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SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean the EPA.

I. Background

On October 1, 2015, the EPA promulgated a revised 8-hour ozone NAAQS of 0.070 parts per million (ppm). See80 FR 65292 (October 26, 2015). Upon promulgation of a new or revised NAAQS, section 107(d)(1)(B) of the CAA requires the EPA to designate as nonattainment any areas that are violating the NAAQS, based on the most recent three years of quality-assured ozone monitoring data. On June 4, 2018 (83 FR 25776), the EPA designated the St. Louis, MO-IL area as Marginal nonattainment for the 2015 ozone NAAQS. The current nonattainment area includes Boles Township of Franklin County, Jefferson County, St. Charles County, St. Louis County, and St. Louis City in Missouri, and Madison County, Monroe County, and St. Clair County in Illinois.

On October 7, 2022 (87 FR 60897), the EPA determined that the St. Louis area did not attain the standards by the Marginal attainment date, and the area was reclassified as Moderate by operation of law. More recently, on November 25, 2024 (89 FR 92816), and December 17, 2024 (89 FR 101901), the EPA determined the Missouri portion of the area and the Illinois portion of the area, respectively, did not attain the standards by the Moderate attainment date, and both portions of the area were reclassified as Serious by operation of law.^[1]

On February 26, 2026 (91 FR 9519), the EPA proposed to determine that the St. Louis area attained the 2015 ozone NAAQS, based upon complete, quality-assured, and certified ambient air monitoring data for the 2023 through 2025 design period. Such a determination, based upon the EPA's Clean Data Policy, is known informally as a clean data determination. As noted in the Notice of Proposed Rule Making, the Illinois portion of the St. Louis area's 3-year ozone design value for 2023 through 2025 is 0.070 ppm, which meets the 2015 ozone NAAQS.

The EPA's proposed clean data determination relied upon the EPA's concurrence on an exceptional events demonstration submitted by Illinois on December 18, 2025. The EPA concurred on Illinois' demonstration on January 12, 2026.

In the February 26, 2026, proposed clean data determination, the EPA proposed to take final agency action on the exceptional events concurrence, which removed the event-influenced data from the design value, and opened an opportunity for public comment on the EPA's concurrence.

This action does not constitute a determination of attainment by the attainment date under CAA section 181(b)(2). In this action, the EPA is considering the area's design value for the 2023 through 2025 period, however the 2023 through 2025 design value does not serve as the area's most recent complete and quality-assured design value available as of the August 3, 2027, applicable attainment date.

This action does not constitute a redesignation of the area to attainment of the 2015 ozone NAAQS, nor does it constitute approval of a maintenance plan for any portion of the area as required under section 175A of the CAA, nor does it find that any portion of the area has met all other requirements for redesignation. A nonattainment area may constitute a redesignation to attainment as it has met requirements of section 107(d)(3)(E) of the CAA. The Illinois portion of the St. Louis area will remain designated nonattainment for the 2015 ozone NAAQS unless and until the EPA determines that the area meets CAA requirements for redesignation to attainment and takes a separate action to redesignate the area.

II. The EPA's Responses to Comments

The public comment period on the EPA's proposed rule opened February 26, 2026, the date of its publication in the Federal Register and closed on March 30, 2026. During this period, the EPA received one supportive comment.

III. What action is the EPA taking?

The EPA is making a determination under the CAA that the Illinois portion of the St. Louis area has achieved clean data for the 2015 ozone NAAQS. This determination is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023 through 2025 design value period showing that the area achieved attainment of the 2015 ozone NAAQS. The EPA is also taking final agency action on an exceptional events request submitted by the Illinois EPA on December 18, 2025, and concurred on by the EPA on January 12, 2026. In a separate action, the EPA is taking action on a similar determination for the Missouri portion of the St. Louis area. As a result of these determinations, EPA suspends the requirements for the area to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable progress, and other planning SIPs related to attainment of the 2015 ozone NAAQS, for as long as the area continues to attain the 2015 ozone NAAQS. In a separate action, the EPA is finalizing a similar determination for the Missouri portion of the St. Louis area.

IV. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at www.epa.gov/​lawsregulations/​laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

This action is not a significant regulatory action and was therefore not submitted to the Office of Management and Budget (OMB) for review. This action issues a clean data determination for the Illinois portion of the St. Louis area for the 2015 ozone NAAQS.

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

This action is not an Executive Order 14192 regulatory action because this action is not significant under Executive Order 12866.

C. Paperwork Reduction Act (PRA)

This rule does not impose an information collection burden under the provisions of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.)

D. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA. This action will not impose any requirements on small entities beyond those imposed by State law. The clean data determination does not create any new requirements and does not directly regulate any entities.

E. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect small governments. This action imposes no enforceable duty on any State, local or Tribal governments or the private sector.

F. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial ( printed page 52248) direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. This action issues a clean data determination for the Illinois portion of the St. Louis area under the CAA.

G. Executive Order 13175: Coordination With Indian Tribal Governments

This rule does not have Tribal implications, as specified in Executive Order 13175. It will not have substantial direct effects on Tribal governments. Thus, Executive Order 13175 does not apply to this rule.

H. Executive Order 13045: Protection of Children From Environmental Health Risks and Safety Risks

The EPA interprets Executive Order 13045 as applying only to those regulatory actions that concern environmental health or safety risks that the EPA has reason to believe may disproportionately affect children, per the definition of “covered regulatory action” in section 2-202 of the Executive Order. This action is not subject to Executive Order 13045 because it merely issues a clean data determination.

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution, or Use

This action is not subject to Executive Order 13211, because it is not a significant regulatory action under Executive Order 12866.

J. National Technology Transfer Advancement Act

This rulemaking does not involve technical standards.

K. Congressional Review Act

This action is subject to the Congressional Review Act, and the EPA will submit a rule report to each House of the Congress and to the Comptroller General of the United States. This action is not a “major rule” as defined by 5 U.S.C. 804(2).

L. Judicial Review

Under section 307(b)(1) of the CAA, petitions for judicial review of this action must be filed in the United States Court of Appeals for the appropriate circuit by October 13, 2026. Filing a petition for reconsideration by the Administrator of this final rule does not affect the finality of this rule for the purposes of judicial review nor does it extend the time within which a petition for judicial review may be filed, and shall not postpone the effectiveness of such rule or action. This action may not be challenged later in proceedings to enforce its requirements. See section 307(b)(2).

For the reasons stated in the preamble, 40 CFR part 52 is amended as follows:

91 FR 52246