How Florida Electric and SSM Litigation Group reshaped EPA’s perspective on SIP reviews and whether emission standards need to be met during all periods of operation.

By Matt Morrison, Partner; Mark Talty, Senior Counsel; and Natalie Buechler, Associate

EPA’s September 1, 2026 SSM memorandum gives air agencies a framework for reviewing SIP provisions after two D.C. Circuit decisions reset the agency’s approach to startup, shutdown, malfunction, and affirmative defenses. The memorandum supersedes EPA’s December 19, 2024 memorandum on the same subject, but does not itself amend a SIP, permit, or enforcement rule. It does, however, signal how EPA may evaluate future SIP submissions and how operators should assess existing limits, SSM provisions, and compliance strategies before a state submits revisions or EPA opens a notice-and-comment proceeding.

Key Takeaways

• EPA will generally presume continuity where a provision is required to be an emission limitation or standard under another CAA provision, is used expressly to attain or maintain a NAAQS or comparable requirement, or the state provides no relevant information. States may rebut that presumption with rationale and analysis.


• Alternative emission limitations can provide operating flexibility: a continuous emission limitation may include different numerical limits, technology requirements, or work practices for startup and shutdown. The memorandum does not categorically eliminate SSM flexibility.


• The memorandum is non-binding guidance, not a new rule. EPA says it will decide SIP questions through notice-and-comment proceedings, case by case.

Background: Why SSM Provisions Matter

Under the CAA, SIPs are the primary mechanism through which states implement, maintain, and enforce the NAAQS for six criteria pollutants: carbon monoxide, lead, ozone, nitrogen dioxide, particulate matter, and sulfur dioxide. Once EPA approves a SIP provision, it becomes federally enforceable.

CAA Section 302(k) defines an “emission limitation” as a requirement that limits emissions “on a continuous basis.” Importantly, the continuity requirement is triggered only when a SIP provision is “necessary or appropriate” to function as an “emission limitation” under CAA § 110(a)(2)(A). This requirement is central to the SSM debate because industrial facilities inevitably experience periods of startup, shutdown, and malfunction during which emissions may exceed normal operating limits. The question is whether, and to what extent, SIP provisions may accommodate those events.

On June 12, 2015, EPA issued the 2015 SSM SIP Action, 80 Fed. Reg. 33,840, and a separate CAA section 110(k)(5) SIP Call to 36 air agencies covering 45 statewide and local jurisdictions. EPA identified four types of SSM-related provisions as inconsistent with the CAA: automatic exemptions (categorical exemptions for excess emissions during SSM events), director’s discretion provisions (case-specific regulatory exceptions), overbroad enforcement discretion provisions (limits on enforcement authority), and affirmative defenses (defenses to liability or remedies for otherwise proven violations). The agency required revisions because, in its view, those provisions could allow excess emissions to escape otherwise applicable SIP requirements or interfere with the CAA’s enforcement structure. Florida Electric later vacated part of that action, but the SIP Call remains in effect for overbroad enforcement discretion and affirmative defenses that limit specific forms of relief.

Two D.C. Circuit Decisions Changed the Landscape

In Environmental Committee of the Florida Electric Power Coordinating Group, Inc. v. EPA, 94 F.4th 77 (D.C. Cir. 2024), the D.C. Circuit reviewed EPA's 2015 SIP Call and delivered a mixed ruling with far-reaching implications:

• Vacated in part: The court vacated the 2015 SIP Call as to automatic exemptions, director's discretion provisions, and complete affirmative defenses. EPA had not first determined that it was “necessary or appropriate” under CAA section 110(a)(2)(A) for the specific provisions to function as emission limitations subject to section 302(k)'s continuity requirement.


• Upheld in part: The court upheld the SIP Call as to overbroad enforcement discretion and affirmative defenses that limit only specific forms of relief, such as monetary penalties while leaving injunctive relief available.


• Key holding: Florida Electric did not decide whether automatic exemptions or director's discretion provisions belong in SIPs. It held that EPA needed the threshold “necessary or appropriate” determination before using the continuity requirement as the basis for the SIP Call, leaving open a later, properly supported action.

After Florida Electric, EPA partially withdrew its Findings of Failure to Submit for 13 states that had not responded to the SIP Call. The withdrawal stopped sanctions and Federal Implementation Plan (FIP) clocks for provisions affected by the vacatur. See 91 Fed. Reg. 35,628 (June 12, 2026).

In SSM Litigation Group v. EPA, 150 F.4th 593 (D.C. Cir. 2025), the court further clarified the treatment of affirmative defenses and significantly shifted the legal landscape:

• Complete affirmative defenses are permissible: The court held that an affirmative defense that, if proven, precludes any form of liability or remedy for violations of emission limitations is valid. It reasoned that a complete affirmative defense is a “defense to liability, not a limitation on judicial remedies,” and does not make an emission limitation non-continuous under section 302(k).


• EPA's prior rescission was vacated: The court vacated EPA's rescission of longstanding Title V affirmative defense provisions, which EPA had adopted based on its rejected view that all affirmative defenses were impermissible. The rescission appeared at 88 Fed. Reg. 47,029 (July 21, 2023).


• Scope and timing: Although the case arose in the Title V context, EPA expects the D.C. Circuit's reasoning to apply broadly to affirmative defenses that states may include in SIP provisions outside Title V permits. The D.C. Circuit denied rehearing en banc on January 2, 2026.

