SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean the EPA. This SUPPLEMENTARY INFORMATION section is arranged as follows:

Table of Contents

I. What is being addressed in this document?

II. The EPA's Responses to Comments

III. What action is the EPA taking?

IV. Statutory and Executive Order Reviews

I. What is being addressed in this document?

The EPA has determined that ground-level ozone is detrimental to human health. On October 1, 2015, the EPA promulgated a revised 8-hour ozone NAAQS of 0.070 parts per million (ppm). See 80 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 standard 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 standard 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 9159), the EPA published a notice of proposed rulemaking (NPRM) which proposed to determine that the Missouri portion of the St. Louis area attained the 2015 ozone NAAQS, based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period.^[2] Such a determination, based upon the EPA's Clean Data Policy, is known informally as a clean data determination (CDD). As noted in the NPRM, the Missouri portion of the St. Louis area's 3-year ozone design value for 2023-2025 is 0.070 ppm, which meets the 2015 ozone NAAQS.

The EPA's proposed CDD relied upon EPA's concurrence on an exceptional events demonstration submitted by Missouri on November 3, 2025. Missouri posted the demonstration for public comment after engagement with EPA consistent with the process outlined in the EPA's Exceptional Events Rule. The EPA concurred on a portion of the state's demonstration on January 27, 2026. The EPA may defer action on requested events which do not currently have regulatory significance. See81 FR 68216, 68269 (Oct. 3, 2016).

In the February 26, 2026, NPRM, the EPA proposed to take final agency action to issue a CDD based on the concurred dates, which removed the event-influenced data from the design value, and opened an opportunity for public comment on the EPA's exceptional events concurrence. The EPA responds to comments received in section II. of this document.

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 2023-2025 period, however the 2023-2025 design value does not serve as the area's most recent complete and quality-assured design value available as of the applicable attainment date.

This action does not constitute a redesignation of any portion of the area to attainment of the 2015 ozone NAAQS under section 107(d)(3)(E) of the CAA, 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. The Missouri portion of the St. Louis area will remain designated nonattainment for the 2015 ozone NAAQS until such time as the EPA determines that the Missouri portion of the area meets CAA requirements for redesignation to attainment and takes a separate action to redesignate the Missouri portion of 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 comments from two entities: the Midwest Ozone Group and the Washington University School of Law Interdisciplinary Environmental Clinic on behalf of the Sierra Club (hereinafter referred to as the Sierra Club). The Midwest Ozone Group supported the proposed action and requested its finalization. In this section, the EPA provides a high-level summary of the comments received from the Sierra Club and our responses. The EPA's full responses to comments are included in a separate Response to Comment (RTC) document included in the docket for this action.

The EPA's RTC summarizes the Sierra Club's concerns, including assertions about the procedural adequacy of Missouri's exceptional events demonstration, the timing and scope of the State's submission, and the degree of public participation afforded during the State's review process. In response, the EPA reiterates that Missouri complied with all requirements of the Exceptional Events Rule, including public notice, refinement of the technical documentation following the EPA's initial feedback, and appropriate sequencing of the demonstration once the data became regulatorily significant under 40 CFR 50.14. The EPA therefore concludes that procedural concerns about the State's development of its exceptional events demonstration do not undermine the validity of the State's submission.

The EPA also responds to the Sierra Club's comments asserting that the EPA selectively concurred on only a subset of the wildfire influenced exceedance days submitted- by Missouri and that this concurrence was intended to produce a desired regulatory outcome. The EPA does not find merit in these claims and explains that only data ( printed page 52245) relevant to the regulatory determination in this action were subject to review. The EPA's concurrence was based on a full technical evaluation of each day that had regulatory significance to the CDD, and the EPA's review of and concurrence on additional days was not required. As explained in the NPRM and RTC, a design value at or below 0.070 ppm demonstrates attainment under the 2015 ozone NAAQS and certified data for all ozone monitors in the Missouri portion of the area for 2023 through 2025 meet that standard.

