# Control strategy: Ozone.
**AGENCY:**
Environmental Protection Agency (EPA).
**ACTION:**
Proposed rule.
**SUMMARY:**
The Environmental Protection Agency (EPA) is proposing to determine under the Clean Air Act (CAA) that the Missouri portion of the St. Louis, MO-IL nonattainment area (hereafter also referred to, respectively, as the “St. Louis area” or “area”) has achieved clean data for the 2015 ozone National Ambient Air Quality Standards (NAAQS or standard). This determination of clean data is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period showing that the Missouri portion of the area achieved attainment of the 2015 ozone NAAQS. The 2023-2025 design value relies upon EPA concurrence on a portion of the exceptional events request as submitted by the Missouri Department of Natural Resources (MoDNR) on November 3, 2025, and concurred on by the EPA on January 27, 2026. Therefore, the EPA is proposing to approve Missouri's November 3, 2025, Clean Data Determination (CDD) request. If finalized, this proposed CDD would suspend the obligations of the State of Missouri to submit certain nonattainment area planning requirements for as long as the Missouri portion of the St. Louis area continues to attain the 2015 ozone NAAQS. In a separate action, the EPA is proposing a similar determination for the Illinois portion of the St. Louis area.
**DATES:**
Comments must be received on or before March 30, 2026.
**ADDRESSES:**
Submit your comments, identified by Docket ID No. EPA-R07-OAR-2026-0463 to *https://www.regulations.gov.* Follow the online instructions for submitting comments.
Once submitted, comments cannot be edited or removed from *Regulations.gov* . The EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission ( *i.e.,* on the web, cloud, or other file sharing system). For additional submission methods, the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit *https://www.epa.gov/dockets/commenting-epa-dockets.*
**FOR FURTHER INFORMATION CONTACT:**
Ashley Keas, Environmental Protection Agency, Region 7 Office, Air and Radiation Division, 11201 Renner Boulevard, Lenexa, Kansas 66219; telephone number: (913) 551-7629; email address: *[email protected].*
**SUPPLEMENTARY INFORMATION:**
Throughout this document “we,” “us,” and “our” refer to the EPA.
**Table of Contents**
I. Background
II. Exceptional Events Demonstration
III. Clean Data Determination
IV. Proposed Action
V. Statutory and Executive Order Reviews
**I. Background**
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). Under EPA's regulations at 40 CFR part 50, the 2015 ozone NAAQS is attained in an area when the 3-year average of the annual fourth highest daily maximum 8-hour average concentration is equal to or less than 0.070 ppm, when truncated after the thousandth decimal place, at all of the ozone monitoring sites in the area. See 40 CFR 50.19 and appendix U to 40 CFR part 50.
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 quality-assured ozone monitoring data from 2015-2017, the EPA designated the St. Louis, MO-IL bi-state area as Marginal nonattainment for the 2015 Ozone NAAQS with an effective date of August 3, 2018 (83 FR 25776). At that time, the area included Boles Township of Franklin County, St. Charles County, St. Louis County, and St. Louis City in Missouri, and Madison and St. Clair Counties in Illinois. As part of that same action, the EPA designated Jefferson County and the remaining portion of Franklin County, in Missouri, and Monroe County in Illinois, as attainment/unclassifiable. On July 10, 2020, the U.S. Court of Appeals for the District of Columbia remanded the Jefferson County, Missouri, and Monroe County, Illinois, designations (among other designations) to the EPA. The Court upheld the EPA's designation of Boles Township as nonattainment and the remainder of Franklin County as attainment/unclassifiable. In response to the Court's remand, the EPA revised the Jefferson County, Missouri, and Monroe County, Illinois designation to nonattainment on June 14, 2021 (86 FR 31438). The St. Louis, MO-IL nonattainment area for the 2015 ozone NAAQS consists of Boles Township in 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]
[^1] EPA's rules dated November 24, 2024, and December 17, 2024, are the subject of ongoing litigation in the U.S. Court of Appeals for the Eighth Circuit and the U.S. Court of Appeals for the Seventh Circuit, respectively. Both cases are currently held in abeyance. The U.S. Court of Appeals for the Eighth Circuit granted Missouri's motion to stay the effectiveness of the underlying action pending judicial review on June 6, 2025.
