM2.5 and PM10 National Ambient Air Quality Standards (NAAQSs). Effective December 18, 2006, EPA is strengthening the 24-hour PM2.5 standard from the 1997 level of 65 micrograms per cubic meter (µg/m³) to 35 µg/m³, and retaining the current annual PM2.5 standard at 15 µg/m³. Under the revised PM2.5 standards, EPA designated new nonattainment areas on November 13, 2009 with an effective date of December 14, 2009. As provided by the Clean Air Act, transportation conformity applied in these areas one year after the designations become effective.
EPA is retaining the existing national 24-hour PM10 standard of 150 µg/m³. However, EPA is revoking the annual PM10 standard.