Rescission of the Greenhouse Gas Endangerment Finding and Motor Vehicle Greenhouse Gas Emission Standards Under the Clean Air Act

The Endangerment Finding announced an interpretation of CAA section 202(a)(1) that permitted the EPA to prescribe standards in response to global climate change concerns rather than air pollution that threatens public health or welfare through local or regional exposures. We asserted that the statute's “silence” granted us “procedural discretion” to issue standalone findings without considering the regulatory response required by those findings. In setting out our standalone findings, we severed the endangerment analysis (based on health and welfare harms attributed primarily to trends in GMST and GSLR) from the cause or contribution analysis (based on the estimated share of domestic GHG emissions from all new and existing motor vehicles and engines in global GHG emissions from all anthropogenic sources). In the endangerment analysis, we acknowledged that none of the health effects of concern were associated with direct exposure to GHGs, and in the contribution analysis, we acknowledged that combatting the identified risks would require all contributors--both domestic and international and from all anthropogenic sources--to “do their part.” Throughout, we assumed that the Supreme Court's decision in Massachusetts compelled us to read the statute as authorizing the regulation of GHG emissions under CAA section 202(a)(1).

In important respects, the Endangerment Finding and the Supreme Court's decision in Massachusetts straddled a transitional period regarding the standards for statutory interpretation and understandings of agency authority. The breadth of agency discretion, and the question whether Congress reserves major policy questions for itself, were sharply disputed. Judicial decisions in the intervening fifteen years have significantly clarified the law. In Loper Bright, the Supreme Court overruled the Chevron doctrine of deference to agency statutory interpretation, ruling that statutes “have a single, best meaning” that is “`fixed at the time of enactment”' and informed, but not dictated, by Executive Branch practice. 603 U.S. at 400-01 (quoting Wis. Cent. Ltd. v. United States, 585 U.S. 274, 284 (2018)). And in West Virginia, the Supreme Court built upon its decisions in UARG and Brown & Williamson, among others, by confirming that an agency must have more than “a colorable textual basis” to claim authority to decide major questions of policy that Congress generally reserves for itself. 597 U.S. at 723.

In this subsection, we explain that the best reading of CAA section 202(a)(1), as informed by Loper Bright and principles of statutory interpretation, does not authorize the EPA to assert jurisdiction over GHG emissions based on global climate change concerns in a standalone endangerment finding. Scientific understanding of environmental issues may be continuously evolving, but the scope of the EPA's authority under CAA section 202(a)(1) is fixed by the terms Congress used when enacting and amending the language of CAA section 202(a)(1) from 1965 to 1977. Regardless whether GHGs are “agents of air pollution” under the Act-wide definition of “air pollutant” in CAA section 302(g), we cannot regulate under CAA section 202(a) unless emissions of the air pollutant by new motor vehicles and engines “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” Because the ordinary meaning, structure, and history of CAA section 202(a)(1) and related provisions demonstrate that this language targets “air pollution” that threatens public health or welfare through local or regional exposure, the “six well- mixed” GHGs defined by reference to global climate change concerns cannot satisfy this standard. The futility of GHG emission standards in addressing the

health and welfare impacts attributed to global climate change further reinforces this interpretation. For these reasons, and on account of the additional procedural and analytical errors discussed below, we are rescinding the Endangerment Finding. 1. Final Rationale

Congress originally enacted the language of CAA section 202(a) in the Motor Vehicle Pollution Control Act of 1965 and retained it, with minor revisions, in 1967, the 1970 CAA, and the 1977 amendments. The key language in CAA section 202(a)(1) provides:

The Administrator shall by regulation prescribe (and from time to time revise) in accordance with the provisions of this section, standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines, which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.\97\

\97\ 42 U.S.C. 7521(a)(1). The key terms “cause, or contribute,” “air pollution,” “endanger,” and “health or welfare” were introduced in 1965. Public Law 89-271, section 101, 79 Stat. 992, 992-93. The phrase “may reasonably be anticipated to” was added to the earlier phrase “which endangers the public health or welfare” in 1977. Public Law 95-95, section 401(d)(1), 91 Stat. 685, 791.

Since 1977, CAA section 302(g) has defined the term “air pollutant” throughout the statute as “any air pollution agent or combination of such agents . . . which is emitted into or otherwise enters the ambient air.” \98\ CAA section 302(h) also provides that any reference to “effects on welfare includes, but is not limited to, effects on” the environment, property, transportation hazards, and “on economic values and on personal comfort and well-being.” \99\

\98\ 42 U.S.C. 7602(g). Notably, the statute does not separately define “air pollution.”

\99\ 42 U.S.C. 7602(h). This definition took its current form in the 1970 CAA and was amended in part in the 1990 CAA Amendments to add the final clause “whether caused by transformation, conversion, or combination with other air pollutants.” See Public Law 91-604, 84 Stat. 1676, 1710; Public Law 101-549, 104 Stat. 2399, 2470.

The EPA concludes that this statutory language is best read as authorizing the Agency to identify and regulate, as an integral part of a rulemaking prescribing emission standards, emissions that cause or contribute to air pollution that endangers public health and welfare through local or regional exposure. This reading is consistent with the ordinary meaning of key terms and the statutory structure, our decades- long implementation of the statute prior to 2009, and background principles of statutory interpretation, including default rules for proximate cause. This reading is also consistent with the Supreme Court's decision in Massachusetts, which addressed distinct issues arising out of the denial of a petition for rulemaking and must, as a matter of stare decisis, be read in harmony with subsequent decisions bearing on the EPA's authority and statutory interpretation, including UARG, West Virginia, and Loper Bright.

Air Pollution. The EPA is finalizing as proposed that CAA section 202(a)(1) is best read as authorizing the Agency to regulate emissions that cause or contribute to air pollution that endangers public health or welfare through local or regional exposure. For the purposes of this final action, we use the phrase local or regional exposure to distinguish air pollution that impacts public health and welfare by its presence in the ambient air from “air pollution” consisting of six “well-mixed” GHGs that, as conceptualized in the Endangerment Finding, impacts public health and welfare only indirectly and not by its mere presence in the ambient air. As discussed below, this aspect of the final action effectively returns the EPA to its interpretation of CAA section 202(a)(1) prior to 2009 and the ordinary meaning of the terms Congress selected.

In CAA section 202(a)(1), Congress identified the object of the regulatory authority conferred in the remainder of the section--“air pollution which may reasonably be anticipated to endanger public health or welfare.” The EPA's emission standards for new motor vehicles and engines were a key part of the congressional design for combatting air pollution problems impacting the Nation throughout the 1960s and 1970s, particularly in high-population areas. Congress debated these issues extensively in advance of the 1970 CAA by reference to the air pollution impacting Americans every day, with smog, criteria pollutants, and air toxics taking center stage.\100\ To address the perceived need for a rapid response, Congress paired the preexisting language imported into CAA section 202(a)(1) \101\ with new language in CAA section 202(b)(1) requiring that emission standards contain significant, short-term reductions in CO, HC, and NOX emissions from new LD vehicles and engines.\102\ As discussed elsewhere in this preamble, Congress repeatedly returned to this strategy in the subsequent decades by adding language to CAA section 202 requiring that emission standards achieve further reductions for additional pollutants and classes of new motor vehicles and engines.

\100\ See, e.g., S. Rep. 91-1196, at 1, 7 (1970) (expressing “concern with direct adverse effects upon public health” and the need for “definitive knowledge of the causal relationships between exposure to air pollution agents . . . and health or welfare under varying environmental conditions,” particularly by reference to SOx, PM, CO, HC, and oxidants and the role of mobile sources in urban pollution); id. at 18 (describing the three general categories of air pollution as criteria pollutants, hazardous air pollutants, and certain emissions unique to stationary sources); H.R. Rep. 91-1146, at 6 (1970) (explaining that mobile-source air pollution “is particularly dangerous in the highly urbanized areas of our country”); 116 Cong. Rec. 32902 (1970) (statement of Sen. Muskie) (explaining that the draft legislation targeted mobile- source contribution to urban pollution, including by requiring “emission standards for carbon monoxide, hydrocarbons, and nitrogen oxides”); see also 111 Cong. Rec. 10782 (1965) (statement of Sen. Muskie) (similarly emphasizing in advance of the original 1965 legislation that mobile sources accounted for “50 percent of our national air pollution problem” and focusing in particular on “carbon monoxide,” “hydrocarbons,” and “nitrogen oxides”).

\101\ See, e.g., S. Rep. 91-1196, at 24 (“The regulatory authority in section 202(a) would be essentially the same as existing law . . . .”); H.R. Rep. 91-1783 (1970) (conf. report) (explaining that the House largely acceded to the Senate bill in relevant part).

\102\ Public Law 91-604, section 6(a), 84 Stat. 1676, 1690. In subsequent amendments, Congress modified and expanded upon the provisions in CAA section 202(b)(1) to require that emission standards achieve further reductions for later model years. See 42 U.S.C. 7521(b)(1).

Particularly in light of this history, the term “air pollution” as used in CAA section 202(a)(1) must be construed in context with the specific air pollutants and air pollution concerns identified in the remainder of CAA section 202. Each of these listed pollution control targets share the common quality of causing or contributing to air pollution that adversely impacts public health or welfare through local or regional exposure to the air pollution itself. CAA section 202 specifically requires the EPA to prescribe emission standards with various minimum content for HCs, CO, NOX, and PM, all of which harm human health and the environment through exposure (e.g., inhalation and dermal contact) or by causing or contributing to air pollution that harms health and the environment through exposure (e.g., smog and acid rain).\103\ CAA section 202(l) also requires prescribing emission standards under CAA section 202(a)(1) for certain air pollutants that qualify as “toxic” or “hazardous” air pollutants, including benzene and formaldehyde.\104\ Neither GHGs nor any of the individual “six well-mixed” GHGs defined in the Endangerment Finding by reference to global climate change concerns appear

anywhere in CAA section 202.\105\ That pattern holds for the criteria pollutants identified in the CAA--CO, lead, ozone (O3), nitrogen dioxide (NO2), PM, and SO2--as well as the initial list of hazardous air pollutants in CAA section 112(b)(1).\106\

\103\ See, e.g., 42 U.S.C. 7521(a)(3)(A)(i), (b), (g), (h), (j), (k).

\104\ 42 U.S.C. 7521(l). Such regulations may include fuel standards under issued under the EPA's fuel and fuel additive authority in CAA section 211.

\105\ Notably, in the last major amendments to the Clean Air Act in 1990, Congress specified “nonmethane hydrocarbons (NMHC)” when adding additional minimum requirements for HC, CO, NOX, and PM emission standards at CAA section 202(g) and (h). Public Law 101-549, section 203, 104 Stat. 2399, 2474 (emphasis added) (codified at 42 U.S.C. 7521(g), (h)).

\106\ 42 U.S.C. 7412(b)(1).

We find it significant that in subjecting a number of air pollutants emitted by new motor vehicles and engines to regulation under CAA section 202, Congress did not include substances that are potentially indirectly harmful to public health or welfare based on elevated global concentrations in the upper atmosphere. That conspicuous omission supports the conclusion that emissions subject to regulation under CAA section 202(a) are those that cause or contribute to air pollution which itself endangers public health or welfare through local or regional exposure.\107\ For certain regulated air pollutants, the emissions themselves are the air pollution that endangers public health or welfare, i.e., emissions are the air pollution with adverse health and welfare impacts. An example is CO, which can be harmful, and even fatal, to humans at sufficient localized concentrations.\108\ For other regulated air pollutants, emissions contribute to air pollution that endangers public health or welfare by interacting with other airborne chemicals or environmental factors such as sunlight to create the air pollution that endangers public health or welfare, i.e., the emitted air pollutants are ingredients that create the air pollution that endangers public health or welfare in combination. An example is acid rain, in which air pollutants such as SO2 interact locally and regionally with additional airborne chemicals to form acidic precipitation.\109\ Another example is NOX, which reacts with VOCs in the presence of heat and sunlight to create ground-level ozone as the airborne chemicals are carried by wind over geological features amenable to ground-level ozone formation.\110\

\107\ As discussed herein, the references to GHGs in the CAA are in non-regulatory contexts in which Congress authorized funding for various forms of research and grant programs and the Renewable Fuel Standard (RFS) program. The choice to limit such references to non- regulatory solutions and the RFS program, which applies to refiners and importers, further supports the conclusion that the CAA section 202(a) regulatory authority for responding to endangerment does not encompass GHG emissions in connection with global climate change concerns.

