United States vehicle emission standards

In the United States, vehicle emissions standards are set through a combination of legislative mandates enacted by Congress through Clean Air Act (CAA) amendments of 1970 and beyond, and executive regulations managed nationally by the Environmental Protection Agency (EPA), and more recently along with the National Highway Traffic Safety Administration (NHTSA). These standard cover common motor vehicle air pollution, including carbon monoxide, nitrogen oxides, 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 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 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 (CAA) was passed as an extension of the Air Pollution Control Act of 1955, encouraging the federal government via the United States Public Health Service under the old Department of Health, Education, and Welfare (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 (MVAPCA) which gave the HEW Secretary authority to set federal standards for vehicle emissions as early as 1967.[1]

California Air Resources Board (CARB)
Smog in the Los Angeles valley in 1972

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 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 which contributed to the formation of the smog and caused the physicological effects on humans and plants. California established the California Air Resources Board (CARB) in 1967, with Haagen-Smit as its first chairman, which among other activities set stringent vehicle emission standards by that year.[2]

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.[3] However, because California has already established its program, it was granted a waiver and allowed to keep its standards. This Act did give states authority to perform vehicle inspections programs beyond the requirements for new vehicles, though few states took their own action on this.[4]

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

Air pollution has 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 (EPA) was formed out of an executive order under President Richard Nixon 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.[1] 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, nitrogen dioxide, sulfur dioxide, particulate matter, hydrocarbons and photochemical oxidants (such as ozone). Other pollutants, such as lead, 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 optional establish I/M programs for existing but were required to follow the EPA's specifications.[4]

New vehicle emission standards

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. The states that have adopted the California standards are: Colorado, Connecticut, Delaware, Maine, Maryland, Massachusetts, New Jersey, New Mexico (2011 model year and later), New York, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington, as well as the District of Columbia.[5][6][7] 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 Californian emissions standards as a national standard by the 2016 model year[8] and is collaborating with Californian regulators on stricter national emissions standards for model years 2017–2025.[9]

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 city driving test (averaging about 21 MPH) and the HWFET highway driving test (averaging about 48 MPH) are used for measuring both emissions and fuel economy.

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 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 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: 1994–1999

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 SUVs and minivans) divided up based on the vehicle weight and cargo capacity.

California's Low-emission vehicle (LEV) program defines six automotive emission standards 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 14,000 pounds (6,350 kg). Listed in order of increasing stringency, the standards are:[10]

  • 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.

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: 2004–2009

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 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 diesel 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: 2010–2016

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 10,000 lbs 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.[11] 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.[12] This is in addition to any gas guzzler tax, if applicable.[13]

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 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.

Heavy-duty vehicles
Further information: Not-To-Exceed – EPA diesel standards

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.[14] Since 2007 only diesel models are allowed in the heavy duty class; the EPA banned gasoline models in 2007.

Greenhouse gases
Further information: Global Warming Solutions Act of 2006 (California)

Federal emissions regulations 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. 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 passed strict greenhouse gas emission standards[15] which are being challenged in the courts.[16]

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.[17][18][19]

Consumer ratings

Air pollution score

EPA's air pollution score[20] 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[20] 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.

California emission standards

Under the CAA Section 209, California is given the ability to apply for special waivers to apply its own emission standards for new motor vehicles that are as least as stringent as the federal standards. [21] 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.[22]

All electric/battery cars, like those of Tesla, are considered zero-emissions vehicles (ZEV) under California's ZEV mandate.

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 (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),[23] 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.[24]
  • California had requested to regulate greenhouse gas emissions (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 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.[25]

State adaption of California Standards

Section 177 of the CAA grants the ability for states to adopt California emission standards instead of federal ones.[26] As of August 2019, thirteen states have adopted the California standards, including their standards for ZEV and GHG:[27]

State Adopted (by model year)
Criteria Pollutant RegulationGHG RegulationZEV Program
California199220091990
New York199320091993
Massachusetts199520091995
Vermont200020092000
Maine200120092001
Pennsylvania20012009N/A
Connecticut200820092008
Rhode Island200820092008
Washington20092009N/A[lower-alpha 1]
Oregon200920092009
New Jersey200920092009
Maryland201120112011
Delaware20142014N/A
Colorado202220222023
  1. Washington passed legislation to meet California's ZEV standards by model year 2022 in March 2020[28]

Revoking of waivers under the Trump administration

President Donald Trump has 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 with Trump's conservative ideology conflicts 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. [29]

Short 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,[30] Trump announced his intention to rollback California's waivers.[31] 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 counter. 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.[32][33]

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 to challenge the EPA's revoking.[24] 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.[34]

Non-road engines

Non-road engines, 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.[35]

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 standards.

