Train v. Natural Resources Defense Council, Inc.

delivered the opinion of the Court.

We granted certiorari in this case, 419 U. S. 823 (1974), to review a judgment of the Court of Appeals for the Fifth Circuit which required petitioner Administrator of the Environmental Protection Agency to disapprove a portion of the implementation plan submitted to him by the State of Georgia pursuant to the Clean Air Amendments of 1970. 1 The case presents an issue of statutory construction which is illuminated by the anatomy of the statute itself, by its legislative history, and by the history of congressional efforts to control air pollution.

I

Congress initially responded to the problem of air pollution by offering encouragement and assistance to the States. In 1955 the Surgeon General was authorized to study the problem of air pollution, to support research, training, and demonstration projects, and to provide technical assistance to state and local governments attempting to abate pollution. 69 Stat. 322. In 1960 Congress directed the Surgeon General to focus his attention on the health hazards resulting from motor vehicle emissions. Pub. L. 86-493, 74 Stat. 162. The Clean Air Act of 1963, 77 Stat. 392, authorized federal authorities to expand their research efforts, to make grants to state air pollu *64 tion control agencies, and also to intervene directly to abate interstate pollution in limited circumstances. Amendments in 1965, § 101, 79 Stat. 992, and in 1966, 80 Stat. 954, broadened federal authority to control motor vehicle emissions and to make grants to state pollution control agencies.

The focus shifted somewhat in the Air Quality Act of 1967, 81 Stat. 485. It reiterated the premise of the earlier Clean Air Act “that the prevention and control of air pollution at its source is the primary responsibility of States and local governments.” Ibid. Its provisions, however, increased the federal role in the prevention of air pollution, by according federal authorities certain powers of supervision and enforcement. But the States generally retained wide latitude to determine both the air quality standards which they would meet and the period of time in which they would do so.

The response of the States to these manifestations of increasing congressional concern with air pollution was disappointing. Even by 1970, state planning and implementation under the Air Quality Act of 1967 had made little progress. Congress reacted by taking a stick to the States in the form of the Clean Air Amendments of 1970, Pub. L. 91-604, 84 Stat. 1676, enacted on December 31 of that year. These Amendments sharply increased federal authority and responsibility in the continuing effort to combat air pollution. Nonetheless, the Amendments explicitly preserved the principle: “Each State shall have the primary responsibility for assuring air quality within the entire geographic area comprising such State . . . .” § 107 (a) of the Clean Air Act, as added, 84 Stat. 1678,42 U. S. C. § 1857c-2 (a). The difference under the Amendments was that the States were no longer given any choice as to whether they would meet this responsibility. For the first time they were required to *65 attain air quality of specified standards, and to do so within a specified period of time.

The Amendments directed that within 30 days of their enactment the Environmental Protection Agency should publish proposed regulations describing national quality standards for the “ambient air,” which is the statute’s term for the outdoor air used by the general public. After allowing 90 days for comments on the proposed standards, the Agency was then obliged to promulgate such standards. § 109 (a)(1) of the Clean Air Act, as added, 84 Stat. 1679, 42 U. S. C. § 1857c-4 (a)(1). The standards were to be of two general types: “primary” standards, which in the judgment of the Agency were “requisite to protect the public health,” §109 (b)(1), and “secondary” standards, those that in the judgment of the Agency were “requisite to protect the public welfare from any known or anticipated adverse effects associated with the presence of such air pollutant in the ambient air.” §109 (b)(2).

