Petitioner Natural Resources Defense Council (“NRDC”) challenges the Environmental Protection Agency’s (“EPA” or “agency”) withdrawal of proposed regulations governing the emissions of vinyl chloride. The NRDC claims that section 112 of the Clean Air Act, 42 U.S.C. § 7412 (1982), pursuant to which the EPA regulates hazardous pollutants such as vinyl chloride, allows consideration of no factors other than health in setting the level of regulation. Because the Administrator relied on economic and technological factors in withdrawing the proposed regulations, the NRDC contends that the withdrawal was arbitrary and capricious and asks that we vacate the agency’s action and remand for further proceedings. We believe, however, that the statute vests the Administrator with some discretion in setting regulations under section 112, but does not specify precisely how that discretion is to be exercised. Accordingly, under Chevron U.S.A. v. Natural Resources Defense Council, 467 U.S. 837,104 S.Ct. 2778, 81 L.Ed.2d 694 (1984), we must uphold the agency’s selection of factors to employ in fleshing out its authority if we find the agency’s choice a reasonable one. Because we believe that the agency’s choice of economic and technological feasibility was reasonable, we affirm the agency’s action.
I.
Section 112 of the Clean Air Act provides for regulation of hazardous air pollutants, which the statute defines as “air pollutant[s] to which no ambient air quality standard is applicable and which in the judgment of the Administrator cause[ ], or contribute^ to, air pollution which may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness.” 42 U.S.C. § 7412(a)(1) (1982). The statute requires the Administrator to publish a list containing each hazardous pollutant for which he intends to adopt an emission standard, to publish proposed regulations and a notice of public hearing for each such pollutant, and then, within a specified period, either promulgate an emission standard or make a finding that the *712particular agent does not amount to a hazardous air pollutant. See 42 U.S.C. § 7412(b) (1982). The statute directs the Administrator to set any emission standard promulgated under section 112 “at the level which in his judgment provides an ample margin of safety to protect the public health.” 42 U.S.C. § 7412(b)(1)(B) (1982). The question before us is whether this standard permits the Administrator to consider economic and technological feasibility.
This case concerns vinyl chloride regulations. Vinyl chloride is a gaseous synthetic chemical used in the manufacture of plastics and is a strong carcinogen. In late 1975, the Administrator issued a notice of proposed rulemaking to establish an emission standard for vinyl chloride. 40 Fed. Reg. 59,532 (1975). In the notice, the EPA asserted that available data linked vinyl chloride to carcinogenic, as well as some noncarcinogenic, disorders and that “[r]easonable extrapolations” from these data suggested “that present ambient levels of vinyl chloride may cause or contribute to ... [such] disorders.” Id. at 59,533. In so deciding, the agency noted that vinyl chloride was “an apparent non-threshold pollutant,” which means that it appeared to create a risk to health at all non-zero levels of emissions, but that scientific uncertainty, due to the unavailability of dose-response data, made it impossible to establish any definite threshold level of adverse effects to human health. Id. at 59,534. In the face of this uncertainty, the EPA decided that setting emissions at the level achievable by the best available technology would substantially reduce emissions and provide stringent regulation that satisfied the command of providing “an ample margin of safety.” Id.
On October 21, 1976, the EPA promulgated final rules for vinyl chloride, expected to reduce emissions to 5% of unregulated levels. 41 Fed.Reg. 46,560 (1976). In promulgating these standards, the EPA stated that the “purpose of the standard is to minimize vinyl chloride emissions ... to the level attainable with best available control technology.” Id. The EPA also noted that it believed section 112 permits the Administrator to “assure that the costs of control technology are not grossly disproportionate to the level of emission reduction achieved.” Id. at 46,562. The Environmental Defense Fund (“EDF”) filed suit challenging the rules on the basis that section 112 required the Administrator to rely exclusively on health, and not at all on technological, considerations in standard setting. The EDF and EPA settled the suit, however, upon EPA’s agreement to propose new and more stringent rules for vinyl chloride and to establish an ultimate goal of zero emissions.
The EPA satisfied its obligations under the settlement agreement by proposing new regulations on June 2, 1977. While the proposal sought to impose more strict regulation and establish an aspirational goal of zero emissions, the EPA made it clear that it considered its previous regulations valid and reemphasized its view that the inability scientifically to identify a threshold of adverse effects did not require prohibition of all emissions, but rather permitted regulation at the level of best available technology. 42 Fed.Reg. 28,154 (1977). The EPA received comments on the proposal, but for over seven years took no final action. On January 9, 1985, the EPA withdrew the proposal. Noting that certain aspects of the proposed regulations imposed “unreasonable” costs and that no control technology “has been demonstrated to significantly and consistently reduce emissions to a level below that required by the current standard,” 50 Fed.Reg. 1182, 1184 (1985), the EPA concluded that it should abandon the 1977 proposals and propose in their place only minor revisions to the 1976 regulations.
This appeal followed.
II.
We must address at the outset two procedural challenges to the NRDC’s bringing this petition for review. First, an industry intervenor, the Vinyl Institute, argues that the petition for review is not timely filed. Second, the EPA argues that NRDC has *713failed to exhaust its administrative remedies and that we must, therefore, dismiss its petition for review. We address these contentions in turn.
A.
The Vinyl Institute argues that this court has no jurisdiction in this case because the statute provides that “[a]ny petition for review ... must be filed within sixty days from the date notice of [the] promulgation, approval, or action appears in the Federal Register, except that if the petition is based on grounds arising after such sixtieth day, then any petition for review ... shall be filed within sixty days after such grounds arise.” 42 U.S.C. § 7607(b)(1) (1982). According to the intervenor, the NRDC seeks to review in this case not the 1985 withdrawal of the proposed amendments, but the 1976 standards themselves. Because that statutory issue did not arise within sixty days before the filing of review, the intervenor claims the petition is untimely. Under Montana v. Clark, 749 F.2d 740, 744 (D.C. Cir.1984), cert. denied, - U.S. -, 106 S.Ct. 246, 88 L.Ed.2d 255 (1985), “an agency decision not to amend long-standing rules after a notice and comment period is reviewable agency action.” Thus, if the petition for review, filed within sixty days of the withdrawal of the proposed amendments, amounts to a genuine challenge to the withdrawal of the proposed regulations, it was timely filed. If, by contrast, Vinyl Institute is correct in asserting that this appeal in fact constitutes a substantive attack on the 1976 regulations, we must dismiss the suit as untimely filed. See Professional Drivers Council v. Bureau of Motor Carrier Safety, 706 F.2d 1216, 1217-18 n. 2 (D.C. Cir.1983). We believe the former is the more accurate characterization of this lawsuit.
