EPA Grants Clean Data Determination for St. Louis Illinois Area
The agency is suspending state planning requirements after monitoring data shows the region met 2015 ozone standards.
The Environmental Protection Agency has determined that the Illinois portion of the St. Louis nonattainment area achieved clean data for the 2015 ozone National Ambient Air Quality Standards (NAAQS). This finding follows a review of certified ambient air monitoring data from the 2023-2025 design value period, which recorded a value of 0.070 ppm.
The determination relies on the EPA's concurrence of an exceptional events request submitted by the Illinois Environmental Protection Agency on December 18, 2025. By removing event-influenced data from the design value, the EPA finalized its concurrence on January 12, 2026.
This action suspends the requirement for the area to submit attainment demonstrations and associated Reasonably Available Control Measures (RACM). It also halts the need for Reasonable Further Progress (RFP) plans, contingency measures for failure to attain, and other planning State Implementation Plans (SIPs) related to the 2015 ozone NAAQS.
These suspensions remain in effect for as long as the area continues to attain the 2015 ozone NAAQS. This action does not constitute a formal redesignation of the area to attainment, nor does it approve a maintenance plan.
The final rule becomes effective on September 14, 2026.