EPA Backs Kentucky's Ozone Maintenance Plan for Cincinnati Region
Published Date: 8/27/2026
Proposed Rule
Summary
The U.S. Environmental Protection Agency (EPA or Agency) is proposing to approve a State Implementation Plan (SIP) revision submitted by the Commonwealth of Kentucky through the Energy and Environment Cabinet (Cabinet), Division of Air Quality (Division), on July 23, 2024. The SIP revision consists of a Limited Maintenance Plan (LMP) for the Kentucky portion of the tri-state Cincinnati-Hamilton 1997 8-hour ozone nonattainment area (the "tri-state Cincinnati-Hamilton Area"). The Kentucky portion of the tri-state Cincinnati-Hamilton Area is composed of Boone, Campbell, and Kenton Counties in Kentucky (hereinafter also referred to as the "Northern Kentucky Area"). The EPA is proposing to approve Kentucky's LMP for the Northern Kentucky Area because it provides for the maintenance of the 1997 8-hour ozone National Ambient Air Quality Standards (NAAQS) within the tri-state Cincinnati-Hamilton Area through the end of the second 10-year portion of the maintenance period. The effect of this proposed action would be to make certain commitments related to maintenance of the 1997 8-hour ozone NAAQS in the Northern Kentucky Area federally enforceable as part of the Kentucky SIP.
Analyzed Economic Effects
4 provisions identified: 2 benefits, 1 costs, 1 mixed.
Triggers Could Prompt New Controls
Kentucky's plan sets clear triggers that would prompt review and possible new controls for the tri-state Cincinnati-Hamilton Area. An 8-hour ozone monitoring reading of 0.085 ppm or greater or emissions inventory updates showing more than a 10 percent increase in NOX or VOC emissions over the attainment inventory will trigger an evaluation; controls must be implemented as quickly as possible but no later than 12 months after the end of the ozone season in which the indicator occurred. If a three-year average violation (three-year average of annual fourth-highest values) of 0.085 ppm or greater occurs, contingency measures must be selected and in place within 24 months of that monitored violation.
List of Possible Contingency Measures
The plan lists specific potential contingency measures that Kentucky may adopt if triggers occur, including: adopting new or revised Reasonably Available Control Technology (RACT) rules (Groups II–IV Control Technique Guidelines) for point sources; enhanced inspection of stationary sources; fuel programs and incentives for alternative fuels; road or lane restrictions for buses or high-occupancy vehicles; trip-reduction ordinances; employer-based transportation management plans; construction of pedestrian or non-motorized paths; a modern vehicle inspection/maintenance program; diesel retrofit programs with incentives for fleet vehicles; and additional engine idling reduction programs.
EPA Would Make LMP Federally Enforceable
The EPA is proposing to approve Kentucky's Limited Maintenance Plan (LMP) for the Northern Kentucky portion of the tri-state Cincinnati-Hamilton Area (Boone, Campbell, and Kenton Counties). If approved, Kentucky's July 23, 2024 commitments to maintain the 1997 8-hour ozone standard through August 5, 2030 would become federally enforceable as part of the Kentucky State Implementation Plan. Comments on the proposal are due by September 17, 2026.
Partial Area Still Subject to 1997 Conformity Rules
Parts of the 1997 maintenance area that are not maintained under the later 2008/2015 ozone standards (the 'partial orphan area'—the southern portions of Boone, Campbell, and Kenton Counties) remain subject to transportation conformity for the 1997 standard. Within that partial orphan area, transportation plans and programs for the 1997 ozone NAAQS can be demonstrated without a regional emissions analysis under 40 CFR 93.109(c).
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