The contention that this case amounts to a back-door challenge to the 1976 regulations is refuted by the substance of petitioner’s brief and the relief requested. The petitioner states that “[i]n withdrawing the proposed amendments EPA violated the law by employing cost-benefit and technological feasibility tests that are prohibited by the Clean Air Act.” Brief for NRDC at 3. Indeed, the brief makes explicit that the petitioner is specifically challenging the EPA’s reliance on cost and technological feasibility tests in its withdrawal of the proposed amendments. Id. at 12-13. Most importantly, the petitioner does not seek to have us overturn the 1976 standards, but rather seeks to have us overturn the 1976 standards, but rather seeks to have us vacate the EPA’s decision to withdraw the amendments and order agency action consistent with our opinion. See Brief for NRDC at 36-37. While, if we were to agree with the NRDC that the statute prohibits any cost or technological considerations in standard setting, our rationale might indicate the invalidity of the 1976 regulations, our decision would not of itself invalidate those regulations. We think it clear, therefore, that the NRDC has challenged the 1985 withdrawal of the proposed amendments. The petition for review is timely.
B.
The EPA argues that the petitioner has failed to exhaust available administrative remedies and asks us to dismiss the petition for that reason. Congress included in section 307(d) of the Clean Air Act a statutory requirement of exhaustion, providing that “[o]nly an objection to a rule or procedure that was raised with reasonable specificity during the period for public comment (including any public hearing) may be raised during judicial review.” 42 U.S.C. § 7607(d)(7)(B) (1982). This statutory requirement of exhaustion, however, does not apply here. The statute states that “[t]he requirements of ... subsection [307(d) of the Act] shall take effect with respect to any rule the proposal of which occurs after ninety days after August 7, 1977.” 42 U.S.C. § 7607(d)(ll) (1982). The withdrawal of the proposed amendments now before us constitutes final agency action on the notice of proposed rulemaking that came out on June 2, 1977. See 42 Fed.Reg. 28,154 (1977). Thus, even if we assume that the action of withdrawing a proposed *714rule amounts to a “rule” for the purposes of section 307(d)’s timing provision, the proposal withdrawn here was issued before the date section 307(d) took effect. Accordingly, we must look to the common law doctrine of exhaustion of remedies. See Safir v. Kreps, 551 F.2d 447, 452 (D.C. Cir.), cert. denied, 434 U.S. 820, 98 S.Ct. 61, 54 L.Ed.2d 76 (1977). The result, however, is the same.
Courts have long required that a party seeking review of agency action exhaust its administrative remedies before seeking judicial relief. See, e.g., Myers v. Bethlehem Shipbuilding Corp., 303 U.S. 41, 50-51, 58 S.Ct. 459, 463, 82 L.Ed. 638 (1938). In the case before us, the administrative remedy was participation in the rulemaking proceedings during the comment period. Indeed, this court generally requires such participation as a prerequisite to petitioning for direct review of the resulting regulations. See Environmental Defense Fund v. EPA, 598 F.2d 62, 91 (D.C. Cir.1978).
The NRDC did not participate in the rule-making proceedings in this case, but argues that we should not dismiss its petition for review because the agency in fact considered the statutory issue pressed on appeal. The NRDC is correct. This court has excused litigants from their exhaustion obligations as to a particular issue so long as the agency in fact considered the issue. See Washington Association for Television & Children v. FCC, 712 F.2d 677, 682 n. 10 (D.C. Cir.1983); Etelson v. Office of Personnel Management, 684 F.2d 918, 923 (D.C. Cir.1982); ASARCO, Inc. v. EPA, 578 F.2d 319, 320-21 n. 1 (D.C. Cir.1978); Safir v. Kreps, 551 F.2d at 452. Thus, courts have waived exhaustion if the agency “has had an opportunity to consider the identical issues [presented to the court] ... but which were raised by other parties,” see Buckeye Cablevision, Inc. v. United States, 438 F.2d 948, 951 (6th Cir.1971), or if the agency’s decision, or even a dissenting opinion, makes it clear that the agency had “the opportunity to consider” “the very argument pressed” by the petitioner on judicial review. Office of Communication of the United Church of Christ v. FCC, 465 F.2d 519, 523 (D.C. Cir.1972).
In this case, the issue whether the clear meaning of section 112 precluded the consideration of cost or technological feasibility in standard setting was adequately raised before the agency. First, the 1977 proposed amendments were the product of the settlement of a lawsuit challenging the previous vinyl chloride standards as having impermissibly taken consideration of feasibility into account. This demonstrates that the agency had notice of the argument that the statute precluded such considerations, and that the agency did or should have taken this into account in reaching a final decision on the proposed amendments. Indeed, in its notice of proposed rulemaking, the EPA remarked that “[t]he [1976] vinyl chloride standard has been criticized for allegedly placing unwarranted emphasis on technological rather than health considerations.” 42 Fed.Reg. 28,154 (1977). The notice then continued by discussing the “ample margin of safety” language, the potential problem, under this standard, of having to shut down an entire industry that produces a non-threshold pollutant, and the way the proposed amendments resolved the problem by moving toward zero emissions without banning vinyl chloride. Id. Thus, it is clear that the EPA considered the question whether the language of section 112 requires a purely health-based standard.
Moreover, EDF explicitly raised the issue before the EPA in its comments on the proposed amendments. In this respect, the EDF stated:
The proposed amendments represent a true compromise between what EDF could have pressed for in court and the existing standard. Section 112 of the Clean Air Act requires that emission standards for hazardous air pollutants be set “at a level which in the judgment of the Administrator provides an ample margin of safety to protect the public health from such hazardous air pollutants.” It clearly requires a health-*715linked, not a technology-based standard. Yet, inconsistent with the statutory requirement, the original standards were based on what EPA believed industry could accomplish with best available technology____ EPA recognized that vinyl chloride is “an apparent non-threshold pollutant” which creates a risk to public health at all levels. Had the case gone to trial, EDF would have taken the position that § 112 required a zero emission standard, the only standard adequate to provide the required margin of safety for a non-threshold pollutant. Instead, EDF settled for a compromise which establishes a goal of zero emissions and requires industry to move one step closer to that goal.
J.A. at 72-73. EDF’s comments contain other, similar references, such as the assertion, in response to cost arguments raised by the industry, that “the statute EPA operates under requires regulations based on protection of health and not cost and technology concerns.” Id. at 83. Thus, the EPA had before it the question whether the statute permits considerations of cost and technology in setting standards, and it had the opportunity to consider that question in deciding to withdraw the proposed amendments. Accordingly, we may not dismiss on the grounds of failure to exhaust.
The EPA also suggests, however, that we should be “especially” loath to allow this petition for review because the “NRDC chose not to participate at all in any of the administrative proceedings on vinyl chloride.” Brief for EPA at 12 (emphasis in original). This merely restates the proposition that the NRDC has failed to exhaust its administrative remedies. None of the cases relied upon by the EPA suggests that exceptions to exhaustion have any less applicability in the case of a wholly absent party than in other exhaustion contexts. See Environmental Defense Fund v. EPA, 598 F.2d 62, 91 (D.C. Cir.1978); Nader v. Nuclear Regulatory Commission, 513 F.2d 1045, 1054-55 (D.C. Cir.1975). This is not a case in which the statute conditions a party’s ability to obtain judicial review upon its participation in the rule-making proceedings. See Gage v. Atomic Energy Commission, 479 F.2d 1214, 1218 (D.C. Cir.1973). The jurisdictional provision of the Clean Air Act imposes no such prerequisite, and, in fact, employs rather permissive language not specifying who may bring review. See 42 U.S.C. § 7607(b) (1982) (“A petition for review of action of the Administrator in promulgating ... any emission standard or requirement under section 7412 ... may be filed ... in the United States Court of Appeals for the District of Columbia.”). The NRDC’s total abstention from participating in the rule-making proceedings does not make the exhaustion requirement more compelling or negate the valid exception to that requirement asserted by the NRDC.
