Blog   |   Sep 20th, 2016 Striving for Insignificance: A Review of EPA'S Ozone and PM2.5 Significance Thresholds


EPA published a draft memo on August 18, 2016 which, among other important issues, provided guidance on the level and use of threshold impact levels for two air pollution species: ozone and fine particulate matter with diameters less than or equal to 2.5 microns (PM2.5). These thresholds serve as a critical measure of significance in air quality modeling for projects requiring a PSD level review of air emissions, so it is important to understand what EPA’s recommendation means.

What is PSD?

Prevention of Significant Deterioration, or PSD, applies to major sources of air pollution, per the EPA New Source Review (NSR) program. A project requiring a PSD review and construction permit must demonstrate quantitatively that the emissions from a proposed new source or modification of an existing source of air pollution in the project will not “cause or contribute to a violation” of EPA’s air quality standards. If the project can show that its new or modified emissions are what EPA considers “insignificant”, a PSD review and permitting path is considerably easier. 

What is SIL?

EPA defines what they think is a significant contribution to predicted air quality concentrations using a pollutant-specific threshold called a Significant Impact Level (SIL). A SIL is a statistically-based and quantifiable limit of modeled impacts below which a new or modified source is considered by EPA to be insignificant. Within a PSD review, modeled air quality impacts of a new or modified source are compared against SILs for each pollutant under evaluation.

For example, if one models the PM2.5 air quality impacts from a new or modified source (say a new lumber mill) and the maximum offsite concentrations at ground level are less than the recommended SILs for PM2.5, then the new mill's PM2.5 emissions are considered to not significantly affect air quality with respect to PM2.5.  No further investigation of PM2.5 air quality impacts would be necessary and the permitting process would continue.

However, if the lumber mill’s modeled PM2.5 impacts exceed the SIL, additional modeling will be required to investigate the mill’s broader PM2.5 impacts. Further, the spatial extent to which the SIL is exceeded will dictate what other existing or permitted sources in addition to the new mill would need to be evaluated.

Needless to say, everyone wins when modeled impacts below the SIL. Air emissions are obviously minimized and air permitting and modeling evaluations are much simpler. You can therefore imagine that any change to a SIL by EPA always creates a buzz in the air quality community. In fact, during an August 24, 2016 EPA presentation of the proposal to the community, over 330 participants joined the call to hear what EPA was planning.

What is the Change in Guidance for ozone and PM2.5?

In the August memo, EPA provided guidance on two substantial proposed changes:

  • Lowering the PM2.5 SIL for annual average impacts, and
  • Creating a new ozone SIL for potential future modeling requirements. 

The proposed guidance will be considered discretionary for permitting agencies and regulators (states), but EPA strongly suggested and recommended that states use the new guidance values. The table below describes the changes.

Table 1: Significance Impact Level Changes Proposed in August 18, 2016 EPA Memo

Pollutant / Averaging Period to Use



Ozone / 8 hours


1.0 ppb

PM2.5 / Annual

0.3 ug/m3

0.2 ug/m3

As you can see, the recommended annual average PM2.5 value has dropped 0.1 ug/m3. This may not seem substantial, but it reduces the SIL by 1/3 and could have big implications for your project.

This change means there would be a lower threshold for demonstration of insignificance to the PM2.5 air quality standard.  Power generation and other manufacturing sources (mining, lumber, etc.) already pushing the envelope of controls to get below the current SIL will be challenged to meet the lower revised SIL or face more burdensome modeling analyses, requiring additional time, expense and possible permitting delays. Depending where a project is on the permitting path, this could affect on-going projects and future projects.

A lower SIL could also mean a larger area of significance, requiring additional offsite investigation modeling, as you can see in the following generalized schematic:


Finally, ozone modeling currently has no significance level and is generally not quantitatively addressed in initial air quality investigation of a new or modified source.  With the impending release of the other air modeling procedures from EPA, ozone will now have a specific quantitative target of significance, and ozone modeling will need to be considered within initial air modeling investigations.

The EPA plans to finalize this guidance by the end of 2016 (Appendix W by fall 2016), but there is still time to determine a revised course of action for your projects.

What should you do?

  • Evaluate your impacts: If you are currently in a project permitting process and have not yet gone through a modeling protocol development step with your regulatory authority, you should review your impacts to determine if the new SIL would require changes in the design of your facility.
  • Engage in the process: The public comment period on the draft guidance is open through September 30th. TRC can help you form your comments for EPA review and represent your project to ensure your concerns are heard.
  • Enlist an expert: TRC’s air quality services incorporate all technical aspects of facility permitting, control engineering, regulatory compliance, air quality compliance demonstration and assessment, emission inventory, and measurement and monitoring. Our national presence and dedicated regional teams can support you with all of your project's air quality evaluation and permitting needs. If you need support designing and planning your next project, or even just in understanding how changes in EPA guidance may affect your current project, please contact Matt Jones

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Matthew Jones

Matthew Jones

Matt Jones is a Principal Meteorologist, with 12 years of experience building and testing weather and micro-climate modeling systems and meteorological analysis in complex terrain. At TRC, Matt works in the Air Quality market as a meteorological and dispersion modeling specialist for permitting, litigation, and risk mitigation projects. Contact Matt at