Within nine months after the Agency’s promulgation of primary and secondary air quality standards, each of the 50 States was required to submit to the Agency a plan designed to implement and maintain such standards within its boundaries. § 110 (a) (1) of the Clean Air Act, as added, 84 Stat. 1680, 42 U. S. C. § 1857c-5 (a)(1). The Agency was in turn required to approve each State’s plan within four months of the deadline for submission, if it had been adopted after public hearings and if it satisfied eight general conditions set forth in § 110 (a)(2). 2 *66 Probably the principal of these conditions, and the heart of the 1970 Amendments, is that the plan provide for the attainment of the national primary ambient air *67 quality standards in the particular State “as expeditiously as practicable but ... in no case later than three years from the date of approval of such plan.” § 110 (a) (2)(A). In providing for such attainment, a State’s plan must include “emission limitations, schedules, and timetables for compliance with such limitations”; it must also contain such other measures as may be necessary to insure both timely attainment and subsequent maintenance of national ambient air standards. § 110 (a)(2)(B).

Although the Agency itself was newly organized, the States looked, to it for guidance in formulating the plans they were required to submit. On April 7,1971 — scarcely three months after the enactment of the Clean Air Amendments — the Agency published proposed guidelines for the preparation, adoption, and submission of such plans. 36 Fed. Reg. 6680. After receiving numerous comments, including those from respondent Natural Resources Defense Council, Inc. (NRDC), it issued final guidelines on August 14, 1971, 36 Fed. Reg. 1586. See 40 CFR Part 51 (1974). The national standards themselves were timely promulgated on April 30, 1971, 36 Fed. Reg. 8186. See 40 CFR Part 50 (1974).

*68 No one can doubt that Congress imposed upon the Agency and States a comprehensive planning task of the first magnitude which was to be accomplished in a relatively short time. In the case of the States, it was soon realized that in order to develop the requisite plans within the statutory nine-month deadline, efforts would have to be focused on determining the stringent emission limitations necessary to comply with national standards. This was true even though compliance with the standards would not be necessary until the attainment date, which normally would be three years after Agency approval of a plan. The issue then arose as to how these stringent limitations, which often could not be satisfied without substantial research and investment, should be applied during the period prior to that date.

One approach was that adopted by Florida, under which the plan’s emission limitations would not take effect until the attainment date. Under this approach, no source is subject to enforcement actions during the preattainment period, but all are put on notice of the limitations with which they must eventually comply. 3 Since the Florida approach basically does not require preattainment date pollution reductions on the part of those sources which might be able to effect them, 4 the Agency encouraged an alternative approach. Under it a State’s emission limitations would be immediately effective. The State, how *69 ever, would have the authority to grant variances to particular sources which could not immediately comply with the stringent emission limitations necessary to meet the standards.

Georgia chose the Agency’s preferred approach. 5 Its plan provided for immediately effective categorical emission limitations, but also incorporated a variance procedure whereby particular sources could obtain individually tailored relief from general requirements. This variance provision, Ga. Code Ann. § 88-912 (1971), 6 was one of the *70 bases upon which the Agency’s approval of the Georgia plan was successfully challenged by respondents in the Court of Appeals. It is the only aspect of that court’s decision as to which the Agency petitioned for certiorari.

II

The Agency’s approval of Georgia’s variance provision was based on its interpretation of § 110 (a)(3), 7 which provides that the Agency shall approve any revision of an implementation plan which meets the §110 (a) (2) requirements applicable to an original plan. The Agency concluded that § 110 (a) (3) permits a State to grant individual variances from generally applicable emission standards, both before and after the attainment date, so long as the variance does not cause the plan to fail to comply with the requirements of § 110 (a)(2). Since that section requires, inter alia, that primary ambient air standards be attained by a particular date, it is of some consequence under this approach whether the period for which the variance is sought extends beyond that date. If it does not, the practical effect of treating such preattainment date variances as revisions is that they can be granted rather freely.

This interpretation of §110 (a) (3) was incorporated in the Agency’s original guidelines for implementation *71 plans, 40 CFR §§ 51.6 (c), 51.32 (f) (1973). 8 Although a spokesman for respondent NRDC had earlier stated that the Agency’s guideline in this regard “correctly provides that variances which do not threaten attainment of a national standard are to be considered revisions of the plan,” 9 that organization later developed second thoughts on the matter. Its present position, in which it is joined by another environmental organization and by two individual respondents who reside in affected air quality control regions within the State of Georgia, is that variances, applicable to individual sources may be approved only if they meet the stringent procedural and substantive standards of § 110 (f). 10 This section permits one-year “postponements” of any requirement of a plan, subject to conditions which will be discussed below.

