Based on its review of the air quality criteria and the national ambient air quality standards (NAAQS) for particulate matter (PM), the EPA proposes to make revisions to the primary and secondary NAAQS for PM to provide requisite protection of public health and welfare, respectively, and to make corresponding revisions to the data handling conventions for PM and ambient air monitoring, reporting, and network design requirements. The EPA also proposes revisions to the prevention of significant deterioration (PSD) permitting program with respect to the proposed NAAQS revisions. With regard to primary standards for fine particles (generally referring to particles less than or equal to 2.5 micrometers (mm) in diameter, PM2.5), the EPA proposes to revise the annual PM2.5 standard by lowering the level to within a range of 12.0 to 13.0 micrograms per cubic meter (mg/m3), so as to provide increased protection against health effects associated with long- and shortterm exposures (including premature mortality, increased hospital admissions and emergency department visits, and development of chronic respiratory disease) and to retain the 24-hour PM2.5 standard. The EPA proposes changes to the Air Quality Index (AQI) for PM2.5 to be consistent with the proposed primary PM2.5 standards. With regard to the primary standard for particles generally less than or equal to 10 mm in diameter (PM10), the EPA proposes to retain the current 24-hour PM10 standard to continue to provide protection against effects associated with short-term exposure to thoracic coarse particles (i.e., PM10-2.5). With regard to the secondary PM standards, the EPA proposes to revise the suite of secondary PM standards by adding a distinct standard for PM2.5 to address PM-related visibility impairment and to retain the current standards generally to address non-visibility welfare effects. The proposed distinct secondary standard would be defined in terms of a PM2.5 visibility index, which would use speciated PM2.5 mass concentrations and relative humidity data to calculate PM2.5 light extinction, translated to the deciview (dv) scale, similar to the Regional Haze Program; a 24-hour averaging time; a 90th percentile form averaged over 3 years; and a level set at one of two options—either 30 dv or 28 dv.