III.
The NRDC mounts only a narrow challenge: that the statute adopts an exclusive focus on health-based considerations and that the EPA, therefore, relies on statutorily impermissible factors in using cost and technological feasibility as the basis for withdrawing the 1977 proposed amendments. According to petitioner, the EPA’s action was “arbitrary and capricious” under Motor Vehicle Manufacturers Association v. State Farm Mutual Automobile Insurance Co., 463 U.S. 29, 43, 103 S.Ct. 2856, 2866, 77 L.Ed.2d 443 (1983). See Brief for NRDC at 17-18 & n. 32. We turn then to the question whether the NRDC has met its burden of making this showing. See San Luis Obispo Mothers for Peace v. United States Nuclear Regulatory Com-. mission, 789 F.2d 26, 37 (D.C. Cir.1986) (en banc), cert. denied, — U.S. —, 107 S.Ct. 330, 93 L.Ed.2d 302 (1986). We think it has not.
A.
This being a question of statutory interpretation, the inquiry of course begins with “the language employed by Congress.” Reiter v. Sonotone Corp., 442 U.S. 330, 337, 99 S.Ct. 2326, 2330, 60 L.Ed.2d 931 *716(1979). The statute commands the Administrator to set an “emission standard” for a particular “hazardous air pollutant” so as to “provide[ ] an ample margin of safety to protect the public health.” Petitioner argues that these terms are plain and direct the Administrator to consider only factors relating to health. We find the statute less clear than does the petitioner. The mandate “to protect the public health” unambiguously evinces a health-based goal as the primary aim of section 112 of the Act. The complementary standard of providing an “ample margin of safety,” however, contemplates some discretion in the regulatory process, thus raising the possibility that the Administrator may properly consider non-health-based factors in deciding upon an appropriate level of regulation.
The statute nowhere defines “ample margin of safety.” The Senate Report, however, in discussing a similar requirement in the context of setting ambient air standards under section 109 of the Act, explained the purpose of the “margin of safety” standard as one of affording “a reasonable degree of protection ... against hazards which research has not yet identified.” S.Rep. No. 1196, 91st Cong., 2d Sess. 10 (1970) (emphasis added). This view comports with the historical use of the term in engineering as “a safety factor ... meant to compensate for uncertainties and variabilities.” See Hall, The Control of Toxic Pollutants Under the Federal Water Pollution Control Act Amendments of1972, 63 Iowa L.Rev. 609, 629 (1978). In a discussion of the use of identical language in the Federal Water Pollution Control Act, this court has recognized that, in discharging the responsibility to assure “an ample margin of safety,” the Administrator faces “a difficult task, indeed, a veritable paradox— calling as it does for knowledge of that which is unknown — [but] ... the term ‘margin of safety’ is Congress’s directive that means be found to carry out the task and to reconcile the paradox.” Environmental Defense Fund v. EPA, 598 F.2d 62, 81 (D.C. Cir.1978). And while Congress used the modifier “ample” to exhort the Administrator not to allow “the public [or] the environment ... to be exposed to anything resembling the maximum risk” and, therefore, to set a margin “greater than ‘normal’ or ‘adequate,’ ” Congress still left the EPA “great latitude in meeting its responsibility.” See id.
Petitioner’s assertion that health-based factors constitute the only permissible factors to consider in standard-setting under section 112 appears implausible precisely because the statute brings the Administrator’s discretion and judgment to bear on scientific uncertainty. If health were the only permissible consideration, no such discretion would be necessary, for deciding how much uncertainty to allow from a strictly health-based perspective would always lead to the same answer — none. Whenever any scientific uncertainty existed about the ill effects of a non-zero level of hazardous air pollutants — and we find it unthinkable that science may ever yield absolute certainty of safety in an area so complicated and replete with problems of measurement, modeling, long latency, and the like — any decision informed solely by health, but no other, values would require a prohibition of any emissions. Had Congress intended that result, it could very easily have said so by writing a statute that states that no level of emissions shall be allowed as to which there is any uncertainty. But Congress chose instead to deal with the pervasive nature of scientific uncertainty and the inherent limitations of scientific knowledge by vesting in the Administrator the discretion to deal with uncertainty in each case.
B.
Petitioner also argues that the legislative history makes clear Congress’ intent to foreclose reliance on non-health-based considerations in standard setting under section 112. The NRDC directs us to the hazardous air pollutants provision of the House bill, which states that “[i]f ... emissions [from any class of new stationary sources] are extremely hazardous to health, no new source of such emissions *717shall be constructed or operated, except where (and subject to such conditions as he deems necessary and appropriate) the [administrator] makes a specific exemption with respect to such construction or operation.” See H.R. 17255, 91st Cong., 2d Sess. § 5(a), 116 Cong.Rec. 19,226 (1970). Thus, as to extremely hazardous emissions, the House bill granted the Administrator a rather open-ended power to exempt a source from the regulation imposed, a power that petitioner presumes to have allowed for exemptions on the basis of non-health considerations.1 By contrast, petitioner notes, the Senate bill had a tight focus on health, prohibiting emissions “hazardous to the health of persons” and allowing only health-based exceptions to that prohibition. See S. 4358, 91st Cong., 2d Sess. § 6(b), 116 Cong.Rec. 32,375 (1970). Because the final version that emerged from conference more closely resembled the Senate than the House bill, and because no express provision for any “specific exemption” survived, the NRDC argues that any feasibility considerations must have been deliberately eliminated. We find this reading of the legislative history strained.
While the original Senate bill is closer than the House bill to the final legislation, neither the House nor the Senate version closely resembles in the aspect relevant here the compromise that emerged from conference. H.R. 17255 dealt only with new stationary sources and, with respect to those, only half of the regulatory scheme dealt with emissions considered “extremely hazardous to health.” See H.R. 17255, 91st Cong., 2d Sess. § 5(a), 116 Cong.Rec. 19,-225-26 (1970). The bill also dealt with new sources the emissions of which could “contribute substantially to endangerment of the public health or welfare,” but which were not “extremely hazardous to health,” providing for control of such emissions “to the fullest extent compatible with the available technology and economic feasibility.” Id. In effect, therefore, the House bill amounted to a comprehensive measure generically aimed at dealing with new sources, and only incidentally treated the problem of “extremely hazardous” agents.
