NOx RACT and the St. Louis Metro: Why a New Illinois Rule Is a Missouri Story Too

On August 6, 2026, EPA proposed approving new NOx control requirements for industrial boilers, furnaces, and kilns in two Illinois ozone nonattainment areas. Read past the Illinois dateline and there’s a Missouri story inside it: one of those two areas, Metro-East St. Louis, is not an Illinois area at all. It’s a single bi-state nonattainment area that reaches across the Mississippi River into St. Louis County, St. Louis City, St. Charles County, Jefferson County, and part of Franklin County, Missouri. The Missouri half carries its own Serious-area NOx RACT obligation, on its own August 3, 2027 attainment clock, and Illinois just published the best available preview of what that obligation is going to look like.

That matters more than a routine SIP notice would, because a plant manager’s first reaction to a facility-wide “major source” designation is usually the wrong question. The right question isn’t whether your facility is big enough to be covered. It’s whether any single unit inside it meets a narrower, conjunctive test that most coverage of RACT rules doesn’t explain correctly.

What did EPA propose on August 6, 2026?

EPA proposed approving revisions to three Illinois regulations, 35 Illinois Administrative Code (IAC) parts 217, 218, and 219, into the Illinois State Implementation Plan (91 FR 50746). Parts 218 and 219 satisfy Moderate-area VOC RACT. Part 217, the one that matters here, satisfies both Moderate and Serious-area NOx RACT under Clean Air Act sections 182(b)(2), 182(c), and 182(f). The action covers two nonattainment areas: Chicago and Metro-East St. Louis.

This is a proposed rule, not a final one. Comments are due September 8, 2026, at regulations.gov under dockets EPA-R05-OAR-2024-0617, EPA-R05-OAR-2024-0618, and EPA-R05-OAR-2025-0259. Nothing here is settled yet, and a plant manager reading a summary that treats it as final is reading a summary that overstates the current status.

One more scope note before the substance: Serious-area VOC RACT for these same two areas is explicitly deferred to a separate, future EPA action, stated twice in the notice. A facility that clears this round on NOx should expect a second round on VOC later, not treat this notice as the last word.

Why is an Illinois rule a Missouri compliance question?

Because the nonattainment area it covers isn’t only in Illinois. Per the Federal Register notice, the Metro-East area includes “Madison, St. Clair, Monroe counties in Illinois, and St. Louis, St. Louis City, St. Charles, and Jefferson counties, as well as Boles Township in Franklin County in Missouri.” That’s one nonattainment area straddling a state line, the same way a single airshed doesn’t stop at a river.

The Missouri portion was reclassified from Moderate to Serious effective December 31, 2024, but the Eighth Circuit stayed that reclassification on June 6, 2025, leaving Moderate-area requirements (100 tpy RACT threshold) in force on the Missouri side while the stay holds. See our St. Louis RACT breakdown for the full timeline. If the stay lifts, Missouri moves to the same Serious classification and August 3, 2027 attainment date as the Illinois portion. Missouri DNR has been developing the Serious-area attainment plan since at least April 2025. Illinois hasn’t finished its own NOx RACT rule yet either, but its proposed rule text, now public and moving through EPA approval, is the closest thing available to a preview of the control level Missouri regulators and EPA will expect on the Missouri side of the same nonattainment area.

That’s a materially different framing than “an out-of-state rule that doesn’t affect us.” A plant in Sauget or Granite City, Illinois, and a plant in south St. Louis County, Missouri, sit inside the same designated nonattainment area and are working toward the same statutory deadline. The rule that lands first tells you what the second one is likely to look like.

Does my facility’s NOx RACT obligation depend on being a “major source”?

Not by itself, and this is where most coverage of RACT rules oversimplifies the rule. Illinois’s new part 217 test is conjunctive: two separate conditions both have to be true before a unit is captured.

The individual unit is one of eight named equipment categories, and it emits at least 15 tons per year of NOx.
That unit sits at a source located in the nonattainment area with facility-wide potential to emit at least 50 tons per year of NOx.

