Air Plan Approval; Illinois; Clean Data Determination for the Illinois Portion of the St. Louis Area for the 2015 Ozone Standard
Key claim: EPA finds the Illinois side of the St. Louis area now meets the 2015 ozone air-quality standard, temporarily relieving Illinois of certain smog-related planning requirements.
Abstract
(Rule · Environmental Protection Agency) The Environmental Protection Agency (EPA) is determining under the Clean Air Act (CAA) that the Illinois portion of the St. Louis, MO- IL nonattainment area (hereafter also referred to 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 is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period showing that the Illinois portion of the area achieved attainment of the 2015 ozone NAAQS. This determination also relies on the EPA’s concurrence of an exceptional events request submitted by the Illinois Environmental Protection Agency (Illinois EPA) on December 18, 2025, and concurred on by the EPA on January 12, 2026. Therefore, the EPA is taking final agency action on Illinois’ exceptional events request. In a separate action, the EPA is finalizing a similar determination for the Missouri portion of the St. Louis area. This final determination suspends the requirements for the area to submit attainment demonstrations and associated Reasonably Available Control Measures (RACM), Reasonable Further Progress (RFP) plans, contingency measures for failure to attain or make reasonable progress, and other planning State Implementation Plans (SIPs) related to attainment of the 2015 ozone NAAQS, for as long as the area continues to attain the 2015 ozone NAAQS.
Why this matters
Clean data determinations pause parts of the Clean Air Act’s nonattainment SIP treadmill—attainment demonstrations, RFP plans, and contingency measures—reducing near-term planning costs for states and regulated sources while monitoring stays clean. Because the St. Louis area straddles Illinois and Missouri, paired determinations on both sides signal region-wide ozone improvement but leave underlying nonattainment designation and permitting constraints (e.g., NNSR) in place until formal redesignation.