Unlike the House bill, the Senate version dealt only with hazardous air pollutants and did so with respect to all stationary sources. The bill proposed a relatively narrow definition of hazardous agents, restricting this category to pollutants “whose presence, chronically or intermittently; in trace concentrations in the ambient air, either alone or in combination with other agents, causes or will cause, or contribute to, an increase in mortality or an increase in serious irreversible or incapacitating reversible damage to health.” S. 4358, 91st Cong., 2d Sess. § 6(b), 116 Cong.Rec. 32,-375 (1970). Under the scheme set up by the bill, the Administrator was to publish a list of hazardous agents, and follow it by a “proposed prohibition of emissions of each such agent or combination of agents from any stationary source.” Id. The bill then *718provided for a hearing, after which the Administrator had to promulgate the prohibition unless a preponderance of evidence demonstrated either “that such agent is not hazardous to the health of persons” or “that departure from prohibition for [a] stationary source will not be hazardous to the health of persons.” Id. If the Administrator found either such condition to exist, he would then implement an emission standard in lieu of a prohibition. Id.
Given these starting points, the inference the petitioner draws from the changes made at conference appears tenuous at best. The final version defines a “hazardous air pollutant” as
an air pollutant to which no ambient air quality standard is applicable and which in the judgment of the Administrator may cause, or contribute to, air pollution which may reasonably be anticipated to result in an increase in mortality or an increase in serious irreversible, or incapacitating reversible, illness.
Clean Air Amendments of 1970, Pub.L. No. 91-604, § 4(a), 84 Stat. 1676, 1685.2 The statute instructs the Administrator to publish a list of such pollutants, to conduct hearings, and, unless the hearings show a particular agent not to be a “hazardous air pollutant,” to promulgate emissions standards. Id. The Administrator must “establish any such standard at the level which in his judgment provides an ample margin of safety to protect the public health from such hazardous air pollutant.” Id. Petitioner has correctly observed that the law enacted has a closer structural resemblance to the Senate bill, but this offers little, if any, support to petitioners’ claim because the Senate bill and the final legislation both were measures to deal with the problem of hazardous air pollutants from all sources. That they would resemble each other more than a House bill meant to deal generically with the problem of all pollution from new sources seems a natural outcome for that reason alone and thus sheds no light on the intent to adopt particular aspects of the Senate version in the final bill.
Nor do we accept petitioner’s contention that the final legislation’s failure to allow for “specific exemptions” forecloses consideration of non-health factors. The House bill made such “specific exemptions” available only where the bill prohibited the construction or operation of new sources. Because the final version enacted contains no prohibitory provision of any kind, failure to include the power to exempt a source from a non-existent blanket prohibition tells us little. It seems more likely that the power to exempt was omitted because the considerations that might have gone into deciding whether and under what conditions to exempt a source, which we think did relate to cost and technology, see infra pp. 722-23, were now to be taken into account in deciding on the margin of safety to provide.
Moreover, we believe that the petitioner’s reliance on the Senate bill to support its claim about the exclusivity of health-based considerations is misplaced. The emphasis on factors of health in the Senate bill, to the extent that it reveals anything, seems actually to cut against the petitioner’s argument. The Senate bill sought to prohibit emissions which were hazardous even in “trace” concentrations and allowed the imposition of emissions standards in other cases. An “emission standard” “limits the quantity, rate, or concentration of emissions of air pollutants on a continuous basis.” 42 U.S.C. § 7602(k) (1982) (emphasis added). And the definition of “hazardous air pollutants” in the final legislation is clearly broad enough to include substances hazardous to health even in “trace” concentrations. One might conclude, therefore, that Congress was aware of the distinction between a “prohibition” and an “emission standard,” and that the ultimate abandonment of the former in the final version of *719the statute may evince an intent not to prohibit emissions of even those hazardous pollutants known to pose a danger to public health in trace amounts. Such a conclusion, of course, would flatly contradict the petitioner’s position by indicating a retreat in the final legislation from the exclusive concern with health in the Senate bill.
We do not believe that the final bill precludes a flat prohibition of emissions of a hazardous pollutant. A summary of the provisions of the conference agreement presented by Senator Muskie, the principal Senate sponsor of the Clean Air Amendments of 1970 and the chairman of the Senate conferees, made it clear that the “ample margin of safety” standard might require “a plant ... to close” or “could include emission standards which allowed for no measurable emissions.” 116 Cong. Rec. 42,385 (1970). This suggests that, rather than backing away from the protection provided in the Senate bill, the “ample margin of safety” standard provided the flexibility to provide for such protection. A hazardous pollutant demonstrated to have ill effects at trace concentrations may clearly be prohibited under this standard. Absent the explanatory statement by a reliable source about what the conferees intended, however, we have no way of knowing the meaning of the change from specific regulatory commands to a different and more general pattern.
And so it is with the House bill. At conference, the specific provisions of the House bill gave way to a more amorphous standard in which the sources and types of pollution covered were different and the Administrator was given the power to regulate in his sound judgment to protect the public health with a reasonable degree of caution for scientific unknowns. We know of no statement, nor has petitioner directed us to any, that reliably indicates what the conferees intended in replacing the specific regulatory scheme for new sources with the broad standard covering all hazardous pollutants in the final bill. At most, therefore, we find the legislative history of the 1970 Clean Air Amendments ambiguous with respect to what the Administrator may consider in setting standards for hazardous air pollutants. Some of the history supports an inference that factors other than health might appropriately be taken into account at the Administrator’s discretion. Petitioner has not met its burden of showing that section 112(b) permits only the consideration of health.
C.
On the other side of this controversy, the EPA argues that the 1977 amendments of the Clean Air Act, in the light of Congress’ awareness of the 1976 vinyl chloride regulations, amounts to a ratification of the use of cost and technological feasibility considerations in standard setting under section 112. We think this overstates the significance of the legislative history leading up to the Clean Air Act Amendments of 1977. To understand why this is the case, and to appreciate what significance, if any, the 1977 amendments have, we turn to an examination of the history of those amendments.
In 1976, both houses of Congress passed bills purporting to amend the Clean Air Act. The first section of the House bill sought to spur the EPA to take action with respect to specified unregulated pollutants, including vinyl chloride. Within one year of the enactment of the amendments unless the Administrator found after notice and a hearing that the enumerated “substance [would] not cause or contribute to air pollution which [could] reasonably be anticipated to endanger public health,” he was to include such substance on the list of pollutants subject to regulation under an ambient air standard pursuant to sections 108 through 110 or under the hazardous air pollutant provisions of section 112, or to include sources of such pollutants on the list of stationary sources governed by section Ill’s new source performance standards, or to implement some combination of such regulation. H.R. 10498, 94th Cong., 2d Sess. § 101(a), 122 Cong.Rec. 29,-219 (1976).