\108\ U.S. Environmental Protection Agency. (Last updated Oct. 7, 2025). Carbon Monoxide's Impact on Indoor Air Quality: https://www.epa.gov/indoor-air-quality-iaq/carbon-monoxides-impact-indoor-air-quality.

\109\ U.S. Environmental Protection Agency. (Last updated Mar. 4, 2025). What is Acid Rain?: https://www.epa.gov/acidrain/what-acid-rain.

\110\ U.S. Environmental Protection Agency. (Last updated Mar. 11, 2025). Ground-level Ozone Basics: https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics.

We also emphasize that expanding CAA section 202(a)(1) to encompass global climate change concerns required the EPA to take the admittedly “unique” approach of finding endangerment and contribution where the overwhelming majority of relevant emissions hails from international sources. Although we justified this approach by concluding as a policy matter that all sources must “do their part” to avoid a collective action problem, Congress has specifically provided in the CAA when and how the EPA may consider international emissions. For example, CAA section 115 authorizes the EPA to require controls for domestic emissions that contribute to air pollution that endangers public health or welfare in another country only when, among other things, that country has adopted reciprocal protections for emissions into the United States.\111\ CAA section 179B authorizes the EPA to account for the impact of international emissions on NAAQS attainment under certain conditions.\112\ Most importantly, Congress adopted a new regulatory regime in 1990--Title VI--in response to global concerns about depletion of the ozone layer, which contains its own findings, policies, and regulatory authorities that required the EPA to phase out domestic use of ozone-depleting substances.\113\ None of these provisions encompass GHG emissions, and all support the conclusion that Congress does not presume that general authorities in the CAA encompass international emissions. Rather, Congress knows how to provide for the consideration of and regulation in response to international emissions, and has not done so for GHG emissions in the CAA section 202 provisions governing new motor vehicle and engine emissions.

\111\ 42 U.S.C. 7415.

\112\ 42 U.S.C. 7509a.

\113\ 42 U.S.C. 7671 et seq.

The definition of “air pollutant” in CAA section 302(g) and the ordinary meaning of the undefined terms pollutant, pollution, and air pollution support this reading. At the time Congress added these terms to CAA section 202(a)(1), the term “pollutant” was defined as “[a]nything that pollutes; especially, any gaseous, chemical, or organic waste that contaminates air, soil, or water,” \114\ and “pollution” was defined as “[t]he contamination of soil, water or the atmosphere by the discharge of noxious substances.” \115\ The definition of the root word “pollute”--“[t]o dirty, contaminate,” confirms the relationship of these terms to concepts of contamination and toxicity.\116\ The central concept is the addition of a contaminant, something that “make[s] impure by contact or mixture.” \117\ CAA section 302(g) defines “air pollutant” is any “air pollution agent or combination of such agents” that “is emitted into or otherwise enters the ambient air.” \118\ Read together with CAA section 202(a)--as the Supreme Court held we must in UARG--the underlying concept of dangerousness and contamination reinforces the conclusion that air pollution which endangers public health or welfare is air pollution (caused or contributed to by air pollutants) that itself endangers public health or welfare through local or regional exposures.

\114\ Pollutant, Am. Heritage Dictionary 1015 (1970); see also Pollutant, 3 Webster's Third New Int'l Dictionary 1756 (1966) (“something that pollutes: a polluting substance, medium or agent”).

\115\ Pollution, Am. Heritage Dictionary 1015 (1970); see also Pollution, 3 Webster's Third New Int'l Dictionary 1756 (1966) (“the action of polluting or the state of being polluted: defilement, desecration, impurity, uncleanness”).

\116\ Pollute, Am. Heritage Dictionary 1015 (1970); see also Pollute, Black's Law Dictionary 1043 (5th ed 1979) (“To corrupt or defile. The contamination of soil, air and water by noxious substances and noises.”); Pollute, 3 Webster's Third New Int'l Dictionary 1756 (1966) (“to make physically impure or unclean: befoul, dirty, taint”).

\117\ Contaminate, Am. Heritage Dictionary 156 (1970); see also Contaminate, 1 Webster's Third New Int'l Dictionary 491 (1966) (“to soil, stain, corrupt, or infect by contact or association”).

\118\ 42 U.S.C. 7602(g).

Contemporaneous usage of the term “air pollution” in the 1960s and 1970s further indicate the term was understood in this way when Congress adopted it into Title II of the CAA. Judicial decisions issued close in time to the public debates and enactment of the CAA Amendments of 1970 used the term exclusively in reference to local and regional exposure.\119\ News reports

and legislative debates leading up to the 1970 Amendments similarly attacked air pollution problems arising from local and regional exposure, including smog and health and welfare impacts related to inhalation and physical contact.\120\ This pattern of usage is consistent with subsequent legislative amendments to CAA section 202, which added provisions specific to criteria pollutants and air toxics fitting this profile, and with the EPA's course of mobile-source regulation until 2009. In reviewing the relevant history, including materials received during the public comment period, we have not identified an authoritative source suggesting that the ordinary meaning of “air pollution” would have included, without additional modifying language, gases that may endanger public health or welfare only on a global scale and through an attenuated and indirect causal chain.

\119\ See, e.g., Washington v. GM Corp., 406 U.S. 109, 115-16 (1972) (declining to exercise original jurisdiction over complaint alleging conspiracy to restrain the development of air pollution control devices for motor vehicles because, although “Congress has largely preempted the field with regard to `emissions from new motor vehicles,' . . . geophysical characteristics which define local and regional airsheds are often significant considerations in determining the steps necessary to abate air pollution”); Friends of Earth v. FCC, 449 F.2d 1164, 1165-66 (D.C. Cir. 1971) (addressing challenge to the FCC's treatment of automobile advertisements that petitioners alleged took a position on motor vehicle air pollution worsening local conditions in New York City, including “dangerous hydrocarbons in the air”).

\120\ See, e.g., Coal. for Responsible Regulation, 2012 U.S. App. LEXIS 25997, at *32-37 (Brown, J., dissenting from denial of rh'g en banc) (summarizing relevant history).

The “air pollution” addressed in the Endangerment Finding is different in kind. In that decision, the Administrator defined the relevant “air pollutant” as six “well-mixed GHGs” and the relevant “air pollution” as total global concentrations of “the combined mix of” these GHGs “which together, constitute the root cause of human- induced climate change and the resulting impacts on public health and welfare.” 74 FR 66516. In contrast to the air pollution addressed expressly in CAA section 202 and elsewhere in the statute, GHGs do not endanger public health or welfare through local or regional exposure. Rather, the Endangerment Finding asserted that GHG “air pollution” would lead to increases in global temperature and change to ocean pH that, in turn, would lead to environmental phenomena, in combination with an open-ended universe of additional factors, which would potentially have adverse health and welfare impacts of varying severity in certain regions. Indeed, the Administrator expressly admitted at the time that the circumstances were “unique” because “[n]one of th[e] human health effects” identified in the Endangerment Finding “are associated with direct exposure to greenhouse gases.” 74 FR 66527. With respect to welfare effects, the Administrator acknowledged that the primary effects of concern could be considered health or welfare impacts \121\ and that certain welfare impacts were “effects on people that do not rise to the level of health effects” but utilize the same causal chain. 74 FR 66527; see 74 FR 66531 (explaining that the Endangerment Finding considered the same causal “pathways” in analyzing “public health” and “public welfare”).\122\ Regulating GHG emissions based on global climate change concerns requires reading an additional instance of “cause, or contribute” into the statute, such that CAA section 202(a) encompasses the `emission of air pollutants that cause, or contribute to, air pollution that causes, or contributes to, endangerment of public health or welfare.'

\121\ For example, the EPA in the Endangerment Finding understood impacts on “well-being” as used in the CAA section 302(h) definition of “welfare” to be relevant “whether [the impacts] resul[t] directly or indirectly from the pollution in the air.” 74 FR 66528.

\122\ The Agency acknowledged that difficult questions about the distinction between health and welfare impacts was something the “EPA has not had to resolve” in the past, “as it has been clear whether the effects relate to public health or relate to public welfare, with no confusion over what category was at issue.” 74 FR 66527. Rather than take this analytical difficulty as a sign that the causal chain was different in kind from the type of “air pollution” addressed by CAA section 202(a)(1), however, we proceeded to finalize a novel invocation of authority to regulate in response to global climate change concerns.

This interpretation is also supported by the best reading of the terms “cause,” “contribute,” and “reasonably be anticipated to endanger.” In enacting and amending CAA section 202(a)(1), Congress legislated against background legal principles, including principles of causation and proximate cause.\123\ These “default rules” are “presumed to have [been] incorporated, absent an indication to the contrary in the statute itself,” \124\ and nothing in the text of CAA section 202(a)(1) indicates that Congress intended to depart from ordinary legal meaning. Indeed, Congress affirmatively incorporated proximate cause principles when it added the phrase “may reasonably be anticipated” to the statute in 1977 amendments to the CAA. That phrasing is another way of saying “reasonably foreseeable,” a longstanding touchstone of proximate cause.\125\ As a general matter, there is a point at which harm no longer has a sufficiently close connection to the relevant conduct to reasonably draw a causal link. Emissions from new motor vehicles and new motor vehicle engines in the United States do not have a sufficiently close connection to the adverse impacts identified in the Endangerment Finding to fit within the legal meaning of “cause” or “contribute.” This reading is complemented by the term “reasonably” in the phrase “air pollution which may reasonably be anticipated to endanger public health or welfare.” Like the terms “cause” and “contribute,” the term “reasonably” places an outer legal limit on the authority to anticipate dangers to public health and welfare from air pollution. The greater the number of causal links involved in anticipating such endangerment, the more difficult it is to qualify that anticipation as “reasonable.”

\123\ See, e.g., Bank of Am. Corp. v. City of Miami, 581 U.S. 189, 201 (2017); Lexmark Int'l, Inc. v. Static Control Components, Inc., 572 U.S. 118, 132 (2014); Univ. of Tex. Sw. Med. Ctr. v. Nassar, 570 U.S. 338, 347 (2013); City of Oakland v. Wells Fargo & Co., 14 F.4th 1030 (9th Cir. 2021) (en banc).

\124\ Nassar, 570 U.S. at 347.

\125\ Foreseeable, 1 Webster's Third New Int'l Dictionary 890 (1966) (“being such as may reasonably be anticipated”); see, e.g., Hicks v. United States, 511 F.2d 407, 421 (D.C. Cir. 1975) (finding “proximate cause” satisfied because it was “foreseeable” that a hospital's release without warning of an alcoholic patient with a history of abusing his wife could result in harm to the patient's wife).

Notably, contemporary understandings of terms used in the CAA section 302(h) definition of “welfare” also support the understanding that CAA section 202(a)(1) encompasses air pollution with adverse impacts from local or regional exposure. The statute provides that references to “effects on welfare” include “effects on soils, water, crops, vegetation, manmade materials, animals, wildlife, weather, visibility, and climate,” damage to property, transportation hazards, and effects on economic values and personal comfort and well-being. The ordinary meaning of “climate,” an undefined term, was “[t]he prevailing weather in a particular region” or “[a] region manifesting particular meteorological conditions.” \126\ Similarly, “weather” meant “[t]he state of the atmosphere at a given time and place, described by temperature, moisture, wind velocity, and pressure.” \127\ Both terms must also be read together in context, including by reference to the other terms enumerated in the list.\128\ Each of the other terms in

the definition refers to things and mechanisms of action that occur in a particular place or under regionally bounded conditions. The terms Congress used to define “welfare” speak to air pollution with adverse impacts from local and regional exposure, not global climate change concerns that require a very different and much longer causal chain. The definition is broad enough to encompass the various air pollutants and air pollution of concern, each of which interacts differently with the environment--smog, particulate matter, and the like. Congress understood that air pollution challenges varied from State-to-State and region to region, while, at the same time, recognizing that the most acute challenges--smog in highly populated urban areas, for example-- had similarities that would benefit from national standards.\129\ But none of the many terms listed in the definition of welfare would have been understood, absent modifying terms, to refer to global considerations. Nor has Congress added terms like “global” or “change” that would have expanded the scope of the effects on welfare encompassed within the definition.\130\

\126\ Climate, Am. Heritage Dictionary 136 (1970); see, e.g., Alameda Cons. Ass'n v. California, 437 F.2d 1087, 1096 (9th Cir. 1971) (using “climate” to discuss local environmental conditions in San Francisco Bay); Levenson's Case, 194 N.E.2d 103, 105 (Mass. 1963) (using “climate” to address whether moving to another state with a different climate is a covered medical expense).

