If you operate a large non-Electric Generating Unit (non-EGU) in Indiana that's subject to the NOX SIP Call, the EPA just made a significant move that affects your monitoring, reporting, and recordkeeping obligations. On May 14, 2026, the EPA published a final rule in the Federal Register approving Indiana's revised State Implementation Plan (SIP) under the Clean Air Act — and the practical compliance implications are real enough to warrant a close look before your next operating cycle.
The lesson here isn't complicated, but it matters: when a state SIP revision gets federally approved, it doesn't just update a state document. It becomes federally enforceable. That changes the stakes.
What the EPA Just Approved — and Why It Matters
The EPA's action (Federal Register Document 2026-09612, published May 14, 2026) approves a request from the Indiana Department of Environmental Management (IDEM) to revise Indiana's SIP under Section 110 of the Clean Air Act. The revision incorporates updated nitrogen oxides (NOX) emissions monitoring, reporting, and recordkeeping requirements specifically for new and existing large non-EGUs affected by the NOX SIP Call.
The NOX SIP Call, originally promulgated under 40 CFR Part 51, has been a foundational piece of EPA's ozone transport control strategy for years. Indiana's revision brings the state's implementation framework into closer alignment with current federal expectations — and once the EPA approves a SIP revision, enforcement is no longer just a state-level matter. It carries federal Clean Air Act authority.
In practical terms: if your facility falls under the NOX SIP Call as a large non-EGU, your monitoring program now needs to match what Indiana's revised SIP requires. The window to get your house in order is now, not later.
Who Is Affected by This Rule?
The scope here is specific. This rule targets large non-Electric Generating Units — industrial boilers, cement kilns, glass furnaces, and similar large combustion or process sources — that are covered under Indiana's NOX SIP Call obligations. Electric generating units (EGUs) operate under a separate framework and are not the subject of this revision.
If you're uncertain whether your facility qualifies as a "large non-EGU" under Indiana's SIP, that determination hinges on your facility's NOX emission rate and whether it's been identified by IDEM as a covered source. According to the EPA's NOX SIP Call framework, large non-EGUs are generally those emitting NOX above the applicable threshold during the ozone season (May 1 through September 30).
A few categories worth reviewing against the new requirements:
| Source Type | Likely Coverage | Key Monitoring Obligation |
|---|---|---|
| Industrial boilers (large) | High — if above NOX threshold | Continuous Emissions Monitoring System (CEMS) or approved alternative |
| Cement kilns | High | CEMS or PEMS, per approved monitoring plan |
| Glass melting furnaces | Moderate to High | Monitoring plan required; CEMS often required |
| Chemical process units | Situational — size-dependent | Consult IDEM for applicability determination |
| Small industrial boilers | Generally excluded | Confirm with IDEM based on actual NOX rates |
If your facility has been operating under an existing monitoring plan approved under Indiana's prior SIP language, this is a good time to pull that plan and compare it against the revised requirements. SIP revisions don't automatically grandfather old monitoring plans when the underlying requirements change.
What Actually Changed in Indiana's NOX Monitoring Requirements
The SIP revision approved here addresses three interconnected areas: monitoring, reporting, and recordkeeping. Each one has day-to-day compliance implications.
Monitoring. Indiana's revised SIP aligns its monitoring requirements for large non-EGUs with EPA's expectations under 40 CFR Part 75 or approved alternative monitoring methodologies. Facilities subject to this rule are generally required to use Continuous Emissions Monitoring Systems (CEMS) or receive approval for an alternative approach. The revision clarifies what "new" and "existing" sources must do, which matters for facilities that have recently undergone modifications or are planning capital projects.
Reporting. The reporting framework under the revised SIP establishes how facilities must submit NOX emissions data — including what data elements are required, submission timelines, and the format IDEM expects. Ozone season NOX emissions data is central here. Facilities should confirm their current data management systems can produce reports that match Indiana's revised specifications.
Recordkeeping. This is the area that often gets underestimated. Recordkeeping requirements tie directly to audit readiness. Under the revised SIP, large non-EGUs must maintain documentation that supports every data point submitted — monitoring system calibrations, quality assurance records, and audit trails for any data substitution or missing data events.
Effective Dates and Compliance Deadlines
The rule was published May 14, 2026 in the Federal Register. Federal SIP approvals typically carry a 30-day effective date after publication, placing the effective date at approximately June 13, 2026, absent any specific delayed-effective-date language in the rule.
For facilities already operating under Indiana's NOX SIP Call framework, this doesn't mean you have until June 13 to start fresh. If IDEM's revised requirements have been in effect at the state level ahead of the EPA's federal approval — which is common in SIP revision sequencing — your state-law obligations may predate the federal approval. The EPA's approval simply federalizes those requirements.
Key dates to flag:
- May 14, 2026 — Federal Register publication date (Document 2026-09612)
- ~June 13, 2026 — Expected federal effective date (30 days post-publication)
- May 1 – September 30 — Annual ozone season; NOX monitoring and reporting obligations are most active during this window
- Ongoing — CEMS quality assurance testing, data substitution documentation, and quarterly/annual reporting cycles continue under the revised framework
If your facility is in the middle of the ozone season when this rule takes effect, that's not a reason to wait. Compliance gaps discovered during an ozone season audit carry heavier consequences than gaps identified and corrected proactively.
