delivered the opinion of the Court.

After the Administrator of the Environmental Protection Agency (EPA) approves a state implementation plan under the Clean Air Act, the plan may be challenged in a court of appeals within 30 days, or after 30 days have run if newly discovered or available information justifies subsequent review. We must decide whether the operator of a regulated emission source, in a petition for review of an EPA-approved state plan filed after the original 30-day appeal period, can raise the claim that it is economically or technologically infeasible to comply with the plan.

I

We have addressed the history and provisions of the Clean Air Amendments of 1970, Pub. L. 91-604, 84 Stat. 1676, in detail in Train v. Natural Resources Defense Council (NRDC), 421 U. S. 60 (1975), and will not repeat that discussion here. Suffice it to say that the Amendments reflect congressional dissatisfaction with the progress of existing air pollution programs and a determination to “tak[e] a stick to the States,” id., at 64, in order to guarantee the prompt attainment and maintenance of specified air quality standards. The heart of the Amendments is the requirement that each State formulate, subject to EPA approval, an implementation plan designed to achieve national primary ambient air quality standards — those necessary to protect the public health — “as expeditiously as practicable but . . . in no case later than three years from the date of ap*250proval of such plan.” § 110 (a) (2) (A) of the Clean Air Act, as added, 84 Stat. 1680, 42 U. S. C. § 1857&-5 (a) (2) (A). The plan must also provide for the attainment of national secondary ambient air quality standards'— those necessary to protect the public welfare — within a “reasonable time.” Ibid. Each State is given wide discretion in formulating its plan, and the Act provides that the Administrator “shall approve” the proposed plan if it has been adopted after public notice and hearing and if it meets eight specified criteria. § 110 (a) (2).1

*251On April 30, 1971, the Administrator promulgated national primary and secondary standards for six air pollutants he found to have an adverse effect on the public health and welfare. 40 CFR pt. 50 (1975). See § 108 (a) of the Act, as added, 84 Stat. 1678, 42 U. S. C. § 1857c-3 (a). Included among them was sulfur dioxide, at issue here. 40 CFR §§ 50.4-50.5 (1975). After the promulgation of the national standards, the State of Missouri formulated its implementation plan and submitted it for approval. Since sulfur dioxide levels exceeded national primary standards in only one of the *252State’s five air quality regions — the Metropolitan St. Louis Interstate region, 40 CFR § 52.1321 (1975)- — the Missouri plan concentrated on a control strategy and regulations to lower emissions in that area. The plan’s emission limitations were effective at once, but the State retained authority to grant variances to particular sources that could not immediately comply.2 Mo. Rev. Stat. §203.110 (1972). The Administrator approved the plan on May 31, 1972. See 40 CFR § 52.1320 et seq. (1975).

Petitioner is an electric utility company servicing the St. Louis metropolitan area, large portions of Missouri, and parts of Illinois and Iowa. Its three coal-fired generating plants in the metropolitan St. Louis area are subject to the sulfur dioxide restrictions in the Missouri implementation plan. Petitioner did not seek review of the Administrator’s approval of the plan within 30 days, as it was entitled to do under § 307 (b)(1) of the Act, as added, 84 Stat. 1708, 42 U. S. C. § 1857h-5 (b) (1), but rather applied to the appropriate state and county agencies for variances from the emission limitations affecting its three plants. Petitioner received one-year variances, which could be extended upon reapplication. The variances on two of petitioner’s three plants had expired and petitioner was applying for extensions when, on May 31, 1974, the Administrator notified petitioner that sulfur dioxide emissions from its plants violated the emission limitations contained in the Missouri plan.3 See 40 Fed. Reg. 3566 (1975). Shortly there*253after petitioner filed a petition in the Court of Appeals for the Eighth Circuit for review of the Administrator’s 1972 approval of the Missouri implementation plan.

Section 307 (b)(1) allows petitions for review to be filed in an appropriate court of appeals more than 30 days after the Administrator’s approval of an implementation plan only if the petition is "based solely on grounds arising- after such 30th day.” Petitioner claimed to meet this requirement by asserting, inter alia, that various economic and technological difficulties had arisen more than 30 days after the Administrator’s approval and that these difficulties made compliance with the emission limitations impossible.4 The Court of Ap*254peals ordered briefing on the question of its subject-matter jurisdiction to hear the case and, after argument, granted the motions of the EPA and intervenor-respond-ents, the Attorney General of Missouri and the Missouri Air Conservation Commission, to dismiss the petition for review for lack of jurisdiction.

