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Emission Standards

United States: Cars and Light-Duty Trucks

Federal Standards (US EPA)

The most recent emission standards for light-duty vehicles are the Tier 3 standards, that follow the earlier Tier 2, Tier 1, and pre-Tier 1 federal emission regulations.

Two sets of standards had been defined for light-duty vehicles in the Clean Air Act Amendments (CAAA) of 1990: Tier 1 and Tier 2 standards. Once Tier 2 standards were fully phased-in, the EPA adopted the Tier 3 emission regulations. The succession of these regulations can be summarized as follows:

  • Tier 1 standards were published as a final rule on June 5, 1991 and phased-in progressively between 1994 and 1997.
  • Tier 2 standards were adopted on December 21, 1999, with a phase-in implementation schedule from 2004 to 2009.
  • Tier 3 standards were finalized on March 3, 2014, to be phased-in between 2017 and 2025.

Tier 1 standards applied to all new light-duty vehicles (LDV) of less than 8500 lb gross vehicle weight rating (GVWR). The Tier 2 rule extended the applicability of the light-duty emission standards to medium-duty passenger vehicles (MDPV) with GVWR between 8500 and 10,000 lbs. Tier 3 regulations additionally include emission standards for chassis-certified heavy-duty vehicles up to 14,000 lbs (Class 2b and Class 3).

The successive tiers of emission regulations do not begin with a sharp cut-off date. Rather, each new tier of emission standards is phased-in over a number of years. During the phase-in period, manufacturers are required to certify an increasing percentage of their new vehicle fleet to the new standards, with the remaining vehicles still certified to the preceding tier of emission regulations.

The US emission standards establish (a set of) certification emission limits applicable to each certified vehicle, as well as fleet average standards. The fleet average standards—for NOx at the Tier 1 and Tier 2 stages and for NMOG+NOx at Tier 3—distinguish the US emission regulations from other light-duty emission standards for criteria pollutants across the world, which typically include only certification/type approval limits.

California Standards (CARB)

California emission regulations for light duty vehicles are designated Low Emission Vehicle (LEV) standards. The succession of California standards—including the LEV I, LEV II and LEV III stages—has closely followed (or, more accurately, preceded) the respective federal standards.

The California standards have been mores stringent than the federal ones. The tendency, however, has been that of increasing harmonization—the federal Tier 3 regulations are closely aligned with California LEV III standards.

Under Section 177 of the Clean Air Act, other states have a choice to either follow the federal regulations or to adopt California emission standards. A number of states who needed emission standards more stringent than the federal regulations in order to meet their air quality targets have adopted California standards. The adoption of California requirements was especially strong at the LEV II stage. States that adopted California standards are listed in the following table.

Table 1
States adoption status of California emission standards for light-duty vehicles
StateLegislationYear AdoptedMY Effective*
ColoradoExecutive Order B 2018 00620182022
ConnecticutPublic Act 04-8420042008
DelawareAdministrative Code Title 7, 114020102014
MaineAmendments to Chapter 127a2004
MarylandSenate Bill 10320072011
MassachusettsAmendments to the state’s LEV regulationsa2004
New JerseyP.L. 2003, Chapter 26620042009
New MexicoExecutive Order 2006-6920072016
New YorkChapter III, Subpart 218-8a2004
OregonDivision 257; OAR 340-256-0220; Division 1220062009
PennsylvaniaAmendments to Title 25, Chapters 121 and 12620062008
Rhode IslandAir Pollution Control Regulation No. 3720052008
VermontAmendments to Subchapter XIa2004
WashingtonHouse Bill 139720052009
* Refers to LEV II / LEV III standards for Passenger Cars and Light-Duty Trucks. Dates for Medium-Duty Passenger Vehicles and Medium-Duty Vehicles may be different, or California standards may not be applicable.
a Four states (NY, MA, VT, ME) adopted California LEV requirements in the late 1990s.

Most of the above states also adopted California GHG regulations for light-duty vehicles, as well as California emission regulations for Heavy-Duty Vehicles, HDV (which differed from federal regulations in certain 2005 NTE and 2008 SET, NTE and idle requirements). California HDV requirements were also adopted by some states—including Georgia and North Carolina—that did not adopt California light-duty vehicle standards.