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United States vehicle emission standards are set through a combination of legislative mandates enacted by Congress through Clean Air Act (CAA) amendments from 1970 onwards, and executive regulations managed nationally by the
Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale f ...
(EPA), and more recently along with the
National Highway Traffic Safety Administration The National Highway Traffic Safety Administration (NHTSA ) is an agency of the U.S. federal government, part of the Department of Transportation. It describes its mission as "Save lives, prevent injuries, reduce vehicle-related crashes" rela ...
(NHTSA). These standard cover common
motor vehicle air pollution Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
, including
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
,
nitrogen oxides Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or ...
, and particulate emissions, and newer versions have incorporated fuel economy standards. In nearly all cases, these agencies set standards that are expected to be met on a fleet-wide basis from automobile and other vehicle manufacturers, with states delegated to enforce those standards but not allowed to set stricter requirements. California has generally been the exception, having been granted a waiver and given allowance to set stricter standards as it had established its own via the
California Air Resources Board The California Air Resources Board (CARB or ARB) is the "clean air agency" of the government of California. Established in 1967 when then-governor Ronald Reagan signed the Mulford-Carrell Act, combining the Bureau of Air Sanitation and the Moto ...
prior to the 1970 CAA amendments. Several other states have since also gotten waivers to follow California's standards, which have also become a ''de facto'' standard for vehicle manufacturers to follow.
Vehicle emission standards Emission standards are the legal requirements governing air pollutants released into the atmosphere. Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources ove ...
have generally been points of debate between the government, vehicle manufacturers, and environmental groups, and has become a point of political debate.


Legislative and regulation history


Clean Air Act of 1963 (CAA)

The
Clean Air Act of 1963 The Clean Air Act (CAA) is the United States' primary federal air quality law, intended to reduce and control air pollution nationwide. Initially enacted in 1963 and amended many times since, it is one of the United States' first and most inf ...
(CAA) was passed as an extension of the
Air Pollution Control Act The Air Pollution Control Act of 1955 (, ch. 360, ) was the first U.S. federal law to address the national environmental problem of air pollution. This was "an act to provide research and technical assistance relating to air pollution control". ...
of 1955, encouraging the federal government via the
United States Public Health Service The United States Public Health Service (USPHS or PHS) is a collection of agencies of the Department of Health and Human Services concerned with public health, containing nine out of the department's twelve operating divisions. The Assistant S ...
under the then-
Department of Health, Education, and Welfare The United States Department of Health and Human Services (HHS) is a cabinet-level executive branch department of the U.S. federal government created to protect the health of all Americans and providing essential human services. Its motto is ...
(HEW) to encourage research and development towards reducing pollution and working with states to establish their own emission reduction programs. The CAA was amended in 1965 with the
Motor Vehicle Air Pollution Control Act The Clean Air Act (CAA) is the United States' primary federal air quality law, intended to reduce and control air pollution nationwide. Initially enacted in 1963 and amended many times since, it is one of the United States' first and most infl ...
(MVAPCA) which gave the HEW Secretary authority to set federal standards for vehicle emissions as early as 1967.


California Air Resources Board (CARB)

In the mid-20th century, as California economically grew after the Great Depression, the pollution in the state started to increase, generating a great deal of
smog Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words ''smoke'' and '' fog'' to refer to smoky fog due to its opacity, and odor. The word was then inte ...
particularly in the valleys of southern California, making it difficult to breathe and harming crops in nearby areas. It was not until the 1960s that this smog was tied to emissions of carbon monoxide, hydrocarbons, nitrogen oxides from inefficient fuel combustion from cars and factories by Arie Jan Haagen-Smit. Haagen-Smit found these air pollutants photoreacted with sunlight to form
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
which contributed to the formation of the smog and caused the physicological effects on humans and plants. California established the
California Air Resources Board The California Air Resources Board (CARB or ARB) is the "clean air agency" of the government of California. Established in 1967 when then-governor Ronald Reagan signed the Mulford-Carrell Act, combining the Bureau of Air Sanitation and the Moto ...
(CARB) in 1967, with Haagen-Smit as its first chairman, which among other activities set stringent vehicle emission standards by that year. Other states were also facing similar air pollution issues at the same time, but fearing that setting too strict a standard would drive away automobile manufacturers, they considered implementing standards that were less restrictive compared to California, potentially creating a patchwork of regulations across the United States. The automobile industry lobbied to Congress, and the CAA was modified in 1967 with the National Emissions Standards Act (also known as the Air Quality Act) that expressly prevented states from setting more restrictive emission standards than the federal levels. However, because California has already established its program, it was granted a waiver and allowed to keep its standards. This Act did give states the authority to perform vehicle inspections programs beyond the requirements for new vehicles, though few states took their own action on this.


