Significant Impact Levels for Ozone and Fine Particles

In 2018, the EPA finalized the guidance and supporting documents recommending Significant Impact Levels (SILs) for ozone and fine particle pollution that may be used in the Prevention of Significant Deterioration (PSD) permitting program. SILs have been used in the PSD permitting process for many years to help permit applicants and permitting authorities determine whether proposed construction may be authorized in a Clear Air Act (CAA) permit. The 2018 SILs were developed using a peer-reviewed approach based on the variability in monitored design value data from 2014-2016. 

On February 7, 2024, the EPA strengthened the National Ambient Air Quality Standards for Particulate Matter (PM NAAQS). More recent ambient monitoring data are also now available for ozone and PM_2.5. Considering both of these developments, the EPA re-assessed the SIL values presented in the 2018 guidance to provide updated SILs for ozone and PM_2.5. This analysis supports continuing to use the same values for ozone and 24-hour PM_2.5, but the EPA recommends new SIL values for annual PM_2.5 that are representative of the new level of the annual PM_2.5 NAAQS and more recent design value data from 2020-2022. The updated technical analysis and resulting values are presented in the Supplement to the Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program.

Please note, a detailed appendix to the 2018 Technical Basis for the EPA’s Development of the Significant Impact Thresholds for PM_2.5 and Ozone is available. If you would like to receive a copy of this document, please contact Alyssa Piliero.

2024

• Supplement to the Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program (pdf) (1.56 MB)

2018

• Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program (pdf) (547.42 KB)
• Legal Memorandum Application of Significant Impact Levels in the Air Quality Demonstration for Prevention of Significant Deterioration Permitting under the Clean Air Act (pdf) (112.18 KB)
• Technical Basis for the EPA's Development of the Significant Impact Thresholds for PM2.5 and Ozone (pdf) (2.11 MB)
• Peer Review Report for the Technical Basis for the EPA's Development of Significant Impact Thresholds for PM2.5 and Ozone (pdf) (233.49 KB)