The Court of Appeals agreed with respondents, and ordered the Agency to disapprove Georgia’s variance provision, although it did not specify which of the § 110 (a)(2) requirements were thereby violated. 11 It held *72 that while the revision authority of §110 (a)(3) was available for generally applicable changes of an implementation plan, the postponement provision of § 110 (f) was the only method by which individual sources could obtain relief from applicable emission limitations. In reaching this conclusion the court rejected petitioners’ suggestion that whether a proposed variance should be treated as a “revision” under § 110 (a)(3), or as a “postponement” under § 110 (f), depended on whether it would affect attainment of a national ambient air standard, rather than on whether it applied to one source or to many.

Other Circuits have also been confronted with this issue, and while none has adopted the Agency’s position, all have differed from the Fifth Circuit. The first case was Natural Resources Defense Council v. EPA, 478 F. 2d 875 (CA1 1973). For reasons to be discussed, infra, at 91-94, the First Circuit rejected the revision authority as a basis for a variance procedure. It nonetheless concluded that prior to the three-year date for mandatory attainment of primary standards, a State could grant variances to sources which could not immediately meet applicable emission limitations. The court reasoned:

“We can see value in permitting a state to impose strict emission limitations now, subject to individual exemptions if practicability warrants; otherwise it may be forced to adopt less stringent limitations in order to accommodate those who, notwithstanding reasonable efforts, are as yet unable to comply.

“The Administrator sees his power to allow such exemption procedures as deriving from the 'revision’ authority in § [110] (a)(3). We tend to view it more as a necessary adjunct to the statutory scheme, which anticipates greater flexibility during the preattainment period.” 478 F. 2d, at 887.

*73 The First Circuit’s resolution, which has been described as “Solomonesque,” is not tied to any specific provision of the Clean Air Act. Rather, it is quite candidly a judicial creation providing flexibility which, according to its creators, Congress may be inferred to have intended to provide. Two other Circuits subsequently followed the First Circuit. Natural Resources Defense Council v. EPA, 483 F. 2d 690, 693-694 (CA8 1973); Natural Resources Defense Council v. EPA, 494 F. 2d 519, 523 (CA2 1974). Neither expanded on the First Circuit’s reasoning.

The Ninth Circuit has adopted a third approach to this question, in Natural Resources Defense Council v. EPA, 507 F. 2d 905, 911-917 (1974). After considering legislative history, the Ninth Circuit concluded that Congress did not intend the postponement mechanism to be the exclusive source for variances. But the court also did not adopt the Agency’s view that variances could be authorized as § 110 (a) (3) revisions, although it did not explain its rejection of this interpretation. Rather, the Ninth Circuit agreed with the First Circuit that flexibility was “a necessary adjunct to the statutory scheme.” It explained:

“As long as a possible variance from a state plan will not preclude the attainment or maintenance of such standards, we discern no legislative intent to commit a state, in toto, to its initial plan, without any flexibility whatsoever.” 507 F. 2d, at 913.

The Ninth Circuit, however, rejected the First Circuit’s distinction between the preattainment and postattainment periods. It concluded that statutory support for flexibility was as strong after the attainment date as before, especially in light of the Act’s encouragement of the States to adopt plans even stricter than those required *74 to attain national standards. 12 The court thus adopted an approach which differs from the Agency’s, but which reaches the same result — authorization of variances on standards other than those required for § 110 (f) postponements, both before and after the attainment date, so long as the variance does not prevent timely attainment and subsequent maintenance of national ambient air standards.

