United States emission standards
||This article needs additional citations for verification. (September 2009)|
In the United States, emissions standards are managed on a national level by the Environmental Protection Agency (EPA). State and local governments may apply for waivers to enact stricter regulations.
Motor vehicles 
Due to its preexisting standards and particularly severe motor vehicle 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 adopting the California standards include Arizona (2012 model year), Connecticut, Maine, Maryland, Massachusetts, New Jersey, New Mexico (2011 model year), New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, as well as the District of Columbia. Such states are frequently referred to as "CARB states" in automotive discussions because the regulations are defined by the California Air Resources Board.
The EPA has adopted the California emissions standards as a national standard by the 2016 model year and is collaborating with California regulators on stricter national emissions standards for model years 2017–2025.
Light-duty vehicles 
|This section is outdated. (November 2010)|
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). The limits are defined in grams per mile (g/mi).
Phase 1: 1994–99 
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 lifetime due to California's clean renewable energy infrastructure.
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: 2004–09 
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 to 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 were available for classifying a restricted number of light-duty trucks until the end of 2008, when they were removed along with bin 11 for medium-duty vehicles. As of 2009, light-duty trucks must meet the same emissions standards as passenger cars.
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.
Phase 3A: 2010–16 
In 2009, President Barack Obama announced a new national fuel economy and emissions policy that incorporated California's contested plan to curb greenhouse gas emissions on its own, apart from federal government regulations.
The combined fleet fuel economy for an auto manufacturer of cars and trucks with a GVWR of 10,000 lbs or less will have to average 35.5 mpg. The average for its cars will have to be 42 mpg, and for its trucks will be 26 mpg by 2016, all based upon CAFE Standards. If the manufacturers do not meet these standards, they will be assessed a $5 fee per vehicle made for every .1mpg that they're under the standard for.
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 more stringent exhaust emission standards and requires ultra-low sulfur diesel (ULSD) fuel (15 ppm maximum) beginning in 2007 
Greenhouse gases 
|This section is outdated. (November 2010)|
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.
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 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.
Small engines 
Pollution from small engines, such as those used in gas-powered groundskeeping equipment has an impact on air quality. Emissions from small offroad engines is regulated by the EPA. Specific pollutants subject to limits include hydrocarbons, carbon monoxide, and nitrogen oxides.
Electricity generation 
Performance-based regulation of greenhouse gases from electricity generation has been initiated on the state level. California was the first to implement this standard in January 2007 by adopting Senate Bill 1368, which set a limit of 1,100 lbs. CO2 per megawatt-hour on "new long-term commitments" for baseload power generation. This legislation was intended to apply to new plant investments (new construction), new or renewal contracts with a term of five years or more, or major investments by the electrical utility in its existing baseload power plants. The number of 1,100 lbs. CO2/MWhr corresponds to the emissions per electrical output of a combined cycle gas turbine plant. By comparison, coal-fired steam turbine plants produce 2,200 lbs. CO2/MWhr or more. Other western states followed suit soon after California, with Oregon, Washington, and Montana passing similar bills into law later that year.
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.
State emission testing 
|This section requires expansion. (August 2011)|
|Name||Light duty gasoline||Emissions testing||Light-duty diesel||Emissions testing||Heavy-duty diesel||Emissions testing||Special notes|
|Alabama||No emissions testing required||No emissions testing required||Alabama maintains "volunteer emission testing"|
|Alaska ||≥6 model years old (Anchorage)||Every two years||Exempt||Exempt|
|Arizona ||Model Years ≥1967 but more than 5 model years old (Pima County/ Maricopa County)||Every two years||1967–2007 (Pima County/ Maricopa County)||Pima County: 30 percent opacity
||Maricopa County||J1667 test protocol
|California||Model Years 1976 and newer. New cars are exempt until 6 years old. Hybrid vehicles are exempt.||Every two years|
|Connecticut||more than 4 model years old but less than 25 model years old||Every two years (OBDII)||more than 4 model years old but less than 25 model years old||Every two years||Exempt|
|Delaware||Over 5 model years old||Every two years (OBDII)|
|Georgia||Annually (Atlanta metro area only)|
|Illinois||Every two years|
|Maryland||Hybrid vehicles that achieve an EPA gas mileage rating of at least 50 miles per gallon are exempt from testing for 3 years after their model year, after which they are then tested at the standard two year intervals.||Every two years|
|Massachusetts ||Every year||OBDII||Every year||OBDII||8,501 to 14,000 lbs (Medium duty)
||Medium Duty: OBDII
|New Mexico||Model Years ≥1978 but more than 4 model years old (Bernalillo County)||Every two years and at change of ownership||Model Years ≥1978 but more than 4 model years old (Bernalillo County)||Every two years and at change of ownership||Exempt|
|Rhode Island ||more than 2 models years old (inclusive) or 24,000 miles (inclusive)||Every two years (OBDII)||Exempt||Exempt||Effective July 2012, any state contracted job requiring heavy duty vehicles must be operated with pollution control devices|
|Texas||more than 2 model years old to 24 model years old||annually (both OBDII for 1996–present and ASM to 1995 for motor vehicles except for 4-wheel drive/all wheel drive powertrains (4WD/AWD including vehicles over 8500 GVW are subjected to the two-speed idle (TSI) testing; TSI testing to 1995 only in El Paso, Travis, and Williamson Counties)||Emission testing mandated for motor vehicles registered in Brazoria, Fort Bend, Galveston, Harris, Montgomery, Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, Tarrant, Travis, Williamson and El Paso counties|
|Virginia||more than 2 model years old to 24 model years old||Every two years. First-time registrations exempt if vehicle has received an emissions certificate from certain states within previous 12 months.||newer than 1996 model year||every two years||GVWR > 8500 lbs. exempt||Emission testing mandated for motor vehicles registered in the counties of Arlington, Fairfax, Loudoun, Prince William, or Stafford, and the cities of Alexandria, Fairfax, Falls Church, Manassas or Manassas Park. Certain hybrids exempt.|
See also 
- Regulation of greenhouse gases under the Clean Air Act
- AP 42 Compilation of Air Pollutant Emission Factors
- Emissions standard
- List of low emissions locomotives
- Motor vehicle emissions
- Portable Emissions Measurement System
- Timeline of major U.S. environmental and occupational health regulation
- Vehicle emissions control
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- EPA fuel economy guide for consumers
- EPA Green Vehicles guide
- EPA Climate Change guide
- Dieselnet: Cars and Light-Duty Trucks—Tier 1
- Dieselnet: Cars and Light-Duty Trucks—Tier 2
- Dieselnet: Cars and Light-Duty Trucks—California
- Earthcars: Vehicle Emission Ratings Decoded