\127\ Weather, Am. Heritage Dictionary 785 (1970).

\128\ See Fischer v. United States, 603 U.S. 480, 487 (2024) (“[T]he canon of noscitur a sociis teaches that a word is `given more precise content by the neighboring words with which it is associated.' That `avoid[s] ascribing to one word a meaning so broad that it is inconsistent with' `the company it keeps' ” (citations omitted)); Gustafson v. Alloyd Co., 513 U.S. 561, 575 (1995) (applying canon to interpret the broad term “communication,” as used in a statutory definition of “prospectus,” to mean only public-facing communications that offer securities).

\129\ See, e.g., S. Rep. 91-1196, at 1-8, 24 (1970) (discussing need for and intent of Senate bill that would eventually form much of the 1970 CAA by reference to urban pollution problems and areas in proximity to stationary and mobile sources and recognizing that “protection of the public health and welfare requires definitive knowledge of the causal relationships between exposure to air pollution agents . . . under varying environmental conditions”); H.R. Rep. 91-1146, at 6 (1970) (similar for House bill that informed aspects of the 1970 CAA).

\130\ As discussed further in this section of the preamble and the Response to Comments document, Congress has used such language to specify the relevance of global climate change concerns in more recent amendments to different programs. CAA section 211(o)(2)(B)(ii), for example, provides that the EPA must consider the impact of the production and use of renewable fuels on “climate change” when setting renewable fuel volumes under the RFS program. 42 U.S.C. 7545(o)(2)(B)(ii) (emphasis added); see id. 7545(o)(1) (defining various renewable fuels in part by reference to GHG emissions).

The Endangerment Finding largely avoided addressing these interpretive problems by severing the question whether GHG emissions from new motor vehicle engines contribute to GHG concentrations in the atmosphere from the question whether GHG concentrations in the atmosphere endanger public health and welfare. As discussed in further detail below, there is no basis in the statute for severing the inquiry in that way. Nevertheless, even with respect to endangerment and contribution in isolation, global climate change concerns involve causal relationships that are too uncertain, conjectural, remote, and convoluted by intervening and confounding factors to fit within the terms “cause,” “contribute,” and “reasonably be anticipated to endanger” as used in CAA section 202(a)(1). This understanding follows from the position discussed above that CAA section 202(a)(1) and the statute more generally were designed to address air pollution with harmful impacts from local and regional exposure and that are amenable to analysis using ordinary causation standards. In specifying that emissions may “cause, or contribute to” air pollution, and that air pollution need only “be reasonably anticipated to endanger public health or welfare,” Congress signaled that regulation may be appropriate when harm is not yet occurring or is not certain to occur. But that language bearing on the degree of certainty required does not override ordinary background principles governing the limits of an attenuated causal chain.

Ultimately, the Endangerment Finding did not reflect consideration of the interpretive principles or ordinary meaning of the relevant terms discussed above. With respect to “air pollution,” the Administrator in 2009 asserted an unlimited discretion to decide what the EPA may target through regulation by defining “air pollution” without reference to the best reading of the statutory term. 74 FR 66516-17. Neither the factors used to select the six GHGs--that they are (a) “directly-emitted,” (b) “long-lived,” and (c) “well- mixed”--nor the reasons used to support this definition--that they (1) “share common properties,” (2) are “estimated to be the primary cause of human-induced climate change,” (3) are “the common focus of climate change science research and policy analyses,” (4) have not been “assessed on an individual gas approach,” and (5) that the Agency had combined certain pollutants in the past--are rooted in the ordinary meaning of “air pollution” or any other statutory term in CAA section 202(a)(1). Id. Instead, the Administrator extended discussion in Massachusetts of the CAA section 302(g) definition of “air pollutant” to the undefined term “air pollution,” reasoning that because the EPA could group multiple air pollutants into a “combination of such agents,” there was no relevant statutory limit to the Agency's discretion to identify subjects for regulation. 74 FR 66537. Nor did the Administrator in 2009 grapple with the ordinary meaning of the terms used in the CAA section 302(h) definition of welfare, including “climate,” consider the full range of evidence bearing on the ordinary meaning of “reasonably be anticipated to endanger,” or appropriately evaluate the full context and structure relevant to CAA section 202(a)(1). In short, we now conclude that the legal analysis conducted in the Endangerment Finding, as well the resulting interpretation, cannot be squared with the longstanding principles that now trump deference to agency statutory interpretation under Loper Bright.

In finalizing a different interpretation, we note that a limiting construction is necessary to avoid absurd results and potential conflict with the nondelegation doctrine. Because Congress cannot delegate legislative powers to the Executive Branch, statutes granting an agency regulatory authority must provide an intelligible principle to guide its exercise.\131\ Our authority under CAA section 202(a)(1) to “prescribe . . . standards” for emissions by any class or classes of new motor vehicles and engines is limited by the requirement that the Administrator find such emissions cause or contribute to air pollution that may reasonably be anticipated to endanger public health and welfare. The best reading of the statute recognized in this final action circumscribes this authority to air pollution that itself endangers health or welfare through local or regional exposure. Under the interpretation adopted in the Endangerment Finding, however, our authority under CAA section 202(a)(1) would have no readily discernible limiting principle, particularly in combination with the authority asserted to sever the analysis of endangerment and causation or contribution. Any “air pollutant” emitted by new motor vehicles or engines at more than de minimis volumes would trigger our authority and obligation to prescribe standards so long as emissions from any and all sources globally contributes to “air pollution” that, in turn, can be said to have any causal relationship to adverse impacts on public health and welfare, broadly defined.\132\ Put another way, the

Administrator in 2009 asserted authority to define the relevant “air pollution” without reference to any statutory limiting principle, leaving the EPA free to redefine the objectives of the regulatory scheme.

\131\ See, e.g., Gundy v. United States, 588 U.S. 128 (2019).

\132\ The consequences of this interpretation are not limited to mobile sources. When issuing the Endangerment Finding, the EPA understood that stationary sources would be subject to a variety of PSD and Title V permitting obligations related to GHG emissions.

That limitless construction of CAA section 202(a)(1) cannot be reconciled with the Supreme Court's instructions regarding the scope of agency authority in Loper Bright. Statutes have a single, best meaning that may include “a degree of discretion.” 603 U.S. at 369. But that discretion does not extend to redefining statutory terms in a manner inconsistent with ordinary meaning. Although “Congress has often enacted” statutes that “`expressly delegate[]' to an agency the authority to give meaning to a particular statutory term,” Loper Bright, 603 U.S. at 394-95 (quoting Batterton v. Francis, 432 U.S. 416, 425 (1977)), there is no such express delegation in CAA section 202.\133\ Nor can extending CAA section 202(a)(1) to the regulation of GHGs in response to global climate change concerns plausibly be understood as “`fill[ing] up the details' of a statutory scheme.” Id. (quoting Wayman v. Southard, 23 U.S. (10 Wheat.) 1, 43 (1825)). And “air pollution” is not a discretion-conferring “term or phrase that `leaves agencies with flexibility, such as `appropriate' or `reasonable.”' Id. (quoting Michigan, 576 U.S. at 752). Under these circumstances the ordinary meaning of “air pollution” controls. The EPA has a degree of discretion in identifying and regulating emissions that cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare. But that discretion does not extend to redefining “air pollution” from the local and regional exposure problems understood at the time of enactment and addressed throughout the statute to global climate change concerns.\134\

\133\ See, e.g., Batterton, 432 U.S. at 417 n.2 (interpreting statutory phrase “by reason of the unemployment (as determined in accordance with standards prescribed by the Secretary)”); 42 U.S.C. 7410(m) (authorizing the application of sanctions under certain conditions “in relation to any plan or plan item (as that term is defined by the Administrator)”) (emphasis added), 7411(i) (excluding from certain stationary source regulations “country elevators (as defined by the Administrator)”) (emphasis added); 33 U.S.C. 1311(b)(1)(A) (requiring application of “the best practicable control technology currently available as defined by the Administrator”) (emphasis added).

\134\ In reaching this conclusion, we are mindful that the Sixth Circuit recently applied Loper Bright to hold that the FCC exceeded its statutory authority in a 2024 order that subjected broadband internet service providers to “net-neutrality principles.” Ohio Telecom Ass'n, 124 F.4th at 997. With respect to mobile broadband, the FCC had interpreted “the public switched network” to include not only the traditional telephone numbers comprising the network at the time the statute was enacted, but also public internet protocol (“IP”) addresses. Id. at 1011. The court rejected this approach, holding as a matter of statutory interpretation that “delegation is not unfettered” and that “nothing in the statute . . . permits the FCC to effectively change the statute's original meaning of `the public switched network' . . . by adding `public IP addresses' to adapt to new technology.” Id. at 1012 (citing Loper Bright, 603 U.S. at 395).

Indeed, the Endangerment Finding did not even limit the definitions selected for “air pollutant” or “air pollution” to gases emitted by new motor vehicles or engines. Rather, the Administrator defined the terms to include any “climate forcer” that met the identified criteria and expressly reserved the right to add to the six “well- mixed” GHGs in future actions. 74 FR 66520-21. Nor were the identified criteria--that GHGs are long-lived, directly emitted, and well-mixed-- tied to any statutory language that requires the EPA to retain them or prevents the Agency from further expanding the category. Instead, the Administrator asserted “broad discretion to determine appropriate combinations of compounds that should be treated as a single air pollutant.” 74 FR 66537. In other words, under this interpretation of CAA section 202(a)(1), the only limit on our authority to regulate in response to global climate change is the exercise of reasonable discretion.\135\ The best reading of the statute, and the reading we restore in this final action, avoids this concern by giving the terms Congress selected their full and ordinary meaning.\136\

\135\ See Whitman v. Am. Trucking Ass'ns, 531 U.S. 457, 474 (2001) (“The idea that an agency can cure an unconstitutionally standardless delegation of power by declining to exercise some of that power seems to us internally contradictory. The very choice of which portion of the power to exercise--that is to say, the prescription of the standard that Congress had omitted--would itself be an exercise of the forbidden legislative authority.”).

\136\ See Feliciano v. DOT, 605 U.S. 38, 55 n.6 (2025) (recognizing that “considerations of constitutional avoidance might counsel in favor of a narrowing construction of certain laws”); Crowell v. Benson, 285 U.S. 22, 62 (1932) (summarizing constitutional avoidance principles); Hignell-Start v. City of New Orleans, 154 F.4th 353, 360 (5th Cir. 2025) (accepting city's interpretation of an ordinance that avoided constitutional problems).

Under the logic of the Endangerment Finding, water vapor (H2O) emissions from vehicles and engines could meet the standard for regulation because the presence of additional water from all human activities around the world can be said to contribute to water-based disasters. See 74 FR 66520. The EPA would have the authority, and statutory duty, to prescribe standards for water vapor that would then trigger various permitting obligations--indeed, water is a recognized GHG, albeit one the EPA declined to regulate on a discretionary basis in 2009. Nor does this logic recognize any statutory limits to regulating pollutants under the global climate change concerns reading of CAA section 202(a)(1) that are addressed more specifically by other provisions of the statute, including black carbon (a form of the criteria pollutant PM), ground-level ozone (formed by the criteria pollutant NOx), and ozone-depleting substances (including those specifically addressed by Title VI and the Montreal Protocol). The Administrator declined to include these matters in the six “well-mixed” GHGs encompassed within the Endangerment Finding but remained open to future actions treating them as a climate issue. Because that reading effectively converts CAA section 202(a)(1) into a roaming license to “prescribe . . . standards,” the reading finalized in this action is more faithful to the governing principles of statutory interpretation.