After the Courts of Appeals for the First, Eighth, Fifth, and Second Circuits had spoken, but prior to the decision of the Ninth Circuit, the Agency modified its guidelines to comply with the then-unanimous rulings that after the attainment date the postponement provision was the only basis for obtaining a variance. 39 Fed. Reg. 34533-34535, adding 40 CFR §§ 51.11 (g), 51.15 (d) and revising § 51.32 (f). At the same time, the Agency formally disapproved variance provisions to the extent they authorized variances extending beyond attainment dates, unless the standards of § 110 (f) were met. 39 Fed. Reg. 34535, adding 40 CFR § 52.26.

Because the Agency has conformed its regulations to the decisions of the First, Eighth, and Second Circuits, this case on its facts is now limited to the validity of the Georgia variance provision insofar as it authorizes variances effective before Georgia’s attainment date, which is in July 1975. 13 The Agency nonetheless has not abandoned its original view that the revision section authorizes variances which do not interfere with the attainment or maintenance of national ambient air standards. Moreover, the Agency is candid in admitting that should we *75 base our decision on its interpretation of § 110 (a)(3), the decision would support the approval of implementation plans which provide for variances effective after the attainment date.

The disparity among the Courts of Appeals rather strongly indicates that the question does not admit of an easy answer. Without going so far as to hold that the Agency’s construction of the Act was the only one it permissibly could have adopted, we conclude that it was at the very least sufficiently reasonable that it should have been accepted by the reviewing courts.

Ill

Both of the sections in controversy are contained in § 110 of the amended Clean Air Act, which is entitled “Implementation Plans.” Section 110 (a) (3) provides in pertinent part:

“(A) The Administrator shall approve any revision of an implementation plan applicable to an air quality control region if he determines that it meets the requirement of paragraph (2) and has been adopted by the State after reasonable notice and public hearings.”

Section 110 (f) provides:

“(1) Prior to the date on which any stationary source or class of moving sources is required to comply with any requirement of an applicable implementation plan the Governor of the State to which such plan applies may apply to the Administrator to postpone the applicability of such requirement to such source (or class) for not more than one year. If the Administrator determines that—

“(A) good faith efforts have been made to comply with such requirement before such date,

“(B) such source (or class) is unable to comply *76 with such requirement because the necessary technology or other alternative methods of control are not available or have not been available for a sufficient period of time,

“(C) any available alternative operating procedures and interim control measures have reduced or will reduce the impact of such source on public health, and

“ (D) the continued operation of such source is essential to national security or to the public health or welfare,

“then the Administrator shall grant a postponement of such requirement.” 14

*77 As previously noted, respondents contend that “variances” applicable to individual sources — for example, a particular factory — may be approved only if they meet the stringent procedural and substantive standards set forth in § 110 (f). As is apparent from the text of § 110 (f), its postponements may be for no more than one year, may be granted only if application is made prior to the date of required compliance, arid must be supported by the Agency’s determination that the source’s continued operation “is essential to national security or to the public health or welfare.” Petitioners, on the other hand, rely on the revision authority of § 110 (a)(3) for the contention that a state plan may provide for an individual variance from generally applicable emission limitations so long as the variance does not cause the plan to fail to comply with the requirements of §110 (a)(2). Since a variance would normally implicate only the § 110 (a) (2) (A) requirement that plans provide for attainment and maintenance of national ambient air standards, treatment as revisions would result in variances being readily approved in two situations: first, where the variance does not defer compliance beyond the attainment date; 15 and second, where the national standards have been attained and the variance is not so great that a plan incorporating it could not insure their continued maintenance. Moreover, a§110(a)(3) revision may be granted on the basis of hearings conducted by the State, whereas a § 110 (f) *78 postponement is available only after the Agency itself conducts hearings.

There is thus considerable practical importance attached to the issue of whether variances are to be treated as revisions or as postponements, or for that matter, as the First Circuit would have it, as neither until the mandatory attainment date but as postponements thereafter. This practical importance reaches not merely the operator of a particular source who believes that circumstances justify his receiving a variance from categorical limitations. It also reaches the broader issue of whether Congress intended the States to retain any significant degree of control of the manner in which they attain and maintain national standards, at least once their initial plans have been approved or, under the First Circuit’s approach, once the mandatory attainment date has arrived. To explain our conclusion as to Congress’ intent, it is necessary that we consider the revision and postponement sections in the context of other provisions of the amended Clean Air Act, particularly those which distinguish between national ambient air standards and emission limitations.

