Significant Impact Levels: the screening ladder

A permit modeling demonstration is staged. The Guideline (40 CFR Part 51, Appendix W §9.2.3) describes a two-stage approach: model the project alone first, and only escalate to the full cumulative analysis when the project-alone impact reaches a Significant Impact Level (SIL):

a. As described in this subsection, the recommended procedure for conducting either a NAAQS or PSD increments assessment under PSD permitting is a multi-stage approach that includes the following two stages: i. The EPA describes the first stage as a single-source impact analysis, since this stage involves considering only the impact of the new or modifying source. … ii. The EPA describes the second stage as a cumulative impact analysis, since it takes into account all sources affecting the air quality in an area. In addition to the project source impact, this stage includes consideration of background, which includes contributions from nearby sources and other sources (e.g., natural, minor, and distant major sources).

Source · 40 CFR 51 App. W §9.2.3(a) — captured verbatim in the source library.

The logic of the screen: if the project's own impact is below the SIL at every receptor, the source is treated as not causing or contributing to a violation, and the demonstration ends there — no inventory of neighboring sources, no background analysis. Crossing the SIL anywhere triggers the cumulative stage, whose design value (modeled + background) is compared to the NAAQS or increment under the PSD rule's required demonstration:

(k) Source impact analysis — (1) Required demonstration. The owner or operator of the proposed source or modification shall demonstrate that allowable emission increases from the proposed source or modification, in conjunction with all other applicable emissions increases or reductions (including secondary emissions), would not cause or contribute to air pollution in violation of: (i) Any national ambient air quality standard in any air quality control region; or (ii) Any applicable maximum allowable increase over the baseline concentration in any area.

Source · 40 CFR §52.21(k)(1) — captured verbatim in the source library.

Where the SIL numbers come from

An important distinction: Appendix W codifies the approach, not a table of numbers. With one exception, the numeric SILs live in EPA guidance:

• Codified in regulation: a source constructing within 10 km of a Class I area whose 24-hour impact on that area is ≥ 1 µg/m³ is significant by definition — 40 CFR §52.21(b)(23)(iii).
• EPA guidance (2018 SILs guidance + the April 2024 supplement): the recommended values for ozone and PM2.5, revised after the 2024 annual PM2.5 NAAQS. From the supplement's Table 1, for use "on a case-by-case basis, in the manner described in the 2018 guidance":

• For PM2.5 increment comparisons, the supplement's Table 2 recommends the same values for Class II/III areas and lower values for Class I (calculated from the Class I : Class II increment ratios), "to uphold a higher level of protection."

Source · EPA, Supplement to the Guidance on Significant Impact Levels for Ozone and Fine Particles in the PSD Permitting Program (April 30, 2024), Tables 1–2 — held in the platform's source library; 2018 SILs guidance + supplement at EPA NSR guidance.

SILs for the other criteria pollutants trace to earlier EPA guidance documents; the platform's reference tables state each value's specific provenance rather than treating them as codified.

Two cautions the guidance itself gives

The 2024 supplement is explicit that SILs are recommendations, applied case-by-case by the permitting authority — not a bright-line entitlement; and that a below-SIL result is the basis for concluding no significant contribution, not an automatic exemption in every circumstance. Both cautions carry into practice: agencies can and do ask for cumulative analyses despite sub-SIL screens.

Source · 2024 SILs supplement, §I (use "on a case-by-case basis … subject to the limitations described in that guidance").

In PlumeSmart

The compliance view runs the ladder in this order automatically: the project-alone design value against the SIL first (with each SIL's provenance labeled), and the cumulative design value against the NAAQS and applicable increments when the analysis includes inventory and background.