On Friday, November 21, 2025, the U.S. Environmental Protection Agency (EPA) approved off-model adjustment factors for EMFAC2021 that can be used to remove the emissions benefits of the Advanced Clean Trucks (ACT), Zero-Emission Airport Shuttle, Heavy-Duty Vehicle and Engine Emission Warranty and Maintenance Provisions (Warranty Phase 1), and Heavy-Duty Omnibus (Omnibus) regulations.

CARB developed the off-model adjustment factors in response to three Congressional Resolutions that purported to disapprove U.S. EPA’s decisions to grant California waivers for several regulations on June 12, 2025. While those illegal actions are being contested, CARB staff provided factors to remove these regulations from the EMFAC2021 baseline solely for the purposes of State Implementation Plan (SIP) development and conformity determinations, including regional emissions analysis for transportation plan and transportation improvement program conformity determinations, as well as hot-spot analysis for project-level conformity determinations.

The U.S. EPA approval letter, EMFAC2021 adjustment factors, and documentation are now on the Mobile Source Emissions Inventory (MSEI) website. 

EPA Approval Letter & Adjustment Factors