On January 4, 2017, EPA proposed to redesignate the Ohio portion of the Cincinnati-Hamilton, OH-IN-KY, nonattainment area to attainment for the 1997 fine particulate matter (PM2.5) annual National Ambient Air Quality Standard (NAAQS). The Ohio portion of the Cincinnati-Hamilton area includes Butler, Clermont, Hamilton and Warren Counties. EPA has determined the Cincinnati-Hamilton area is attaining the annual PM2.5 standard and is proposing to redesignate the area to attainment.

On July 14, 2015, the United States Court of Appeals for the Sixth Circuit vacated EPA’s redesignation of the Indiana and Ohio portions of the Cincinnati-Hamilton area to attainment of the 1997 PM2.5 NAAQS in Sierra Club v. EPA, 793 F.3d 656 (6th Cir. 2015). In that case, the Sixth Circuit held that EPA erred when it designated the Ohio and Indiana portions as in attainment without adequately addressing Reasonably Available Control Measures (RACM) and Reasonably Available Control Technology (RACT). Ohio subsequently provided more information and modeling data to EPA regarding why RACM and RACT were unnecessary. EPA agreed with Ohio’s RACM/RACT analysis in its Cincinnati-Hamilton area attainment plan. EPA now proposes to approve an update to the Ohio State Implementation Plan (SIP) for maintaining the 1997 annual PM2.5 NAAQS through 2027.