The court held that “only matters which, if known to the Administrator at the time of his action [in approving a state implementation plan], would justify setting aside that action are properly re viewable after the initial 30 day review period.” 515 F. 2d 206, 216 (1975). Since, in the court’s view, claims of economic and technological infeasibility could not properly provide a basis for the Administrator’s rejecting a plan, such claims could not serve — at any time — as the basis for a court’s overturning an approved plan. Accordingly, insofar as petitioner’s claim of newly discovered or available information was grounded on an assertion of economic and technological infeasibility, the court held itself to be without jurisdiction to consider the petition for review, and so dismissed the petition. In so holding the Court of Appeals considered and rejected the contrary or partially contrary holdings of three other Circuits. Buckeye Power, Inc. v. EPA, 481 F. 2d 162, 168-169 (CA6 1973) (but see id., at 173); Appalachian Power Co. v. EPA, 477 F. 2d 495, 505-507 (CA4 1973); Duquesne Light Co. v. EPA, 481 F. 2d 1 (CA3 1973); Getty Oil Co. v. Ruckelshaus, 467 F. 2d 349 (CA3 1972), cert. denied, 409 U. S. 1125 (1973). See also St. Joe Minerals Corp. v. EPA, 508 F. 2d 743, 746-749 (CA3 1975), vacated and remanded, *255425 U. S. 987 (1976); Duquesne Light Co. v. EPA, 522 F. 2d 1186 (CA3 1975), cert. pending, No. 75-736. On the other hand, the Eighth Circuit found support for its position in the decisions of several other Circuits. South Terminal Corp. v. EPA, 504 F. 2d 646, 675-676 (CA1 1974); Texas v. EPA, 499 F. 2d 289, 317 (CA5 1974); Natural Resources Defense Council v. EPA, 507 F. 2d 905, 914 (CA9 1974). See also Indiana & Michigan Electric Co. v. EPA, 509 F. 2d 839, 843-845 (CA7 1975). Cf. Buckeye Power, Inc. v. EPA, 525 F. 2d 80 (CA6 1975). We granted certiorari to resolve the conflict among the Circuits, 423 U. S. 821 (1975), and we now affirm.

II

A

We reject at the outset petitioner’s suggestion that a claim of economic or technological infeasibility may be considered upon a petition for review based on new information and filed more than 30 days after approval of an implementation plan even if such a claim could not be considered by the Administrator in approving a plan or by a court in reviewing a plan challenged within the original 30-day appeal period. In pertinent part § 307 (b)(1) provides:

“A petition for review of the Administrator’s action in approving or promulgating any implementation plan under section 110 .. . may be filed only in the United States Court of Appeals for the appropriate circuit. Any such petition shall be filed within 30 days from the date of such promulgation or approval, or after such date if such petition is based solely on grounds arising after such 30th day.”

Regardless of when a petition for review is filed under i 307 (b)(1), the court is limited to reviewing “the Ad*256ministrator’s action in approving . . . [the] implementation plan . . . Accordingly, if new “grounds” are alleged, they must be such that, had they been known at the time the plan was presented to the Administrator for approval, it would have been an abuse of discretion for the Administrator to approve the plan. To hold otherwise would be to transfer a substantial responsibility in administering the Clean Air Act from the Administrator and the state agencies to the federal courts.

B

Since a reviewing court — regardless of when the petition for review is filed — may consider claims of economic and technological infeasibility only if the Administrator may consider such claims in approving or rejecting a state implementation plan, we must address ourselves to the scope of the Administrator’s responsibility. The Administrator’s position is that he has no power whatsoever to reject a state implementation plan on the ground that it is economically or technologically infeasible, and we have previously accorded great deference to the Administrator’s construction of the Clean Air Act. See Train v. NRDC, 421 U. S., at 75. After surveying the relevant provisions of the Clean Air Amendments of 1970 and their legislative history, we agree that Congress intended claims of economic and technological infeasibility to be wholly foreign to the Administrator’s consideration of a state implementation plan.