*720In addressing this section of the bill, the Report of the Committee on Interstate and Foreign Commerce discussed the vinyl chloride problem in some detail, emphasizing the dangerous nature of the substance. H.R.Rep. No. 1175, 94th Cong., 2d Sess. 23 (1976). During the development of the bill, the Report noted, the EPA had “proposed emission standards for vinyl chlorides under section 112 of the Act for major sources in the plastics industry,” but the Committee retained vinyl chlorides in the proposed legislation to underscore “the Committee’s concern that the standards be promulgated without delay and that standards be promulgated for any other significant sources of vinyl chlorides which may exist.” Id. at 23-24. The Committee also noted, however, that it “did not intend to specify the degree of emission reduction which should be required,” but that “the Administrator should apply the appropriate means and extent of regulation under the existing statutory criteria.” Id. at 26. The House passed the bill, leaving section 101 intact.
The Senate bill contained nothing similar. The Conference Committee, however, decided to adopt the House provision regarding unregulated pollutants in relevant part. See H.R.Rep. No. 1742, 94th Cong., 2d Sess. 25-26 (1976). The threat of a total filibuster, however, prevented the Senate from voting on the recommendations of the conferees, and the bill died.
In 1977, both houses reintroduced legislation to amend the Clean Air Act. In the interim between the abandonment of the 1976 amendments and the introduction of the new legislation, the EPA promulgated emission standards for vinyl chloride under section 112 of the Act. See 48 Fed.Reg. 46,560 (1976). In so doing, the EPA had clearly articulated that the regulations adopted reduced vinyl chloride emissions “to the level attainable with best available technology” and that, while “section 112 does not explicitly provide for consideration of costs,” the agency believed it could take them into account for the limited purpose of “assurpng] that the costs of control technology are not grossly disproportionate to the amount of emission reduction achieved.” Id. at 46,560, 46,562. The 1977 House bill contained a provision “nearly identical” to section 101 of the 1976 House bill, differing primarily in its inclusion of radioactive materials and deletion of vinyl chloride from the unregulated pollutants specified. H.R.Rep. No. 294, 95th Cong., 1st Sess. 3 (1977), U.S.Code Cong. & Admin.News 1977, pp. 1077, 1080. The Committee explained the deletion of vinyl chloride on the ground that “[d]uring the past year the Administrator [had] promulgated final regulations for the control of vinyl chloride.” Id.
The bill passed the House with the unregulated pollutants provision intact. Once again, the Senate bill had no such provision, and, at conference, the House’s provision was adopted in relevant part. See H.R.Rep. No. 564, 95th Cong., 1st Sess. 141-42 (1977). The conference recommendations regarding unregulated pollutants passed both houses intact, and President Carter signed the Clean Air Act Amendments into law on August 7, 1977. See Pub.L. No. 95-95, 91 Stat. 685 (1977).
The 1977 legislation comprehensively amended the Clean Air Act, and, in fact, amended the very section that is the subject of this lawsuit. Indeed, that amendment added a further subsection employing substantially the language that EPA had construed in the vinyl chloride regulations to allow consideration of economic and technological feasibility. The relevant amendment to section 112 empowered the Administrator under certain circumstances to forgo use of an emission standard and instead to “promulgate a design, equipment, work practice, or operational standard, or combination thereof, which in his judgment is adequate to protect the public health ... with an ample margin of safety.” 42 U.S.C. § 7412(e)(1) (1982) (emphasis added); see also Pub.L. No. 95-95, 91 Stat. 685, 703 (1977).3 Thus, at the time of *721the 1977 amendments, Congress expressly considered the kind of regulation the EPA should apply to hazardous air pollutants, once identified, and, in so doing, reenacted the standard construed by the EPA in the vinyl chloride regulations.
The House knew of the 1976 regulations,4 and the failure to clarify the “ample margin of safety” requirement when adopting that language anew in the amendment adding section 112(e) may, therefore, indicate that the EPA has correctly discerned legislative intent. See United States v. Rutherford, 442 U.S. 544, 554 n. 10, 99 S.Ct. 2470, 2476 n. 10, 61 L.Ed.2d 68 (1979). Indeed, if the House’s and the House Committee’s awareness of what was taking place could confidently be attributed to the entire Congress, the history recited of reactions in 1976 and 1977 would make a considerable case for ratification. But we cannot be certain that Congress was aware of the content of the vinyl chloride regulations,5 and, therefore, we give the failure to repudiate the EPA’s substantive interpretation of section 112 in those regulations only “modest weight,” see National Wildlife Federation v. Gorsuch, 693 F.2d 156, 167 (D.C. Cir.1982), and we certainly cannot construe Congress’ failure to act in these circumstances as amounting to ratification of the EPA’s construction.6 Congress, in confronting the problem of unregulated pollutants, sought only to provoke some action by the EPA, and in no way aimed to specify the appropriate degree of emission control. That the House knew of the existence of the 1976 standard for vinyl chloride and decided to remove it from the unregulated pollutants list does not, therefore, tell us whether the House examined and became aware of the content of those regulations or the theory or level of the controls imposed. The history of the 1977 amendments may give a scintilla of evidence in support of the agency position here, but it is far short of legislative ratification of the EPA’s construction.
*722D.
Because the statute does not seem to restrict the Administrator to health-based considerations, and furthermore is ambiguous in that it does not specify what additional factors the Administrator may permissibly take into account in “provid[ing] an ample margin of safety to protect the public health,” we may not “simply impose [our] own construction on the statute.” Chevron U.S.A. v. Natural Resources Defense Council, 467 U.S. 837, 843, 104 S.Ct. 2778, 2782, 81 L.Ed.2d 694 (1984). As the Supreme Court said, “if the statute is silent or ambiguous with respect to the specific issue, the question for the court is whether the agency’s answer is based on a permissible construction of the statute.” Id. Accordingly, we must uphold the Administrator’s construction of the section if it is “a reasonable one,” that is, if the Administrator’s decision represents “a reasonable policy choice for the agency to make.” Id. at 845, 104 S.Ct. at 2783.
We believe that the agency in this case has made a reasonable interpretation. Contrary to the Dissent’s view of what is at issue here, see Dissent infra at 734-35, the EPA has not taken the position that it may consider cost and technological feasibility to set a standard that allows a level of emission at or above which evidence has indicated adverse health effects to occur. Perhaps if the evidence positively demonstrated that a given substance had ill effects and endangered the public health in trace amounts, the EPA could permit no emissions of that pollutant. Only when an area of uncertainty exists does the Administrator “reconcile the paradox’’ of having to protect against dangers he cannot know by setting standards as strict as possible given both available technology and the requirement that the cost of reduction not be grossly disproportionate to the level achieved. That the area of uncertainty, as with vinyl chloride, covers all non-zero levels of emission does not alter our conclusion.7
Since the Administrator has no way of knowing health effects in the range of uncertainty, such considerations as technological and economic feasibility seem natural, perhaps inevitable, choices to inform the Administrator’s decision whether he has amply provided for a reasonable degree of safety from the unknown. By emphasizing available technology, the EPA has ensured the maximum regulation against uncertainty without the economic and social displacements that would accompany the closing of an industry or any substantial part of an industry. By ensuring that costs do not become grossly disproportionate to the level of reduction achieved, the EPA guarantees that the consuming public does not *723pay an excessive price for the marginal benefits of increasing increments of protection against the unknown. We cannot say that this represents an unreasonable weighing of values, especially when no other value readily suggests itself and petitioner supplies none apart from health effects, which in the range of uncertainty are by definition unknowable.
