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Missouri Narrowed the Air Permit Exemption for Emergency Generators in 2026. One Unit Still Passes.
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Missouri Narrowed the Air Permit Exemption for Emergency Generators in 2026. One Unit Still Passes.

Missouri narrowed its construction permit exemption for emergency generators on January 30, 2026. One generator still qualifies. Two or more is a calculation.

by iSi Environmental

Missouri rewrote the air construction permit exemption for emergency generators on January 30, 2026, and the regulated community said almost nothing about it. The Missouri Department of Natural Resources (MDNR) received no comments during its state rulemaking, which ran from June 16 to August 7, 2025 with a hearing on July 31. EPA received two comments on the federal side, both anonymous (91 FR 64295). Nine months on, most Missouri plants adding backup power still work from the old text.

For a single generator, the old text is still a fair description. The exemption at 10 CSR 10-6.061(3)(A)2.BB used to be simple arithmetic: emergency use only, 500 hours a year, a non-resettable hour meter on the unit, and no construction permit before installation. No horsepower cap. No limit on how many units you put in. The amended rule keeps that clean pass for a project with one generator and adds a second test for everything else. Install two or more in the same construction or modification and the exemption now turns on a combined potential-emissions calculation expressed in pounds per hour.

Two dates matter here and they are not the same date. The narrower rule has been Missouri state law since January 30, 2026 (91 FR 48032). On October 8, 2026 EPA published final approval of the revision into the Missouri State Implementation Plan, with a federal effective date of November 9, 2026 (91 FR 64295, FR Doc. 2026-20634, docket EPA-R07-OAR-2026-5182). November 9 is the day the provision becomes federally enforceable. It is not the day the obligation starts. A Missouri facility that added a second generator in February 2026 on the old reading was already outside the exemption.

Does Missouri require an air permit for an emergency generator?

Not if the unit qualifies for the construction permit exemption at 10 CSR 10-6.061(3)(A)2.BB. The unit’s sole function has to be backup power during a utility interruption, a non-resettable hour meter has to be installed, operation is limited to emergencies plus maintenance and readiness testing, and annual operating hours stay at or below 500.

That exemption is one entry on a categorical list. Missouri’s construction permit program itself sits at 10 CSR 10-6.060, with a permit-by-rule track at 10 CSR 10-6.062. The exemption list exists so that a facility installing low-emitting equipment does not have to run a permit application for every small source. Generators have always been on that list. What changed is how many of them you can put in at once and still be on it. For the broader set of federal programs a backup generator can trigger, including SPCC, EPCRA and the engine NESHAP and NSPS rules, see our overview of environmental regulations for emergency generators.

What changed in Missouri’s emergency generator permit exemption in 2026?

The exemption was restructured into three mandatory conditions plus an either/or test. A project with only one emergency generator still qualifies. A project with two or more must now show that combined potential emissions fall at or below Missouri’s insignificance thresholds. State effective January 30, 2026; federally effective November 9, 2026.

Here is the operative text from the amendment as published in the Missouri Register, Vol. 50, No. 12. An emergency generator or generators are exempt only if they meet all three of these:

(I) The sole function is to provide backup power when electric power from the utility is interrupted; (II) The generator is equipped with a non-resettable meter to measure hours of operation; (III) The generator is operated only during emergency situations and for short periods of time to perform maintenance and operational readiness testing;

along with one or both of these:

(IV) There is only one (1) emergency generator included in the construction or modification; or (V) The combined potential emissions of all generators included in the construction or modification are less than or equal to the levels specified in subparagraphs (3)(A)3.A. and (3)(A)3.B.

Conditions (I) through (III) are close to the old requirements. Condition (IV) is the one that keeps most real-world projects whole. Condition (V) is the new work. The 500-hour annual cap applies on top of all of it. Compare the prior text effective September 30, 2020 and the structural change is obvious: there was no unit count and no emissions test in the old paragraph at all.

