EPA proposed revising its regulations at 40 CFR § 51.1403, to eliminate the "leftover SIP elements policy" codified in a January 2025 final rule, under which states whose ozone nonattainment areas are reclassified to a higher classification (for example from Moderate to Serious) also retain most state-implementation-plan (SIP) obligations tied to their prior classification, including reasonably available control technology (RACT) requirements and reasonable further progress demonstrations, on their original timetable. EPA now proposes that upon reclassification, prior-classification obligations are extinguished, and states need only fulfill the SIP requirements for their new higher classification, potentially deferring the implementation of emissions reductions. Tropospheric ozone is a potent greenhouse gas.