Fail either condition and the unit isn’t captured, no matter how the other one comes out. A facility with 60 tons per year of facility-wide NOx potential to emit, comfortably over the 50 tpy gate, can still have zero captured units if every individual boiler, heater, or furnace sits under 15 tpy on its own. Run the arithmetic the other way and a single 40-tpy unit at a facility whose total potential to emit is only 45 tpy isn’t captured either, because the facility-wide gate isn’t met. “My facility is a major source” and “my unit is a RACT unit” are two different questions, and treating them as the same one is the single most common way this kind of rule gets misapplied.

Which equipment categories does the new rule name?

Eight, not nine, despite how some early summaries of the rule have described it. The Federal Register text names, verbatim: any industrial boiler, process heater, glass melting furnace, cement kiln, lime kiln, iron and steel reheat, annealing, or galvanizing furnace, aluminum reverberatory or crucible furnace, or fossil fuel-fired stationary boiler (91 FR 50748).

Counted as named categories, that’s eight. Two of them, the iron and steel furnace category and the aluminum furnace category, each bundle multiple furnace subtypes, so counting distinct unit types instead of categories gets to eleven. Either count is defensible depending on how you’re using it; what isn’t defensible is a count of nine, which doesn’t match the rule text under either reading.

Two of the eight categories, industrial boilers and process heaters, aren’t sector-specific. That’s the pair worth flagging for a broader audience than metals, glass, and cement producers: any facility running boilers or process heaters at scale, in food processing, chemicals, pulp and paper, or general manufacturing, needs to run the applicability test rather than assume the rule is limited to heavy industry.

What are the exact boundaries of the Metro-East nonattainment area?

Madison, St. Clair, and Monroe counties in full on the Illinois side. On the Missouri side: St. Louis County, St. Louis City, St. Charles County, and Jefferson County, plus only Boles Township in Franklin County, not the entire county. That township-level limit on Franklin County is the detail most likely to trip up a self-check: a facility in western or southern Franklin County, Missouri, outside Boles Township, sits outside the nonattainment area even though it shares a county name with a covered township.

One live iSi post already covers a related but distinct Missouri boundary question. Our St. Louis petroleum vapor recovery piece describes the broader rule-defined St. Louis metropolitan area used for a different program (VOC vapor recovery), which includes the entirety of Franklin County rather than just Boles Township. Different program, different pollutant, different boundary. Don’t use one boundary definition to answer a question about the other.

Can I average NOx emissions across units instead of meeting each unit’s limit?

Yes, but only voluntarily and only at a cost. Illinois’s Economic Incentive Program lets a source meet its NOx RACT obligation in aggregate across units rather than unit by unit. Using EPA’s 2001 presumptive approach, that flexibility comes with a price: an additional 10 percent emissions reduction beyond what unit-by-unit compliance would require (91 FR 50748 to 50749). A source can also simply meet each captured unit’s individual limit and skip averaging entirely. Model both paths before choosing one; the averaging option isn’t automatically the cheaper route once the 10 percent premium is priced in.

When do the new NOx limits actually take effect?

That’s the honest gap in this notice, and it’s worth stating plainly rather than guessing. The Federal Register notice supplies a comment deadline, September 8, 2026, and two state rule effective dates, April 23, 2025 for part 217 and December 9, 2024 for parts 218 and 219. It does not supply a compliance date for the actual numeric NOx limits, and a state rule effective date is not the same thing as a compliance deadline. The compliance schedule lives in the Illinois Pollution Control Board’s rule text, which this brief did not pull and which any facility relying on a specific date should verify directly before treating it as final.

If EPA says the air is clean, why are new limits coming?

Because those are two different legal questions, and the tension between them is real enough that it deserves a direct answer rather than a dismissal. On February 26, 2026, EPA proposed Clean Data Determinations for both the Missouri and Illinois portions of the St. Louis nonattainment area, and finalized them on August 13, 2026 (91 FR 52243, effective September 14, 2026), a status that on its face suggests the area is currently meeting the ozone standard.

A Clean Data Determination suspends specific planning obligations: attainment demonstrations, reasonable further progress plans, reasonably available control measures, and contingency measures. RACT is not one of those. It’s an emission control program, not a planning document, and a federally approved RACT limit stays enforceable regardless of a Clean Data Determination. The area can be reading clean today and still be legally required to adopt and enforce control-technology limits set for its classification. Both things are true at once, and neither cancels the other out.