Formation of the Environmental Protection Agency and the Clear Air Act Amendment of 1970

Air pollution had become a major national focal point by 1970, leading to a major amendment to the CAA. Near the end of 1970, the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it be ...
(EPA) was formed out of an
executive order In the United States, an executive order is a directive by the president of the United States that manages operations of the federal government. The legal or constitutional basis for executive orders has multiple sources. Article Two of t ...
under President
Richard Nixon Richard Milhous Nixon (January 9, 1913April 22, 1994) was the 37th president of the United States, serving from 1969 to 1974. A member of the Republican Party, he previously served as a representative and senator from California and was ...
with ratification by Congress to consolidate all of the environmental-related executive-branch programs to a single entity; the new agency was reflect as the primary agency for administering the CAA going forward. Among the provisions related to vehicle emissions: * The 1970 Amendment required EPA to define the National Ambient Air Quality Standards (NAAQS), to be set and updated by the EPA. The NAAQS, at passage of the 1970 amendment, included the pollutants
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
,
nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . It is one of several nitrogen oxides. is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year for use primarily in the producti ...
,
sulfur dioxide Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
, particulate matter,
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s and photochemical oxidants (such as
ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
). Other pollutants, such as
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
, were added later based on EPA review of current conditions. Along with the other activities set by the EPA, the states were to create State Implementation Plans (SIP)s to bring their air quality within the NAAQS by 1975. * The Amendment also required the EPA to define emission standards for new vehicles to help with the NAAQS emission reduction goals, including standards for fuel and testing of new vehicles to make sure these standards are met. Additionally, the 1970 CAA Amendment continued California's waiver program through which California can seek exemptions from the EPA's emissions requirements as long as theirs are at least as strict as the EPA's vehicle standards.


Clear Air Act Amendment of 1977

The EPA's assessment of the state of the country meeting the target NAAQS goals by 1975 was poor, having identified numerous nonattainment areas in the country. With the 1977 Amendment to the CAA, a new deadline of December 31, 1982 for meeting the NAAQS was fixed with no allowance for extending the deadline unless specific control measures were established. Among other key provisions was the establishment of required vehicle inspection and maintenance programs (I/M) in nonattainment states and optional in other areas. This required that states establish emission testing facilities for in-use vehicles to make sure they meet emissions requirements, maintained and repaired as necessary to correct any problems before their license was renewed. The EPA was tasked to establish the basic protocols for these facilities. Other states that had met the NAAQS attainment goals could optionally establish I/M programs for existing but were required to follow the EPA's specifications.