As we have previously recognized, the 1970 Amendments to the Clean Air Act were a drastic remedy to what was perceived as a serious and otherwise uncheckable problem of air pollution. The Amendments place the primary responsibility for formulating pollution control strategies on the States, but nonetheless subject *257the States to strict minimum compliance requirements. These requirements are of a "technology-forcing character,” Train v. NRDC, supra, at 91, and are expressly designed to force regulated sources to develop pollution control devices that might at the time appear to be economically or technologically infeasible.

This approach is apparent on the face of § 110 (a)(2). The provision sets out eight criteria that an implementation plan must satisfy, and provides that if these criteria are met and if the plan was adopted after reasonable notice and hearing, the Administrator "shall approve” the proposed state plan. The mandatory "shall” makes it quite clear that the Administrator is not to be concerned with factors other than those specified, Train v. NRDC, supra, at 71 n. 11, 79, and none of the eight factors appears to permit consideration of technological or economic infeasibility.5 Nonetheless, if a basis is to be found for allowing the Administrator to consider such claims, it must be among the eight criteria, and so it is here that the argument is focused.

It is suggested that consideration of claims of technological and economic infeasibility is required by the first criterion — that the primary air quality standards be met “as expeditiously as practicable but... in no case later than three years . . .” and that the secondary air *258quality standards be met within a “reasonable time.” §110 (a)(2)(A). The argument is that what is “practicable” or “reasonable” cannot be determined without assessing whether what is proposed is possible. This argument does not survive analysis.

Section 110 (a)(2)(A)’s three-year deadline for achieving primary air quality standards is central to the Amendments’ regulatory scheme and, as both the language and the legislative history of the requirement make clear, it leaves no room for claims of technological or economic infeasibility. The 1970 congressional debate on the Amendments centered on whether technology forcing was necessary and desirable in framing and attaining air quality standards sufficient to protect the public health, standards later termed primary standards. The House version of the Amendments was quite moderate in approach, requiring only that health-related standards be met “within a reasonable time.” H. R. 17255, 91st Cong., 2d Sess., § 108 (c)(1)(C)(i) (1970). The Senate bill, on the other hand, flatly required that, possible or not, health-related standards be met “within three years.” S. 4358, 91st Cong., 2d Sess., § 111 (a)(2) (A) (1970).

The Senate’s stiff requirement was intended to foreclose the claims of emission sources that it would be economically or technologically infeasible for them to achieve emission limitations sufficient to protect the public health within the specified time. As Senator Muskie, manager of the Senate bill, explained to his chamber:

“ ‘The first responsibility of Congress is not the making of technological or economic judgments— or even to be limited by what is or appears to be technologically or economically feasible. Our responsibility is to establish what the public interest requires to protect the health of persons. This may *259mean that people and industries will be asked to do what seems to be impossible at the present time.5 ” 116 Cong. Rec. 32901-32902 (1970).

See also id., at 32919 (remarks of Sen. Cooper); 33115 (remarks of Sen. Prouty). This position reflected that of the Senate committee:

“In the Committee discussions, considerable concern was expressed regarding the use of the concept of technical feasibility as the basis of ambient air standards. The Committee determined that 1) the health of people is more important than the question of whether the early achievement of ambient air quality standards protective of health is technically feasible; and 2) the growth of pollution load in many areas, even with application of available technology, would still be deleterious to public health.

“Therefore, the Committee determined that existing sources of pollutants either should meet the standard of the law or be closed down ....” S. Rep. No. 91-1196, pp. 2-3 (1970).

The Conference Committee and, ultimately, the entire Congress accepted the Senate’s three-year mandate for the achievement of primary air quality standards, and the clear import of that decision is that the Administrator must approve a plan that provides for attainment of the primary standards in three years even if attainment does not appear feasible. In rejecting the House’s version of reasonableness, however, the conferees strengthened the Senate version. The Conference Committee made clear that the States could not procrastinate until the deadline approached. Rather, the primary standards had to be met in less than three years if possible; they had to be met “as expeditiously as practica*260ble.” § 110 (a)(2)(A). Whatever room there is for considering claims of infeasibility in the attainment of primary standards must lie in this phrase, which is, of course, relevant only in evaluating those implementation plans that attempt to achieve the primary standard in less than three years.