On November 3, 2025, MoDNR submitted an exceptional events (EE) demonstration to show that the ozone concentration at certain Missouri monitors during the 2023 ozone season were influenced by wildfire smoke events originating in Canada. On January 27, 2026, the EPA concurred on a portion of the state's November 3, 2025, EE demonstration. Ambient air monitoring data from 2023 to 2025, which pursuant to the EPA's concurrence on a portion of the Missouri EE demonstration now excludes exceptional event influenced monitor days, indicates that the Missouri portion of the St. Louis Area has attained the 2015 ozone NAAQS. The EPA's Exceptional Events Rule and MoDNR's EE demonstration are discussed in more detail in section II. of this document. The EPA's Technical Support Document (TSD) detailing the EPA's rationale for concurring with a portion of Missouri's EE demonstration is included in the docket for this action.
**II. Exceptional Events Demonstration**
Congress has recognized that it may not be appropriate for the EPA to use certain monitoring data collected by the ambient air quality monitoring network and maintained in the EPA's Air Quality System (AQS) database in certain regulatory determinations. Thus, in 2005, Congress provided the statutory authority for the exclusion of data influenced by “exceptional events” meeting specific criteria by adding section 319(b) to the CAA and granted the EPA with the authority to propose regulations to review and manage air quality monitoring data influenced by exceptional events. [^2]
[^2] Under CAA section 319(b), an exceptional event means an event that (i) affects air quality; (ii) is not reasonably controllable or preventable; (iii) is an event caused by human activity that is unlikely to recur at a particular location or a natural event; and (iv) is determined by EPA under the process established in regulations promulgated by EPA in accordance with section 319(b)(2) to be an exceptional event. For the purposes of section 319(b), an exceptional event does not include (i) stagnation of air masses or meteorological inversions; (ii) a meteorological event involving high temperatures or lack of precipitation; or (iii) air pollution relating to source noncompliance.
To implement this 2005 CAA amendment, on March 22, 2007, (72 FR 13560), the EPA promulgated the 2007 Exceptional Events Rule. The 2007 Exceptional Events Rule created a regulatory process codified at 40 CFR parts 50 and 51 (§§ 50.1, 50.14, and 51.930). These regulatory sections, which superseded the EPA's previous guidance on handling data influenced by events, contain definitions, procedural requirements, requirements for air agency demonstrations, criteria for the EPA's approval of the exclusion of event-affected air quality data from the data set used for regulatory decisions, and requirements for air agencies to take appropriate and reasonable actions to protect public health from exceedances or violations of the NAAQS. On October 3, 2016, (81 FR 68216), the EPA promulgated a comprehensive revision to the 2007 Exceptional Events Rule. The 2016 Exceptional Events Rule revision included the requirement that, if a state demonstrates that emissions from a wildfire smoke event caused a specific air pollution concentration in excess of the NAAQS at a particular air quality monitoring location and otherwise satisfies the requirements of 40 CFR 50.14, the EPA must exclude that data from use in determinations of exceedances and violations. [^3]
[^3] 40 CFR 50.14(b)(4).
The CAA provides for the exclusion of air quality monitoring data from design value calculations when there are NAAQS exceedances caused by events, such as wildfires, that meet the criteria for an exceptional event identified in the EPA's Exceptional Events Rule at 40 CFR 50.1, 50.14, and 51.930. For the purposes of this proposed action, on November 3, 2025, MoDNR submitted an exceptional events demonstration to show that ozone concentrations recorded at the West Alton monitor with Site ID 29-183-1002 on May 24, June 3, June 6, June 10, June 17, and June 24, 2023, and the Maryland Heights monitor with Site ID 29-189-0014 on June 6, 2023, were caused by the 2023 Canadian wildfires in the provinces of Alberta, Quebec, Nova Scotia, the Northwest Territories and British Columbia. The EPA concurred on this request on January 27, 2026.
The EPA found that Missouri's demonstration met the Exceptional Events Rule criteria by establishing a clear, causal relationship between these wildfire events and the monitoring data which have regulatory significance for purposes of calculating the area's most recent design value (2023-2025 monitoring data). The removal of this exceptional event-impacted data, consistent with the CAA and the implementing regulations, supports a clean data determination (CDD) for the 2015 ozone NAAQS. As such, the EPA proposes to take final agency action to approve this CDD based on the concurred dates, summarized in table 1 of this document and detailed in the docket, and remove from the data set used for regulatory purposes those monitor days that the EPA concurred on as influenced by an exceptional event consistent with CAA section 319(b) and the implementing regulations. [^4]
[^4] At this time, the EPA defers action on other monitor-days included in the November 3, 2025, Missouri submitted demonstration, as referenced in the EPA's TSD and associated concurrence letter included in the docket for this action.