As we have already noted, primary ambient air standards deal with the quality of outdoor air, and are fixed on a nationwide basis at levels which the Agency determines will protect the public health. It is attainment and maintenance of these national standards which § 110 (a) (2) (A) requires that state plans provide. In complying with this requirement a State’s plan must include “emission limitations,” which are regulations of the composition of substances emitted into the ambient air from such sources as power plants, service stations, and the like. They are the specific rules to which operators of pollution sources are subject, and which if enforced should result in ambient air which meets the national standards.

*79 The Agency is plainly charged by the Act with the responsibility for setting the national ambient air standards. Just as plainly, however, it is relegated by the Act to a secondary role in the process of determining and enforcing the specific, source-by-source emission limitations which are necessary if the national standards it has set are to be met. 16 Under § 110 (a)(2), the Agency is required to approve a state plan which provides for the timely attainment and subsequent maintenance of ambient air standards, and which also satisfies that section’s other general requirements. The Act gives the Agency no authority to question the wisdom of a State’s choices of emission limitations if they are part of a plan which satisfies the standards of § 110 (a)(2), and the Agency may devise and promulgate a specific plan of its own only if a State fails to submit an implementation plan which satisfies those standards. § 110 (c). Thus, so long as the ultimate effect of a State’s choice of emission limitations is compliance with the national standards for ambient air, the State is at liberty to adopt whatever mix of emission limitations it deems best suited to its particular situation.

This analysis of the Act’s division of responsibilities is not challenged by respondents insofar as it concerns the process of devising and promulgating an initial imple *80 mentation plan. Respondents do, however, deny that the States have such latitude once the initial plan is approved. Yet the third paragraph of § 110 (a), and the one immediately following the paragraphs which specify that States shall file implementation plans and that the Agency shall approve them if they satisfy certain broad criteria, is the section which requires the Agency to “approve any revision of an implementation plan” if it “determines that it meets the requirements” of § 110 (a)(2). On its face, this provision applies to any revision, without regard either to its breadth of applicability, or to whether it is to be effective before or after the attainment date; rather, Agency approval is subject only to the condition that the revised plan satisfy the general requirements applicable to original implementation plans. Far from evincing congressional intent that the Agency assume control of a State’s emission limitations mix once its initial plan is approved, the revision section is to all appearances the mechanism by which the States may obtain approval of their developing policy choices as to the most practicable and desirable methods of restricting total emissions to a level which is consistent with the national ambient air standards.

In order to challenge this characterization of § 110 (a)(3), respondents principally rely on the contention that the postponement provision, § 110 (f), is the only mechanism by which exceptions to a plan’s requirements may be obtained, under any circumstances. Were this an accurate description of § 110 (f), we would agree that the revision authority does not have the broad application asserted by the Agency. Like the Ninth Circuit, 17 however, we believe that § 110 (f) serves a function different from that of supervising state efforts to modify the initial *81 mix of emission limitations by which they implement national standards.

In our view, § 110 (f) is a safety valve by which may be accorded, under certain carefully specified circumstances, exceptions to the national standards themselves. That this is its role is strongly suggested by the process by which it became a part of the Clean Air Act. The House version of the Amendment, H. R. 17255, 91st Cong., 2d Sess., contained no provisions for either postponements or, most significantly, mandatory deadlines for the attainment of national ambient air standards. The Senate bill, S. 4358, 91st Cong., 2d Sess., did contain both the three-year deadline, which now appears in §110 (a)(2), and the predecessor of the present §110 (f). That predecessor 18 permitted the governor of a *82 State to petition a three-judge district court for “relief from the effect” of expiration of the three-year deadline as to a region or persons, and provided for the grant of such relief upon a showing of conditions similar to those *83 now appearing in § 110 (f). Under its language the postponement provision plainly applied only when deferral of a national deadline was sought. 19