Is there an active enforcement trend behind this rule?

No, and stating that plainly matters more than manufacturing urgency that isn’t supported by the record. A search across EPA and Federal Register enforcement records covering 2024 through 2026 found no NOx RACT-specific enforcement action. Every recent RACT-related item in the record is a rulemaking, a state implementation plan approval, not a penalty case.

What the record does show is that Illinois air-permit enforcement generally is active, and that federally approved limits are what actually gets enforced once a state rule is in the SIP. Heidtman Steel in Granite City, Illinois, settled for $75,000 in March 2026 over a Title V permit gap and Boiler NESHAP noncompliance on a natural gas-fired industrial boiler, the closest live analogue to what a NOx RACT enforcement case would look like once these limits are approved and in effect. That’s a permit and standards case, not a NOx RACT case specifically, but it’s the right reference point for what federal enforceability means in practice once a state rule clears SIP approval.

What should a Metro-East or St. Louis-area facility do now?

Pull facility-wide NOx potential to emit from the current Title V or FESOP permit and confirm whether it clears 50 tons per year.
Pull unit-level NOx emissions for every boiler, process heater, kiln, and furnace on site, and check each one against the 15 tpy threshold and the eight named categories, matching the rule’s category language rather than the vernacular name for the equipment.
Confirm the facility’s location against the nonattainment boundary at township resolution if Franklin County, Missouri, or Grundy or Kendall County, Illinois, is involved.
Track the Illinois rule as a preview, not a final answer, for Missouri obligations. Missouri’s own Serious-area plan is still in development; Illinois’s proposed part 217 text is the best current indicator of the control level Missouri sources should expect to be measured against.
Comment by September 8, 2026 if a specific limit, boundary, or the averaging premium affects your facility and is worth contesting or clarifying while the rule is still proposed.

Where iSi fits

Running this applicability test correctly, facility-wide potential to emit against the 50 tpy gate, then each individual unit against the 15 tpy trigger and the eight-category list, at township-level boundary precision, is exactly the kind of cross-referencing that gets skipped when it’s someone’s fourth priority on a Tuesday. iSi supports manufacturers across Missouri and the greater St. Louis area with air permitting applicability determinations, emissions inventories, and SIP-tracking for exactly this kind of two-part test, so a facility knows whether it has a RACT obligation before EPA or Missouri DNR tells them so directly.

If your facility sits in the St. Louis metro and runs boilers, process heaters, kilns, or furnaces at meaningful scale, that applicability question is worth answering now, while the Illinois rule is still in comment and before Missouri’s own Serious-area plan firms up further.

Update, October 2026: Missouri’s Own NOx Rule for Engines Above 1,300 Horsepower

Everything above tracks a RACT rulemaking that is still moving. Missouri already has a NOx rule on the books for large engines, and this article did not name it until now. It is 10 CSR 10-6.390, in force at state level since May 30, 2020, and on October 1, 2026 EPA approved a set of revisions to it into the Missouri State Implementation Plan (91 FR 62320). No facility picks up a new obligation from that approval. The rule underneath it carries real limits, and it reaches a far wider slice of Missouri than the Metro-East nonattainment area does.

Which Missouri rule sets NOx limits on large stationary engines?

10 CSR 10-6.390, “Control of NOX Emissions From Large Stationary Internal Combustion Engines.” It has been effective at state level since May 30, 2020, and EPA approved revisions to it into the Missouri SIP on October 1, 2026, federally effective November 2 (91 FR 62320). It covers engines greater than 1,300 horsepower in 36 counties plus the City of St. Louis.

The rule names the engine, the horsepower, the county and the in-service date, then sets a gram-per-horsepower-hour limit against that combination (10 CSR 10-6.390). It runs on its own terms and has for six years. A facility that has been filing annual NOx reports under it since 2020 keeps filing them the same way.

Does the rule apply to my engine, and what is the horsepower threshold?

