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Details of US emission standard

In the United States, emissions standards are managed on a national level by the Environmental Protection Agency (EPA). State and local governments play a subsidiary role.

Motor vehicles
Due to its pre-existing standards and particularly severe automobile-driven air pollution problems in the Los Angeles metropolitan area, the U.S. state of California has special dispensation from the federal government to promulgate its own automobile emissions standards. Other states may choose to follow either the national standard or the stricter California standards. States following the California standards include Maine, Massachusetts, New York, Oregon, Vermont, and Washington and are frequently referred to as "CARB states" in automotive discussions since the regulations are defined by the California Air Resources Board.

Light-duty vehicles
Two sets, or Tiers, of emission standards for light-duty vehicles in the United States were defined as a result of the Clean Air Act Amendments of 1990. The Tier I standard was adopted in 1991 and was phased in from 1994 to 1997. Tier II standards are being phased in from 2004 to 2009.

Within the Tier II ranking, there is a subranking ranging from BIN 1-10, with 1 being the cleanest (Zero Emission vehicle) and 10 being the dirtiest. The former Tier 1 standards that were effective from 1994 until 2003 were different between automobiles and light trucks (SUVs, pickup trucks, and minivans), but Tier II standards are the same for both types.

These standards specifically restrict emissions of carbon monoxide (CO), oxides of nitrogen (NOx), particulate matter (PM), formaldehyde (HCHO), and non-methane organic gases (NMOG) or non-methane hydrocarbons (NMHC). Confusingly, the limits are defined in the mixed-system unit of grams per mile (g/mi).

Phase 1 - 1994-1999
The national Tier 1 regulations were phased in from 1994 to 1997, and are being phased out in favor of the national Tier 2 standard, from 2004 to 2009.
Tier I standards cover vehicles with a gross vehicular weight rating (GVWR) below 8,500 pounds (3,856 kg) and are divided into five categories: one for passenger cars, and four for light-duty trucks (which include SUVs and minivans) divided up based on the vehicle weight and cargo capacity.

California's Low Emission Vehicle (LEV) program defines automotive emission standards which are stricter than the United States' national "Tier" regulations. It contains various emissions levels, one of which is confusingly named "Low Emission Vehicle (LEV)". In increasing stringency, these are:

The LEV standard created six major emission categories, each with several targets available depending on vehicle weight and cargo capacity. Vehicles with a test weight up to 14,000 pounds (6,350 kg) were covered by the regulations. The major emission categories were:
* 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. However, a battery-powered electric vehicle charged from the California power grid will still be up to ten times cleaner than even the cleanest gasoline vehicles over their respective lifetimes.

Transitional NLEV - 1999-2003
A set of transitional and initially voluntary National Low Emission Vehicle (NLEV) standards were in effect starting in 1999 for northeastern states and 2001 in the rest of the country until Tier II, adopted in 1999, began to be phased in from 2004 onwards. The National Low Emission Vehicle program covered vehicles below 6,000 pounds GVWR and adapted the national standards to accommodate California's stricter regulations.

Phase 2 - 2004 and later
More stringent national Tier 2 standards are being phased in from 2004 to 2009.
Instead of basing emissions on vehicle weight, Tier II standards are divided into several numbered "bins". Eleven bins were initially defined, with bin 1 being the cleanest (Zero Emission Vehicle) and 11 the dirtiest. However, bins 9, 10, and 11 are temporary. Only the first ten bins were used for light-duty vehicles below 8,500 pounds GVWR, but medium-duty passenger vehicles up to 10,000 pounds (4,536 kg) GVWR and can be classified into all 11 bins. Manufacturers can make vehicles which fit into any of the available bins, but still must meet average targets for their entire fleets.

The two least-restrictive bins for passenger cars, 9 and 10, were phased out at the end of 2006. However, bins 9 and 10 will remain available for classifying light-duty trucks until the end of 2008, when they will be removed along with bin 11 for medium-duty vehicles.

Tier II regulations also defined restrictions for the amount of sulfur allowed in gasoline and diesel fuel, since sulfur can interfere with the operation of advanced exhaust treatment systems such as selective catalytic converters and particulate filters. Sulfur content in gasoline was limited to an average of 120 parts-per-million (maximum 300 ppm) in 2004, and this was reduced to an average 30 ppm (maximum 80 ppm) for 2006. Ultra-low sulfur diesel began to be restricted to a maximum 15 ppm in 2006 and refiners are to be 100% compliant with that level by 2010.