It is argued that when such a state plan calls for proceeding more rapidly than economics and the available technology appear to allow, the plan must be rejected as not “practicable.” Whether this is a correct reading of § 110 (a) (2) (A) depends on how that section’s “as expeditiously as practicable” phrase is characterized. The Administrator’s position is that § 110 (a) (2) (A) sets only a minimum standard that the States may exceed in their discretion, so that he has no power to reject an infeasible state plan that surpasses the minimum federal requirements — a plan that reflects a state decision to engage in technology forcing on its own and to proceed more expeditiously than is practicable. On the other hand, petitioner and amici supporting its position argue that § 110 (a) (2) (A) sets a mandatory standard that the States must meet precisely, and conclude that the Administrator may reject a plan for being too strict as well as for being too lax. Since the arguments supporting this theory are also made to show that the Administrator must reject a state plan that provides for achieving more than the secondary air quality standards require, we defer consideration of this question in order to outline the development and content of the secondary standards provision of § 110 (a)(2)(A).

Secondary air quality standards, those necessary to protect the public welfare, were subject to far less legislative debate than the primary standards. The House version of the Amendments treated welfare-related standards together with health-related standards, and *261required both to be met "within a reasonable time.” H. R. 17255, 91st Cong., 2d Sess., §§ 107 (e)(1), 108 (c) (l)(C)(i) (1970). The Senate bill, on the other hand, treated health- and welfare-related standards separately and did not require that welfare-related standards be met in any particular time at all, S. 4358, 91st Cong., 2d Sess., §§ 110 (a)(3), 110 (b), 111 (a)(2)(A) (1970), although the Committee Report expressed the desire that they be met “as rapidly as possible.” S. Rep. No. 91-1196, p. 11 (1970). The final Amendments also separated welfare-related standards from health-related standards, labeled them secondary air quality standards, and adopted the House’s requirement that they be met within a “reasonable time.” §§ 109 (b), 110 (a)(2)(A). Thus, technology forcing is not expressly required in achieving standards to protect the public welfare.6

It does not necessarily follow, however, that the Administrator may consider claims of impossibility in assessing a state plan for achieving secondary standards. As with plans designed to achieve primary standards in less than three years, the scope of the Administrator’s power to reject a plan depends on whether the State itself may decide to engage in technology forcing and adopt a plan more stringent than federal law demands.7

Amici Appalachian Power Co. et al. argue that the Amendments do not give such broad power to the States. *262They claim that the States are precluded from submitting implementation plans more stringent than federal law demands by § 110 (a) (2)’s second criterion — that the plan contain such control devices “as may be necessary” to achieve the primary and secondary air quality standards. §110(a)(2)(B).8 The contention is that an overly restrictive plan is not “necessary” for attainment of the national standards and so must be rejected by the Administrator.9

The principal support for this theory of amici lies in the fact that while the House and Senate versions of §110 (a)(2) both expressly provided that the States could submit for the Administrator’s approval plans that were stricter than the national standards required, see H. R. 17255, 91st Cong., 2d Sess., § 108 (c) (1970); S. 4358, 91st Cong., 2d Sess., § 111 (a)(1) (1970), the section as enacted contains no such express language. Amici argue that the Conference Committee must have decided to require state implementation plans simply — and precisely — to meet the national standards. The argument of amici proves too much. A Conference Committee lacks power to make substantive changes on matters about *263which both Houses agree. 2 U. S. C. § 190c (a) (Senate Conference Reports); Rule XXVIII (3), Rules of the House of Representatives, and § 546, Jefferson’s Manual, H. R. Doc. No. 384, 92d Cong., 2d Sess., 526, 270-271 (1972); National Coal Operators’ Assn. v. Kleppe, 423 U. S. 388, 401 n. 10 (1976). Here the Conference Report expressly notes that both the Senate and House bills would allow States to submit plans more stringent than the national standards demand, and offers no suggestion that the Conference bill intended to change that result, even if it could. H. R. Conf. Rep. No. 91-1783, p. 45 (1970). And while the final language of § 110 (a) (2)(B) may be less explicit than the versions originally approved by the House and the Senate, the most natural reading of the “as may be necessary” phrase in context is simply that the Administrator must assure that the minimal, or “necessary,” requirements are met, not that he detect and reject any state plan more demanding than federal law requires.10

This reading is further supported by practical considerations. Section 116 of the Clean Air Act, as added, 84 Stat. 1689, 42 U. S. C. § 1857d-l (1970 ed., Supp. IV), provides that the States may adopt emission standards *264stricter than the national standards.11 Amici argue that such standards must be adopted and enforced independently of the EPA-approved state implementation plan. This construction of §§ 110 and 116, however, would not only require the Administrator to expend considerable time and energy determining whether a state plan was precisely tailored to meet the federal standards,12 but would simultaneously require States desiring stricter standards to enact and enforce two sets of emission standards, one federally approved plan and one stricter state plan. We find no basis in the Amendments for visiting such wasteful burdens upon the States and the Administrator, and so we reject the argument of amici.