The EPA is also finalizing that the futility of GHG emission standards in addressing the adverse health and welfare impacts predicted in the Endangerment Finding support this interpretation of CAA section 202(a)(1). At proposal, we sought comment on whether the EPA must consider the potential impact of regulation when applying CAA section 202(a)(1) and, if so, how this interpretation should inform any final action. We received significant comments on the efficacy of the EPA's GHG emission standards to date, particularly with respect to their limited impact on projected trends in GMST and GSLR and the relevance of the impacts of regulation on the interpretation of CAA section 202(a)(1). As discussed further in section V.C of this preamble, we conclude that even the complete elimination of GHG emissions from all new and existing LD, MD, and HD vehicles would have a de minimis impact on these values as a proxy for adverse health and welfare impacts. When accounting for the emissions reduction potential of GHG emission standards and their application only to new vehicles and engines, the de minimis nature of these impacts becomes even clearer. The trivial impacts of eliminating GHG emissions on trends in GMST and GSLR--which are less than one percent of the projected changes through 2050 and 2100 once the nature of the GHG emission standards are taken into account--are squarely in line with regulatory and judicial precedents treating values of approximately one percent or more as de minimis.

Courts have long recognized the “background” legal principle “against

which all enactments are adopted” that general language does not encompass de minimis concerns. Wis. Dep't of Rev. v. William Wrigley Jr., Co., 505 U.S. 214, 231 (1992); see UARG, 573 U.S. at 309 n.1. Unless the statute provides otherwise, agencies have implied authority to exempt de minimis concerns “when the burdens of regulation yield a gain of trivial or no value.” Ala. Power Co. v. Costle, 636 F.2d 323, 360-61 (D.C. Cir. 1979). This conclusion informs our interpretation of CAA section 202(a)(1) by suggesting that the provision does not encompass the attenuated chain of causation required to invoke the authority to regulate GHG emissions where regulations cannot have more than a trivial impact on the identified dangers to health and welfare. Nothing in the statutory language suggests that Congress intended to overcome this background principle, and the both the Supreme Court and the D.C. Circuit have recognized its applicability in comparable environmental contexts.\137\ Put another way, the inability of new motor vehicle and engine GHG emission standards to have any material impact on the global climate change concerns relied upon by the Agency in the 2009 Endangerment Finding suggests that it is unreasonable to conclude that GHG emissions from new motor vehicles and engines cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare. For further discussion, see section V.C of this preamble and the Response to Comments document.

\137\ See UARG, 573 U.S. at 309 n.1; Ala. Power, 636 F.2d at 360-61; see also EPA v. EME Homer City Generation, L.P., 572 U.S. 489 (2014) (approving of approach that did not require additional emissions reductions from States that contributed trivially to nonattainment in other States); Ohio v. EPA, 997 F.2d 1520, 1534-35 (D.C. Cir. 1993) (accepting de minimis approach to CERCLA five-year risk reviews because the statute did not clearly prohibit the approach and anything less would be contrary to legislative design).

Finalizing this interpretation effectively returns the EPA to its longstanding practice prior to 2009 of applying CAA section 202(a)(1) and related statutory endangerment provisions to air pollution that adversely impacts public health and welfare through local or regional exposure. As discussed further in sections III.A and V.B of this preamble, we historically utilized this authority on a relatively infrequent basis to prescribe standards for pollutants identified in the CAA itself, including NOX, PM, HCs and other VOCs, and CO, and then only as a backstop when more specific CAA section 202 authorities were unavailable. The distinction between air pollution that harms public health and welfare through local and regional exposure and global “air pollution” consisting of GHG concentrations without any such direct impacts also played a role in our evaluation of waiver requests under CAA section 209.\138\ Even in the Endangerment Finding, the Administrator recognized that “[n]one” of the identified health impacts were “associated with direct exposure” and that we had previously applied CAA section 202(a)(1) to the “more typical local or regional air pollution problem.” 74 FR 66527, 66538 (emphases added); see 74 FR 66531 (explaining that the Agency considered the same causal “pathways” in assessing public health and welfare impacts). In adopting a novel analytical approach in the Endangerment Finding, we failed to adequately address this prior practice and improperly relied on the Supreme Court's decision in Massachusetts for the proposition that CAA section 202(a)(1) authorizes emission standards in response to air pollution raising global climate change concerns. As discussed below, Massachusetts did not separately construe the scope of the EPA's authority to regulate under CAA section 202(a)(1), and the Court has since made clear in UARG and West Virginia that our authority to regulate an “air pollutant” encompassed within the Act-wide definition must be evaluated in the context of the particular statutory provision that confers authority to regulate.

\138\ See, e.g., “California State Motor Vehicle Pollution Control Standards; Notice of Decision Denying a Waiver of Clean Air Act Preemption for California's 2009 and Subsequent Model Year Greenhouse Gas Emission Standards for New Motor Vehicles,” 73 FR 12156, 12161 (Mar. 6, 2008) (denying California's waiver request for GHG emission standards on the ground that “the different, and global, nature of the pollution at issue” requires a different conceptual approach); see also “The Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule Part One: One National Program,” 84 FR 51310, 51328-52 (Sept. 27, 2019) (summarizing and applying this interpretation).

In Massachusetts, the Supreme Court rejected the argument that GHGs are not “air pollutants” under the Act-wide definition, reasoning that CAA section 302(g)'s use of the word “any” in connection with “air pollutant agent or combination of such agents, including any physical [or] chemical . . . substance” was sufficiently broad to encapsulate the combination of GHGs at issue. 549 U.S. at 530. On this basis, the Court stated that the EPA “has the statutory authority to regulate the emission of such gases from new motor vehicles.” Id. at 532. The Court did not, however, separately decide whether including GHGs within the definition of “air pollutant” meant that we must find that GHGs meet the statutory standard for regulation under CAA section 202(a) because they cause or contribute to air pollution which endangers the public health or welfare. Rather, the Court emphasized that its review of the denial of the rulemaking petition was “extremely limited” and concluded its opinion by clarifying that it “need not and do[es] not reach the question whether on remand EPA must make an endangerment finding.” Id. at 527, 534.

Consistent with Massachusetts, and reading that decision in harmony with UARG, we interpret the CAA as setting out a broad, threshold definition of “air pollutant” on an Act-wide basis that must be interpreted in the context of each applicable, particular provision granting regulatory authority in order to determine whether that provision authorizes the EPA to regulate an air pollutant under that particular authority. For purposes of CAA section 202(a)(1), that means that even if GHGs are “air pollutant[s]” as defined on an Act-wide basis, they must meet the statutory standard for regulating emissions from new motor vehicles and engines before we may invoke our regulatory authority. Put simply, regardless whether GHGs are “air pollutants” as defined in CAA section 302(g), they must satisfy the same standard as any other emitted “air pollutant” by causing or contributing to “air pollution which may reasonably be anticipated to endanger public health or welfare.”

This understanding is necessary to account for UARG, in which the Supreme Court distinguished between “the Act-wide definition” of air pollutant and the application of that definition to the Act's regulatory provisions. 573 U.S. at 320. The Court specifically addressed the holding in Massachusetts, adopting the argument that “while Massachusetts rejected EPA's categorical contention that [GHGs] could not be air pollutants for any purposes of the Act, it did not embrace EPA's [then] current, equally categorical position that [GHGs] must be air pollutants for all purposes regardless of the statutory context.” Id. (cleaned up).

In sum, CAA section 202(a)(1) does not provide authority to regulate GHGs based on global climate change concerns because that provision authorizes regulating only emissions that “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” The EPA must “ground its reasons for action or inaction in the statute,” Massachusetts, 549 U.S. at 535, and

“possess[es] only the authority that Congress has provided,” NFIB v. DOL, 595 U.S. 109, 117 (2022). In finalizing this interpretation, we note that our actions must be consistent with “the single, best meaning” of the statute, “ `fixed at the time of enactment”' and resolved through application of “all relevant interpretive tools,” and cannot expand our authority in response to pressing concerns based on statutory silence or ambiguity. Loper Bright, 603 U.S. at 400, 411 (quoting Wis. Cent., 585 U.S. at 284). Properly interpreted, the statute confers “regulatory flexibility” to respond to “changing circumstances and scientific developments,” Massachusetts, 549 U.S. at 532, while bounding the scope of the EPA's authority to “air pollution” as that term was understood at the time of enactment.

Findings and Standards. The EPA is also finalizing as proposed that CAA section 202(a)(1) requires issuing emission standards together with the findings necessary to invoke our regulatory authority, rather than severing the regulatory action into separate endangerment and standards-setting proceedings. The statute begins by providing that the Administrator “shall prescribe . . . standards applicable to the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines,” and follows this requirement by describing the scope of the duty to regulate air pollutant emissions “which, in his judgment cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” The best reading of the statute requires the Administrator, when prescribing any emission standard for new motor vehicles or engines, to find that the air pollutant or air pollutants emitted by the class or classes of new motor vehicles or engines subject to the standard cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare.

The Endangerment Finding severed this statutory language by finding endangerment and contribution in the abstract for all potential CAA section 202 sources with respect to GHGs. In so doing, the Administrator vastly increased the Agency's authority by removing the restrictions Congress placed on the issuance of emission standards. As a result of this new conception of authority, the EPA may issue a single endangerment finding in the abstract with respect to emissions from all sources potentially subject to CAA section 202 (and their existing-source counterparts) without addressing the danger posed by any particular source category or the causal role of that particular source category in any identified danger. The EPA relied on the Endangerment Finding to prescribe emission standards for various classes of new motor vehicles and engines, as well as a variety of other sources under distinct statutory authorities, without making the requisite findings or assessment of factors necessary to regulate the sources in question.\139\ Congress enacted CAA section 202(a)(1) as an integrated regulatory provision for a reason, and giving effect to the language of the statute requires the issuance of emission standards only when the Administrator has made an integrated finding of both endangerment and cause or contribution. Put another way, it is impermissible for the Administrator to make findings that trigger a duty to regulate without prescribing the emission standards required in response to such a finding, just as the Administrator may not prescribe emission standards without making the findings required by the statute.

\139\ See sections III.D and VII of this preamble for a summary of the EPA's rulemaking activities in response to the Endangerment Finding.

This interpretation is consistent with the EPA's implementation of CAA section 202(a)(1) and similar provisions of the CAA prior to 2009. In the Endangerment Finding, the Administrator acknowledged that “typically endangerment and cause or contribute findings have been proposed concurrently with proposed standards under various sections of the CAA, including CAA section 201(a).” 74 FR 66501. That has also been our approach to other similarly worded provisions in the statute, including in response to petitions seeking findings and action under CAA section 115.\140\ We believe that our historical practice under CAA section 202(a)(1) reflects the better reading of the statute and is entitled to greater weight. As the Supreme Court explained in Loper Bright, such weight is “especially warranted when an Executive Branch interpretation was issued roughly contemporaneously with enactment of the statute and remained consistent over time.” 603 U.S. at 386.

\140\ 42 U.S.C. 7415(a); see Her Majesty the Queen v. EPA, 912 F.2d 1525, 1533-34 (D.C. Cir. 1990) (deferring to the EPA's interpretation of CAA section 115(a) as requiring an integrated action because the statute's text and structure “creates a specific linkage between the endangerment finding and the remedial procedures”).