New vehicle emission standards

Due to its preexisting standards and particularly severe
motor vehicle air pollution Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
problems in the
Los Angeles Los Angeles ( ; es, Los Ángeles, link=no , ), often referred to by its initials L.A., is the largest city in the state of California and the second most populous city in the United States after New York City, as well as one of the world' ...
metropolitan area, the U.S. state of
California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
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. The following states that have adopted the California standards are:
Colorado Colorado (, other variants) is a state in the Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the western edge of t ...
,
Connecticut Connecticut () is the southernmost state in the New England region of the Northeastern United States. It is bordered by Rhode Island to the east, Massachusetts to the north, New York to the west, and Long Island Sound to the south. Its cap ...
,
Delaware Delaware ( ) is a state in the Mid-Atlantic region of the United States, bordering Maryland to its south and west; Pennsylvania to its north; and New Jersey and the Atlantic Ocean to its east. The state takes its name from the adjacent Del ...
,
Maine Maine () is a state in the New England and Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Canadian provinces of New Brunswick and Quebec to the northeast and north ...
,
Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean to ...
,
Massachusetts Massachusetts (Massachusett language, Massachusett: ''Muhsachuweesut assachusett writing systems, məhswatʃəwiːsət'' English: , ), officially the Commonwealth of Massachusetts, is the most populous U.S. state, state in the New England ...
,
New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
,
New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...
(2011 model year and later), New York,
Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...
,
Oregon Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...
,
Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
,
Rhode Island Rhode Island (, like ''road'') is a U.S. state, state in the New England region of the Northeastern United States. It is the List of U.S. states by area, smallest U.S. state by area and the List of states and territories of the United States ...
,
Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...
,
Vermont Vermont () is a state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York to the west, and the Canadian province of Quebec to ...
, and
Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered o ...
(2009 model year and later), as well as the
District of Columbia ) , image_skyline = , image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
. Such states are frequently referred to as "CARB states" in automotive discussions because the regulations are defined by the
California Air Resources Board The California Air Resources Board (CARB or ARB) is the "clean air agency" of the government of California. Established in 1967 when then-governor Ronald Reagan signed the Mulford-Carrell Act, combining the Bureau of Air Sanitation and the Moto ...
. The EPA adopted the Californian fuel economy and green house gas standard as a national standard by the 2016 model year and is collaborating with Californian regulators on stricter national emissions standards for model years 2017–2025.


Criteria pollutants


Light-duty vehicles

Light-duty vehicles are certified for compliance with emission standards by measuring their tailpipe emissions during rigorously-defined driving cycles that simulate a typical driving pattern. The
FTP-75 The EPA Federal Test Procedure, commonly known as FTP-75 for the city driving cycle, are a series of tests defined by the US Environmental Protection Agency (EPA) to measure tailpipe emissions and fuel economy of passenger cars (excluding lig ...
city driving test (averaging about ) and the HWFET highway driving test (averaging about ) are used for measuring both emissions and fuel economy. Two sets, or tiers, of
emission standard Emission standards are the legal requirements governing air pollutants released into the atmosphere. Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources over ...
s for light-duty vehicles in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
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 were 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 truck Light truck or light-duty truck is a US classification for vehicles with a gross vehicle weight up to and a payload capacity up to 4,000 pounds (1,815 kg). Similar goods vehicle classes in the European Union, Canada, Australia, and New Zeala ...
s (
SUV A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive. There is no commonly agreed-upon definiti ...
s,
pickup truck A pickup truck or pickup is a light-duty truck that has an enclosed cabin, and a back end made up of a cargo bed that is enclosed by three low walls with no roof (this cargo bed back end sometimes consists of a tailgate and removable covering) ...
s, and
minivan Minivan (sometimes called simply as van) is a North American car classification for vehicles designed to transport passengers in the rear seating row(s), with reconfigurable seats in two or three rows. The equivalent classification in Europe i ...
s), but Tier II standards are the same for both types. These standards specifically restrict emissions of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(CO),
oxides of nitrogen Nitrogen oxide may refer to a binary compound of oxygen and nitrogen, or a mixture of such compounds: Charge-neutral *Nitric oxide (NO), nitrogen(II) oxide, or nitrogen monoxide *Nitrogen dioxide (), nitrogen(IV) oxide * Nitrogen trioxide (), or n ...
(NOx), particulate matter (PM),
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
(HCHO), and non-
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
organic gases (NMOG) or non-methane hydrocarbons (NMHC). The limits are defined in
gram The gram (originally gramme; SI unit symbol g) is a Physical unit, unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram. Originally defined as of 1795 as "the absolute weight of a volume of pure wate ...
s per mile (g/mi).


