On January 15, 2013, EPA published in the Federal Register (78 FR 3085) a final rule strengthening the primary annual PM2.5 standard from the 1997 level of 15.0 micrograms per cubic meter (µg/m³) to 12.0 µg/m³, and retaining the current primary 24-hour PM2.5 standard at 35 µg/m³. The secondary standards for both the annual and 24-hour PM2.5 standards were retained at 15.0 µg/m³ and 35 µg/m³ respectively. EPA also retained the existing 24-hour PM10 primary and secondary standard at 150 µg/m³. These revisions will become effective on March 18, 2013.
Further information regarding the revised NAAQS can be found on EPA's website.