The Conference Committee adopted the Senate’s general approach to the deadline issue. Its report states:

“The conference substitute follows the Senate . amendment in establishing deadlines for implementing primary ambient air quality standards but leaves the States free to establish a reasonable time period within which secondary ambient air quality standards will be implemented. The conference substitute modifies the Senate amendment in that it allows the Administrator to grant extensions for good causes shown upon application by the Governors.” H. R. Conf. Rep. No*91-1783, p. 45 (1970). (Emphasis added.)

Nowhere does the report suggest that other changes in the Senate’s proposed § 111 (f) were intended to dramatically broaden its reach, such that it would not merely be available to obtain deferral of the strict deadlines for compliance with national standards, but would also be the exclusive mechanism for any ameliorative modification of a plan, no matter how minor.

*84 That the postponement provision was intended merely as a method of escape from the mandatory deadlines becomes even clearer when one considers the summary of the conference’s work which Senator Muskie presented to the Senate. The summary referred to a provision under which a single two-year extension of the deadline could be obtained were it shown to be necessary at the time a State’s initial plan was submitted. It then immediately discussed the postponement provision, as follows:

“A Governor may also apply for a postponement of the deadline if, when the deadline approaches, it is impossible for a source to meet a requirement under an implementation plan, interim control measures have reduced (or will reduce) the adverse health effects of the source, and the continued operation of the source is essential to national security or the public health or welfare of that State.” 116 Cong. Rec. 42384-42385. (Emphasis added.)

This limited view of the role of § 110 (f) is reinforced by comparison with the section which immediately precedes it in the statute, § 110 (e). 20 This is the provision *85 to which Senator Muskie’s summary was obviously referring when it stated that the three-year deadline could be extended for up to two years if proper application were made at the time a State first submitted its plan. Like i 110 (f), § 110 (e) is available only if an emission source is unable to comply with plan requirements because “the necessary technology or other alternatives are not available or will not be available soon enough to permit compliance.” Section 110 (e) also contains a requirement parallel to that of § 110 (f)(1)(C), that available alternative procedures and control measures have been considered and utilized. Unlike § 110 (f), however, § 110 (e) contains no requirement that “the continued operation of such source is essential to national security or to the public health or welfare.” Section 110 (e) thus permits a two-year extension on a showing considerably less stringent than that required for a § 110 (f) one-year postponement. This disparity is quite logical, however, because the relief under § 110 (e) is limited to an initial two-year period, whereas that under § 110 (f)- is available at any time, so long as application is made prior to the effective date of the relevant requirement. 21

*86 On the other hand, the disparity between the standards of § 110 (e) and those of § 110 (f) would be inexplicable were § 110 (f) also the sole mechanism by which States could modify the particular emission limitations mix incorporated in their initial implementation plans, even though the desired modifications would have no impact on the attainment or maintenance of national standards. Respondents’ interpretation requires the anomalous conclusion that Congress, having stated its goal to be the attainment and maintenance of specified ambient air standards, nonetheless made it significantly more difficult for a State to modify an emission limitations mix which met those standards both before and after modification than for a State to obtain a two-year deferral in the attainment of the standards themselves. The interpretation suffers, therefore, not only from its contrariety to the revision authority which Congress provided, but also from its willingness to ascribe inconsistency to a carefully considered congressional enactment.

