Fleets Explained: Emissions regulations

One of EPA’s latest major emission regulations is “Control of Air Pollution From New Motor Vehicles: Heavy-Duty Engine and Vehicle Standards.” The final rule adopted by EPA in December 2022 introduces many requirements. Notably, it tightens nitrogen oxides, particulate matter, carbon monoxide, and hydrocarbon emission standards in heavy-duty engines for model year 2027 and beyond. The rule is also known as Heavy-Duty NOx—and it could soon be changed by the new administration.

See also: Fleets Explained: DEF and NOx

EPA’s other recent major emission regulation focuses on greenhouse gases.

How does the Clean Air Act regulate greenhouse gases?

It wasn’t always the case, but currently the Clean Air Act requires the EPA to regulate emissions of greenhouse gases (this will likely be changed—more information below).

This requirement became official following the 2007 Supreme Court decision in Massachusetts v. EPA. In its decision, the Court ruled that greenhouse gases—gases that affect Earth’s atmosphere and contribute to climate change—are air pollutants, and thus EPA can and must regulate vehicles producing those gases under the Clean Air Act.

In 2009, the EPA formally recognized that six primary greenhouse gases, including carbon dioxide, threaten public health. Since then, EPA has published several rules to regulate greenhouse gas emissions.

EPA’s latest major greenhouse gas rulemaking is “Final Rule: Greenhouse Gas Emissions Standards for Heavy-Duty Vehicles – Phase 3.” Adopted as a final rule in March 2024, the standard introduces tightening carbon dioxide emission standards for heavy-duty vehicles from Model Year 2027 through 2032 and beyond. Critics call the regulation an EV mandate, though that is a misconception.

The emission standard varies by equipment type and model year, ranging from 103.4 grams per ton-mile for MY2027 high roof Class 7 tractors to 45.4 g/t-m for MY2032 high roof Class 8 day cab tractors.

What is CARB, and how does it regulate trucking?

EPA is not the only major regulatory authority for vehicle emissions. The California Air Resources Board sets emissions standards for California. Other states have the option to adopt CARB’s standards.

The Clean Air Act prohibits states from setting their own emission standards—but with one key exception. The Clean Air Act allows states with pre-existing emissions standards to seek a waiver to set their own emissions standards. California was the only state with existing emissions standards before the Clean Air Act, due to the state’s severe air quality problems.

Los Angeles established the first air pollution control program in the nation in 1947. In 1959, the state legislature founded the California Motor Vehicle Pollution Control Board, which regulated vehicle emissions and would later become the California Air Resources Board. The Board established the first tailpipe emissions standards in the nation in 1966.

The federal government consistently granted California’s emissions waivers since the passage of the Clean Air Act until recently. In 2009, after EPA recognized the public health threat posed by greenhouse gases, the agency granted California a waiver to regulate vehicles’ greenhouse gas emissions.

Other states have the option to adopt California’s emissions standards without asking for authorization. These states are sometimes called CARB states.

Federal lawmakers knock down CARB regulations

Until this year, the biggest CARB regulations affecting heavy-duty vehicles were Heavy-Duty Omnibus, which established strict engine emissions standards, and Advanced Clean Trucks, which introduced a zero-emissions vehicle sales requirement. In June, Congress and the president used the Congressional Review Act to revoke the EPA waivers that authorized CARB's Omnibus and ACT regulations.

EPA is working to weaken vehicle emissions rules

In March, EPA Administrator Lee Zeldin announced the agency would "reconsider" its emissions standards. Zeldin made clear that he means to replace GHG3, heavy-duty NOx, and beyond with more business-friendly rules.

EPA in July proposed to revoke the 2009 endangerment finding and all regulations that depend on it. If the agency can remove the endangerment finding, it could nullify all of EPA's GHG emissions standards.

It is unclear how far or how quickly the rollback could occur. Jim Mullen, an industry leader familiar with the first Trump administration, said he expects EPA to revert to GHG Phase 2 standards for a year or more. Manufacturers have expressed hope that EPA would either delay the NOx rule or weaken its waiver requirements.

Legal battles further muddy regulatory outlook

The Trump administration's emissions rule rollback is pursuing—and defending—several legal battles with uncertain fates. Some legal experts expect battles to travel all the way to the Supreme Court.

• California is contesting its waiver revocations, arguing that the Congressional Review Act does not apply to these waivers.
• The Department of Justice is pressuring manufacturers to leave the Clean Truck Partnership, and manufacturers in turn sued California to exit the partnership.
• Revoking the endangerment finding has steep bureaucratic requirements and would almost certainly provoke further lawsuits