On April 17, 2010 EPA issued a guidance document on the implementation of significant impact levels (“SIL”) for ozone and fine particles. Under EPA’s air pollution permitting regime known as “New Source Review,” SIL values are one way to demonstrate that a new facility or modification of an existing facility will not cause a violation of the National Ambient Air Quality Standards (“NAAQS”) or Prevention of Significant Deterioration (“PSD”) increments for a regulated pollutant. In short, if a source’s “projected impact on air quality” is below the “SIL,” the source is deemed to have no significant impact on air quality. If a source’s impacts are above the SIL, far more extensive modeling analyses are needed to demonstrate compliance, so the SIL helps streamline the permitting process for projects that can meet it.

The guidance document sets the following recommended SILs for NAAQs and PSD increments:

Recommended SIL Values for Ozone and PM2.5 NAAQS

Criteria Pollutant (NAAQS level) NAAQS SIL Concentration
Ozone 8-hour (70 ppb) 1.0 ppb
PM2.5 24-hour (35 µg/ m3) 1.2 µg/ m3
PM2.5 annual (12 µg/ m3 or 15 µg/ m3) 0.2 µg/ m3

Recommended SIL Values for PM2.5 PSD Increments

Criteria Pollutant

(averaging period)

PSD increment SIL concentration
Class 1 Class 2 Class 3
PM2.5 (24-hour) 0.27 µg/ m3 1.2 µg/ m3 1.2 µg/ m3
PM2.5 (annual) 0.05 µg/ m3 0.2 µg/ m3 0.2 µg/ m3

EPA developed the SILs using a different methodology than has been used in the past. In essence, EPA evaluated the variability of ambient concentrations of each pollutant across the country to determine what level of increase would be considered “statistically significant” in light of that natural variability. In other words, if the expected impact of a new source or project would be less than the normal variation typically observed in the ambient air, the source’s impact would be deemed insignificant. Importantly, EPA’s guidance claims that this approach allows SILs to be used even in areas that are already very close to exceeding a standard, something EPA’s prior guidance declined to cover.

In the guidance, EPA emphasized that permitting authorities will always have the discretion to use the SIL values on a case-by-case basis, even if EPA later decides to officially promulgate the SILs via regulation. Thus, while it may provide sources an opportunity to seek a more streamlined permitting approach, it leaves to the states the discretion to decide whether a streamlined approach is appropriate in each individual case.