The statute, moreover, explicitly endorses economic values, suggesting that the use of such values to inform ambiguous provisions is not unreasonable. Section 112(e) specifically allows for the use of design, equipment, work practice, and operational standards in lieu of emissions standards where “the application of measurement methodology to a particular class of sources is not practicable due to technological or economic limitations.” 42 U.S.C. § 7412(e)(1) & (2) (1982). Where measurement is impractical because of technological or economic factors, uncertainty as to health effects necessarily exists. It is significant both that uncertainty need not he dissipated if the cost is prohibitive and that requirements other than emissions limits may then be imposed. Congress has thus explicitly expressed its belief that technological and economic values have a place in the regulation of hazardous pollutants.8 We believe, therefore, that the EPA has acted reasonably in relying on economic and technological feasibility, and we cannot
overturn the withdrawal of the proposed regulations on the basis that the EPA so relied.
E.
We think this analysis of the statute demonstrates that Congress did not preclude consideration of economic and technological considerations in the range of emissions where health effects are uncertain. The language Congress used is quite inappropriate to support any such conclusion. Had that result been desired, Congress could easily have stated that where health effects at any level are unknown there shall be zero emissions. That does not even remotely resemble the statute we have before us.
IV.
The cases cited by petitioner in support of its claim do not alter our conclusion. No case has squarely addressed what the Administrator may consider in the context of section 112(b)’s command to “provide an ample margin of safety to protect the public health.” While some cases have construed similar or identical language to foreclose consideration of technological and economic feasibility, we do not find the reasoning of those cases compelling. Nor do the various dicta petitioner pulls from oth*724er, related cases persuade us that we have reached the wrong conclusion. It must be remembered that each statute and provision has its own structure and legislative history so that a decision about one cannot be considered to control the interpretation of another. Both similarities and dissimilarities must be considered in deciding how persuasive the analogy is.
Petitioner’s strongest case is Lead Industries Association v. EPA, 647 F.2d 1130 (D.C.Cir.), cert. denied, 449 U.S. 1042, 101 S.Ct. 621, 66 L.Ed.2d 503 (1980). In Lead Industries, this court reviewed a challenge by representatives and members of the regulated industry to the promulgation of primary air quality standards for lead under section 109 of the Clean Air Act.9 Petitioners in that case argued that the Administrator must consider economic impact and technological feasibility in determining the appropriate margin of safety to set under statutory language requiring that the standards “allow[] an adequate margin of safety ... to protect the public health,” 42 U.S.C. § 7409(b)(1) (1982). See 647 F.2d at 1148. The court not only rejected this argument, but also went on to state that section 109 of the Act affirmatively precluded consideration of feasibility in standard setting. The NRDC argues, therefore, that Lead Industries, which involved the more permissive language of “adequate,” rather than “ample,” “margin of safety,” compels the conclusion that section 112 precludes consideration of economic and technological feasibility. We think not.
The dicta in Lead Industries on which petitioner relies rest upon statutory premises and legislative history inapposite to this case. The court, significantly, did not assert that the statutory language precluded consideration of feasibility. In this respect, the opinion stated merely that “[njothing in its language suggests that the Administrator is to consider economic or technological feasibility in setting ambient air quality standards.” 647 F.2d at 1148-49. This says only that the “margin of safety” language does not demand consideration of such factors, something that no party in the case before us disputes.
The Lead Industries court did state that the statute on its face does not allow consideration of technological or economic feasibility, but the court based its conclusion on structural aspects of the ambient air pollution provisions not germane here. The court relied on sections of the Act closely related to section 109 in reaching its determination. First, besides “allowing an adequate margin of safety,” ambient air standards set under section 109(b) must be based on so-called “air quality criteria,” which section 108 defines as comprising several elements, all related to health. See 42 U.S.C. § 7408(a)(2)(A), (B), & (C) (1982). The court reasoned that the exclusion of economic and technological feasibility considerations from air quality criteria also foreclosed reliance on such factors in setting the ambient air quality standards based on those criteria. 647 F.2d at 1149 n. 37. The court also relied on the fact that state implementation plans, the means of enforcement of ambient air standards, could not take into account economic and technological feasibility if such consideration interfered with the timely attainment of ambient air standards, and that the Administrator could not consider such fea*725sibility factors in deciding whether to approve the state plans. Id.; see 42 U.S.C. § 7410 (1982). This provided further grounds for the court to believe that Congress simply did not want the economics of pollution control considered in the scheme of ambient air regulations. See 647 F.2d at 1149 n. 37.
Moreover, the relevant Senate Report stated flatly that “existing sources of pollutants either should meet the standard of the law or be closed down.” 647 F.2d at 1149. This is a far clearer statement than anything in the present case that Congress considered the alternatives and chose closing down sources or even industries rather than allow risks to health.
The substantive standard imposed under the hazardous air pollutants provisions of section 112, by contrast, is not based on criteria that enumerate specific factors to consider, yet pointedly exclude feasibility. Section 112(b)(l)’s command “to provide an ample margin of safety to protect the public health” is self-contained, and the absence of enumerated criteria may well evince a congressional intent for the Administrator to supply reasonable ones. And while the hazardous pollutants provision does all for state implementation plans, section 112, in marked contrast to the regime of ambient air standards, operates through nationally enforced standards; the state plans are permissive and may not interfere with national enforcement of any hazardous pollutant standard. 42 U.S.C. § 7412(d) (1982). No detailed provisions preclusive of technological and economic considerations govern the state plans allowed under section 112; indeed, the Administrator must delegate enforcement and implementation authority to the state (subject to his continuing ability to enforce national standards) if he finds the state plan “adequate.” Id. Thus, nothing in the scheme of state implementation plans under section 112 demonstrates disfavor for feasibility considerations, and this further distinguishes section 112 from the Lead Industries court’s interpretation of section 109.
Before turning from Lead Industries, we must also address another aspect of that opinion. The Lead Industries court did not “discern” in the margin of safety requirement “any congressional intent to require, or even permit, the Administrator to consider economic or technological factors in promulgating air quality standards.” 647 F.2d at 1150. In so concluding, the court was fully aware of the nature of the margin of safety standard and, indeed, quoted the Senate Report’s description of its purpose as one of providing “a reasonable degree of protection” from the unknown. Id. Thus, at first blush, the reasoning of Lead Industries may seem at odds with our conclusion that the margin of safety requirement is what allows for such consideration.