EPA's New Framework: A Presumption, Not a Prohibition

Against that backdrop, the memorandum tells air agencies how EPA intends to evaluate SIP submissions raising SSM issues. Any provision required by law to be an “emission limitation,” or that EPA determines is “necessary or appropriate” to function as one, must be continuous under section 302(k) and may not contain an exemption for SSM emissions. States make the initial judgment; EPA makes the final determination through the notice-and-comment process. The memorandum provides a review framework, not a categorical rule invalidating existing SIP provisions.

1. The provision is required to function as an emission limitation or emission standard under a CAA provision other than section 110(a)(2)(A).


2. The state is using the provision expressly to attain or maintain a NAAQS or to meet a comparable CAA requirement.

When either category applies, or the state provides no relevant information, EPA says it will generally presume that the provision is an “emission limitation” under section 302(k) and must apply continuously. This is a rebuttable presumption: a state may include rationale and analysis explaining why the provision should not be an emission limitation, and may challenge EPA's proposed action during the public-comment period.

For example, if a provision was assumed to operate continuously in an attainment demonstration, EPA will generally presume that the SIP provision is an emission limitation and that its application in practice must match the modeling assumptions. Category 1 likewise reaches provisions intended to constitute BACT or NSPS emission limitations when submitted to satisfy a SIP obligation. These are presumptions for EPA's review—not automatic findings that every such provision is unlawful.

The framework does not require all SIP controls to be numerical or continuously applicable. EPA recognizes non-numerical emission limitations and other controls, and explains that a continuous emission limitation may include alternative emission limitations (AELs)—different numerical levels, technological control requirements, or work practices for startup and shutdown—so long as the overall limitation is continuous and meets applicable CAA stringency and enforceability requirements. Other controls may apply only at particular times or under particular circumstances, but EPA expects at least some restrictions to apply continuously when a state relies on the provision to achieve or maintain a NAAQS or comparable requirement.

The memorandum is non-binding and does not itself resolve the status of any particular SIP provision. EPA has not systematically reviewed all SIPs for SSM provisions that may be inconsistent with its policy or the D.C. Circuit decisions. Future evaluations will proceed case by case through notice and comment. For pending submissions, air agencies may contact the relevant Regional Office, withdraw or supplement a submission, or resubmit it; for SIPs still under development, agencies may characterize provisions themselves or rely on EPA's presumptions.

What Operators Should Watch

The updated framework creates several areas of uncertainty and potential exposure for industrial operators:

• SIP revision risk. As states submit new or revised SIPs, EPA may apply its continuity presumption. Provisions containing SSM exemptions or unbounded discretion may be rejected or modified if the state cannot establish that they need not function as continuous limits. Facilities should not assume that the memorandum has categorically invalidated existing provisions.


• Enforcement exposure. The 2015 SIP Call remains in effect for overbroad enforcement discretion and affirmative defenses that limit specific forms of relief. If a SIP or permit requires continuous compliance, excess emissions during SSM may create enforcement risk; complete affirmative defenses may remain available where validly adopted.


• Citizen suit risk. Changes to SSM provisions may affect how exceedances during startup, shutdown, or malfunction are treated in citizen suits. Operators should assess both defenses to liability and provisions that limit only particular judicial remedies.


• Permit and operational impacts. Facilities may need to revise operating procedures, invest in additional controls, or modify startup and shutdown sequences to achieve continuous compliance. AELs may allow different requirements during those modes while preserving an overall continuous limitation.


• Case-by-case uncertainty. The memorandum is non-binding, and EPA has not conducted a systematic review of all existing SIPs for SSM consistency. The timing and outcome of review will depend on the state's submission, EPA's proposed action, public comments, and the facts presented.

What Companies Should Do Now

Companies should use the memorandum as a screening tool, not as a substitute for permit-specific analysis. They should also consider the following steps:

1. Audit existing air permits and SIP-based emission limitations. Identify all SSM-related provisions in current air permits, state SIP provisions, and applicable consent decrees. Look for automatic exemptions, director's discretion provisions, overbroad enforcement discretion, and affirmative defenses, and document how each provision operates during an SSM event.


2. Classify provisions under EPA's framework. For each provision, determine whether it falls within Category 1, Category 2, or neither. Also determine whether the state has supplied relevant information; no information triggers the same general continuity presumption. If the state disagrees with that presumption, develop the rationale and analysis needed to rebut it.


3. Engage proactively with state air agencies. Work with state regulators on pending or upcoming SIP revisions to ensure that SSM provisions are properly characterized and that the state's submissions explain whether each provision is intended to be an emission limitation. For pending submissions, consider whether withdrawal, supplementation, or resubmission would provide a better record.


4. Evaluate affirmative defense and AEL options. Distinguish complete affirmative defenses, which SSM Litigation Group held permissible, from defenses that limit specific remedies, which Florida Electric upheld the SIP Call against. Evaluate whether AELs with different numerical limits, technological control requirements, or work practices during startup and shutdown can provide flexibility while satisfying applicable CAA requirements.


5. Strengthen SSM event protocols. Review and enhance internal procedures for startup, shutdown, and malfunction events, including operational protocols, emissions monitoring, recordkeeping, and reporting systems. Robust documentation is essential for demonstrating compliance and supporting any affirmative defense or agency submission.


6. Monitor EPA and state actions. Track EPA actions on SIP submissions in relevant jurisdictions, including the 13-state withdrawal and any proposed approvals or disapprovals. Consider submitting comments on SIP-related rulemakings that could affect facility operations.

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Baker Botts will continue to monitor EPA and state actions implementing the memorandum. If you have questions about how this guidance may affect your facilities, permits, or pending SIP revisions, or would like to discuss SSM-related compliance strategy, please contact the authors or your usual Baker Botts contact.

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