Does the exemption still apply if I have more than one emergency generator?

Only if combined potential emissions of all generators in that construction or modification stay at or below the 10 CSR 10-6.061(3)(A)3.A. Table 1 levels and the (3)(A)3.B. HAP level. Condition (IV), the clean single-unit pass, applies only when one generator is included in the project.

Read “included in the construction or modification” carefully, because it is a project test and not a site inventory test. Adding one generator to a plant that already has three is a single-generator project and lands on (IV). Adding two generators at once to a plant that has none lands on (V). The question is what the project includes, not what the fence line holds.

This is where N+1 backup designs get interesting. A hospital, a cold-storage building or a data center backup generator installation typically specifies redundant units in one construction package, which is exactly the shape of project that condition (V) now captures.

What are Missouri’s insignificance emission levels for generators?

At maximum design capacity: 1.0 lb/hr PM10, 2.75 lb/hr SO2, 2.75 lb/hr NOx, 2.75 lb/hr VOC and 6.88 lb/hr CO under Table 1 at (3)(A)3.A., plus 0.5 lb/hr for any hazardous air pollutant, or the applicable screening model action level at 10 CSR 10-6.060(5)(F)6.A., whichever is less.

PollutantInsignificance levelCitation
PM101.0 lb/hr(3)(A)3.A., Table 1
SO22.75 lb/hr(3)(A)3.A., Table 1
NOx2.75 lb/hr(3)(A)3.A., Table 1
VOC2.75 lb/hr(3)(A)3.A., Table 1
CO6.88 lb/hr(3)(A)3.A., Table 1
Any single HAP0.5 lb/hr, or the SMAL at 10 CSR 10-6.060(5)(F)6.A., whichever is less(3)(A)3.B.

All figures from the Missouri Register, Vol. 50, No. 12.

Two further ceilings sit nearby and get missed. Subparagraph (3)(A)3.C. caps actual emissions at 876 lb/yr per criteria pollutant, and (3)(A)3.D. caps non-HAP VOC at 4 tons per year.

When we run a potential-to-emit calculation on a pair of standby diesels, NOx is the number that fails first. 2.75 pounds per hour is a low bar for compression-ignition engines at maximum design capacity, and two mid-size gensets will commonly clear it on combined uncontrolled potential emissions. Operators who think in tons per year read 2.75 and assume it is generous. It is not. If you only have time to run one number before a project meeting, run combined NOx.

When does the Missouri generator exemption change take effect?

Two dates. The narrower rule has been Missouri state law since January 30, 2026. EPA’s SIP approval makes it federally enforceable on November 9, 2026 (91 FR 64295). November 9 adds EPA and citizen-suit exposure. It is not the date the obligation begins.

The practical consequence of the gap between those two dates is a nine-month window of installations judged against the amended text with only MDNR able to act on them. After November 9, the same installations sit inside the federally approved SIP, which is a different enforcement posture with different parties able to bring a case.

How many hours can an exempt emergency generator run in Missouri?

A maximum of 500 hours per year, and only during genuine emergencies plus short periods for maintenance and operational readiness testing. A non-resettable meter must be installed to measure hours of operation. Running the unit for peak shaving or demand response falls outside the exemption entirely.

That last point is worth a second read if your utility has an interruptible rate or a demand-response program. Condition (I) limits the generator’s sole function to backup power when utility power is interrupted. A unit dispatched by a utility program is not performing that function, whatever the contract calls it, and running it that way puts the exemption at risk for the whole installation rather than for the hours involved.

Do I need a construction permit before installing a generator, or after?

Before. A construction permit under 10 CSR 10-6.060 authorizes construction or modification, so the applicability question has to be answered while the project is still on paper. A unit installed outside the exemption is an unpermitted construction from the day it goes in.