| EPA decision | Site name | AQS ID | Event date |
| --- | --- | --- | --- |
| Concur | West Alton | 29-183-1002 | May 24, 2023. |
| Concur | West Alton | 29-183-1002 | June 3, 2023. |
| Concur | West Alton | 29-183-1002 | June 6, 2023. |
| Concur | West Alton | 29-183-1002 | June 10, 2023. |
| Concur | West Alton | 29-183-1002 | June 17, 2023. |
| Concur | West Alton | 29-183-1002 | June 24, 2023. |
| Concur | Maryland Heights | 29-189-0014 | June 6, 2023. |
For this proposed action, the EPA will rely on the calculated design values that exclude the concurred dates for the purpose of demonstrating attainment of the 2015 8-hour ozone NAAQS. Further details on Missouri's analyses and the EPA's concurrence can be found in the docket for this regulatory action. While the EPA has concurred on a portion of Missouri's request to exclude exceptional event-influenced air quality monitoring data from regulatory decisions, the EPA must provide an opportunity for public comment on the concurred exceptional events, as noted in table 1, and all supporting data prior to the EPA taking a final clean data determination action which relies on the revised data set. This proposed action provides the public with an opportunity to comment on the exceptional events, all supporting documents, and the EPA's concurrence with Missouri's request for the events listed in table 1.
**III. Clean Data Determination**
Following enactment of the CAA Amendments of 1990, the EPA discussed its interpretation of the requirements for implementing the NAAQS in the General Preamble for the Implementation of title I of the CAA Amendments of 1990 (General Preamble), 57 FR 13498, 13564 (April 16, 1992). On November 29, 1995, (70 FR 71612), the EPA set forth what has become known as its “Clean Data Policy” for the 1-hour ozone NAAQS. Under the Clean Data Policy, for a nonattainment area that can demonstrate attainment of the standard before implementing CAA nonattainment measures, the EPA interprets the requirements of the CAA that are specifically designed to help an area achieve attainment, such as the requirements for such area to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable progress, and other planning state implementation plans (SIPs) related to attainment of the ozone NAAQS, to be suspended for as long as air quality continues to meet the standard. Such a determination of attainment under the Clean Data Policy is known informally as a clean data determination. On December 6, 2018, (83 FR 62998), in the final rule updating implementing regulations for the 2015 ozone NAAQS, the EPA codified this policy at 40 CFR 51.1318.
An area is attaining the 2015 ozone NAAQS if it meets the 2015 ozone NAAQS based on three complete, consecutive calendar years of quality-assured air quality data for all monitoring sites in the area. To attain the 2015 ozone NAAQS, the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentrations (ozone design values) at each monitor must not exceed 0.070 ppm. [^5] The air quality data must be collected and quality-assured in accordance with 40 CFR part 58 and recorded in AQS. Ambient air quality monitoring data for the 3-year period must also meet data completeness requirements. An ozone design value is valid if daily maximum 8-hour average concentrations are available for at least 90% of the days within the ozone monitoring seasons, [^6] on average, for the 3-year period, with a minimum data completeness of 75% during the ozone monitoring season of any year during the 3-year period. See section 4 of appendix U to 40 CFR part 50.
[^5] The rounding convention in 40 CFR part 50, appendix U dictates that concentrations shall be reported in “ppm” to the third decimal place, with additional digits to the right being truncated. Thus, a computed 3-year average ozone concentration of 0.071 ppm is greater than 0.070 ppm and would exceed the standard, but a DV of 0.0709 is truncated to 0.070 and attains the 2015 ozone NAAQS.
[^6] The ozone season is defined by state in 40 CFR part 58, appendix D. The ozone season for Missouri and Illinois is March-October. *See* 80 FR 65292, at 65466-67 (October 26, 2015).
On November 3, 2025, the MoDNR sent a clean data determination request to the EPA. [^7] The EPA has reviewed the available ozone monitoring data from MoDNR's monitoring sites in the St. Louis area for the 2023-2025 period. These data have been quality assured, are recorded in the AQS, and were certified in advance of the EPA's publication of this proposal. Specifically, Missouri notified the EPA on December 4, 2025, that the 2025 ozone season data for all monitors in the Missouri portion of the St. Louis area had been quality assured and certified in AQS. [^8] These data demonstrate that the St. Louis area is attaining the 2015 ozone NAAQS. The annual fourth-highest 8-hour ozone concentrations and the 3-year average of these concentrations (monitoring site ozone design values) for all Missouri monitoring sites are summarized in table 2. As explained in section II. of this action, this data excludes the exceptional event impacted monitoring days.
[^7] The State's request is included in the docket for this action. The State's CDD request relies on EPA concurrence on six of the days included in the State's exceptional events demonstration for the West Alton ozone monitor and one of the days included in the demonstration for the Maryland Heights ozone monitor.