In departing from the EPA's historical practice in the Endangerment Finding, the Administrator reasoned that “[t]he text of CAA section 202(a) is silent on this issue” and “invoked the procedural discretion that is provided by CAA section 202(a)'s lack of specific direction.” 74 FR 66501. We no longer maintain that CAA section 202(a)(1) is silent on the issue, as the statute sets out an integrated process that requires the EPA to prescribe standards when the Administrator finds certain conditions are met. When Congress intends a multi-step inquiry in the environmental context, it typically says so expressly. In the NAAQS program, for example, the CAA separates our authority to establish air quality criteria under CAA section 108 from our obligation to promulgate and revise NAAQS based on the criteria under CAA section 109, in addition to separating both of these regulatory steps from our duties to implement the NAAQS by reviewing State Implementation Plans (SIPs) or promulgating Federal Implementation Plans (FIPs) under CAA section 110 and related statutory provisions.\141\ A particularly relevant analogy is Clean Water Act section 303(c)(4), which pairs the Administrator's authority to “determin[e] that a revised or new [water quality standard] is necessary to meet the requirements of this chapter” with the requirement that the Administrator “shall promptly prepare and publish proposed regulations” after making such a determination and “promulgate any revised or new standard . . . not later than ninety days after he publishes such proposed standards.” \142\ Even if CAA section 202(a)(1) were ambiguous or silent in this respect, agencies may no longer assert delegated discretionary authority when the statute is amenable to a single, best reading under ordinary tools of statutory interpretation. As the Supreme Court held in Loper Bright, “statutory ambiguity . . . is not a reliable indicator of actual delegation of discretionary authority to agencies.” 603 U.S. at 411.

\141\ See 42 U.S.C. 7408, 7409, 7410.

\142\ 33 U.S.C. 1313(c)(4), (c)(4)(B). Various provisions of the SDWA and the Toxic Substances Control Act (TSCA) similarly articulate multi-step processes for determining risk and addressing risk through regulation using language that Congress did not include in CAA section 202. See, e.g., NRDC, 67 F.4th at 398-402 (discussing the two-step process for promulgating national primary drinking water regulations under SDWA section 1412).

Severing the EPA's standards-setting authority from the findings that trigger a duty to exercise that authority shaped the analysis in the Endangerment Finding in a manner that ran counter to the statute. The Endangerment Finding first projected adverse public health and welfare impacts of global climate change and attributed those adverse impacts to

all manmade sources of GHG emission around the world and then, separately, used data from existing CAA section 202(a) sources in the United States to find that new motor vehicles and engines in the United States contributed to global GHG air pollution. The Administrator treated adaptation (adjustments to the effect of climate change that lessen impacts) and mitigation (reductions in emissions and global GHG concentrations unrelated to CAA section 202(a)(1) regulation) as outside the scope. 74 FR 66512. Moreover, the Administrator declined to consider cost, asserting that the Endangerment Finding imposed no regulatory requirements as a standalone action and relying on the Supreme Court's decision in Whitman v. American Trucking Associations, 531 U.S. 457 (2001), that the EPA cannot consider cost in setting the NAAQS under CAA section 109(b)(1). 74 FR 66515. Nor did the Administrator consider potential beneficial impacts from climate change with respect to whether and which standards would be appropriate. See 74 FR 66524 (purporting to compare “risks and benefits” only with respect to endangerment).

Severance also shaped all subsequent standards prescribed and revised in reliance on the Endangerment Finding in a manner we now conclude was unlawful. The EPA asserted in subsequent rulemakings that there was no need to make particularized findings for the relevant source category because the Endangerment Finding identified public health and welfare dangers and contribution for all CAA section 202 source categories. Nor did we consider the impacts of adaptation or mitigation when prescribing standards--considerations that the Endangerment Finding also treated as out of scope. As a result, the decision to sever meant that the EPA has never meaningfully considered or invited public comments on the cost, effectiveness, and continued propriety of its GHG regulatory program.

These considerations should have been taken into account when the EPA triggered a duty to regulate in the Endangerment Finding by invoking our CAA section 202(a)(1) authority. CAA section 202(a)(2) expressly provides that “[a]ny regulation prescribed under paragraph (1) of this subsection . . . shall” provide adequate time for “the development and application of the requisite technology, giving appropriate consideration to the cost of compliance within such period.” \143\ CAA section 202(a)(1) authorizes the Administrator to “by regulation prescribe” standards “in accordance with the provisions of this section” and does not separately authorize standalone findings, meaning any action taken “under paragraph (1) of this subsection” is subject to the considerations in paragraph (2). In addition, the Supreme Court explained in Michigan that “agency action is lawful only if it rests `on a consideration of the relevant factors,' ” 576 U.S. at 750 (quoting State Farm, 463 U.S. at 43), including “at least some attention to cost,” id. at 752.

\143\ 42 U.S.C. 7521(a)(2).

Accordingly, we now conclude that the Administrator erred in analogizing the NAAQS program and the Supreme Court's decision in Whitman to avoid considering costs in the Endangerment Finding. Unlike CAA section 202(a)(1), the language in CAA section 109(b)(1) makes no reference to cost or implementation and focuses solely on the protection of public health. Nor does CAA section 109(b) include the lead time and technical feasibility concepts embedded in CAA section 202(a). And whereas CAA section 202(a)(1) sets out an integrated authority to prescribe emission standards when the provision's triggering condition is satisfied, CAA section 109(b)(1) uses mandatory language requiring the EPA to establish certain standards, the content and implementation of which are specified in various provisions throughout Title I of the Act. We further note that the Supreme Court's decision in Massachusetts did not address the question whether the EPA could issue standalone findings or bar the Administrator from taking cost and implementation concerns into account when exercising CAA section 202(a) authority. Rather, Massachusetts must be read together with Michigan, and the language of CAA section 202(a)(1) must be read in context to “produc[e] a substantive effect that is compatible with the rest of the law.” UARG, 573 U.S. at 321 (quoting United Sav. Ass'n of Tex. v. Timbers of Inwood Forest Assocs., 484 U.S. 365, 371 (1988)).

Endangerment and Cause or Contribute. The EPA is also finalizing as proposed that CAA section 202(a)(1) requires the Agency to evaluate whether source emissions cause or contribute to air pollution and whether that air pollution poses endangerment in a single causal chain, rather than considering these issues in isolation by severing the inquiries. The relevant inquiry is whether “the emission of any air pollutant from any class or classes of new motor vehicles or new motor vehicle engines,” in the judgment of the Administrator, “cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare.” As explained in this section, the emission must cause or contribute to the danger posed by the air pollution to a sufficient extent to satisfy the standard for regulation.

In the Endangerment Finding, the Administrator made two distinct findings based on two distinct sets of assumptions. In the first, the Administrator found that the “air pollution,” defined as the combined global concentrations in the upper atmosphere of six “well-mixed GHGs,” CO2, methane, N2O, HFCs, PFCs, and SF6, endangered public health or welfare by playing a causal role in global temperature increases, sea level rise, and other phenomena (including ocean pH changes), which, in turn, were then asserted to play a causal role in environmental phenomena with adverse impacts on public health and welfare. 74 FR 66516. In the second, the Administrator found that the quantity of the “air pollutant” (defined as the combination of same six “well-mixed GHGs”) emitted by new motor vehicles and engines annually contributed to the “air pollution.” 74 FR 66536. The Administrator did not consider the extent to which emissions from CAA section 202(a)(1) sources have a more than de minimis effect on the danger identified with respect to elevated concentrations of GHGs in the upper atmosphere--let alone whether emissions from any particular class or classes of sources that the EPA intended to regulate had such an effect. Nor did the Administrator recognize the mismatch between “air pollution” consisting of global concentrations formed by GHG emissions past, present, and future and “air pollutant” emissions from new motor vehicles and engines on an annual basis, or the problems associated with measuring domestic contribution against an air pollution problem that necessarily requires global emissions to result in the identified danger.

Upon review, we no longer believe that the approach taken in the Endangerment Finding was consistent with the language of CAA section 202(a)(1) and the structure of the CAA, which requires making distinct findings for regulating distinct types of emission sources and authorizing different regulatory tools when such standards are met. For example, CAA section 111(b)(1)(A) authorizes the EPA to regulate emissions from listed categories of stationary sources if the Administrator determines those sources emit air pollutants that “significantly contribute” to air pollution that

endangers public health or welfare.\144\ When that standard is met, CAA section 111(b)(1)(B) requires the EPA to regulate such emissions from such sources by setting standards of performance that, among other things, reflect the best system of emission reduction that has been adequately demonstrated in practice.\145\ The CAA similarly sets out distinct standards for regulating and distinct modes of regulation for additional major source categories, including vehicles in use, aircraft engines, and separately addresses when and how to respond to international emissions that impact the United States. The Endangerment Finding effectively attributed the total GHG emissions coming from all of these various distinct sources within the United States, as well as from all international sources, to the mobile sources regulated under CAA section 202 without having made the requisite determinations for any of those sources and without considering the different regulatory tools Congress authorized for those sources as compared to CAA section 202(a) sources. Although the statute anticipates that “air pollution” may reflect contributions from multiple source categories, application of the global climate change concerns reading of CAA section 202(a)(1) leads to impermissible gaps between the contribution and endangerment analyses that the Endangerment Finding failed to address.

\144\ 42 U.S.C. 7411(b)(1)(A).

\145\ 42 U.S.C. 7411(a)(1), (b)(1)(B). CAA section 111 also differentiates between new and existing stationary sources in a listed source category and limits the EPA's role with respect to existing sources by authorizing only emission guidelines implemented by the States. See id. 7411(d).

Whereas the identified “air pollution” leads to endangerment because of the sum total of all emissions, past, current, and projected, from all source categories foreign and domestic, the identified contribution of “air pollutant emissions” from new motor vehicles and engines was measured in annual terms. In other words, the Endangerment Finding compared the wrong figures in tying contribution to endangerment. The Administrator found contribution based on the conclusion that existing vehicles and engines constituted 4.3 percent of annual global GHG emissions. But the Administrator found endangerment based on the theory that “air pollution” consisting of total global concentrations of the six “well-mixed” GHGs endangered public health and welfare. This mismatch is not presented when analyzing the air pollution addressed expressly by the CAA because the mechanism of harm does not depend on centuries-long time horizons. Annual emissions of airborne lead, for example, are readily measurable against the total annual concentrations of airborne lead in areas of concern, and the health and welfare impacts of air pollution in the form of airborne lead can be analyzed on the same scale. By completely severing the contribution and endangerment analyses for the six “well- mixed” GHGs, the Endangerment Finding avoided grappling with this disconnect. The difficulties in analyzing the nexus between contribution and endangerment was not a problem to be avoided, but a further reason to conclude that CAA section 202(a)(1) was not designed to address global climate change concerns.

The Administrator also defined the relevant “air pollution” as the combined global concentration of six “well-mixed GHGs” but found that CAA section 202(a) sources emitted only four of them: CO2, methane, NOX, and HFCs. 74 FR 66538. As a result, the “air pollution” identified as endangering public health or welfare included PFCs and SF6, and the “air pollution” used to conclude that CAA section 202(a) sources satisfy the regulatory standard did not. Contrary to the EPA's conclusion at the time, 74 FR 66541, that difference is material, as PFCs and SF6 are asserted to have many times the global warming potential of CO2.\146\ Severing the endangerment and cause-or-contribute analysis allowed the Agency to compare apples and oranges in a manner inconsistent with the best reading of the statute.

\146\ U.S. Environmental Protection Agency. (Last updated Jan. 16, 2025). Understanding Global Warming Potentials: https://www.epa.gov/ghgemissions/understanding-global-warming-potentials.

The Endangerment Finding also did not limit the analysis of contribution to “new motor vehicles or new motor vehicle engines” in the United States, which are the only sources covered by the EPA's CAA section 202(a) authority.\147\ Because the Administrator considered all sources in analyzing the danger posed by elevated concentrations of GHGs in the upper atmosphere, the endangerment analysis necessarily included emissions from foreign and domestic vehicles that had been in use for years or decades and were not “new.” Even when analyzing contribution, the Administrator used emission estimates from “the entire fleet of motor vehicles in the United States for a certain calendar year” rather than projecting emissions from new motor vehicles and engines over time. 74 FR 66543. That decision increased the absolute contribution figure by orders of magnitude, including because newer vehicles and engines tend to be more efficient and emit less.\148\ Difficulties in disaggregating emission data from emission sources, however reasonable, do not license us to read the term “new” out of the statutory text.