This is the sequencing error that turns a routine capital project into a disclosure problem. The determination is cheap while the equipment is still a line item on a drawing and expensive after the pad is poured. Budget for the permit if you need one: Missouri’s 2026 fee schedule carries a $6,000 filing fee for major New Source Review, PSD and HAP permits plus $100 per hour of MDNR processing time, uncapped. We covered the full schedule in Missouri’s 2026 air construction permit fees.

What is the penalty for installing a generator without a Missouri construction permit?

Missouri’s Air Conservation Law authorizes a civil penalty of up to $10,000 per violation per day for each day the violation continues (RSMo 643.151). Once the rule is federally enforceable on November 9, 2026, a federal civil judicial action under CAA Section 113(b) can reach $124,426 per day per violation (40 CFR 19.4).

Two details on the state figure. The Air Conservation Commission may institute the civil action itself or ask the attorney general to bring it, and a penalty cannot be assessed under 643.151 where an administrative penalty was already imposed under 643.085 (RSMo 643.151). On the federal figure, the $124,426 amount applies to penalties assessed on or after January 8, 2025 and remains operative through 2026, because the annual inflation adjustment for 2026 was not issued (40 CFR 19.4).

Does this apply to generators already installed before January 2026?

The rule governs construction permit exemptions, so it decides whether a new or modified unit needed a permit before installation. Units installed under the prior text were judged against that text. EPA’s notice does not address existing installations, and this reading is an inference from how 10 CSR 10-6.060 and 6.061 operate rather than a statement in the rulemaking record.

Flagging that as an inference is deliberate. Neither 91 FR 64295 nor the Missouri Register notice says anything about retroactive application to units already in the ground, so anyone telling you that pre-2026 generator banks have to be re-permitted is reading past the record. The exposure that does exist is narrower and more specific: projects completed on or after January 30, 2026 that included more than one generator and never ran the (V) calculation.

Does the Kansas side of the metro work the same way?

No. Kansas uses a two-tier structure under K.A.R. 28-19-300 et seq., a construction permit plus a lower construction-approval tier, where Missouri uses a categorical exemption list. A project under Kansas’s tons-per-year permit thresholds can still land in the pounds-per-hour approval tier, so comparing the two states’ exemption language directly is misleading.

For a Kansas City operator with plants on both sides of State Line Road, the same generator model runs through two different applicability tests. Kansas City, Missouri adds a third: the city runs its own construction and operating permit program under KCMO Code of Ordinances Chapter 8, layered on top of MDNR. Our breakdown of Kansas City air permitting requirements walks the jurisdictional split in more detail.

Also worth knowing for multi-state programs: no comparable 2026 emergency-generator exemption amendment was located in Oklahoma, Nebraska, Texas or Colorado. Missouri moved on its own here.

What records do I have to keep to hold the exemption?

Records in sufficient detail to show compliance with the exemption, beginning on the date of the construction or modification. Under the amended paragraph (4), failing to keep them is itself a violation, voids the exemption, and triggers a requirement to apply for a construction permit under 10 CSR 10-6.060 (Missouri Register, Vol. 50, No. 12).

The wording is self-executing and that is what makes it sharp. The amended paragraph states that noncompliance with the recordkeeping requirement “constitutes a violation and is grounds for enforcement action and the exemption will no longer apply,” and that an installation found not in compliance “shall be required to apply for a construction permit under 10 CSR 10-6.060.” MDNR does not have to win an argument about emissions math first. A missing hour log converts into an unpermitted-construction problem on its own.

Treat the recordkeeping as a condition of the exemption, which is what the text makes it, and keep the hour meter readings, the emergency and testing logs, and the potential-emissions calculation that put the project on (IV) or (V) in the same file.

Did EPA loosen anything in this rule?

No. EPA states plainly that “the revisions to the rule do not add additional exemptions to construction permitting requirements in Missouri’s SIP” (91 FR 64295). The amendment also deletes language that let lead escape the 876 lb/yr actual-emissions cap and tightens the lead-compound exemption threshold.