[^8] Materials related to Missouri's certification of ozone season monitoring data is included in the docket for this action.
| County | Monitor name | Monitor | 2023 | 2024 | 2025 | 2023-2025 |
| --- | --- | --- | --- | --- | --- | --- |
| Jefferson | Arnold West | 29-099-0019 | 0.078 | 0.061 | 0.071 | 0.070 |
| Saint Charles | West Alton | 29-183-1002 | 0.075 | 0.068 | 0.069 | 0.070 |
| Saint Charles | Orchard Farm | 29-183-1004 | 0.073 | 0.067 | 0.070 | 0.070 |
| Saint Louis | Pacific | 29-189-0005 | 0.077 | 0.064 | 0.069 | 0.070 |
| Saint Louis | Maryland Heights | 29-189-0014 | 0.078 | 0.066 | 0.066 | 0.070 |
| Saint Louis City | Blair Street | 29-510-0085 | 0.077 | 0.066 | 0.066 | 0.069 |
The Missouri portion of the St. Louis area's 3-year ozone design value for 2023-2025 is 0.070 ppm, [^9] which meets the 2015 ozone NAAQS. Therefore, in this action, the EPA proposes to find that the Missouri portion of the St. Louis area is attaining the 2015 ozone NAAQS.
[^9] The monitor ozone design value for the monitor with the highest 3-year averaged concentration.
The EPA will not take final action to determine that the St. Louis area is attaining the NAAQS if the design value of a monitoring site in the area violates the NAAQS prior to final approval of the clean data determination. Additionally, the EPA will not take final action to determine that the Missouri portion of the St. Louis area is attaining the NAAQS, absent a final action to determine that the Illinois portion of the St. Louis area is also attaining the NAAQS. Under the EPA's Clean Data Policy, a clean data determination only has meaning and effect when an entire area is attaining the NAAQS. In the instance of a multi-state area like the St. Louis area, this means that all monitors in both states must have attaining data.
Should this action be finalized, the requirements for MoDNR to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable further progress, and planning elements related to attainment of the 2015 ozone NAAQS for the Missouri portion of the St. Louis area, would be suspended for as long as the area continues to attain the 2015 ozone NAAQS. 40 CFR 51.1318.
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.
**IV. Proposed Action**
The EPA is proposing to determine that the Missouri portion of the St. Louis area has attained the 2015 ozone NAAQS and meets the requirements under the Clean Data Policy for a CDD. This determination of clean data is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period showing that the Missouri portion of the area achieved attainment of the 2015 ozone NAAQS. The 2023-2025 design value relies upon EPA concurrence on a portion of the exceptional events request as submitted by the Missouri Department of Natural Resources (MoDNR) on November 3, 2025, and concurred on by the EPA on January 27, 2026. Therefore, the EPA is proposing to approve Missouri's November 3, 2025, CDD request. As provided in 40 CFR 51.1318, if the EPA finalizes this CDD, the requirements for the MoDNR to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable further progress, and planning elements related to attainment of the 2015 ozone NAAQS for the Missouri portion of the St. Louis area, are suspended for as long as the Missouri portion of the area continues to attain the 2015 ozone NAAQS. In a separate action, the EPA is proposing a similar determination for the Illinois portion of the St. Louis area.
**V. Statutory and Executive Order Reviews**
Additional information about these statutes and Executive Orders can be found at * https://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 proposes to issue a 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 proposed 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 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 proposes 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 merely proposes 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.
**List of Subjects in 40 CFR Part 52**
Environmental protection, Air pollution control, Incorporation by reference, Intergovernmental relations, Nitrogen dioxide, Ozone, Reporting and recordkeeping requirements, Volatile organic compounds.
Dated: February 18, 2026.
James Macy,
Regional Administrator, Region 7.
For the reasons stated in the preamble, the EPA proposes to amend 40 CFR part 52 as set forth below:
**PART 52—APPROVAL AND PROMULGATION OF IMPLEMENTATION PLANS**
1. The authority citation for part 52 continues to read as follows:
**Authority:**
42 U.S.C. 7401 *et seq.*
**Subpart AA—Missouri**
2. In § 52.1342, add paragraph (f) to read as follows:
§ 52.1342
(f) *Determination of attainment.* The EPA has determined, as of [date of publication of the final rule in the *Federal Register* ], that the St. Louis Ozone nonattainment area has attained the 2015 8-hour Ozone NAAQS. This determination suspends the requirements for this area to submit an attainment demonstration, associated reasonably available control measures, reasonable further progress, contingency measures, and other plan elements related to attainment of the standards for as long as the area continues to meet the 2015 8-hour Ozone NAAQS.