\147\ 42 U.S.C. 7521(a)(1) (emphases added); see, e.g., City of New York v. Chevron Corp., 993 F.3d 81, 101 (2d Cir. 2021) (“Together, the statute's silence on the issue of extraterritorial reach, the fact that the Act contemplates the need for reciprocal protections from foreign nations, and the State Department's lead role in setting foreign policy on environmental matters, all plainly demonstrate that the Clean Air Act regulates only domestic emissions.”).

\148\ For additional discussion of improvements in new motor vehicles and engines relative to older vehicles and engines, see section VI.D of the preamble to the proposed rule.

We further conclude that severing the endangerment and cause or contribution findings leads to untenable results and lacks any limiting principle. To illustrate the problem, the same logic would allow the EPA to issue emission standards for water vapor (H2O), another substance emitted by new motor vehicles and engines that is also considered a GHG. Considered in isolation, increased H2O concentrations in the atmosphere from all human activities can be said to endanger public health or welfare by resulting in rain that leads to slip-and-fall injuries, drownings, and damage to crops, livestock, and property, including through pools, rivers, and floodwater, although water vapor is not itself harmful and is necessary to sustain life. Also considered in isolation, CAA section 202 sources can be said to “contribute” to elevated H2O concentrations in the atmosphere from all anthropogenic sources, and these emissions of water vapor would thereby assertedly “contribute” to global climate effects similar to those attributed to other GHGs. CAA section 202(a)(1) does not contemplate prescribing emission standards for such an omnipresent, naturally occurring, and essential component of the ambient air because the text requires a unified analysis that ensures a nexus between the extent of contribution and the resulting danger. The logic of regulating water vapor appears absurd, but it is the same logic required to regulate GHGs under CAA section 202(a)(1). And the Administrator acknowledged in the Endangerment Finding that the statutory interpretation adopted in that action could support adding water vapor to the defined regulatory for “climate forcing” GHGs.

The decision to sever the analysis of endangerment from the analysis of contribution, combined with the decision to sever the Administrator's

findings from any standards prescribed as a result, produced an analysis that is incompatible with the statute. In the Endangerment Finding, the Administrator concluded that anything more than a trivial or de minimis contribution to elevated global GHG concentrations by CAA section 202(a) sources was sufficient to trigger regulation because the “unique, global aspects of the climate change problem tend to support contribution at lower percentage levels of emissions than might otherwise be considered appropriate when addressing a more typical local or regional air pollution problem.” 74 FR 66538. Because the Endangerment Finding did not consider the standards that the statute requires when the Administrator makes such a finding, we did not consider whether emission standards for new motor vehicles would be futile as a means to address the identified dangers of GHG emissions from all anthropogenic sources. As discussed in section V.C of this preamble, available modeling indicates that reducing GHG emissions from all vehicles and engines in the United States to zero would not have a measurable, material impact on trends in global temperature or sea level. Because our GHG emission standards apply only to new vehicles and engines and have not, to date, mandated the elimination of all emissions, their impact is only a fraction of the already de minimis impacts identified in the modelled scenario. It was foreseeable at the time that issuing the Endangerment Finding would trigger a duty to regulate and that stringent measures would be necessary under all of the EPA's separate statutory authorities, and not just CAA section 202(a), to have any potentially material impact on the identified harm. Refusing to consider these foreseeable consequences was inconsistent with the statutory scheme and, as explained further below, an unreasonable exercise of the authority we asserted.

Finally, the Administrator did not adequately consider the meaning in context of the statutory term “endanger” and failed to identify with sufficient rigor the purported danger linked to GHG emissions from new motor vehicles and engines. As used in CAA section 202(a)(1), “endanger” is not best read as meaning any predicted negative impact to any public health or welfare value, as that interpretation would render the constraint placed on the EPA's authority to prescribe standards essentially meaningless, thereby violating ordinary principles of statutory interpretation and raising constitutional nondelegation concerns. Severing the endangerment and contribution inquiries improperly allowed the Administrator to avoid this concern by concluding that new motor vehicle and engine emissions included more than de minimis GHG emissions, even if those emissions did not themselves contribute to a danger in any meaningful sense. See 74 FR 66543 (asserting that “contributors must do their part even if their contributions to the global problem, measured in terms of percentage, are smaller than typically encountered”). 2. Summary of Comments and Updates Since Proposal

The EPA received comments from a variety of stakeholders supporting and criticizing the legal rationale set out in the proposed rule. Commenters supporting the rescission and repeals pointed to the Supreme Court's decisions in West Virginia, UARG, and Loper Bright as strongly supportive of what we proposed to be the best reading of CAA section 202(a)(1) and generally agreed that the Endangerment Finding erred in severing the statutory analysis in various ways. Commenters opposing the rescission and repeals generally argued that the Supreme Court's decision in Massachusetts and several subsequent precedents must be read as requiring the EPA to regulate GHG emissions and that the statute must be interpreted broadly to accomplish what they described as the preventative purposes of the statute. The final rationale set out in the preceding section of this preamble reflects this input by including certain interpretive evidence identified by commenters and additional analysis developed in response to arguments raised during the public comment period. In this subsection, we summarize major themes presented in the comments received along with our high-level responses. For detailed comment summaries and our full responses thereto, please see the Response to Comments document in the docket for this rulemaking.

Comment: Commenters supportive of the proposal generally agreed that the EPA exceeded its statutory authority under CAA section 202(a)(1) by issuing the Endangerment Finding and resulting standards. Some of these commenters emphasized agreement with our proposed interpretation of the term “air pollution” and the role that term plays in the provision, while others further agreed with our proposed understanding of the nature of the statutory analysis and the ways in which the Endangerment Finding erred in severing the analysis.

With respect to “air pollution,” commenters offered additional legislative history, regulatory history, or other support for interpreting the term as referring to pollution that adversely impacts health or welfare through local or regional exposure, such as smog. Several commenters recounted the air pollution concerns leading up to the 1965, 1970, and 1977 enactments in particular and emphasized that Congress and the public understood the problem in terms of increased urbanization, including in cities that crossed over State lines and made pollution control strategies by individual States and localities difficult with respect to mobile sources. These commenters provided further evidence in contemporary legislative history and other public materials that Congress understood the national air pollution problem being addressed in legislation as one related to criteria pollutants that lead to smog, primarily in urban areas, as well as air toxics. Several also pointed to additional provisions of the CAA, including general statements of purpose and the structure of the statute as a whole, to argue that Congress designed a regulatory scheme for regulating domestic emissions and domestic impacts in a manner that does not contemplate or authorize regulation in response to global climate change concerns. Several commenters also cited case law to argue that the CAA does not regulate extraterritorially. With respect to the ways in which the Endangerment Finding severed the statutory analysis, several commenters agreed that these considerations were relevant to statutory interpretation and authority as well as the quality or validity of the underlying analysis in the Endangerment Finding.

Response: The EPA agrees with these comments and is finalizing, as proposed, that the Endangerment Finding exceeded the Agency's statutory authority under CAA section 202(a)(1) in multiple respects. In addition to the further discussion incorporated into section V.A.1 of this preamble, we agree that viewed as a whole, the legislative history and other materials contemporary to the 1965, 1970, and 1977 enactments most relevant to interpreting the key statutory language in CAA section 202(a)(1) tend to undermine the interpretation adopted in the Endangerment Finding and support the interpretation we are finalizing in this action. While legislative history cannot trump the statutory text, widely publicized materials and evidence of common understanding at the time of enactment can be relevant to the

ordinary meaning of undefined terms. Here, that material supports the conclusion that “air pollution” as used in CAA section 202(a)(1) meant pollution that harms public health or welfare through local or regional exposure, rather than gases that are not harmful in that sense but may contribute to global phenomena on a far more attenuated chain of causation. We further agree that other provisions of the statute, including the findings and declarations of purpose in CAA section 101, support the interpretation finalized in this action by indicating that while Congress referenced and addressed local and regional problems, it did not reference global climate change concerns at all through the 1970s and even today uses express terms in the relatively few provisions that address GHGs, such as in the RFS and provisions authorizing certain grants and financial or technical assistance.

Comment: Adverse commenters argued that the EPA's proposed interpretation of CAA section 202(a)(1) is foreclosed in whole or in part by precedent. Many of those commenters argued that the Supreme Court's decision in Massachusetts unambiguously held that the EPA has authority to prescribe GHG emission standards for new motor vehicles and engines in response to global climate change concerns. Others also cited to subsequent cases, including the Supreme Court's decisions in American Electric Power Co. v. Connecticut, 564 U.S. 410, 426 (2011), UARG, and West Virginia, as well as the D.C. Circuit's decisions in Coalition for Responsible Regulation and American Lung Association, as individually or collectively precluding the EPA from evaluating and applying the best reading of CAA section 202(a)(1) and related provisions.

Response: The EPA disagrees with these comments, many of which significantly overread relevant precedent and misunderstand principles governing the scope of judicial decisions and statutory interpretation. Fundamentally, commenters' arguments stem from the flawed proposition that the Supreme Court held in Massachusetts that the EPA can or must regulate GHG emissions from new motor vehicles and engines in response to global climate change concerns. As detailed in section V.A.1 of this preamble, we no longer believe that this reading is accurate on its own terms, nor does it reflect the Court's subsequent holdings and rationale in UARG, West Virginia, and, more generally, Michigan and Loper Bright. The Court in Massachusetts rejected the policy reasons the Agency offered for declining to regulate and the interpretation of the statutory definition of “air pollutant” in CAA section 302(g) that the Agency relied upon to deny petitions for rulemaking in 2003. Contrary to the framing presented by some commenters, the Court found that the statute “foreclose[d]” the Agency's reading and is “unambiguous” only with respect to the “air pollutant” definition, holding that “the definition embraces all airborne compounds of whatever stripe.” 549 U.S. at 529 (citing 42 U.S.C. 7602(g)). Nor do commenters offer persuasive reasons to conclude that the Court's subsequent decision in UARG, which held that the term “air pollutant” as defined in the statute and construed in Massachusetts must be read in context of the regulatory provision in which it appears, applies to the entirety of the CAA except for CAA section 202(a)(1). 573 U.S. at 318-20 (“[Massachusetts] did not hold that EPA must always regulate [GHGs] as an `air pollutant' everywhere that term appears in the statute, but only that EPA must `ground its reasons for action or inaction in the statute,' rather than on `reasoning divorced from the statutory text.' ” (quoting 549 U.S. at 532, 535)).

Similarly, we disagree with commenters' suggestions that additional precedents since Massachusetts purported to decide the interpretive issues addressed in this final action. In American Electric Power, for example, the Supreme Court held that federal common law was not the appropriate avenue for deciding “whether and how to regulate carbon- dioxide emissions from powerplants.” 564 U.S. at 426. Indeed, the Court has since confirmed in West Virginia that it “said nothing about the ways in which Congress intended EPA to exercise its power” under the CAA, particularly with respect to the regulation of stationary sources under CAA section 111(d). 597 U.S. at 730. Commenters' attempt to repeat similar arguments for UARG and West Virginia lack credibility given the questions presented in those cases and the reasoning adopted by the Court with respect to the questions presented. These comments largely did not engage with the interpretation of “air pollution” presented at proposal and finalized in this action, and the relatively small number that did failed to offer persuasive evidence that rebuts the ordinary meaning of the term or relevant contextual or structural indicators in the statutory text. For additional discussion of these cases, the D.C. Circuit's decisions in Coalition for Responsible Regulation and American Lung Association, and other issues bearing on statutory interpretation, see the Response to Comments document.

In this final action, the EPA is acting consistently with Massachusetts by “ground[ing] its reasons for action or inaction in the statute” and concluding that, given the best reading of the language in CAA section 202(a)(1), we lack authority to issue an affirmative finding that triggers our regulatory authority in response to global climate change concerns. 549 U.S. at 535.

Comment: Adverse commenters also asserted that the EPA's proposed interpretation gave inadequate weight to the statutory terms “public health” and “welfare.” These commenters generally argued that Congress delegated broad authority to the EPA to regulate any air pollutant emissions in response to any air pollution that may arise in the future, so long as we conclude such regulation further public health or welfare. Several of these commenters focused particularly on the statutory definition of welfare in CAA section 302(g), and particularly on the term “climate,” to argue that Congress wrote these concepts into the statute to give the Agency such broad authority.

