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The Plain English Guide to Tailpipe Standards

The federal Clean Air Act (CAA) provides the framework for regulating emissions from motor vehicles. In 1970, it 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. Finally, the CAA granted California, which has some of the worst air quality in the nation, the authority to set it's own vehicle emission standards. As of 1990, other states may adopt the California program as their own (and several have done so), but are otherwise prohibited from setting their own emission standards.

Vehicle emission standards have successfully reduced pollution from cars and trucks by about 90 percent since the 1970s. But Americans are driving more miles each year, partially offsetting the environmental benefits of individual vehicle emissions reductions. That's why even tougher emission standards for conventional vehicles, and zero and near-zero emissions vehicles are essential for achieving and maintaining clean air.

What pollutants and vehicles are regulated?

Federal and California tailpipe standards limit exhaust emissions of five pollutants: hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM, for diesel vehicles only), and formaldehyde (HCHO). Hydrocarbons and NOx are the major contributors to urban smog.

The standards regulate emissions from cars and "light-duty trucks," which includes sport utility vehicles (SUVs), pickups, and minivans. Currently, light trucks are allowed to emit up to five and a half times more smog-forming pollution than cars.

Federal Standards (Tier 1, Tier 2, and NLEV)

Tier 1 refers to the current federal tailpipe standards for passenger cars and light trucks. Under this program there is one emission category (called the Tier 1 category), but SUVs, minivans, pickup trucks, and diesel vehicles are allowed to pollute more than gasoline cars.

Tier 2. Tier 2 is a fleet averaging program, modeled after the California LEV II standards. Manufacturers can produce vehicles with emissions ranging from relatively dirty to zero, but the mix of vehicles a manufacturer sells each year must have average NOx emissions below a specified value. This provides automakers with flexibility for meeting the standards and is a cost-effective method of reducing overall pollution from automobiles.

The Tier 2 program will cut vehicle pollution significantly, but UCS is working to strengthen several aspects of the program. Find out what you can do to help on our Tier 2 campaign page.

NLEV stands for "National Low-Emission Vehicle Program." It is the same as the California LEV program (see below), with two notable differences.

  • NLEV reduces emissions from cars and the lightest trucks, but the standards for bigger SUVs and pickup trucks remained unchanged.

  • NLEV does not include a zero-emission vehicle sales requirement.

California Standards (LEV and LEV II)

The LEV and LEV II programs are both fleet average programs, much like the new federal Tier 2 standards (see above), but are programs are based on a fleet hydrocarbon, rather than NOx, standard. Manufacturers can certify vehicles to one of several emissions categories as long as the average hydrocarbon emissions of all new vehicles sold meets a specified standard. This standard becomes more stringent each year, forcing manufacturers to move toward a cleaner overall mix of vehicles.

The LEV programs also include a 10 percent zero-emission vehicle (ZEV) production requirement, starting in 2003. Under the LEV II program, some hybrid electric, extremely low-emission gasoline and methanol fuel-cell vehicles can qualify for "partial ZEV" credits. Partial credits are given based on several criteria, including low emissions associated with refining and distribution of the fuel, the all-electric vehicle range (for hybrids), and near-zero evaporative and tailpipe emissions.

LEV: Under the LEV program, vehicles may certify to one of the following emission categories:

TLEV = Transitional Low-Emission Vehicle
LEV = Low-Emission Vehicle
ULEV = Ultra Low-Emission Vehicle
ZEV = Zero-Emission Vehicle

A note on emissions categories: all LEVs are not alike.

While light trucks and cars can all certify as "LEVs", the numerical standards are actually different between the two types of vehicles. As a result, a LEV truck (such as an SUV) is not as clean as a LEV car; it can emit up to three times more smog-forming pollution. This difference will be eliminated under LEV II.

LEV II: Under the LEV II program, vehicles may certify to one of the following emission categories:

LEV = Low-Emission Vehicle
ULEV = Ultra Low-Emission Vehicle
SULEV = Super Ultra Low-Emission Vehicle
ZEV = Zero-Emission Vehicle

Although the category names are virtually the same names as under the LEV program, the actual standards are much more stringent and emissions loopholes for light trucks are eliminated.

Tailpipe Tables

Craving more detailed information? The following pdf files include the nitty-gritty details -- everything you ever wanted to know about passenger vehicles but were afraid to ask.


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