= Phase 1: 1994–1999

= These standards were phased in from 1994 to 1997 and were 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
SUV A sport utility vehicle (SUV) is a car classification that combines elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive. There is no commonly agreed-upon definiti ...
s and
minivan Minivan (sometimes called simply as van) is a North American car classification for vehicles designed to transport passengers in the rear seating row(s), with reconfigurable seats in two or three rows. The equivalent classification in Europe i ...
s) divided up based on the vehicle weight and cargo capacity.
California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...
's Low-emission vehicle (LEV) program defines six automotive
emission standard Emission standards are the legal requirements governing air pollutants released into the atmosphere. Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources over ...
s which are stricter than the United States' national Tier regulations. Each standard has several targets depending on vehicle weight and cargo capacity; the regulations cover vehicles with test weights up to . Listed in order of increasing stringency, the standards are: * TLEV – Transitional low-emission vehicle * LEV – Low-emission vehicle * ULEV – Ultra-low-emission vehicle *
SULEV Super ultra-low emissions vehicle (SULEV) is a U.S. classification for passenger vehicle emissions. The classification is based on producing 90% fewer emissions than the average gasoline-powered vehicle. The SULEV standard is stricter than the stan ...
– Super-ultra low-emission vehicle * ZEV – Zero-emission vehicle The last category is largely restricted to electric vehicles and
hydrogen car A hydrogen vehicle is a vehicle that uses hydrogen fuel for motive power. Hydrogen vehicles include hydrogen-fueled space rockets, as well as ships and aircraft. Power is generated by converting the chemical energy of hydrogen to mechanical energ ...
s, 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 A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the generation of electric power. Power stations are generally connected to an electrical grid. Many pow ...
or hydrogen reforming center, unless such sites run on
renewable energy Renewable energy is energy that is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, the movement of water, and geothermal heat. Although most renewable energy ...
.


=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 GVWR and adapted the national standards to accommodate California's stricter regulations.


= Phase 2: 2004–2009

= 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 A zero-emission vehicle, or ZEV, is a vehicle that does not emit exhaust gas or other pollutants from the onboard source of power. The California definition also adds that this includes under any and all possible operational modes and conditions ...
) and 11 the dirtiest. However, bins 9, 10, and 11 are temporary. Only the first ten bins were used for light-duty vehicles below GVWR, but medium-duty passenger vehicles up to 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 Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
allowed in
gasoline Gasoline (; ) or petrol (; ) (see ) is a transparent, petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines (also known as petrol engines). It consists mostly of organic co ...
and
diesel fuel Diesel fuel , also called diesel oil, is any liquid fuel specifically designed for use in a diesel engine, a type of internal combustion engine in which fuel ignition takes place without a spark as a result of compression of the inlet air and t ...
, since sulfur can interfere with the operation of advanced exhaust treatment systems such as selective catalytic converters and
diesel particulate filter A diesel particulate filter (DPF) is a device designed to remove diesel particulate matter or soot from the exhaust gas of a diesel engine.Tom Nash (May 2003) "Diesels: The Smoke is clearing", ''Motor '' Vol.199 No. 5, p. 54, Hearst Business Publ ...
s. 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 Ultra-low-sulfur diesel (ULSD) is diesel fuel with substantially lowered sulfur content. Since 2006, almost all of the petroleum-based diesel fuel available in Europe and North America has been of a ULSD type. The move to lower sulfur content allo ...
began to be restricted to a maximum 15 ppm in 2006 and refiners are to be 100% compliant with that level by 2010.


= Tier III

= The phase-in of new tailpipe and evaporative emission standards begin to phase-in beginning with the 2017 model year along with new fuel standards.


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 model year.


