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Emission standards limit the amount of pollution that can be released into the atmosphere. Emissions come from many places including industry, power plants, vehicles (from trains to automobiles to mopeds), and small equipment such as lawn mowers. Many emissions standards focus on regulating the amount of material that can be released by automobiles, which use the largest portion of energy in most places around the world. Regulations limit mandate the types of fuels that can be used and the amount of smog-forming material that can be released, but they generally do not directly limit fuel economy—the amount of fuel that can be consumed.

Standards generally regulate the amount of carbon monoxide (CO), oxides of nitrogen (NOx), sulfur, hydrocarbons, and particulate matter (PM) or soot that can be released. The main components of automobile exhaust, carbon dioxide (CO2) and water vapor (H2O), are not considered to be emissions in most cases. Carbon monoxide and other chemicals can generally be reduced by modifying engines to more completely burn fuel, and by using catalytic converters to convert the chemicals into less noxious compounds. Hydrocarbons sometimes leak out of the fuel system, so redesigning that to prevent fuel vapors from escaping can reduce emissions. Reducing carbon dioxide emissions can be accomplished by reformulating fuels and by reducing overall fuel consumption.

EPA standards in the United States

In the United States, emissions standards are managed by the Environmental Protection Agency as well as some state governments. Some of the strictest standards in the world are enforced in California by the California Air Resources Board (CARB). Currently, vehicles sold in the United States must meet "Tier 1" standards that went into effect in 1994. Additional "Tier 2" standards have been optional from 2001 to 2003, and are currently being phased in—a process that should be complete by 2009. The current Tier 1 standards are different between automobiles and light trucks (SUVs, pickup trucks, and minivans), but Tier 2 standards will be the same for both types. A common measurement system for American standards is the somewhat confusing mixed-standard unit of gram per mile.

There are several ratings that can be given to vehicles. A certain percentage of the cars produced by major manufacturers must meet these different levels in order for the company to sell their products in affected regions. Tier 1 has been the baseline used. Beyond Tier 1, in increasing stringency, there are

• TLEV – Transitional Low Emission Vehicle
• LEV – Low Emission Vehicle
• ULEV – Ultra-Low Emission Vehicle
• SULEV – Super-Ultra Low Emission Vehicle
• ZEV – Zero Emission Vehicle

The last category is largely restricted to electric vehicles and hydrogen cars, although such vehicles are usually not entirely non-polluting. In those cases, the other emissions are transferred to another site, such as a power plant or hydrogen reforming center, unless such sites run on renewable energy.

The above standards are being made even more stringent. Tier 2 variations are appended with "II", such as LEV II or SULEV II. There are other categories that have also been created.

• ILEV – Inherently Low-Emission Vehicle
• PZEV – Partial Zero Emission Vehicle
• AT-PZEV – Advanced Technology Partial Zero Emission Vehicle
• NLEV – National Low Emission Vehicle

PZEV vehicles do not produce emissions for part of the time that they are in operation, and only produce very low levels the rest of the time. This and the AT-PZEV category are generally restricted to hybrid electric vehicles, like the Toyota Prius, and cars with internal combustion engines that turn off when the car is stopped, such as with Volkswagen's Lupo.

European Standards

Europe has its own set of standards that vehicles must meet. The tiers are

• Euro I (1992-1995)
• Euro II (1995-1999)
• Euro III (1999-2005)
• Euro IV (2005-2008)
• Euro V (2008)

See also

• Corporate Average Fuel Economy