On April 17, 2018 EPA released a memo titled “Guidance on Significant Impact Levels for Ozone and Fine Particles in the Prevention of Significant Deterioration Permitting Program.” In this guidance EPA recommended a 24-hour significant impact level (SIL) of 1.2 µg/m3 for particulate matter less than or equal to 2.5 microns in diameter (PM2.5), which is the same level as the previously proposed draft SIL