Small engines

Pollution from small engines, such as those used in gas-powered groundskeeping equipment reduces air quality. Emissions from small offroad engines are regulated by the EPA.[36] Specific pollutants subject to limits include hydrocarbons, carbon monoxide, and nitrogen oxides.[37]

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.[38] 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.[38] 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.[39] Other western states followed suit soon after California, with Oregon, Washington, and Montana passing similar bills into law later that year.[40]

Existing vehicle emissions standard
See also: Vehicle inspection in the United States

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.[4]

A representative dynamometer-based emissions test station

For model years prior to 1996, emissions tests were performed using a chassis dynamometer-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.


State Emissions
Name Light duty gasoline Emissions testing Light-duty diesel Emissions testing Heavy-duty diesel Emissions testing Special notes
Alabama[41] No emissions testing required No emissions testing required Alabama maintains "volunteer emission testing"
Alaska [42] ≥6 model years old (Anchorage) Biennially Exempt Exempt
Arizona [43][44] Model Years ≥1967 but more than 5 model years old (Pima County/ Maricopa County) Biennially 1967–2007 (Pima County/ Maricopa County) Pima County: 30 percent opacity


Maricopa County: 20 percent opacity

 Maricopa County J1667 test protocol


Model year <1991: 55 percent opacity
Model year ≥1991: 40 percent opacity

 Only state which still conduct emission testing dating back to the 1967 model year unlike other states using EPA classification (a few still conduct test for 1968–present (1968+ testing is for jurisdictions using defined EPA standards for vehicle classification since the '68 model year and beyond automobiles must have an exhaust emission system - PCV system, air injection pump, controlled spark module) e.g. the absence of a rolling chassis exemption - in some states when an automobile reaches an age limit (as in 25 model years old) the vehicle is exempted or pertaining to motor vehicle laws e.g. classic automobile registration and displaying vintage license plates
Arkansas
California vehicles 1976 and newer model-year gas-powered, hybrid, and flex-fuel vehicles; exemptions: new car = ≤6 model-years old are exempt; change of ownership = ≤4 model-years old are exempt required biennially for registration renewal; 2000 model-year and newer receive only visual and OBDII inspections vehicles 1998 and newer model-year diesel-powered vehicles with a GVWR ≤14,000 pounds; exemptions: none applicable to diesel-powered vehicles required biennially for registration renewal; 1998 model-year and newer receive only visual and OBDII inspections California-based fleets of 2 or more heavy-duty vehicles with a GVWR >14,000 pounds Periodic Smoke Inspection Program (PSIP) an annual smoke and tamper self-inspection; exemptions: new vehicle = ≤4 model-years old are exempt Prior to 2004 emission testing included 1966 and newer model year automobiles - 1984 calendar year phased in the use of an acceleration simulated mode test criteria - a 1998 state law made pre-1973 automobiles exempt from emission testing where it temporarily included a 30-year rolling chassis exemption which was repealed in 2006 where 1976 and newer are subjected to state mandated emission testing - California visual inspection strictest in the USA (also similar to Japanese 'Shaken' vehicle laws which is their equivalent of vehicle inspection) which serves as a model for other states implementing I/M (inspection/maintenance) programs
Colorado[45] Vehicles older than 4 years required, this will change to 7 years in 2015. Specially registered cars made before 1975 and special vehicles may be exempted. Biennially when required Exempt for first seven model years of gasoline vehicles. When outside exemption, vehicles up to 32 years old - biennially, else annually. OBD testing done when newer than 10 years Exempt for first four model years of diesel vehicles. When outside exemption, diesel trucks less than 10 years - Biennially else annually. Only on high population corridor encompassing Castle Rock to Wellington, including Boulder and Greeley. Required for vehicles used in this area. Dynamometer testing used.
Connecticut[46] more than 4 model years old but less than 25 model years old Biennially (OBDII) more than 4 model years old but less than 25 model years old Biennially Exempt
Delaware[47] Over 5 model years old Biennially (OBDII)
Florida
Georgia Annually (Atlanta metro area only)
Hawaii
Idaho
Illinois Biennially, after the vehicle is four years old Required only in the Chicago metropolitan area and eastern suburbs of St. Louis, Missouri
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland Biennially 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.
Massachusetts [48][49] Every year OBDII Every year OBDII 8,501 to 14,000 lbs (Medium duty)


14,000 lbs+ (Heavy duty)