We read the “as may be necessary” requirement of § 110 (a) (2) (B) to demand only that the implementation plan submitted by the State meet the “minimum conditions” of the Amendments.13 Train v. NRDC, 421 *265U. S., at 71 n. 11. Beyond that, if a State makes the legislative determination that it desires a particular air quality by a certain date and that it is willing to force technology to attain it — or lose a certain industry if attainment is not possible — such a determination is fully consistent with the structure and purpose of the Amendments, and § 110 (a) (2) (B) provides no basis for the EPA Administrator to object to the determination on the ground of infeasibility.14 See Train v. NRDC, supra, at 79.

In sum, we have concluded that claims of economic or technological infeasibility may not be considered by the Administrator in evaluating a state requirement that primary ambient air quality standards be met in the mandatory three years. And, since we further conclude that the States may submit implementation plans more stringent than federal law requires and that the Administrator must approve such plans if they meet the minimum requirements of § 110 (a)(2), it follows that the language of §110 (a)(2)(B) provides no basis for the Administrator ever to reject a state implementation plan on the ground that it is economically or technologically infeasible. Accordingly, a court of appeals reviewing an *266approved plan under § 307 (b) (1) cannot set it aside on those grounds, no matter when they are raised.

Ill

Our conclusion is bolstered by recognition that the Amendments do allow claims of technological and economic infeasibility to be raised in situations where consideration of such claims will not substantially interfere with the primary congressional purpose of prompt attainment of the national air quality standards. Thus, we do not hold that claims of infeasibility are never of relevance in the formulation of an implementation plan or that sources unable to comply with emission limitations must inevitably be shut down.

Perhaps, the most important forum for consideration of claims of economic and technological infeasibility is before the state agency formulating the implementation plan. So long as the national standards are met, the State may select whatever mix of control devices it desires, Train v. NRDC, supra, at 79, and industries with particular economic or technological problems may seek special treatment in the plan itself. Cf. 40 CFR §§51.2 (b), (d) (1975); S. Rep. No. 91-1196, p. 36 (1970). Moreover, if the industry is not exempted from, or accommodated by, the original plan, it may obtain a variance, as petitioner did in this case; and the variance, if granted after notice and a hearing, may be submitted to the EPA as a revision of the plan.15 § 110 (a) (3) (A), as amended, 88 Stat. 256, 42 U. S. C. § 1857c-5 (a) (3) (A) (1970 ed., Supp. IY). Lastly, an industry denied an exemption from the implementation plan, or denied a subsequent variance, may be able to take its claims of *267economic or technological infeasibility to the state courts. See, e. g., Mo. Rev. Stat. §203.130 (1972); Cal. Health & Safety Code § 39506 (West 1973); Pa. Stat. Ann., Tit. 71, § 1710.41 (1962).16

While the State has virtually absolute power in allocating emission limitations so long as the national standards are met, if the state plan cannot meet the national standards, the EPA is implicated in any postponement procedure. There are two ways that a State can secure relief from the EPA for individual emission sources, or classes of sources, that cannot meet the national standards. First, if the Governor of the State so requests at the time the original implementation plan is submitted, and if the State provides reasonable interim controls, the Administrator may allow a two-year extension of the three-year deadline for attainment of primary air quality standards if he finds, inter alia, that it is technologically infeasible for the source to comply. § 110 (e).17 Second, again upon application of the Governor of the State, the Administrator may allow a one-year postponement of any compliance date in an implementation plan if he finds, inter alia, that compliance is technologically *268infeasible and that “the continued operation of [the emission source] is essential to national security or to the public health or welfare . . . § 110 (f). See Train v. NRDC, 421 U. S., at 81.

