On July 8, 2024, the Illinois Environmental Protection Agency (Illinois EPA) submitted its initial proposal for amendments to Illinois’ air pollution rules for nitrogen oxide (NOX) emissions, found at Title 35 of the Illinois Administrative Code (35 IAC) Part 217. These rules are commonly referred to as “NO_X RACT” (where “RACT” is an acronym for “Reasonably Available Control Technology”). The proposed amendments potentially impact NOX-emitting units located at major stationary sources of NO_X emissions in areas designated as nonattainment for the 2015 eight-hour (8-hr) ozone National Ambient Air Quality Standard (NAAQS). The amendments include lowering the applicability threshold for facilities that are subject, more stringent emission limitations for some categories of emission units, and increased recordkeeping and reporting requirements. 
On April 17, 2025, the Illinois Pollution Control Board (IPCB) issued its final order adopting the proposed amendments. The amendments to the rule are scheduled to take effect on July 1, 2025. However, there may be additional actions required prior to this date in order for affected sources to establish compliance beginning on this date. The following is a brief history of the rule and a summary of the major changes to be aware of.

NO_X RACT Background and Timeline

In 2015, the United States Environmental Protection Agency (USEPA) revised the primary and secondary NAAQS for ozone to 0.070 parts per million. USEPA designated Chicago and Metro East as “marginal” nonattainment areas (NAAs) for the 2015 ozone NAAQS with an attainment date of August 3, 2021. The Chicago area includes the counties of Cook, DuPage, Kane, Lake, McHenry, and Will, the Townships of Aux Sable and Goose Lake in Grundy County, and the Township of Oswego in Kendall County. The Metro East area consists of the counties of Madison, Monroe, and St. Clair. In 2022, USEPA determined that these two areas failed to attain the standard by that date, and by operation of law reclassified them as “moderate” nonattainment for the 2015 ozone NAAQS. NO_X is regulated in conjunction with the ozone NAAQS because NO_X is an ozone precursor, as ground-level ozone forms when NO_X and volatile organic compounds react in the atmosphere in the presence of sunlight.
35 IAC contains rules and regulations approved as part of the Illinois State Implementation Plan (SIP). USEPA reviews SIPs to confirm they provide a path to compliance with the NAAQS. In 2007, 2009, and 2011, the Board amended its NO_X rules under Part 217 to satisfy the NO_X RACT requirements under the Clean Air Act (CAA). However, not all of these amendments received SIP approval. On October 18, 2023, USEPA found that eleven states, including Illinois, had failed to submit SIP revisions required by the CAA deadline of May 1, 2023, for areas classified as moderate nonattainment for the 2015 ozone NAAQS. This finding triggered deadlines to impose mandatory sanctions for states which did not submit a complete SIP before May 19, 2025, addressing the outstanding requirements.     
The Illinois EPA states that its proposal intends to address NO_X RACT regulations for moderate nonattainment areas as well as deficiencies identified in USEPA’s October 2023 finding. Additionally, the Chicago and Metro East areas failed to attain the NAAQS by the designated date and were subsequently “bumped up” from “moderate” to “serious” NAAs by the USEPA on January 16, 2025. Therefore, the Illinois EPA’s initial proposal also proactively addressed Illinois’ obligation to adopt NO_X RACT regulations for major stationary sources of NOX in serious ozone NAAs. This imposes additional requirements, including lowering the major source threshold from 100 to 50 tons per year of NO_X.

Summary of Changes to NO_X RACT Rules

The proposed changes to the NO_X RACT rules include reducing applicability thresholds, changing compliance averaging periods, changing emission limitations for some unit categories, sunsetting provisions allowing for sources to exclude periods relating to shutdowns of units or control equipment from compliance, and updates to recordkeeping and reporting requirements. Not every change will be covered in detail in this article, so it is recommended that facilities review the rule in detail and/or contact Trinity’s Chicago office for additional assistance. Contact information may be found at the end of this article.
Overall, the following subparts to 35 IAC Part 217 are affected by the adopted amendments. Facilities with equipment in these categories—especially those sources with units which are already subject to NO_X RACT requirements—should be aware of the amendments.

• Subpart D – NO_X General Requirements
• Subpart E – Industrial Boilers
• Subpart F – Process Heaters
• Subpart G – Glass Melting Furnaces
• Subpart H – Cement and Lime Kilns
• Subpart I – Iron, Steel and Aluminum Manufacturing
• Subpart M – Electrical Generating Units
• Subpart Q – Stationary Reciprocating Internal Combustion Engines (RICEs) and Turbines

Lower Source Applicability Threshold 

One of the primary proposed changes concerns the threshold for overall source applicability. The amendments align the threshold for triggering NO_X RACT requirements with the threshold for major sources in serious ozone NAAs. Sources in the serious ozone NAAs that have the potential to emit NO_X in an amount equal to or greater than 50 tons per year will be required to evaluate their emission units for applicability. Please be aware that with the amendments, the amended rule retains the “Once In-Always In” general applicability for sources. This means that any emission unit which was ever subject to NO_X RACT provisions shall remain subject for the entire lifetime of the unit, even if the unit no longer actively meets the applicability criteria.

