Under the Clean Air Act, EPA is required to review the science upon which the primary (health-based) and secondary (welfare-based) national ambient air quality standards (NAAQS) are based and the standards themselves every 5 years. On October 17, 2006, EPA published a final rule to revise the primary and secondary NAAQS for particulate matter to provide increased protection of public health and welfare. With regard to the primary standards for fine particles (PM2.5 generally referring to particles with an aerodynamic diameter less than or equal to 2.5 micrometers, µm), EPA revised the level of the 24-hour PM2.5 standard to 35 micrograms per cubic meter (µg/m3) and retained the level of the annual PM2.5 standard at 15 µg/m3. With regard to the primary standards for coarse particles (PM10, generally referring to particles with an aerodynamic diameter less than or equal to 10 µm), EPA retained the 24-hour PM10 standard and revoked the annual PM10 standard. With regard to the secondary PM standards, EPA made them identical in all respects to the primary PM standards, as revised. In subsequent litigation, the U.S. Court of Appeals for the District of Columbia Circuit remanded the primary annual PM2.5 standard and secondary PM2.5 standards to EPA. The EPA is responding to the court's remands as part of the current review of the PM NAAQS. EPA initiated the current review in 2007 and issued a proposed rule in June 2012 (77 FR 38890). Based on its review of the air quality criteria and the 2006 standards, EPA proposed to revise the primary annual PM2.5 standard by lowering the level to within a range of 12.0 to 13.0 µg/m3 in conjunction with retaining the primary 24-hour PM2.5 standard. The EPA also proposed to retain the current primary 24-hour PM10 standard. With regard to the secondary PM standards, EPA proposed to retain the current standards (i.e., 24-hour and annual PM2.5 standards and 24-hour PM10 standard) to provide protection for non-visibility welfare effects and to establish a distinct secondary standard to address PM-related visibility impairment.