Greenhouse gases

Federal emissions regulations cover the primary component of vehicle exhaust,
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(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. The EPA faced a lawsuit seeking to compel it to regulate greenhouse gases as a pollutant, Massachusetts v. Environmental Protection Agency. As of 2007, the
California Air Resources Board The California Air Resources Board (CARB or ARB) is the "clean air agency" of the government of California. Established in 1967 when then-governor Ronald Reagan signed the Mulford-Carrell Act, combining the Bureau of Air Sanitation and the Moto ...
passed strict
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
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
automaker The automotive industry comprises a wide range of companies and organizations involved in the design, development, manufacturing, marketing, and selling of motor vehicles. It is one of the world's largest industries by revenue (from 16 % suc ...
s to block state emissions standards. A group of automakers including
General Motors The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
,
DaimlerChrysler The Mercedes-Benz Group AG (previously named Daimler-Benz, DaimlerChrysler and Daimler) is a German multinational automotive corporation headquartered in Stuttgart, Baden-Württemberg, Germany. It is one of the world's leading car manufacture ...
, and the
Alliance of Automobile Manufacturers The Auto Alliance (AAM) is a defunct trade group of automobile manufacturers that operated in the United States. It was the leading advocacy group for the auto industry, representing 77% of all car and light truck sales in the United States. The ...
had
sued - A lawsuit is a proceeding by a party or parties against another in the civil court of law. The archaic term "suit in law" is found in only a small number of laws still in effect today. The term "lawsuit" is used in reference to a civil acti ...
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.
Environmentalist An environmentalist is a person who is concerned with and/or advocates for the protection of the environment. An environmentalist can be considered a supporter of the goals of the environmental movement, "a political and ethical movement that se ...
s pressed the Administration to grant California a
waiver A waiver is the voluntary relinquishment or surrender of some known right or privilege. Regulatory agencies of state departments or the federal government may issue waivers to exempt companies from certain regulations. For example, a United St ...
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.


2010–2016

In 2009, President 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 or less will have to average 35.5 miles per gallon (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 average fuel economy of a manufacturer's annual fleet of vehicle production falls below the defined standard, the manufacturer must pay a penalty, currently $5.50 USD per 0.1 mpg under the standard, multiplied by the manufacturer's total production for the U.S. domestic market. This is in addition to any gas guzzler tax, if applicable. Should CAFE targets have been extended through to 2026 under the Obama administration, it would have sought a 54 mpg industry-wide average fuel efficiency for cars and light trucks manufactured in 2026 or later, with automobile manufacturers instructed to increase the fuel economy across all of their vehicles by 5% each year. A second round of California standards, known as Low Emission Vehicle II, is timed to coordinate with the Tier 2 rollout. The PZEV and AT-PZEV ratings are for vehicles which achieve a SULEV II rating and also have systems to eliminate
evaporative emission Vehicle emissions control is the study of reducing the Exhaust gas, emissions produced by motor vehicles, especially internal combustion engines. Types of emissions Emissions of many Air pollution, air pollutants have been shown to have Environmen ...
s 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 An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
or high-pressure gaseous fuel tanks for
compressed natural gas Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of , usually in cy ...
(CNG) or
liquified petroleum gas Liquefied petroleum gas (LPG or LP gas) is a fuel gas which contains a flammable mixture of hydrocarbon gases, specifically propane, propylene, butylene, isobutane and n-butane. LPG is used as a fuel gas in heating appliances, cooking ...
(LPG), it qualifies as an AT-PZEV. Diesel particulate filters became a requirement in 2014; gasoline vehicles were exempt.


Trump-era rollback and Biden-era reversal (2017–2021)

After
Donald Trump Donald John Trump (born June 14, 1946) is an American politician, media personality, and businessman who served as the 45th president of the United States from 2017 to 2021. Trump graduated from the Wharton School of the University of Pe ...
was inaugurated as president in 2017, he instructed the NHTSA and EPA to rollback Obama's CAFE standards, increasing the 2026 target to a then-projected 202 g CO2/mi and requiring only an annual 1.5% fleet efficiency improvement. The new rule was issued in March 2020. The Trump administration argued the rollback was required due to the increasing costs of cars on consumers that higher efficiencies would only make more expensive. The move was criticized by several environmentalists as well as the state of California, as the ruling coincided with Trump's efforts to remove the waiver for California emissions exemptions. Following Joe Biden becoming president in 2021, he signed
Executive Order In the United States, an executive order is a directive by the president of the United States that manages operations of the federal government. The legal or constitutional basis for executive orders has multiple sources. Article Two of t ...
14057, "Catalyzing Clean Energy Industries and Jobs Through Federal Sustainability", which in addition to committing the federal government to implement clean transport options such as EVs, also committed to improving the fuel efficiency standards and reversing the Trump administration's actions., the EPA issued a new rule in December 2021 and to become enforceable by February 2022 that effectively restored the Obama-era standards, through decreasing the fleet-wide emissions target to a projected 161 g CO2/mi by the 2026 model year.