The EPA also responds to the Sierra Club's comments regarding long-term ozone trends, precursor emissions trajectories, and projected future increases in emissions. In the RTC, the EPA concludes that such issues fall outside the scope of the CDD, which is based solely on complete, quality-assured monitoring data for the most recent three-year-period. The EPA notes that the exceedances in late 2025 cited by commenters were fully accounted for in the design value calculations for 2023-2025 that demonstrate attainment.

The EPA reviewed the Sierra Club's concerns about potential public health implications associated with excluding wildfire-influenced-data under the Exceptional Events Rule. The EPA acknowledges the health impacts of exposure to high ozone concentrations. The EPA further notes that the Exceptional Events Rule was established under the authority granted by Congress under CAA section 319(b) and implemented by the EPA to ensure that data influenced by uncontrollable natural events are not used in regulatory determinations where doing so would be inappropriate. The Sierra Club also expresses concerns that the State is avoiding important planning and emissions reductions requirements associated with the ozone NAAQS. As explained in the proposed rule and the RTC, Missouri remains responsible for meeting all applicable State Implementation Plan requirements until the area is redesignated to attainment, and the suspension of certain attainment-planning-elements under 40 CFR 51.1318 occurs only upon a final clean data finding and only so long as the area continues to meet the standard.

In the RTC, the EPA also addresses comments questioning whether the exceptional events demonstration sufficiently established a clear causal relationship between wildfire smoke and the ozone exceedances on the days for which the EPA concurred. Based on the complete technical record and weight of evidence approach, including but not limited to HYSPLIT back-trajectory-modeling, satellite observations, surface measurements, statistical analyses, and supporting meteorological evidence, the EPA concludes that the State provided an adequate demonstration to support each concurrence day. The EPA explains in the RTC that wildfire plumes originating from multiple fires can merge into regional-scale smoke masses and that attribution to a single point source is neither technically feasible nor required under the Exceptional Events Rule. In addition, individual data elements such as monitored black carbon concentrations or satellite snapshots must be interpreted within the broader meteorological context and the full weight of evidence, which supports the EPA's concurrence.

The EPA reviewed comment letters that Sierra Club previously submitted to Missouri during the State's public comment process. The Sierra Club incorporated by reference their previous comment letters in the comment letter it submitted to the EPA. While these materials were cited too generally to constitute significant comments requiring individualized response, the EPA nonetheless evaluated them and addressed the relevant issues within the scope of this federal action. To the extent those materials addressed matters unrelated to the proposed CDD, including redesignation requirements or broader air quality planning questions, the EPA finds that they fall outside the scope of this rulemaking.

Overall, the EPA concludes that none of the comments submitted provide evidence to undermine the basis of the EPA's proposed CDD. The record demonstrates that Missouri submitted complete and procedurally valid documentation under the Exceptional Events Rule; that the EPA's technical review was consistent with the CAA, its implementing regulations, and relevant guidance; and that the certified and quality-assured-2023-2025 monitoring data demonstrate attainment of the 2015 ozone NAAQS in the Missouri portion of the St. Louis nonattainment area. Accordingly, the EPA is finalizing the Clean Data Determination as proposed.

III. What action is the EPA taking?

The EPA is making a determination under the CAA that the Missouri portion of the St. Louis area has attained the 2015 ozone NAAQS. This determination is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 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 MoDNR on November 3, 2025, and concurred on by the EPA on January 27, 2026. As provided in 40 CFR 51.1318, through this final determination, 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 certain other planning requirements related to attainment of the 2015 ozone NAAQS, are suspended 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 Illinois 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 final clean data determination for the Missouri portion of the St. Louis area for the 2015 ozone NAAQS.

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

Executive Order 14192 does not apply because it is not a significant regulatory action and is therefore exempted from review 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 ( printed page 52246) 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 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 finalizes a clean data determination for the Missouri 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. Therefore, this action is not subject to Executive Order 13045 because it is merely 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.

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).

Under section 307(b)(1) of the Clean Air Act, 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 action 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, the EPA amends 40 CFR part 52 as set forth below:

91 FR 52243