 Medium Duty: OBDII


Heavy duty: J1667 Protocol
Michigan
Minnesota
Mississippi
Missouri
Montana
Nebraska
Nevada Model years 1968 and newer
New Hampshire
New Jersey Model years ≥ 1995 Must be inspected biennially. Model years ≤ 1995 do not require inspection.
New Mexico[50][51] Model Years ≥1978 but more than 4 model years old (Bernalillo County) Biennially and at change of ownership Model Years ≥1978 but more than 4 model years old (Bernalillo County) Biennially and at change of ownership Exempt
New York
North Carolina
North Dakota
Ohio More than 4 model years old but less than 25 model years old in the Greater Cleveland area[52] Biennially (OBDII) More than 4 model years old but less than 25 model years old in the Greater Cleveland area Biennially (OBDII) Testing was also required in the Cincinnati and Dayton Metropolitan areas from 1995-2005
Oklahoma
Oregon[53][54] Vehicles must be tested every other year for registration renewal in the Portland and Medford areas. Model Years ≥1975 must be tested in Portland area, and those 20 model year and newer in Medford area. Also all vehicles new to Oregon. Biennially Same intervals and exemptions as gasoline cars, 8,500 lbs or less Biennially ≥8,501 lbs Exempt
Pennsylvania
Rhode Island [55][56][57] more than 2 models years old (inclusive) or 24,000 miles (inclusive) Biennially (OBDII) Exempt Exempt Effective July 2012, any state contracted job requiring heavy duty vehicles must be operated with pollution control devices
South Carolina
South Dakota
Tennessee[58] Any gasoline vehicle with a model year of 1975 and newer and a GVWR of 10,500 lbs or less. Motorcycles are exempt. Annual testing. OBDII + catalytic converter tamper check + fuel cap leak test for 1996 or later vehicles. Tailpipe/tampering testing for 1995 and older vehicles. Any diesel vehicle with a model year of 1975 and newer and a GVWR of 10,500 lbs or less. Annual testing. OBDII + catalytic converter tamper check + fuel cap leak test for 2002 or later vehicles. Curb Idle-opacity/tampering testing for 2001 and older vehicles. Exempt Only applies to Hamilton (Chattanooga), Davidson, Rutherford, Sumner, Williamson, and Wilson (all part of Greater Nashville area) Counties. Required for registration renewal. Davidson county runs their own emissions programs/facilities with the assistance of the state. Shelby County (Memphis) formerly required testing, but ended their program in July 2013 due to funding issues and no state support.[59]
Texas more than 2 model years old to 24 model years old; motorcycles, diesel-powered, and automobiles 25 model years or older exempt from emission testing - includes motor vehicles registered as an Antique or Classic under Texas law (either displaying state-mandated vanity license plate or vintage-era license plates (this also includes past general issue plates from 1975–present with validation stickers used prior to the 1995–present windshield sticker; license plate law in Texas (effective since May 1988) requires new general issue plates after it reaches 8 years of age - license plate law was amended 11.1.16 where TxDMV no longer mandates plate replacement every 8 years with the exception that the license plate is considered legible e.g. loss of reflectivity with the paint base and/or if a license plate is unreadable if a police officer does a probable cause traffic stop) annually (both OBDII for 1996–present and ASM to 1995 for motor vehicles registered in the DFW Metroplex and Houston Metro area 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 the Houston Metropolitan area (Brazoria, Fort Bend, Galveston, Harris, Montgomery), DFW Metroplex (Collin, Dallas, Denton, Ellis, Johnson, Kaufman, Parker, Rockwall, Tarrant), Austin Metropolitan area (Travis, Williamson) and El Paso counties; since March 1, 2015 inspection stickers no longer issued where an automobile must pass the safety or emission test prior to renewing automobile registration
Utah
Virginia[60] more than 2 model years old to 24 model years old Biennially. First-time registrations exempt if vehicle has received an emissions certificate from certain states within previous 12 months. newer than 1996 model year Biennially 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.
Washington[61] No emissions testing required No emissions testing required
West Virginia No emissions testing required No emissions testing required
Wisconsin[62] Testing required for vehicles model years 1996 and later in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties (model years 1996 to 2006 up to 8,500 pounds; model years 2007 or newer up to 14,000 pounds) () Every other year before registration renewal; after transfer of ownership; after registration in Wisconsin Testing required for vehicles model years 1996 and later in Kenosha, Milwaukee, Ozaukee, Racine, Sheboygan, Washington and Waukesha counties: Model years 2007 and newer which have a gross vehicle rating up to 14,000 pounds Every other year before registration renewal; after transfer of ownership; after registration in Wisconsin Greater than 14,000 lbs. Exempt
Wyoming
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See also

References
  1. Stern, Arther C. (1982). "History of Air Pollution Legislation in the United States". Journal of the Air Pollution Control Association. 32 (1): 44–61. doi:10.1080/00022470.1982.10465369. PMID 7033323.
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