Response: The EPA disagrees that the references in CAA section 202(a)(1) to “public health” and “welfare” confer discretion broad enough to identify and regulate any form of air pollution, including in the form of global climate change concerns. As discussed in section V.A.1 of this preamble, that interpretation, which we acknowledge is consistent with the interpretation adopted in the Endangerment Finding, is inconsistent with ordinary principles of statutory interpretation and would needlessly give rise to absurdity and nondelegation concerns that the statute itself does not create, properly interpreted. With respect to the statutory definition of “welfare,” we note that the ordinary meaning of the term “climate” at the time of enactment is nowhere near as broad as commenters suggest and that the term, as well as additional terms in the definition such as “weather” and “visibility,” must be read in the context of a much broader list that consists of terms having the physical property of being local or regional. For additional discussion, see the detailed explanation of the term “welfare” and additional statutory terms informed by proximate cause principles, including “cause,” “contribute,” and “reasonably be anticipated to endanger,” in the Response to Comments document.

The EPA is also finalizing as proposed that, in addition to the basis set out above, we lack the “clear congressional authorization” required under the major questions doctrine to decide the Nation's response to global climate change concerns. West Virginia, 597 U.S. at 723 (quoting UARG, 573 U.S. at 324). In this subsection, we conclude that the major questions doctrine applies to the Endangerment Finding because the global climate change concerns addressed in that action, and the mandatory duty to regulate triggered by that action, present a major question of undeniable political and economic significance. Until 2009, we had never used CAA section 202(a)(1) to assert authority over an entirely new subject, instead hewing closely to the air pollution problems that Congress identified in CAA section 202. To break with this longstanding practice, we developed a “unique” framework that broadened our statutory authority to prescribe emission standards in response to air pollution far enough to encompass global climate change concerns. The result was a new policy direction for the United States-- one that Congress had repeatedly and recently declined to adopt--in which the EPA declared that every source and every nation must be required to “do their part” to combat global climate change. Implementation of the Endangerment Finding since 2009 has shown the extraordinary consequences of this assertion of authority, including an increasing trend toward forcing a shift from internal combustion engine (ICE) vehicles to EVs for virtually all classes of LD, MD, and HD vehicles.

Next, we conclude that Congress did not clearly authorize the EPA to decide this question when it empowered the Administrator to “prescribe . . . standards” for new motor vehicle and engine emissions under CAA section 202(a)(1). The general nature of the statutory text and the more specific authorities and commands throughout CAA section 202, as well as additional provisions throughout the CAA, leave no room for doubt that Congress knew how to, and did not, expressly authorize the regulation of vehicle and engine GHG emissions. On that basis, we determine that the Endangerment Finding and resulting GHG emission standards exceeded our statutory authority and must be rescinded. That conclusion follows from the Supreme Court's decisions in UARG and West Virginia and is consistent with Massachusetts, which held that GHGs fell within the definition of “air pollutant” but did not interpret the scope of our authority to regulate air pollutants that cause, or contribute to, air pollution which may reasonably be anticipated to endanger public health or welfare. 1. Final Rationale

Applicability of the Major Questions Doctrine. In recent decisions construing the scope of the EPA's statutory authority to regulate GHGs, the Supreme Court has emphasized that the “ `history and breadth of the authority' ” asserted by an agency and “the `economic and political significance' of that assertion” provide “ `a reason to hesitate before concluding that Congress' meant to confer such authority.” West Virginia, 597 U.S. at 721 (quoting Brown & Williamson, 529 U.S. at 159-60); accord UARG, 573 U.S. at 324. Whether viewed as an ordinary tool of statutory interpretation that looks to the structure of the regulatory scheme \149\ or a clear statement rule that implements nondelegation and separation of power principles,\150\ the major questions doctrine requires us to identify “more than a merely plausible textual basis” when asserting authority to decide a significant policy issue on Congress' behalf. Id. at 723.

\149\ Biden v. Nebraska, 600 U.S. 477, 507-21 (2023) (Barrett, J., concurring).

\150\ West Virginia, 597 U.S. at 735-51 (Gorsuch, J., concurring).

In UARG, the Supreme Court applied the major questions doctrine to reject our attempt to expand the number of stationary sources subject to the CAA's PSD and Title V permitting requirements based on their GHG emissions. 573 U.S. at 310-13.\151\ The Court held that the EPA had “exceeded its statutory authority when it interpreted the Clean Air Act to require PSD and Title V permitting for stationary sources based on their greenhouse gas emissions” and “may not treat greenhouse gases as a pollutant” in this PSD and Title V contexts. Id. at 333. In reaching this conclusion, the Court found that our interpretation of the statute and related “tailoring rule” that exempted many sources to address workability concerns was “unreasonable because it would bring about an enormous and transformative expansion in EPA's regulatory authority without clear congressional authorization.” Id. at 324. Citing earlier major questions doctrine precedents, the Court noted that “a measure of skepticism” is required when “an agency claims to discover in a long-extant statute an unheralded power to regulate `a significant portion of the American economy,' ” id. (quoting Brown & Williamson, 529 U.S. at 159), and that “[w]e expect Congress to speak clearly if it wishes to assign to an agency decisions of vast `economic and political significance,' ” id. (quoting Brown & Williamson, 529 U.S. at 160).

\151\ See 42 U.S.C. 7470-92, 7661 et seq.

In West Virginia, the Supreme Court again applied the major questions doctrine to reject our attempt to shift the power grid away from using fossil fuels through GHG emission guidelines for existing power plants under CAA section 111(d). 597 U.S. at 711-15.\152\ The Court noted that when interpreting a grant of regulatory authority, the inquiry includes the question “whether Congress in fact meant to confer the power the agency has asserted.” Id. at 721. The Court explained that the major questions doctrine applies when “the `history and breadth of the authority that [the agency] has asserted,' and the `economic and political significance' of that assertion, provide `a reason to hesitate before concluding that Congress' meant to confer such authority.” Id. (quoting Brown & Williamson, 529 U.S. at 159-60). In such cases, “both separation of powers principles and a practical understanding of legislative intent make us `reluctant to read into ambiguous statutory text' the delegation claimed to be lurking there,” and “[t]he agency instead must point to `clear congressional authorization' for the power it claims.” Id. at 723 (quoting UARG, 573 U.S. at 324). Applying that standard, the Court held that our statutory authority to establish emission limits under CAA section 111(a)(1) and (d) “is not close to the sort of clear authorization required by our precedents.” Id. at 732.

\152\ See 42 U.S.C. 7411(d). The EPA had also issued GHG performance standards for new and modified fossil fuel-fired power plants under CAA section 111(b) that triggered the Agency's authority to issue guidelines for existing sources under CAA section 111(d). The new source standards were not before the Supreme Court in West Virginia.

The Endangerment Finding implicates the major questions doctrine for many of the same reasons the Supreme Court applied it in UARG and West Virginia. By asserting authority to regulate in response to global climate change concerns, the EPA “ `claim[ed] to discover in a long- extant statute an unheralded power' representing a `transformative expansion in [its] regulatory authority.' ” West Virginia, 597 U.S. at 724 (quoting UARG, 573 U.S. at 324). From 1965 to 2009, we invoked CAA section 202(a)(1) consistent with the more specific direction provided elsewhere in section 202 regarding the

air pollution Congress intended the EPA to address under this authority. As noted in section III.A of this preamble, the 15 final rules we identified as invoking CAA section 202(a)(1) prescribed standards for air pollution problems enumerated in the statute, including HC and other VOCs, NOX, PM, and certain air toxics. Critically, Congress repeatedly amended the statute to instruct the EPA what, when, and how to regulate with respect to vehicle and engine emissions. For example, the 1970 CAA included instructions to regulate CO, HCs, and NOX under CAA section 202(a) now codified as amended in CAA section 202(b).\153\ The 1990 CAA amendments included additional instructions to regulate CO, certain HCs, NOX, and PM.\154\ These final rules carried out Congress' instruction to use CAA section 202 in particular ways and did not purport to use CAA section 202(a)(1) as a blanket authorization to explore new vistas on a discretionary basis.

\153\ Public Law 91-604, section 6, 84 Stat. 1676, 1691.

\154\ Public Law 101-549, section 203, 104 Stat. 2399, 2474.

Given this history, the novel use of CAA section 202(a)(1) in the Endangerment Finding is similar to the use of CAA section 111(d) addressed in West Virginia. There, the Supreme Court found that the EPA's use of the provision in a more limited fashion prior to the Clean Power Plan counseled in favor of applying the major questions doctrine, noting that “ `just as established practice may shed light on the extent of power conveyed by general statutory language, so the want of assertion of power by those who presumably would be alert to exercise it, is equally significant in determining whether such power was actually conferred.' ” 597 U.S. at 725 (quoting FTC v. Bunte Bros., Inc., 312 U.S. 349, 352 (1941)). We further note that the regulatory actions reviewed in UARG and West Virginia were predicated in part on the Endangerment Finding, and the PSD and Title V rules in UARG and existing source emission guidelines in West Virginia are similar in scope, approach, and economic impact as the GHG emission standards for new motor vehicles and engines promulgated to fulfill the mandatory duty triggered by the Endangerment Finding.

Moreover, as a consequence of the novel approach taken in the Endangerment Finding to endangerment and contribution, our GHG emission standards reflect an increasing trend toward mandating a shift from gasoline- and diesel-fueled vehicles to EVs on the theory that a substantial reduction in GHG emissions is necessary to address global climate change concerns.\155\ This trend was evident in our earliest GHG emission standards rulemakings and became increasingly clear over time as the standards increased in stringency to the point where alternative compliance options were increasingly infeasible or unattractive for regulated parties. The underlying policy of forcing such a transition is also evident from the Agency's statements and actions on related issues. For further discussion of relevant regulatory history and implementation details, both of which generated significant public input during the comment period, see the Response to Comments document in the docket for this rulemaking.

\155\ 89 FR 27842, 27844 (Apr. 18, 2024).

Mandating a shift in the national vehicle fleet from one type of vehicle to another is indistinguishable from the emission guidelines at issue in West Virginia, which were calculated to force a shift from one means of electricity generation to another. This increasing regulatory trend has borne out over time given the limits of using GHG emission control technologies applicable to new motor vehicles and engines that comport with the magnitude of the problem identified in the Endangerment Finding. As discussed later in this preamble, even eliminating all GHG emissions from all U.S. vehicles and engines would have only a de minimis impact on GMST and GSLR trends as a proxy for adverse health and welfare impacts. See section V.C of this preamble and the Response to Comments document for further discussion.

It is “ `highly unlikely that Congress would leave' to `agency discretion' the decision” whether and how many consumers and manufacturers in the United States may use the ICE in their vehicles. West Virginia, 597 U.S. at 729 (quoting MCI Telecomms. Corp. v. AT&T Co., 512 U.S. 218, 231 (1994)). As the Supreme Court noted with respect to coal-based electricity generation, such a policy decision involves “basic and consequential tradeoffs,” and “Congress certainly has not conferred a like authority upon EPA anywhere else in the Clean Air Act.” Id. Until the Endangerment Finding, we had never invoked CAA section 202(a)(1) to regulate in response to global climate change concerns, whether through a fuel-shifting strategy or any other means. That history is telling because although CAA section 202(a)(1) has existed in substantially similar form since 1967, “the EPA had never regulated in that manner, despite having issued many prior rules governing” vehicle and engine emissions. Id. When Congress intended the EPA to regulate the type of fuels that propel vehicles, it provided express and detailed authority to do so in other provisions. CAA section 211 authorizes the Agency to regulate fuel and fuel additives, including by requiring registration and controlling or prohibiting the manufacture, distribution, or sale of fuel or fuel additives if the Administrator determines that “any emission product of such fuel or fuel additive causes, or contributes, to air pollution or water pollution . . . that may reasonably be anticipated to endanger the public health or welfare” or significantly impair the performance of any generally used emission control device.\156\ Moreover, CAA section 211(o) sets out detailed requirements for the Agency's RFS program, which involves setting annual renewable fuel volume requirements applicable to refiners, blenders, distributors, and importers of transportation fuel.\157\ Both of these provisions, with respect to the Nation's policy approach to GHGs generally and transportation fuel specifically, indicate that Congress knows how to establish policy on the subject and has declined to empower the EPA to decide for itself whether and how to respond to global climate change concerns.