Closer examination reveals that our conclusion stands. First, the Lead Industries court faced the argument that “margin of safety” required feasibility considerations, and, to the extent that it said the language and history of this requirement did not “even permit” such considerations, the court’s statement was not part of the rationale by which it decided the contention before it but amounted only to dicta. Second, these dicta must be understood in the context of where they appear in the Lead Industries opinion. The court’s discussion of “margin of safety” immediately followed a thoughtful and comprehensive analysis of the legislative history of the ambient air pollution scheme found in sections 108 through 110. This analysis concluded that Congress meant ambient air standards to be affirmatively technology-forcing, and, therefore, consciously excluded any feasibility considerations from their formulation. See 647 F.2d at 1149. Accordingly, we read the court’s “margin of safety” discussion to mean that, in the light of overwhelming evidence that Congress did not want feasibility considered in setting ambient air standards, the court would not read feasibility considerations into whatever discretion the Administrator may have been given under the “margin of safety” requirement. In other words, the court could not discern any intent to permit *726such consideration in the face of other strong evidence to the contrary. No evidence of preclusion of economic and technological considerations has emerged in our examination of section 112 and its history. For that reason, Lead Industries does not control this case. Here, for the reasons given, we believe the Administrator is not foreclosed from including these considerations in the exercise of his discretion under the margin of safety requirement of section 112.
Petitioner also cites Hercules, Inc. v. EPA, 598 F.2d 91 (D.C. Cir.1978), in support of its claim that the “ample margin of safety” language prohibits consideration of cost and technological factors. Hercules involved section 307(a)(4) of the Federal Water Pollution Control Act, which directs the EPA to set standards for toxic water pollutants to provide “an ample margin of safety,” see 33 U.S.C. § 1317(a)(4) (1982). In relevant part, the decision dealt with an industry petitioner’s claim that certain regulations promulgated by the EPA under section 307(a) failed adequately to take feasibility into account. The EPA responded that section 307(a) does not require consideration of any such factor.
The court agreed with the EPA, principally on the ground that section 307(a)(2) enumerated six specific factors to take into account in setting standards for toxic water pollutants, and none involved economic or technological criteria. 598 F.2d at Ill. Reinforcing this interpretation was the fact that “[s]ection 307(a)(4) directs EPA to set standards providing ‘an ample margin of safety’ without any mention of feasibility criteria.” Id. This, however, does not support the NRDC’s position in this case, for Hercules merely stands for the proposition that the unadorned appearance of “ample margin of safety” does not require economic and technological considerations; the case says nothing about what such language may permit.
Nor do we find persuasive the dicta from Hercules that the NRDC cites on the subject of section 112 of the Clean Air Act. The Hercules court noted similarities between the Federal Water Pollution Control Act Amendments and the Clean Air Amendments of 1970. 598 F.2d at 112. The court then discerned a distinction applicable to both statutes positing “health-based” regulation for toxic water and hazardous air pollutants and “technology-based” regulation for other water and air pollutants. See id. The court also noted that “Congress enacted section 112 ... without provision for considerations of feasibility.” Id. All this is beside the point.
We may accept the health-based/technology-based distinction put forth by the Hercules court and still accept the EPA’s use of economic and technological feasibility in standard setting under section 112. Health, not technology, is both the starting point and the overriding consideration under the EPA’s construction of section 112. The EPA must set its standard at a level which eliminates known adverse health effects of the hazardous substance. It is only when those health effects become unknowable that the EPA turns to economic and technological feasibility to decide the level of emissions to permit. We do not think that this incidental consideration of non-health factors makes the withdrawal of the proposed standards pursuant to the EPA’s construction of section 112 technology-based, rather than health-based.
Nor do we believe that the Hercules court’s casual observation that section 112 makes no provision for feasibility changes the analysis. Section 112 provides discretion in standard-setting, does not affirmatively preclude feasibility considerations, and may, we believe, reasonably accommodate such considerations. In these circumstances, the EPA’s construction of section 112 to allow for feasibility considerations is permissible.
In this vein, petitioner sets forth one final argument that we address. Petitioner attempts to erect a policy of clear statement in environmental statutes, such that economic and technological considerations may not enter the calculus of environmental regulation unless the statute expressly so provides. In support of this thesis, peti*727tioner first cites American Textile Manufacturers Institute v. Donovan, 452 U.S. 490, 510, 101 S.Ct. 2478, 2491, 69 L.Ed.2d 185 (1981), which states: “When Congress has intended that an agency engage in cost-benefit analysis, it has clearly indicated such intent on the face of the statute.” Petitioner’s reliance on this statement is misplaced.
The Court in American Textile made the statement quoted in response to an argument that a statute exhorting the Secretary of Labor to consider “feasibplity]” required cost-benefit analysis. In this light, the Court’s statement appears to mean that Congress has clearly stated when it has sought to require an agency to engage in cost-benefit analysis. Moreover, the EPA has not engaged in that form of analysis here. Cost-benefit analysis means weighing the marginal gain against the marginal cost of each increment of further regulation and then setting the level of regulation at the point at which the latter exceeds the former. In this case, to the extent that the Administrator considers cost at all, he does so only to ensure that costs are not grossly disproportionate to benefits and only when health effects are uncertain. So even if American Textile does mean that Congress must clearly announce when it intends to permit cost-benefit analysis, that limitation has no application here.
Petitioner also brings to our notice Union Electric Co. v. EPA, 427 U.S. 246, 96 S.Ct. 2518, 49 L.Ed.2d 474 (1976), a case under the Clean Air Amendments of 1970 in which the Supreme Court stated: “Where Congress intended the Administrator to be concerned about economic and technological infeasibility, it expressly so provided.” Id. at 257 n. 5, 96 S.Ct. at 2525, n. 5; see also Lead Industries, 647 F.2d at 1148. Be that as it may, it does not alter our conclusion in this case. The EPA does not contend that Congress intended to require the Administrator to consider economic and technological feasibility in setting standards under section 112. But this does not mean that Congress intended to preclude the Administrator from employing such considerations. It is enough that Congress vested the Administrator with discretion to deal with scientific uncertainty under the “ample margin of safety” standard, and, that, in the absence of congressional direction as to the values the agency should use in guiding that discretion, the Administrator’s choice of economic and technological feasibility amounted to a reasonable one. Chevron requires no more for us to affirm the agency’s construction of the statute.
Accordingly, the decision of the EPA withdrawing the 1977 proposed regulations for vinyl chloride is
Affirmed.
. Petitioner’s assumption seems correct. The bill prohibited new sources if and because they emitted extremely hazardous pollutants. Allowing exemptions to such prohibitions without specifying the permissible bases for exemption seems to invite consideration of non-health factors, for it would be strange indeed to construct a scheme under which both a prohibition and an exemption from prohibition were available on the basis of the same criterion. To be sure, the Senate bill did set up a system under which the agency was to propose a prohibition of hazardous pollutants, and could refuse to promulgate that prohibition only for health-based reasons, see infra pp. 717-18, but this does not make any more plausible the notion that the House bill set up a prohibition and exemption on the sole basis of health. The Senate provision did not set up a system of prohibitions and exemptions, but rather a procedural system to guide the agency. See S. 4358, 91st Cong., 2d Sess. § 6(b), 116 Cong.Rec. 32,375 (1970). The agency was to publish a list of hazardous pollutants on the basis of "available material evidence.” This list would amount to a proposed prohibition of emissions of the substances on the list. The agency was then to provide notice and a public hearing for each agent or combination of agents included on the list and could refuse to promulgate the prohibition only if a preponderance of the evidence at the hearing refuted the initial basis for the substance’s inclusion. Under the House bill, the exemption applied to something still considered "extremely hazardous.” We do not see how such an exemption could rest on exclusively health-based considerations.