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 Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(CO),
formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section F ...
(HCHO), and various
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
measures – non-methane organic gases (NMOG), and non-methane hydrocarbons (NMHC), and total hydrocarbons (THC). This score does not include emissions of
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es (but see Greenhouse gas score, below).


Greenhouse gas score

EPA's greenhouse gas score reflects the amount of
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse ...
es 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 A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), wa ...
's estimated fuel economy and its fuel type. The lower the fuel economy, the more greenhouse gas is emitted as a
by-product A by-product or byproduct is a secondary product derived from a production process, manufacturing process or chemical reaction; it is not the primary product or service being produced. A by-product can be useful and marketable or it can be consid ...
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 Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
(CO2),
nitrous oxide Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
(N2O) and
methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Eart ...
(CH4) emissions, weighted to reflect each gas's relative contribution to the
greenhouse effect The greenhouse effect is a process that occurs when energy from a planet's host star goes through the planet's atmosphere and heats the planet's surface, but greenhouse gases in the atmosphere prevent some of the heat from returning directly ...
.


California emission standards

Under Section 209 of the Clean Air Act (CAA), California is given the ability to apply for special waivers to apply its own emission standards for new motor vehicles that are at least as stringent as the federal standards. California applies for this waiver through the EPA, which publishes the proposed standards for public review in the Federal Register. Based on its own review and public comments, the EPA then grants the waiver unless it has determined that California's requested standards were "arbitrary and capricious" in their findings, that the standards are not needed to "meet compelling and extraordinary conditions", or otherwise are inconsistent with other aspects of the CAA. Since the CAA's passage in 1967, California has applied and received more than fifty waivers, which include emission standards across various vehicle classes. Among these include two special sets of waivers: * California initiated its
zero-emission vehicle A zero-emission vehicle, or ZEV, is a vehicle that does not emit exhaust gas or other pollutants from the onboard source of power. The California definition also adds that this includes under any and all possible operational modes and conditions ...
(ZEV) mandate in 1990. ZEV are defined as vehicles that have no exhaust or evaporative emissions of any regulated pollutant. Vehicle manufacturers were a percentage of their fleet meeting these ZEV standards over a long-term schedule (2% by model year 1998 at its start), but the mandate schedule has shifted based on the unplanned rate of technology advancement and costs, and as of 2020, its current target is to reach 8% ZEV by 2025 determined by fleet credits that account for vehicle range as well as contributions from any low-emission vehicles or plug-in hybrids. The EPA granted the initial request in 1990 and several updates. * California had requested to regulate
greenhouse gas emissions Greenhouse gas emissions from human activities strengthen the greenhouse effect, contributing to climate change. Most is carbon dioxide from burning fossil fuels: coal, oil, and natural gas. The largest emitters include coal in China and lar ...
(GHG) at stricter levels than federal levels first in 2005 as part of its low-emission vehicle program. The EPA initially refused this waiver based on a decision from the
United States Court of Appeals for the District of Columbia Circuit The United States Court of Appeals for the District of Columbia Circuit (in case citations, D.C. Cir.) is one of the thirteen United States Courts of Appeals. It has the smallest geographical jurisdiction of any of the U.S. federal appellate cou ...
that determined the EPA did not have the authority to regulate GHG under the CAA; this ruling was challenged in the Supreme Court case '' Massachusetts v. Environmental Protection Agency'' (549 U.S. 497 (2007)) which ruled that the EPA did have this authority. In later actions, the EPA granted California its GHG waiver by 2009.