A second round of California standards, known as Low Emission Vehicle II, is timed to coordinate with the Tier 2 rollout.
Under LEV II regulations, the Tier I and TLEV classifications were removed for 2004, and the remaining LEV, ULEV, and SULEV categories were made more stringent. These stricter versions are therefore known as "LEV II", "ULEV II", and "SULEV II".
Tier II's bin 5 roughly defines what fleet averages should be, and is equivalent to California's LEV II classification.
The following new categories were also created:

* ILEV – Inherently Low-Emission Vehicle
* PZEV – Partial Zero Emission Vehicle
* AT-PZEV – Advanced Technology Partial Zero Emission Vehicle
* NLEV – National Low Emission Vehicle
The PZEV and AT-PZEV ratings are for vehicles which achieve a SULEV II rating and also have systems to eliminate evaporative emissions from the fuel system and which have 150,000-mile/15-year warranties on emission-control components. Several ordinary gasoline vehicles from the 2001 and later model years qualify as PZEVs.
If a PZEV has technology that can also be used in ZEVs like an electric motor or high-pressure gaseous fuel tanks for compressed natural gas (CNG) or liquified petroleum gas (LPG), it qualifies as an AT-PZEV. Hybrid electric vehicles like the Toyota Prius can qualify, as can internal combustion engine vehicles that run on natural gas like the Honda Civic GX. These vehicles are called "partial" ZEVs because they receive partial credit in place of ZEVs that automakers would otherwise be required to sell in California.

Heavy-duty vehicles
Heavy-duty vehicles must comply with Tier III and Tier IVa or Tier IVb during the next ten years (2014).

Greenhouse gases
Federal emissions regulations do not cover the primary component of vehicle exhaust, carbon dioxide (CO2). Since CO2 emissions are proportional to the amount of fuel used, the national Corporate Average Fuel Economy regulations are the primary way in which automotive CO2 emissions are regulated in the U.S. However, the EPA is facing a lawsuit seeking to compel it to regulate greenhouse gases as a pollutant.

As of 2007, the California Air Resources Board passed strict greenhouse gas emission standards which are being challenged in the courts.
On September 12, 2007, a judge in Vermont ruled in favor of allowing states to conditionally regulate greenhouse gas (GHG) emissions from new cars and trucks, defeating an attempt by automakers to block state emissions standards. A group of automakers including General Motors, DaimlerChrysler, and the Alliance of Automobile Manufacturers had sued the state of Vermont in order to block rules calling for a 30 percent reduction in GHG emissions by 2016. Members of the auto industry argued that complying with these regulations would require major technological advances and raise the prices of vehicles as much as $6,000 per automobile. U.S. District Judge William K. Sessions III dismissed these claims in his ruling. "The court remains unconvinced automakers cannot meet the challenge of Vermont and California's (greenhouse gas) regulations," he wrote.

Meanwhile, environmentalists continue to press the Bush Administration to grant California a waiver from the EPA in order for its emissions standards to take effect. Doing so would allow Vermont and other states to adopt these same standards under the Clean Air Act. Without such a waiver, Judge Sessions wrote, the Vermont rules will be invalid.

Consumer ratings
Air Pollution Score
EPA's Air Pollution Score represents the amount of health-damaging and smog-forming airborne pollutants the vehicle emits. Scoring ranges from 0 (worst) to 10 (best). The pollutants considered are nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), formaldehyde (HCHO), and various hydrocarbon measures - non-methane organic gases (NMOG), and non-methane hydrocarbons (NMHC), and total hydrocarbons (THC). This score does not include emissions of greenhouse gases (but see Greenhouse Gas Score, below).

Greenhouse Gas Score
EPA's Greenhouse Gas Score reflects the amount of greenhouse gases a vehicle will produce over its lifetime, based on typical consumer usage. The scoring is from 0 to 10, where 10 represents the lowest amount of greenhouse gases.
The Greenhouse Gas Score is determined from the vehicle's estimated fuel economy and its fuel type. The lower the fuel economy, the more greenhouse gas is emitted as a by-product of combustion. The amount of carbon dioxide emitted per liter or gallon burned varies by fuel type, since each type of fuel contains a different amount of carbon per gallon or liter.
The ratings reflect carbon dioxide (CO2), nitrous oxide (N20) and methane (CH4) emissions, weighted to reflect each gas' relative contribution to the greenhouse effect.

Small engines
Pollution from small engines, such as those used in gas-powered groundskeeping equipment has a significant impact on air quality. Emissions from small offroad is regulated by the EPA. Specific pollutants subject to limits include hydrocarbons, carbon monoxide, and nitrogen oxides.

Air quality standards
Individual states with areas that do not attain the targets set by the EPA in the National Ambient Air Quality Standards must promulgate specific regulations which reduce the corresponding emissions from local sources.

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