The Practical Compliance Gap Most Facilities Miss
In my experience working with industrial facilities on air quality compliance, the monitoring hardware tends to get attention while the recordkeeping system quietly falls behind. You can have a fully functional CEMS and still be out of compliance because your QA records are incomplete, your data substitution decisions aren't documented at the level the SIP requires, or your reporting format doesn't match what IDEM expects.
The revised Indiana SIP tightens the loop between these three areas. Monitoring data that isn't properly recorded can't be properly reported. Reported data that doesn't reconcile with your QA records is a finding waiting to happen.
I'd suggest a three-step internal review for any Indiana facility potentially covered by this rule:
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Applicability confirmation. Pull your facility's NOX emission rates and confirm your covered-source status with IDEM directly. Don't assume historical determinations still apply if you've modified your process or fuel sources.
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Monitoring plan review. Compare your existing approved monitoring plan against the revised SIP language. If there's a gap — even a procedural one — initiate the plan amendment process with IDEM before the ozone season puts you in a reporting window.
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Recordkeeping audit. Run a mock audit on your last full ozone season's records. Ask whether each data point you reported can be traced back to a calibration record, QA event log, or documented substitution decision. If it can't, that's a corrective action item.
What This Means in the Broader NOX Regulatory Landscape
Indiana's SIP revision doesn't exist in isolation. The EPA has been systematically pressing states to strengthen their NOX SIP Call implementation as part of its broader ozone transport and NAAQS attainment strategy. According to the EPA, ground-level ozone — formed when NOX and volatile organic compounds react in sunlight — remains one of the most widespread air quality challenges in the U.S., with roughly 120 million Americans living in areas that still exceed the 2015 ozone NAAQS of 70 parts per billion.
Indiana sits downwind of significant industrial activity and has historically been scrutinized under the Good Neighbor Provision of the Clean Air Act (CAA Section 110(a)(2)(D)), which requires states to prevent their emissions from significantly contributing to nonattainment in downwind states. The NOX SIP Call is one of the primary tools EPA uses to enforce this obligation.
This SIP approval is consistent with a broader pattern: the EPA is approving state-level NOX monitoring revisions that increase data quality, close reporting gaps, and strengthen the evidentiary record for enforcement. That last point matters — better data means enforcement actions are easier to pursue and harder to defend against.
A Note on State vs. Federal Enforcement Authority
One thing worth understanding clearly: before the EPA's federal approval, Indiana's revised monitoring requirements were enforceable under state law. After the EPA's approval, those same requirements are also enforceable under the federal Clean Air Act.
That distinction has real consequences. Federal CAA enforcement authority includes civil penalties of up to $70,117 per day per violation (adjusted for inflation under 40 CFR Part 19), potential criminal liability for knowing violations, and citizen suit provisions that allow third parties to sue for violations independent of EPA action. State enforcement authority, while meaningful, typically operates through a narrower set of tools.
The federalization of Indiana's revised NOX monitoring requirements means that any compliance gap is no longer just a conversation with IDEM. It's a potential federal enforcement matter.
How Certify Consulting Can Help
At Certify Consulting, we've worked through air quality compliance programs with clients across regulated industries, including facilities navigating SIP-based monitoring obligations. Whether you're confirming applicability, auditing an existing monitoring plan, or preparing for a CEMS performance audit, the work is always more manageable when you start before the regulatory clock is running.
Our track record — 200+ clients, 100% first-time audit pass rate, 8+ years in compliance consulting — comes from one consistent habit: we don't wait for a finding to start solving the problem. If you want to learn more about how we approach environmental and regulatory compliance work, visit certify.consulting or reach out directly to talk through your situation.
Frequently Asked Questions
What is the NOX SIP Call and why does it apply to Indiana? The NOX SIP Call is an EPA rulemaking under the Clean Air Act that requires certain upwind states — including Indiana — to reduce nitrogen oxides emissions that contribute to ozone nonattainment in downwind states. Indiana's obligations under the NOX SIP Call cover large non-EGU sources above applicable emission thresholds, particularly during the ozone season.
What's the difference between a state SIP requirement and a federally approved SIP requirement? Before EPA approval, SIP requirements are enforceable only under state law. Once EPA approves a SIP revision, those requirements become part of the federally enforceable SIP, meaning EPA and private citizens can also pursue enforcement under the Clean Air Act — not just the state agency.
Does this rule affect Electric Generating Units (EGUs) in Indiana? No. This SIP revision specifically addresses large non-EGUs. EGUs operate under a separate regulatory framework and were not the subject of IDEM's revision request or the EPA's May 14, 2026 approval.
What monitoring method is typically required for large non-EGUs under Indiana's revised SIP? The revised SIP generally requires Continuous Emissions Monitoring Systems (CEMS) or an EPA-approved alternative monitoring methodology for large non-EGUs. Facilities wishing to use an alternative must have that approach approved by IDEM as part of their monitoring plan.
What should I do first if I think my facility might be covered? Start with an applicability determination. Pull your facility's NOX emissions data, review your current permit conditions against Indiana's revised SIP language, and contact IDEM directly if there's any ambiguity. Don't rely on a historical applicability determination if your operations or fuel sources have changed.
Source: Federal Register, Document 2026-09612, "Air Plan Approval; Indiana; Indiana NOX Emissions Monitoring," published May 14, 2026. Available at federalregister.gov.
Last updated: 2026-05-29
Jared Clark
Principal Consultant, Certify Consulting
Jared Clark is the founder of Certify Consulting, helping organizations achieve and maintain compliance with international standards and regulatory requirements.