The threshold is greater than 1,300 horsepower. Below that, 10 CSR 10-6.390 does not reach the engine at all. Above it, location decides. The engine has to sit in one of 36 named counties or the City of St. Louis, a footprint anchored on the St. Louis metro and running down the Mississippi corridor (10 CSR 10-6.390).

The 36 counties: Bollinger, Butler, Cape Girardeau, Carter, Clark, Crawford, Dent, Dunklin, Franklin, Gasconade, Iron, Jefferson, Lewis, Lincoln, Madison, Marion, Mississippi, Montgomery, New Madrid, Oregon, Pemiscot, Perry, Pike, Ralls, Reynolds, Ripley, St. Charles, St. Francois, St. Louis, Ste. Genevieve, Scott, Shannon, Stoddard, Warren, Washington and Wayne, plus the City of St. Louis.

Read what is absent from that list. Kansas City, Springfield, Columbia and western Missouri are outside the rule. A plant in Jackson County or Greene County is not covered by 10 CSR 10-6.390 no matter how large its engines are, which makes the county check the fastest way for most Missouri readers to close the question.

This county list is also a different map from the nonattainment boundary described earlier in this article. Franklin County appears in full on the engine rule’s list. The Metro-East nonattainment area reaches only Boles Township inside Franklin County. Two programs, two boundaries, and no reason to expect them to line up.

What are the actual NOx emission limits?

For engines placed in service after September 1997: 3.0 grams per horsepower-hour for spark-ignited rich-burn and lean-burn units, 2.3 g/hp-hr for diesel, and 1.5 g/hp-hr for dual fuel. Engines in service before September 1997 are held to an individually calculated emission rate instead of any of those flat numbers (10 CSR 10-6.390).

So the in-service date is the second thing to pull off the file, right after horsepower. Applying the 3.0 g/hp-hr figure to a 1980s compressor engine gets the wrong answer, because that engine is on a calculated rate of its own. Dual fuel carries the tightest number on the list at 1.5 g/hp-hr, which is something to check before anyone proposes a fuel-flexibility retrofit as a compliance strategy.

Are emergency generators exempt?

Emergency standby engines are exempt from the emission standards under 10 CSR 10-6.390, with an allowance of up to 100 hours per calendar year for routine maintenance checks. Those 100 hours are an annual budget measured on a calendar year, so maintenance-check runtime has to come off the hour meter rather than out of an estimate (10 CSR 10-6.390).

Running past the maintenance allowance puts the exemption itself at risk, so a generator that also gets used for peak shaving needs its hours split and logged. Engine exemptions tend to turn on definitions this specific, and the definitions move. We covered a federal example of that in EPA’s broadened definition of remote sites for stationary engine air compliance.

Is there a tonnage exemption, and what period does it cover?

Engines in service after September 1997 that emit 25 tons or less of NOx between May 1 and September 30 are exempt from the emission standards. The window is the ozone season, five months, so tonnage has to be accumulated against May through September rather than against a calendar year (10 CSR 10-6.390).

This is the practical trap in the whole rule. A spreadsheet that totals NOx from January to December answers a question the rule never asked. An engine can sit well over 25 tons on the year and still come in at or under 25 tons across the ozone season, and that is the number the exemption turns on.

It also cuts against the impression the rule’s title gives. A post-1997 engine above 1,300 horsepower in a covered county can land entirely outside the emission standards on the strength of intermittent duty. The exemption runs against the emission standards, and recordkeeping can still apply, so the tonnage calculation is something a facility should be able to put in front of an inspector.

When is the Missouri NOx report due?

Total NOx emissions for covered engines must be reported by November 1 each year. Compliance records stay at the facility for five years and cover daily operating hours, maintenance, test results and any exceedance of a standard. Monitoring runs on a continuous emissions monitoring system or an EPA-approved alternate method (10 CSR 10-6.390).

Engine identification and operator contact information also go to the director. Get the ordering of this year’s dates straight. November 1, 2026 falls one day before the federal approval takes effect on November 2, so the 2026 annual report is filed under the rule as it already stood. Nothing about the approval changes what goes in that report.

What changed when EPA approved the rule on October 1, 2026?