. Congress in the Clean Air Act Amendments of 1977 made a minor alteration in the definition of a “hazardous air pollutant," replacing "may cause, or contribute to” with "causes, or contributes to." Pub.L. No. 95-95, § 401(a), 91 Stat. 685, 791 (1977). This change does not affect the outcome of this case.
. The Administrator could employ this alternative kind of regulation when an emission stan*721dard proved infeasible because "(A) a hazardous pollutant or pollutants cannot be emitted through a conveyance designed and constructed to emit or capture such pollutant, or that any requirement for, or use of, such a conveyance would be inconsistent with any Federal, State, or local law, or (B) the application of measurement methodology to a particular class of sources is not practicable due to technological or economic limitations.” 42 U.S.C. § 7412(e)(2) (1982); Pub.L. No. 95-95, § 110, 91 Stat. 685, 703 (1977).
. The NRDC argues that because the EPA had promulgated its 1977 Notice of Proposed Rule-making to amend the vinyl chloride standard, it becomes unclear whether the EPA’s cost and technological feasibility interpretation was wellestablished at the time of the 1977 amendments. We think it was. Although embracing a zero emission goal, the notice explicitly disagreed with criticism that the 1976 regulations placed unwarranted emphasis on technological, rather than health-based goals. 42 Fed.Reg. 28,-154 (1977). More importantly, the proposed rules mandated only "more efficient use of existing control technology at existing plants and more effective controls at new plants." Id. The proposal encouraged new technology but refused to ban vinyl chloride because of the drastic implications such a measure would hold for the industry. Id. Thus, while more stringent, the proposed regulations did not by any means abandon the EPA’s earlier position.
. The EPA also argues that asbestos regulations based on feasibility considerations had been promulgated pursuant to § 112 prior to the 1977 amendments. We give this no weight because the EPA has given us no indication that Congress had any awareness of the existence of, let alone the content of, those regulations.
. A court must only sparingly accept arguments based on acquiescence or ratification, for "the Framers of our Constitution deliberately made the passage of legislation difficult — more difficult, for instance, than in parliamentary democracies — [and] Congress simply cannot be obliged affirmatively to correct subsequent administrative interpretations inconsistent with original legislative intent; that is the responsibility of the courts.” Coalition to Preserve the Integrity of American Trademarks v. United States, 790 F.2d 903, 917 (D.C.Cir.1986). Congressional inaction on a particular point may ”betoken[] unawareness, preoccupation, or paralysis,” rather than tacit assent. See Zuber v. Allen, 396 U.S. 168, 185-86 n. 21, 90 S.Ct. 314, 323-24 n. 21, 24 L.Ed.2d 345 (1969). Of course, in the case before us there was more than mere congressional silence; there was affirmative evidence that members of the House knew what EPA was doing and failed to object and that the House Committee employed language that showed a desire to achieve a reduction, but not necessarily the total elimination, of emissions. This is something more than mere failure to enact corrective legislation.
. Petitioner suggests that a deferential approach may not be appropriate in this case because vinyl chloride is "an apparent non-threshold pollutant,” and the legislative history makes clear that in such a case Congress intended to allow no measurable emissions. We disagree. Petitioner confuses a “non-threshold pollutant” with an "apparent non-threshold pollutant.” The former poses a known hazard at all nonzero levels. The latter may appear to pose a risk at all levels, but, by definition, sufficient data are not available to establish the hazard to public health. We believe that this is precisely the type of uncertainty for which the Administrator was vested with discretion, and even though the Administrator may assume that some risk to health is probable at all concentrations, it is still up to him to decide what constitutes a reasonable degree of protection given that uncertainty does exist.
The Dissent’s conclusion that our explanation of the distinction between "non-threshold pollutants” and "apparent nonthreshold pollutants” is “odd” reflects the Dissent’s misunderstanding of the definition of "apparent non-threshold pollutant.” The Dissent claims that because the EPA recognized that "any atmospheric concentration of VC poses some public health risk,” it has also effectively recognized that vinyl chloride is a non-threshold pollutant. See Dissent infra at 735. This is simply incorrect. The EPA explicitly stated that vinyl chloride was an apparent non-threshold pollutant because it appeared to create a risk to health at all non-zero levels of emissions, but scientific uncertainty made it impossible to establish any definite threshold level of adverse effects to human health. 40 Fed.Reg. at 59,534. Because of the uncertainty as to the threshold level, the EPA "assumed ... that there is no atmospheric concentration that poses absolutely no public health risk.” Id. It did not conclude that vinyl chloride was a non-threshold pollutant.
. We do not accept the NRDC’s argument that Congress' explicitly accounting for economic and technological feasibility in this context suggests a congressional intent that the Administrator consider these values only in deciding what type of regulatory standard to use and not in deciding the level of regulation to apply. Petitioner in effect asserts that Congress knew how to designate such factors and did so expressly where it intended their application. This argument falls short. Congress had very specific notions about when the Administrator could employ design, equipment, work practice, and operational, rather than emission, standards under § 112 and it expressed itself very specifically. See supra note 2. That Congress has explicitly provided for certain specific considerations in one detailed subsection does not seem to us a persuasive reason to conclude that failure to specify such considerations when employing a generalized standard in a different subsection forecloses reliance on those factors in fleshing out that standard. If, elsewhere in the statute, Congress had exhorted the Administrator "to provide an ample margin of safety to protect the public health” and then specifically noted that he could or should consider economic or technological feasibility in making his determination, the failure so to specify in § 112(b)(1) would seem persuasively to foreclose consideration of such factors.
Petitioner makes a similar argument with respect to the provision giving the Administrator the power to grant a two-year waiver for installation of controls imposed under § 112. See 42 U.S.C. § 7412(c)(l)(B)(ii) (1982). This argument has no merit. The provision relied on merely tells the outside limit that the Administrator may allow, if necessary, for the imposition of controls. It says nothing about economic or technological feasibility and can in no way be construed as an exclusive avenue for consideration of such factors, as petitioner suggests.
. The statutory scheme involved in Lead Industries regulates air pollutants the "emissions of which, in [the Administrator’s] judgment, cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare ... [and] the presence of which in the ambient air results from numerous or diverse mobile or stationary sources." 42 U.S.C. § 7408(a)(1) (1982). The Administrator must then publish "air quality criteria” for the pollutants thus listed. Id. § 7408(a)(2). Having done this, the Administrator prescribes primary and secondary ambient air standards based on these criteria. See 42 U.S.C. § 7409 (1982). Finally, states must adopt state implementation plans to meet these ambient air standards and must submit their plans to the EPA for approval. See 42 U.S.C. § 7410 (1982). The scheme under review in this case, regulation of hazardous air pollutants, complements the ambient air standards and covers only those pollutants "to which no ambient air quality standard is applicable." See 42 U.S.C. § 7412 (1982).