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This proposed action will establish Significant Impact Levels (SILs) for ozone and PM2.5 to facilitate implementation of the Prevention of Significant Deterioration (PSD) program in areas attaining the national ambient air quality standards (NAAQS) for Ozone or PM2.5. The SILs for Ozone and PM2.5 would be used as compliance demonstration tools by permitting authorities to help determine whether the projected emissions from a proposed new major source or major modification will cause or contribute to a violation of the NAAQS. This proposed action is, in part, in response to the January 22, 2013, D.C. Circuit's decision that vacated the PM2.5 Significant Monitoring Concentration (SMC) and vacated and remanded two provisions in the EPA's PSD regulations containing SILs that were contained in the 2010 rule promulgating increments, SMCs, and SILs for PM2.5. Furthermore, in August 2011, the Texas Commission on Environmental Quality (TCEQ) filed a Petition for Reconsideration to the Administrator regarding several provisions contained in the 2010 PM2.5 final rule, claiming that EPA did not provide an opportunity for public comment prior to issuing the provisions as part of the final rule. In response to the TCEQ petition, EPA will reconsider the following provisions: 1) The revised definition of "baseline area" that includes a new significance level for PM2.5, which is used for determining whether a particular attainment or unclassifiable area should be included in the baseline area for the PM2.5 increments; and 2) The requirement that PM2.5 precursor emissions be included in the significant impact analysis. The EPA intends to address all of the above mentioned items with this rulemaking. This proposed action will be the first to add SILs for ozone to the PSD regulations.