Automobile emission standards are an alphabet soup of bureaucratic acronyms. In some cases, there are different standards for different states. At other times, the same exact emission designation within the same standard carries different meanings when applied to vehicles in different weight classes. What can we say for certain?

All New Vehicles Must Be Certified

All new vehicles sold in the United States must be certified as meeting emissions standards set by:

  • The U.S. Environmental Protection Agency (EPA)


  • The California standards, set by the California Air Resources Board (CARB)

    or both

The California standards apply to vehicles sold in California and in a growing number of Northeastern states that have chosen to adopt California’s vehicle regulations. The federal standards apply to all the remaining states.

The Clean Air Act Establishes the Framework

In 1970, the federal Clean Air Act (CAA) established nationwide air quality standards to protect public health. Recognizing the large contribution motor vehicles make to air pollution, the Clean Air Act also set the first federal tailpipe standards.

The CAA also granted California, which has some of the worst air quality in the nation, the authority to set its own vehicle emission standards. Other states began adopting the stricter California standards beginning in 1990.

Federal and California tailpipe standards limit exhaust emissions of five pollutants:

  • hydrocarbons (HC)—think smog.
  • nitrogen oxides (NOx)—think lung irritant and smog.
  • carbon monoxide (CO)—think poisonous gas.
  • particulate matter (PM, for diesel vehicles only)—think soot.
  • formaldehyde (HCHO)—think cancer-causing gas.

Carbon dioxide is not directly regulated. Because it’s a byproduct of fuel economy—one gallon of gasoline consistently creates 19 pounds of dioxide—it’s indirectly regulated as part of fuel economy.

Currently, light trucks are allowed to emit up to five and a half times more smog-forming pollution than cars. Beginning in 2007, a new regulation will mark the first time that SUVs and other light-duty trucks—even the largest passenger vehicles— will be subject to the same national pollution standards as cars.

The Federal Standards

The rule of thumb on the federal standards is this: the lower the bin number, the cleaner the vehicle. The Toyota Prius is a very clean Bin 3, while the Hummer H2 is a dirty Bin 11.

  • Tier 2 bin 1: The cleanest Federal Tier 2 standard. A zero-emission vehicle (ZEV)
  • Tier 2 bins 4 – 2: Cleaner than the average standard
  • Tier 2 bin 5: "Average" of new Tier 2 standards, roughly equivalent to a LEVII vehicle
  • Tier 2 bins 9 – 6: Not as clean as the average requirement for a Tier 2 vehicle
  • Tier 2 bin 10: Least-clean Tier 2 bin applicable to cars
  • Tier 1: The former Federal standard; carried over to model year 2004 for those vehicles not yet subject to the phase-in.

The California Standards

A vehicle on the cutting edge of cleanliness for the California standards is a Partial Zero Emission Vehicle, like the Prius or Honda Civic Hybrid (1.3L 4, auto). Electric cars have no emissions and are therefore Zero Emission Vehicles (ZEVs). Meanwhile, the Ford Excursion is a LEV 1; despite the literal translation as “Low Emission Vehicle,” it’s on the dirty side of the spectrum.

  • ZEV: Zero Emission Vehicle, a California standard prohibiting any tailpipe emissions.
  • PZEV: Partial Zero-Emission Vehicle, compliant with the SULEV standard; additionally has near-zero evaporative emissions and a 15-year/150,000-mile warranty on its emission control equipment.
  • SULEV: Super-Ultra-Low-Emission Vehicle, a California standard even tighter than ULEV, including much lower NOx emissions. Roughly equivalent to a Tier 2 bin 2 vehicle.
  • ULEV II: Ultra-Low-Emission Vehicle, a cleaner than average vehicle certified under the Phase II LEV standard. Hydrocarbon and carbon monoxide emissions levels are nearly 50% lower than those of a LEV II-certified vehicle.
  • LEV II: Low-Emission Vehicle, the least stringent of the new, Phase II LEV standards. Equivalent to a Tier 2 bin 5 vehicle. NOx emissions are one-quarter the level of a LEV I-certified vehicle.
  • LEV (a.k.a. LEV I): Low Emission Vehicle, an intermediate California standard about twice as stringent as Tier 1.
  • ULEV (a.k.a. ULEV I): Ultra Low Emission Vehicle, a stronger California standard emphasizing very low HC emissions.

Vehicle emission standards and technological advancements have successfully reduced pollution from cars and trucks by about 90 percent since the 1970s. Unfortunately, Americans are driving bigger vehicles and more miles each year, partially offsetting the environmental benefits of individual vehicle emissions reductions.