We believe that the foregoing analysis of the structure and legislative history of the Clean Air Amendments shows that Congress intended to impose national ambient air Standards to be attained within a specific period of time. It also shows that in §§ 110 (e) and (f) Congress carefully limited the circumstances in which timely attainment and subsequent maintenance of these standards could be compromised. We also believe that Congress, consistent with its declaration that “[e]ach State *87 shall have the primary responsibility for assuring air quality” within its boundaries, § 107 (a), left to the States considerable latitude in determining specifically how the standards would be met. This discretion includes the continuing authority to revise choices about the mix of emission limitations. We therefore conclude that the Agency’s interpretation of §§110 (a)(3) and 110 (f) was “correct,” to the extent that it can be said with complete assurance that any particular interpretation of a complex statute such as this is the “correct” one. Given this conclusion, as well as the facts that the Agency is charged with administration of the Act, and that there has undoubtedly been reliance upon its interpretation by the States and other parties affected by the Act, we have no doubt whatever that its construction was sufficiently reasonable to preclude the Court of Appeals from substituting its judgment for that of the Agency. Udall v. Tollman, 380 U. S. 1, 16-18 (1965); McLaren v. Fleischer, 256 U. S. 477, 480-481 (1921). We are not persuaded to the contrary by any of the arguments advanced by respondents or by the Courts of Appeals whieh have rejected § 110 (a)(3) as authority for granting variances. To these various arguments we now turn.

IV

The principal basis on which the Fifth Circuit rejected the Agency’s view of the revision and postponement sections was its analysis of their language. The court focused first on the fact that § 110 (f) speaks in terms of “any stationary source,” and of the postponement of “any requirement of an applicable implementation plan.” (Emphasis added.) This language, according to the Fifth Circuit, belies the Agency’s contention that the postponement section is inapplicable to those variances which do not jeopardize the attainment or maintenance *88 of national standards. The court went on to state, without citation or supporting reasoning:

“A revision is a change in a generally applicable requirement; a postponement or variance [is a] change in the application of a requirement to a particular party. The distinction between the two is familiar and clear.”' 489 F. 2d 390, 401.

We think that the Fifth Circuit has read more into §110(f), and more out of §110 (a)(3), than careful analysis can sustain. In the first place, the “any stationary source” and “any requirement” language of § 110 (f) serves only to define the matters with respect to which the governor of a State may apply for a postponement. The language does not, as the Fifth Circuit would have it, state that all sources desirous of any form of relief must rely solely on the postponement provision. While § 110 (f) makes its relief available to any source which can qualify for it, regardless of whether the relief would jeopardize national standards, the section does not even suggest that other forms of relief, having no impact on the national goal of achieving air quality standards, are not also available on appropriately less rigorous showings.

As for the Fifth Circuit’s observation that “a revision is a change in a generally applicable requirement,” whereas a “postponement or variance” deals with particular parties, we are not satisfied that the distinction is so “familiar and clear.” While a variance is generally thought to be of specific applicability, 22 whether a revision *89 is general or specific depends on what is being revised. In this instance, it is implementation plans which are being revised, and it is clear that such plans may be quite detailed, both as to sources and the remedial steps required of the sources. Not only does § 110 (a)(2)(B) specify that a plan shall include “emission limitations, schedules, and timetables for compliance,” 23 but respondents themselves have urged that the very specific variances which have already been granted in Georgia should have been, and may still be, treated as “compliance schedules” contained within the original plan. 24

A further difficulty with the Fifth Circuit’s analysis of the language of §§ 110 (a)(3) and 110 (f) is that it entirely overlooks an obvious distinction between revisions and postponements. In normal usage, to “postpone” is to defer, whereas to “revise” is to remake or amend. In the implementation plan context, normal usage would suggest that a postponement is a deferral of the effective date of a requirement which remains a part of the applicable plan, whereas a revision is a change in the plan itself which deletes or modifies the requirement. If by revision a requirement of a plan is removed, then a person seeking relief from that requirement has no *90 need to seek its postponement, and § 110 (f) is by its terms inapplicable. But if such a person cannot obtain a revision, because for example the plan as so revised would no longer insure timely attainment of the national standards, then under the Act he has no alternative but to comply or to obtain a postponement of the requirement’s effective date — if he can satisfy the stringent conditions of §110 (f). This distinction between the two is so straightforward, and so consistent with the structure and history of the Act, as discussed in Part III of this opinion, that we perceive no basis for the Fifth Circuit’s strained line of analysis. 25