\156\ 42 U.S.C. 7545(a)-(c).

\157\ 42 U.S.C. 7545(o).

Both before and since the Endangerment Finding, “ `Congress considered and rejected' multiple times” legislation that would have authorized or required the EPA to regulate GHG emissions from vehicles, engines, and additional sources. West Virginia, 597 U.S. at 731 (quoting Brown & Williamson, 529 U.S. at 144). This history is particularly relevant because of the established pattern through the 1990 CAA amendments of Congress adding additional emissions control authority and obligations to CAA section 202. From 2007 to 2009, Congress considered legislation--supported by the President and Administrator in office at the time of the Endangerment Finding--that would have authorized or required the EPA to prescribe emissions regulations for GHGs. For example, the Safe Climate Act of 2007 would have adopted findings and policies with respect to limiting global temperature increase, required various forms of international cooperation, and added a new Title VII to the CAA instructing the EPA to achieve phased GHG emission reduction targets and regulate GHG emissions

under CAA section 202.\158\ Similarly, the American Clean Energy and Security Act of 2009 would have required international cooperation and added new titles to the CAA requiring the EPA to, among other things, regulate GHG emissions under CAA section 202.\159\ Neither bill was enacted through the legislative process, and Congress has since declined to adopt similar legislation.\160\

\158\ H.R. 1590, 110th Cong. (2007). This bill was presented in the House of Representatives and never received a vote.

\159\ H.R. 2454, 111th Cong. (2009). This bill, introduced on May 15, 2009--a month after the EPA proposed the Endangerment Finding--passed the House of Representatives on June 26, 2009, by a 219-212 margin but never received a vote in the Senate. The President and Administrator at the time expressed a strong preference for legislation but also a willingness to resolve legislative inaction by administrative means, and the Agency ultimately finalized the Endangerment Finding on December 7, 2009.

\160\ Congress's pattern of not providing the EPA such authority extends long before the 2009 Endangerment Finding as well. See Coal. for Responsible Regulation, 2012 U.S. App. LEXIS 25997, at * 36-37 (Brown, J., dissenting from denial of rh'g en banc) (noting Congress expressly rejected proposals offered during the drafting of the 1990 CAA Amendments that would have authorized the EPA to regulate GHGs).

When Congress has addressed GHGs individually or collectively, it has not granted the EPA broad regulatory authority to “prescribe . . . standards” under CAA section 202(a)(1). As noted above, Congress enacted the RFS program to promote energy independence while reducing GHG emissions through a detailed regulatory scheme. With respect to HFCs, Congress enacted a comprehensive phaseout scheme in the 2020 American Innovation and Manufacturing (AIM) Act, which includes detailed instructions, timelines, and requirements for implementation and allows some uses to continue under certain conditions.\161\ With respect to CO2, Congress opted for a carrot rather than a stick by authorizing a tax credit to incentivize underground sequestration that mitigates emissions.\162\ With respect to methane, Congress amended the CAA in 2021 through the Inflation Reduction Act of 2022 (IRA) to require us to establish a waste emissions charge for certain sources structured to incentivize emissions reductions over time.\163\ When addressing GHGs and global climate change concerns more generally, Congress has used non-regulatory tools that incentivize, rather than mandate, changes in manufacturing and consumer choice, including through additional funding provisions in the IRA.\164\ Multiple instances of recent legislation addressing GHGs individually and through distinct regulatory approaches suggests that Congress views such policy decisions as economically and politically significant and not adequately addressed by general statutory authorities enacted in response to different problems.

\161\ Public Law 116-260, Div. S, 134 Stat. 1182, 2255-71 (codified at 42 U.S.C. 7675 et seq.).

\162\ 26 U.S.C. 45Q. In 2020, Congress also instructed us to recommend improvements to SDWA permitting procedures for injection wells used in carbon sequestration and appropriated additional fundings for the “Class VI” permitting process. Public Law 116- 260, Div. G, Title II, 134 Stat. 1182, 1507-16.

\163\ Public Law 117-169, section 60113, 136 Stat. 1818, 2074 (codified at 42 U.S.C. 7436).

\164\ See, e.g., Public Law 117-169, sections 60101-03, 60107, 60114, 60201, 136 Stat. 1818, 2063-66, 2069, 2076, 2078 (codified at 42 U.S.C. 7432-35, 7437-38).

The EPA notes that Congress has continued to revise these air pollutant-specific measures and nonregulatory tools as part of an ongoing national debate over the appropriate response to global climate change concerns. On July 4, 2025, President Trump signed into law significant new legislation enacted by Congress, the One Big Beautiful Bill Act (OBBB),\165\ which repealed several relevant measures adopted in the IRA and rescinded the EPA's appropriations to carry out several funding programs related to GHG emissions. Among other things, Congress prohibited the Agency from collecting the waste emission charge for methane for ten years beyond the original statutory collection date, rescinded funding to administer grant programs in CAA sections 132 and 135-38, and repealed CAA section 134, which had included a section- specific definition of “greenhouse gas” applicable to the grant program set out in that section.\166\ This legislation, which was the product of substantial national debate and revised and rescinding funding for provisions of the IRA that were themselves the product of substantial national debate, indicates that the EPA erred in attempting to resolve significant policy issues on its own accord in the Endangerment Finding.

\165\ Public Law 119-21.

\166\ 42 U.S.C. 7434(c)(2) (2022).

Congress has also recently disapproved several actions taken by the EPA with respect to GHG emissions. On May 19, 2025, President Trump signed into law a resolution adopted by Congress under the Congressional Review Act (CRA) to void our final rule implementing the waste emission charge added to the CAA in 2021.\167\ And on June 12, 2025, President Trump signed into law three resolutions adopted by Congress under the CRA \168\ to void waivers we granted under CAA section 209 that allowed California and participating States to enforce GHG emission regulations for motor vehicles and engines, up to and including zero-emission standards that mandated a shift to electric vehicles.\169\ These disapproval resolutions further demonstrate the economic and political significance of the EPA's GHG emission regulations and reinforce the understanding that Congress intends to reserve such major questions of policy for itself. See West Virginia, 597 U.S. at 731-32.

\167\ Public Law 119-2; see 90 FR 21225 (May 19, 2025).

\168\ H.J. Res. 87; H.J. Res. 88; H.J. Res. 89; see also Diamond Alt. Energy, LLC v. EPA, 606 U.S. 100, 107 n.1 (2025); Statement by the President (June 12, 2025): https://www.whitehouse.gov/briefings-statements/2025/06/statement-by-the-president/.

\169\ For example, California's Advanced Clean Cars II required an increasing amount of EVs to be sold so that by 2035 100 percent of new cars and light trucks sold in California would be zero- emission vehicles, including PHEV. See California Air Resources Board, California moves to accelerate to 100% new zero-emission vehicle sales by 2035, available at https://ww2.arb.ca.gov/news/california-moves-accelerate-100-new-zero-emission-vehicle-sales-2035.

Conclusion. Under the major questions doctrine, we conclude that the EPA lacks the “clear congressional authorization” required for the novel approach taken in the Endangerment Finding and resulting GHG emission standards and must rescind these actions. West Virginia, 597 U.S. at 723 (quoting UARG, 573 U.S. at 324). Our statutory authority under CAA section 202(a)(1) to “prescribe . . . standards” does not clearly authorize the EPA to regulate in response to global climate change concerns or, in issuing such regulations, to trend toward mandating a shift from gas- and diesel-fueled vehicles to EVs. This conclusion follows whether the major questions doctrine is viewed as an ordinary interpretive principle or a protection against violations of the separation of powers. As discussed previously in section V.A.1 of this preamble, an interpretation of CAA section 202(a)(1) that permits the EPA to define and regulate any “air pollution” the Agency believes may harm public health or welfare, broadly defined, would raise serious absurdity and nondelegation concerns. Properly interpreted, the statute does not and need not raise such concerns given the best reading of the statute or application of the major questions doctrine.

In West Virginia, the Supreme Court held that our authority under CAA section 111 “to establish emission caps at a level reflecting `the application of the best system of emission reduction . . . adequately demonstrated' ” did not

clearly authorize the EPA to issue emission guidelines that addressed global climate change concerns by mandating a shift away from coal- generated electricity. 597 U.S. at 732. Similarly, in UARG, the Court held that our PSD and Title V authorities could not fully be extended to GHG emissions because those provisions “are designed to apply to, and cannot rationally be extended beyond, a relative handful of large sources capable of shouldering heavy substantive and procedural burdens.” 573 U.S. at 303. In these and other recent precedents, the Court has made clear that the express statutory authority required by major questions doctrine requires more than general language conferring “a merely plausible textual basis for the agency action.” West Virginia, 597 U.S. at 723.\170\

\170\ See, e.g., Nebraska, 600 U.S. at 506-07 (Department of Education lacked clear authority to forgive student loans under statutory language authorizing the Secretary to “waive or modify any statutory or regulatory provision applicable to the student financial assistance programs . . . deem[ed] necessary in connection with a war or other military operation or national emergency”); Ala. Ass'n of Realtors v. HHS, 594 U.S. 758 (2021) (CDC lacked clear authority to impose eviction moratorium during the COVID-19 pandemic under language permitting “such regulations as in [the Surgeon General's] judgment are necessary to prevent the introduction, transmission, or spread of communicable diseases”).

These cases control the analysis of our authority under CAA section 202(a). As in West Virginia, our statutory authority and the findings required to invoke that authority do not clearly authorize the approach taken in the Endangerment Finding and subsequent regulations. And as in UARG, our statutory authority to “prescribe . . . standards” for emissions of certain air pollutants does not clearly authorize using the CAA's vehicle-emission control scheme to address global climate change concerns. As discussed above, the Endangerment Finding did not limit itself to considering the impacts of GHG emissions from new motor vehicles and engines. Rather, the Endangerment Finding reviewed the totality of adverse impacts from climate change attributed to all anthropogenic sources of GHG emissions worldwide and asserted jurisdiction over CAA section 202(a) sources by finding they contributed to such impacts by emitting more than de minimis quantities of GHGs. That understanding has permeated our GHG emission rulemakings since 2009, and we have attempted to apply that framework to our distinct regulatory authorities across the rest of the CAA.

In Massachusetts, the Supreme Court disagreed with the EPA's argument that GHGs were not “air pollutants” because Congress had not revisited CAA section 202(a) in amending the CAA in 1990. 549 U.S. at 512-13. The Court found that our reliance on Brown & Williamson to support that argument was misplaced because unlike the ban on tobacco products at issue in that case, “EPA jurisdiction would lead to no such extreme measures.” Id. at 531. The Court also found that unlike the FDA's earlier statements on tobacco products, the “EPA had never disavowed the authority to regulate greenhouse gases” and had issued a memorandum in 1998 suggesting that we had such authority. Id.

Massachusetts did not consider or have reason to interpret the scope of the EPA's authority under CAA section 202(a) given our position in the 2003 Denial that GHGs are not “air pollutant[s]” under any provision of the statute. Rather, Massachusetts rejected our position that GHGs are “categorically” excluded from the CAA and remanded for the Administrator to determine whether four GHGs met the standard in CAA section 202(a). UARG, 573 U.S. at 320. Further, Massachusetts must be read together with the Supreme Court's decisions in West Virginia and UARG, which applied the major questions doctrine to statutory provisions similar to CAA section 202(a), as well as other relevant precedents decided since 2007.\171\ The decision in Massachusetts necessarily does not reflect consideration of these precedents or additional legislative and regulatory developments since that time. As noted above, the EPA's rulemakings have not been limited to emission standards as anticipated in Massachusetts, but instead reflect an increasing trend toward mandating a transition toward EVs for virtually all classes of LD, MD, and HD vehicles.

\171\ We note that recent Supreme Court decisions have not cited Massachusetts as a precedent applying, or declining to apply, the major questions doctrine. See, e.g., Nebraska, 600 U.S. 477; West Virginia, 597 U.S. 697.