On August 1, 2016, the EPA proposed ozone (8-hour) and PM2.5 (24-hour and annual) Significant Impact Level (SILs). The EPA finalized the SILs with no changes from the proposed recommendations in a memorandum dated April 17, 2018 as shown below in Table 1.

Table 1. Recommended NAAQS and Increment SIL Values for Ozone and PM2.5

Criteria Pollutant (Averaging Period)
Increment SIL
Class IClass II
Ozone70 ppb1.0 ppb--
PM2.5 (24-hour)35 µg/m31.2 µg/m30.27 µg/m31.2 µg/m3
PM2.5 (Annual)12 µg/m30.2 µg/m30.05 µg/m30.2 µg/m3

Regarding the PM2.5 annual NAAQS SIL, although 40 CFR 51.165(b)(2) currently lists 0.3 µg/m3 as the PM2.5 annual NAAQS SIL, the April 2018 EPA memorandum recommends utilizing 0.2 µg/m3 as listed in Table 1. However, the memorandum does discuss the use of an alternate SIL value. Specifically:

Since 40 CFR 51.165(b)(2) does not address whether an impact below 0.3 μg/m3 causes or contributes to a violation of the NAAQS, the EPA and other permitting authorities retain the discretion under this provision to determine on a case-by-case basis whether an impact between 0.2 μg/m3 and 0.3 μg/m3 will cause or contribute to a violation of the annual PM2.5 NAAQS. However, based on the ambient air quality variability approach, the EPA's judgment is that an impact below 0.2 μg/m3 is not significant and should be considered to not cause or contribute to any violation of the annual PM2.5 NAAQS that is identified.

Based on discussions with the LDEQ, an applicant may propose to use a PM2.5 annual SIL other than 0.2 µg/m3; however, EPA Region 6 would likely request justification in an air dispersion modeling protocol. The EPA and LDEQ will assess every protocol and application on a case-by-case basis. Proposing an alternative SIL will most likely increase the time associated with the protocol approval process. As a result, an applicant should plan accordingly from an air permitting strategy perspective.

If you have any questions, please contact the Baton Rouge office .