State adoption of California Standards

Section 177 of the CAA grants the ability for states to adopt California emission standards instead of federal ones. As of December 2021, the following states have adopted the California standards, including their standards for ZEV and GHG:


Revocation of waivers under the Trump administration

Former President
Donald Trump Donald John Trump (born June 14, 1946) is an American politician, media personality, and businessman who served as the 45th president of the United States from 2017 to 2021. Trump graduated from the Wharton School of the University of Pe ...
stated his concern about California's stricter emission standards and their impact on the costs of manufacturing on the automobile industry, though some political analysts asserted this also tied in with Trump's conservative ideology conflicting with California's more liberal stance. Along with the Obama-era mileage goals, Trump had expressed his intent to revoke California's waivers early on in his presidency. Shortly after Ford, Volkswagen, Honda, and BMW announced their intentions to commit to the Obama-era mileage goals and California's emission standards across their fleets in July 2019, Trump announced his intention to rollback California's waivers. As part of Trump's Safer, Affordable, Fuel-Efficient (SAFE) program, the EPA and NHTSA proposed a new "One National Program Rule" that asserted that only the federal government may set emissions standards on September 19, 2019 as to have one consistent set of fuel emission and mileage standards across the country. This rule would include revoking the last set of California waivers that the EPA had granted California in 2013 for its GHG and ZEV programs. California retained its ability to set emission standards that address ozone-formation under the rule. Subsequent to this rule, California led a collation of 23 states to sue the NHTSA in ''California v. Chao'' (Case 1:19-cv-02826) in the D.C. District Court in September 2019, asserting the agency, in setting the rule, violated the intent of the CAA. The same group of states also filed suit against the EPA once the EPA issued the revoking of the 2013 waiver in November 2019, in ''California v. Wheeler'' (Case 19-1239) in the
United States Court of Appeals for the District of Columbia Circuit The United States Court of Appeals for the District of Columbia Circuit (in case citations, D.C. Cir.) is one of the thirteen United States Courts of Appeals. It has the smallest geographical jurisdiction of any of the U.S. federal appellate cou ...
to challenge the EPA's revoking. Further, both Minnesota and New Mexico, plaintiffs in both cases, stated they would take steps to adapt California's standards in their states as a result. Following the election of Joe Biden as president, the EPA and NHTSA moved to reverse the 2019 rule in April 2021, thus returning to the previous status quo for California. Following the EPA granting California its latest request for exemption, seventeen states sued the EPA in May 2022, arguing that because of the impact of California emission standards on vehicle manufacturing, the EPA's actions violate the equal sovereignty granted to the states by the Constitution, since it gives California more power than other states in setting emissions regulations.


Non-road engines

Non-road engine Non-road engines are engines that are used for other purposes than a motor vehicle that is used on a public roadway. The term is commonly used by regulators to classify the engines in order to control their emissions. Non-road engines are u ...
s, including equipment and vehicles that are not operated on the public roadways, are used in an extremely wide range of applications, each involving great differences in operating characteristics and engine technology. Emissions from all non-road engines are regulated by categories. In the United States, the emission standards for non-road diesel engines are published in the US Code of Federal Regulations, Title 40, Part 89 (40 CFR Part 89). Tier 1-3 Standards were adopted in 1994 and was phased in between 1996 and 2000 for engines over 37 kW (50 hp). In 1998 the regulation included engines under 37 kW and introduced more stringent Tier 2 and Tier 3 standards which was scheduled to be phased in between 2000 and 2008. In 2004, US EPA introduced the more stringent Tier 4 standards which was scheduled to be phased in between 2008 and 2015. The testing cycles used for certification follow the
ISO 8178 ISO 8178 is a collection of steady state test cycles used for defining emission standards for non-road engines in the European Union, United States, Japan and other countries. Test cycle ISO 8178 C1 is also referred to as "Non-Road Steady Cycle" a ...
standards.