Even if the State does not intervene on behalf of an emission source, technological and economic factors may be considered in at least one other circumstance. When a source is found to be in violation of the state implementation plan, the Administrator may, after a conference with the operator, issue a compliance order rather than seek civil or criminal enforcement. Such an order must specify a “reasonable” time for compliance with the relevant standard, taking into account the seriousness of the violation and “any good faith efforts to comply with applicable requirements.” § 113 (a)(4) of the Clean Air Act, as added, 84 Stat. 1686, 42 U. S. C. § 1857o-8 (a) (4). Claims of technological or economic infeasibility, the Administrator agrees, are relevant to fashioning an appropriate compliance order under § 113 (a) (4). Brief for Respondent EPA 36 n. 34.18

In short, the Amendments offer ample opportunity for consideration of claims of technological and economic infeasibility. Always, however, care is taken that consideration of such claims will not interfere substantially with the primary goal of prompt attainment of the national standards. Allowing such claims to be raised by *269appealing the Administrator’s approval of an implementation plan, as petitioner suggests, would frustrate congressional intent. It would permit a proposed plan to be struck down as infeasible before it is given a chance to work, even though Congress clearly contemplated that some plans would be infeasible when proposed. And it would permit the Administrator or a federal court to reject a State’s legislative choices in regulating air pollution, even though Congress plainly left with the States, so long as the national standards were met, the power to determine which sources would be burdened by regulation and to what extent. Technology forcing is a concept somewhat new to our national experience and it necessarily entails certain risks. But Congress considered those risks in passing the 1970 Amendments and decided that the dangers posed by uncontrolled air pollution made them worth taking. Petitioner’s theory would render that considered legislative judgment a nullity, and that is a result we refuse to reach.19

Affirmed.

with whom The Chief Justice joins, concurring.

I join the opinion of the Court because the statutory scheme and the legislative history, thoroughly described in the Court’s opinion, demonstrate irrefutably that Congress did not intend to permit the Administrator of *270the Environmental Protection Agency to reject a proposed state implementation plan on the grounds of economic or technological infeasibility. Congress adopted this position despite its apparent awareness that in some cases existing sources that cannot meet the standard of the law must be closed down.1

The desire to impose strong incentives on industry to encourage the rapid development and adoption of pollution control devices is understandable. But it is difficult to believe that Congress would adhere to its absolute position if faced with the potentially devastating consequences to the public that this case vividly demonstrates.

Petitioner is an electric utility supplying power demands in the St. Louis metropolitan area, a large part of Missouri, and parts of Illinois and Iowa. It alleges that it cannot continue to operate if forced to comply with the sulfur dioxide restrictions contained in the *271Missouri implementation plan approved by the Administrator. Specifically, petitioner alleges that since the Administrator’s approval of the plan, low-sulfur coal has become too scarce and expensive to obtain; rehable and satisfactory sulfur dioxide removal equipment that would enable it to comply with the plan’s requirements simply has not been devised; the installation of the unsatisfactory equipment that is available would cost over $500 million, a sum impossible to obtain by bonds that are contingent on approval by regulatory bodies and public acceptance; and, even if the financing could be obtained, the carrying, operating, and maintenance costs of over $120 million a year would be prohibitive.2 Petitioner further alleges that recent evidence has disclosed that sulfur dioxide in the ambient air is not the hazard to public health that it was once thought to be, and that compliance with the sulfur regulation in the Missouri plan is not necessary to the attainment of national primary and secondary ambient air standards in the St. Louis area.

At the risk of civil and criminal penalties enforceable by both the State and Federal Governments, as well as possible citizens’ suits, 42 U. S. C. §§ 1857c-8, 1857h-2, petitioner is being required either to embark upon the task of installing allegedly unreliable and prohibitively expensive equipment or to shut down. Yet the present Act permits neither the Administrator, in approving the state plan, nor the courts, in reviewing that approval under § 307 of the Act, 42 U. S. C. § 1857h-5, even to consider petitioner’s allegations of infeasibility.

Environmental concerns, long neglected, merit high priority, and Congress properly has made protection of *272the public health its paramount consideration. See S. Rep. No. 91-1196, pp. 2-3 (1970). But the shutdown of an urban area’s electrical service could have an even more serious impact on the health of the public than that created by a decline in ambient air quality. The result apparently required by this legislation in its present form could sacrifice the well-being of a large metropolitan area through the imposition of inflexible demands that may be technologically impossible to meet and indeed may no longer even be necessary to the attainment of the goal of clean air.

I believe that Congress, if fully aware of this Draconian possibility, would strike a different balance.