That lead change has a quiet reach beyond generators. The old (3)(A)3.C. capped actual emissions at 876 lb/yr per criteria pollutant “except lead.” The amendment strikes the exception, so lead now counts against the same annual ceiling as every other criteria pollutant on any project relying on the insignificance levels (Missouri Register, Vol. 50, No. 12). If your exemption file was built before January 30, 2026 and leaned on that carve-out, it needs a second look.

One more trap for multi-state compliance programs: two subsections of the amended rule, (3)(A)2.D. and (3)(A)2.E.(II)(c), were not SIP-approved (91 FR 64295). They are state-enforceable only. A program built off the federally approved SIP alone will have a Missouri-shaped hole in it.

A deregulatory EPA approved a tightening

This cuts against how most people are reading 2026. In the same stretch of the year, EPA proposed rescinding the power-plant greenhouse gas findings (FR Doc. 2026-19072, September 17, 2026), proposed a reconsideration of the gasoline-distribution NESHAP (FR Doc. 2026-19774, September 28, 2026), and announced a forthcoming methane rollback. Then it approved a state amendment that narrows a widely used permit exemption and said in the notice that the revisions add no new exemptions. A facility manager running on “everything is loosening this year” will get Missouri wrong.

Nobody fought it either. No comments at the state level, two anonymous comments at EPA. For a rule that changes the applicability test for every multi-unit generator project in the state, that is close to zero engagement. It is the practical reason so few Missouri sites know this happened. It is also, honestly, a signal in the other direction: the regulated community did not treat this as a major change, and for single-unit projects they were right.

The word “narrowing” oversells it too. Condition (IV) preserves a clean pass for any project with one generator, and a large share of real installations are single-unit replacements or additions. Content telling you Missouri ended the emergency generator exemption is wrong on the text and will not survive a reading of the rule.

What the record does not tell us is what the last nine months mean. The rule has been state law since January 30, 2026 and no enforcement action applying the amended provision was located. That is either low enforcement priority or exposure accumulating undetected, and the available record does not distinguish between them. Anyone who tells you which one it is confidently is guessing. There is also nothing on sos.mo.gov, dnr.mo.gov, federalregister.gov or regulations.gov suggesting Missouri intends to revisit or soften the multi-generator condition.

The five-minute self-check

Run this in order before the next generator project goes to bid.

  1. Is the site in Missouri and subject to MDNR air rules? If not, stop.
  2. Does a single construction or modification project include more than one emergency generator, or does it add a generator to a site as part of a project that includes another one? If one generator was ever involved, condition (IV) is satisfied and nothing has changed for you.
  3. Did that project occur on or after January 30, 2026, or is it still planned? If it predates January 30, 2026, the old text governed it.
  4. If yes to 2 and 3, calculate combined potential emissions in pounds per hour at maximum design capacity for all generators in the project, across PM10, SO2, NOx, VOC, CO and each hazardous air pollutant, and compare against the Table 1 levels above.
  5. If any pollutant exceeds, apply for a construction permit under 10 CSR 10-6.060 before installing, and budget the $6,000 filing fee plus $100 per hour of processing time.
  6. Verify the three mandatory conditions on every unit you are holding as exempt: backup-power-only function, non-resettable meter installed, operation limited to emergencies and short maintenance and readiness tests.
  7. Confirm the exemption file exists and starts on the construction date, because under paragraph (4) the missing file is itself the violation.

If step 4 comes back over on a project you already built, that is a disclosure conversation rather than a calculation problem, and it is worth having before an inspection opens it for you. A voluntary disclosure and a discovered violation are different situations with different outcomes.

Potential-to-emit calculations and construction permit applicability determinations are routine work for iSi’s air compliance group, in Missouri and across the 40 states where we operate. If you have a generator project in design, or one that went in since January and never got the combined-emissions math, the determination takes a set of engine data sheets and a short conversation. Talk to us before the equipment ships.

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