Nothing that creates a new duty. EPA’s final rule at 91 FR 62320 approves revisions that reformat reporting, recordkeeping and compliance requirements, incorporate other state rules by reference, add definitions specific to the rule, remove duplicative language and make administrative wording changes. One substantive change adds an exemption for certain spark-ignited engines (91 FR 62320).

The direction of travel runs opposite to what most readers expect, because a Federal Register notice with “NOx” in the title gets read as a tightening. This one hands something back. No limit changes, no deadline moves, and a narrow class of spark-ignited engines gets relief it did not have before.

The docket is EPA-R07-OAR-2026-4819. Comments closed August 14, 2026, and EPA received one anonymous comment. The federal effective date is November 2, 2026, and it marks the point at which the revised state language becomes federally enforceable as part of the Missouri SIP (Missouri SIP Chapter 6).

Do these engine limits relate to the NOx RACT rule for the St. Louis metro?

They share a pollutant and part of a map, and they work through different mechanisms. 10 CSR 10-6.390 is a Missouri engine-specific rule in force since 2020. The RACT activity covered earlier in this article concerns reasonably available control technology obligations tied to ozone nonattainment. One facility can be reached by both.

The two rules name different equipment, which keeps them separate in practice. The RACT categories described above are boilers, process heaters, kilns and furnaces. The engine rule names stationary internal combustion engines. A St. Louis County site running a 2,000 horsepower gas compressor alongside a large process heater has two applicability tests to run, with different thresholds, different geography and different reporting. Answering one of them says nothing about the other.

Pulling an in-service date off a nameplate, matching a site against the rule’s county list and totaling ozone-season tonnage for a unit that runs on an irregular schedule is file work that sits quietly behind the November 1 report until the year someone has to reconstruct it. iSi supports Missouri facilities with air permitting applicability determinations and emissions inventories, and with the Title V reporting that sits on top of them, including the engine-level records the annual NOx report draws on. If an engine over 1,300 horsepower sits in one of those 36 counties, its in-service date and its May-through-September tonnage are the two numbers worth confirming before next year’s report comes due.

Sources

91 FR 50746, FR Doc 2026-16001, Air Plan Approval; Illinois; 2015 Ozone Moderate and Serious Reasonably Available Control Technology Update (verified 2026-08-06)
FR Doc 2024-27382, Finding of Failure To Attain and Reclassification of the Missouri Portion of the St. Louis Nonattainment Area as Serious (verified 2026-08-06)
FR Doc 2026-03845, Air Plan Approval; Missouri; Clean Data Determination for the 2015 8-Hour Ozone Standard (verified 2026-08-06)
91 FR 52243, FR Doc 2026-16515, Air Plan Approval; Missouri; Clean Data Determination for the 2015 8-Hour Ozone Standard (final rule, August 13, 2026, effective September 14, 2026) (verified 2026-10-03)
Missouri DNR, St. Louis Ozone Serious Nonattainment Area Plan public meeting, April 24, 2025 (verified 2026-08-06)
CAFO CAA-05-2026-0024, Heidtman Steel, Granite City, Illinois, $75,000 (verified 2026-08-06)
EPA, RACT Information (verified 2026-08-06)
EPA, Redesignation and Clean Data Policy (verified 2026-08-06)
40 CFR 19.4, Statutory civil monetary penalties as adjusted for inflation (verified 2026-08-06)
91 FR 62320, FR Doc 2026-20096, Air Plan Approval; Missouri; Control of NOX Emissions From Large Stationary Internal Combustion Engines, final rule, October 1, 2026, docket EPA-R07-OAR-2026-4819 (verified 2026-10-02)
Missouri Code of State Regulations, 10 CSR 10-6 (current), Missouri Secretary of State, containing 10 CSR 10-6.390 (verified 2026-10-02)
10 CSR 10-6.390, Control of NOx Emissions From Large Stationary Internal Combustion Engines, rule text (verified 2026-10-02)
EPA, Missouri SIP, Chapter 6: Air Quality Standards, Definitions, Sampling and Reference Methods, and Air Pollution Control Regulations (verified 2026-10-02)

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