More Frequent Emission Averaging Periods

Another far-reaching change concerns emission averaging periods. The USEPA took issue with the existing NO_X RACT rule establishing compliance with applicable emissions limitations or emissions averaging plans based on ozone season (May 1 through September 30) and annual bases. The proposed revisions to the rule require sources to demonstrate compliance based on a 30-day rolling average. Compliance for existing units must be demonstrated continuously beginning thirty (30) operating days after July 1, 2025. Newly constructed and modified units will have 180 days after initial startup of the new or modified emission unit to demonstrate compliance. Engines and turbines subject to 35 IAC 217 Subpart Q would be required to comply by no more than sixty (60) days after achieving maximum operating rate. 
All emissions averaging plans must be submitted to the Illinois EPA at least thirty (30) days before a facility intends to demonstrate compliance using the plan. (E.g., if a facility intends to implement the emissions averaging plan on July 1, 2025, then the plan must be submitted by June 1, 2025.) With the explicit exception of a few facilities, all emissions averaging plans must also include a 10% emissions reduction compared to the aggregate of total allowable emissions from all units included in the plan.

Proposed Changes for Industrial Boilers and Process Heaters

It is important to be aware of the changes applicable to industrial boilers and process heaters (Subparts E and F) as these units are present at many sources. The proposed changes include NO_X emissions limitations on units with a rated heat input capacity greater than fifty (50) MMBtu/hr. Under the current rule, the lowest rated unit with a specific NO_X emission limitation is 100 MMBtu/hr. Below is a summary table of the limitations and requirements under the proposed changes. These limitations and requirements would be applicable to any unit located in either serious ozone NAA that emits fifteen (15) tons or more of NO_X per calendar year and is located at a source with the potential to emit fifty (50) tons per year or more of NO_X.

The combustion tuning requirements under the amended rule align with the federal Boiler Maximum Achievable Control Technology (Boiler MACT) requirements for combustion tuning at 40 CFR 63.7540(a)(10)(i) through (vi) and are required at least annually. Please note that with the proposed changes above, there would be no impact to the thresholds that require continuous emissions monitoring systems (CEMS) or predictive emissions monitoring systems (PEMS). Industrial boiler or process heaters that have a heat input greater than 50 MMBtu/hr but less than or equal to 100 MMBtu/hr would have the option to install a CEMS or PEMS.  
There are similar changes to limitations in Subpart G, H, I, and Q as well. Facilities with units that fall into those categories will need to be aware of the changes.

Additional Recordkeeping, Reporting, and Compliance Demonstration Requirements

Facilities must demonstrate compliance with the limitations through performance testing or through the use of CEMS or PEMS. Existing current and newly subject units will be required to demonstrate compliance by July 1, 2025. Therefore, facilities should plan to prepare an initial compliance demonstration as soon as possible. The initial compliance determination would include the results of performance tests, if applicable, or certification that the facility is operating a CEMS or PEMS for the unit. Performance tests conducted on or after July 1, 2020, may be used to demonstrate compliance.
There are a number of changes to recordkeeping and reporting requirements throughout the amended rule. The change to averaging periods has already been discussed and will require more frequent recordkeeping. In general, the proposed changes include daily recordkeeping, rather than seasonally or annually. These include daily operating hours, daily fuel usage records, and daily product and material throughput, as applicable. Facilities should prepare to collect daily records prior to the required date of compliance.
For reporting requirements, there are a number of changes to note including the following:

• Facilities will now be required to submit an annual compliance certification report each year, due by May 1. This report would be required for any unit subject to the NO_X RACT rules and would include relevant data to demonstrate compliance with limits, as listed in the proposed rule. 
• Units demonstrating compliance through a CEMS or PEMS will be required to submit an additional report due thirty (30) days after the end of each calendar quarter with information regarding the performance of the system, as listed in the proposed rules. 
• For units that require 30-day rolling average calculations to demonstrate compliance, any exceedance of an applicable emissions limitation must be reported, with an explanation of the causes, within 30 days following the end of the relevant 30-day rolling average period in which the limit was exceeded.

Next Steps

First, determine whether you have units that are newly subject to the rule and/or subject to more stringent emissions standards than are currently required. If you haven’t already done so, schedule the initial performance test and consider engaging with the Illinois EPA on timeline for the test if you don’t think results may be ready to be submitted by July 1, 2025.
Next you will want to consider whether updates are needed to allow you to maintain the required daily records and calculations. If you currently comply using an emissions averaging plan and wish to continue to do so beginning July 1, 2025, revise and submit the plan to the Illinois EPA no later than June 1, 2025. You will also want to set reminders for the new routine reports, deviation reports, and/or notifications required by the amended rule.
For further assistance with or information regarding the Illinois NO_X RACT amendments, please contact Connor Rutledge and Philip Masini from Trinity’s Chicago office, or consider calling the main office line at 630.495.1470.