Small engines

Pollution from small engines, such as those used in gas-powered
groundskeeping equipment Groundskeeping is the activity of tending an area of land for aesthetic or functional purposes, typically in an institutional setting. It includes mowing grass, trimming hedges, pulling weeds, planting flowers, etc. The U.S. Department of Labor e ...
reduces air quality. Emissions from small offroad engines are regulated by the EPA. Specific pollutants subject to limits include
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s,
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
, and nitrogen oxides.40 CFR §90.103


Existing vehicle emissions standard

Testing of existing vehicle emissions, also known as vehicle inspection and maintenance programs (I/M) were introduced as part of the 1977 Amendments to the CAA. The 1970 Amendments introduced target NAAQS goals for air quality which were not met in many parts of the country. With the 1977 Amendments, the CAA required I/M programs in non-attainment states as part of their pollution prevent plans. For model years prior to 1996, emissions tests were performed using a
chassis dynamometer A chassis dynamometer, informally referred to as a rolling road, is a mechanical device that uses one or more fixed roller assemblies to simulate different road conditions within a controlled environment, and is used for a wide variety of vehicle ...
-based test; The vehicle is driven so that the wheels of its main driven axle (front or rear) sit atop the dynamometer rotors, when then are unlocked to rotate freely. A special collections line is attached to the tailpipe, and simulated airflow is pushed across the engine to simulate vehicle movement. The test operator then presses the accelerator of the car through a fixed test schedule: the acceleration from the engine translates to force and torque that are measured through the dynamometer, simultaneously mapped against analysis of the emissions from the tailpipe. After completion of the schedule, the computerized system calculates the emissions from the car and determines if it meets the appropriate specification for its model year. Since model year 1994, all LDV and LDT manufactured for use in the United States are required to use the standard on-board diagnostic OBD-II system. This is a computerized system that continually monitors the performance of the engine and its emission control system. Instead of the dynamometer test, the operator hooks the OBD-II to a standard computer system which downloads the information from the computer. It will warn the operator if the OBD-II determines significant deviations from expected emissions control standards, indicating repairs may be needed.


See also

*
Regulation of greenhouse gases under the Clean Air Act The, United States Environmental Protection Agency (EPA) began regulating greenhouse gases (GHGs) under the Clean Air Act ("CAA" or "Act") from mobile and stationary sources of air pollution for the first time on January 2, 2011. Standards for ...
* Regulation on non-exhaust emissions *
AP 42 Compilation of Air Pollutant Emission Factors The ''AP 42 Compilation of Air Pollutant Emission Factors'' is a compilation of the US Environmental Protection Agency (EPA)'s emission factor information on air pollution, first published in 1968. , the last edition is the 5th from 2010. Histo ...
*
Emissions standard Emission standards are the legal requirements governing air pollutants released into the atmosphere. Emission standards set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources over ...
*
List of low emissions locomotives The following is a list of diesel-electric locomotives that meet or exceed EPA Tier 2 locomotive emissions regulations, sorted by builder. This is a non-exhaustive list of locomotives in North America. Furthermore, green/clean energy is an alway ...
*
Motor vehicle emissions Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through an ...
*
Portable Emissions Measurement System A portable emissions measurement system (PEMS) is a vehicle emissions testing device that is small and light enough to be carried inside or moved with a motor vehicle that is being driven during testing, rather than on the stationary rollers of ...
* Timeline of major U.S. environmental and occupational health regulation *
Vehicle emissions control Vehicle emissions control is the study of reducing the emissions produced by motor vehicles, especially internal combustion engines. Types of emissions Emissions of many air pollutants have been shown to have variety of negative effects on public ...


References


External links


EPA fuel economy guide for consumers

EPA Green Vehicles guide



Dieselnet: Cars and Light-Duty Trucks—Tier 1

Dieselnet: Cars and Light-Duty Trucks—Tier 2

Dieselnet: Cars and Light-Duty Trucks—California

Emissions: Vehicle Emission Ratings Decoded
{{DEFAULTSORT:United States Emission Standards Air pollution in the United States Emission standards Environmental law in the United States