The Fifth Circuit also relied on the “technology forcing” nature of the Clean Air Amendments of 1970. It reasoned that because the statute was intended to force technology to meet specified, scheduled standards, *91 it was essential to insure that commitments made at the planning stage could not be readily abandoned when the time for compliance arrived. According to the Fifth Circuit, § 110 (f) “is the device Congress chose to assure this.” 489 F. 2d, at 401. Clearly § 110 (f) does present a formidable hurdle for those proposed departures from earlier commitments which are in fact subject to its stringent conditions. What the Fifth Circuit failed to consider, however, is that so long as the national standards are being attained and maintained, there is no basis in the present Clean Air Act for forcing further technological developments. Agency review assures that variances granted under § 110 (a) (3) will b.e consistent with the § 110 (a) (2) (A) requirement that the national standards be attained as expeditiously as practicable and maintained thereafter. Thus § 110(a)(3) variances ex hypothesi do not jeopardize national standards, and the technology-forcing character of the Amendments is no reason at all for judging them under the provisions of § 110 (f).

The First Circuit also rejected the Agency’s contention that variances could be handled under the revision procedure, supra, at 72-73, but it did so for reasons different from those relied upon by the Fifth Circuit. 26 It stated:

“Had Congress meant [§ 110 (f)] to be followed only if a polluter, besides violating objective state *92 requirements, was shown to be preventing maintenance of a national standard, it would have said so. To allow a polluter to raise and perhaps litigate that issue is to invite protracted delay. The factual question could have endless refinements: is it the individual variance-seeker or others whose pollution is preventing maintenance of standards? See e. g., Getty Oil Company v. Ruckelshaus, 342 F. Supp. 1006 (D. Del. 1972), remanded with directions, 467 F. 2d 349 (3rd Cir. 1972), . . . where Getty raised this issue in various forums.” 478 F. 2d, at 886.

Respondents also stress this argument: treating variances as revisions rather than as postponements would invite litigation, would be impractical in application, and would therefore result in degradation of the environment. Aside from the fact that it goes more to the wisdom of what Congress has chosen to do than to determining what Congress has done, we believe this argument to be overstated. As made clear in the Getty case cited by the First Circuit, a polluter is subject to existing requirements until such time as he obtains a variance, and variances are not available under the revision authority until they, have been approved by both the State and the Agency. Should either entity determine that granting the variance would prevent attainment or maintenance of national air standards, the polluter is presumably within his rights in seeking judicial review. This litigation, however, is carried out on the polluter’s time, not the public’s, for during its pendency the original regulations remain in effect, and the polluter’s failure to comply may subject him to a variety of enforcement procedures. 27

*93 We are further impressed that the Agency itself has displayed no concern for the purported administrative difficulty of treating variances as revisions. Ordinarily, an agency may be assumed capable of meeting the responsibilities which it contends are placed upon it. Were respondents able to make a contrary showing, that fact might have some weight in interpreting Congress’ intent, although we would doubt its relevance unless Congress were also shown to have been aware of the problem when it drafted legislation which otherwise is consistent with the Agency’s contentions. Respondents have made no such showings. The judgments which the Agency must make when passing on variances under § 110 (a) (3) are whether the ambient air complies with national standards, and if so whether a proposed variance would cause a plan to fail to insure maintenance of those standards. These judgments are little different from those which the Agency had to make when it approved the initial plans into which respondents seek to have the States frozen. In each instance the Agency must measure the existing level of pollution, compare it with the national standards, and determine the effect on this comparison of specified emission modifications. 28 That Congress is of the opinion *94 that the Agency can feasibly and reliably perform these functions is manifest not only in its 1970 legislation, but also in a 1974 amendment designed to conserve energy. The amendment provides that the Agency should report to each State on whether its implementation plan could be revised in relation to fuel burning stationary sources, “without interfering with the attainment and maintenance of any national ambient air quality standard.” § 110