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 (EPA), and more recently along with the National Highway Traffic Safety Administration (NHTSA). These standards cover tailpipe pollution, including carbon monoxide, nitrogen oxides, and particulate emissions, and newer versions have incorporated fuel economy standards. However they lag behind European emission standards, which limit air pollution from brakes and tires.
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 received 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. According to a 2025 study, the 2022 vehicle emission standards increased social welfare by $83 billion (approximately $1,200 per vehicle-buying household), with disproportionately greater benefits for low-income groups ($1,700 per household for the lowest-income group).
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 then-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. By 1967, this authority empowered the federal government to establish enforceable emission standards, setting a precedent for addressing transportation-related air pollution on a national scale.
California Air Resources Board (CARB)
In the mid-20th century, California's economy grew rapidly after the Great Depression, but this economic development was accompanied by an increase in air pollution in the state. As a result, smog started to form in the valleys of Southern California, causing respiratory problems for humans and damaging crops. In the 1960s, Dutch chemist Arie Jan Haagen-Smit identified the air pollutants responsible for the smog: carbon monoxide, hydrocarbons, and nitrogen oxides emitted from cars and factories through inefficient fuel combustion. Haagen-Smit also discovered that these air pollutants react with sunlight to form ozone, a major component of smog. As a response to this situation, the California Air Resources Board (CARB) was established in 1967 with Haagen-Smit as its first chairman. CARB set stringent vehicle emission standards to reduce air pollution in the state. 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.
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 (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 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, 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 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 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, Nevada, Oregon, Pennsylvania, Rhode Island, Vermont, and Washington (2009 model year and later), as well as the District of Columbia. Such states are frequently referred to as "CARB states" in automotive discussions because the regulations are defined by the California Air Resources Board.
The EPA adopted the Californian fuel economy and greenhouse gas standard as a national standard by the 2016 model year and collaborated 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. Starting with model year 1972, the UDDS test cycle was used for criteria pollutant testing; this switched in 1975 to the similar FTP-75. The FTP-75 ("city") driving test averages about ). Additional tests have supplemented the FTP, including the HWFET highway driving test (averaging about ), the high speed, high acceleration US06, and the SC03, which includes air conditioning.
Stricter emission standards for light-duty vehicles in the United States were defined as a result of the Clean Air Act Amendments of 1990. 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). The first of these stricter standards (known as Tier 1) was adopted in 1991 and phased in from 1994 to 1997. Subsequent standards, labelled Tiers 2 through 4, were introduced later.
Tier 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 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 . 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-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.
Standards for carbon monoxide (CO) emissions at cold temperatures () were also adopted and phased in at the same schedule as the other Tier 1 standards. Cold CO emissions are measured over the FTP-75 driving cycle.
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.
Supplemental Federal Test Procedure (SFTP) Between 2000, and 2004, a Supplemental Federal Test Procedure (SFTP) was phased in, requiring vehicles be tested on other cycles in addition to the FTP-75. The additional cycles in the SFTP are: US06: A high speed and high acceleration cycle. SC03: Cycle intended to measure emissions in an urban setting at high temperature with air conditioning. Pollutant standards for NMHC+NOx were a weighted average combining measurements from the FTP-75 and the two SFTP cycles.