Man's ability to alter his environment has developed far more rapidly than his ability to foresee with certainty the effects of his alterations. It is only recently that we have begun to appreciate the danger posed by unregulated modification of the world around us, and have created watchdog agencies whose task it is to warn us, and protect us, when technological "advances" present dangers unappreciated or unrevealed by their supporters. Such agencies, unequipped with crystal balls and unable to read the future, are nonetheless charged with evaluating the effects of unprecedented environmental modifications, often made on a massive scale. Necessarily, they must deal with predictions and uncertainty, with developing evidence, with conflicting evidence, and, sometimes, with little or no evidence at all. Today we address the scope of the power delegated one such watchdog, the Environmental Protection Agency (EPA). We must determine the certainty required by the Clean Air Act before EPA may act to protect the health of our populace from the lead particulate emissions of automobiles.

Section 211(c)(1)(A) of the Clean Air Act1 authorizes the Administrator of EPA to regulate gasoline additives whose emission products "will endanger the public health or welfare * * *." 42 U.S.C. § 1857f-6c(c)(1)(A). Acting pursuant to that power, the Administrator, after notice and comment, determined that the automotive emissions caused by leaded gasoline present "a significant risk of harm" to the public health. Accordingly, he promulgated regulations that reduce, in step-wise fashion, the lead content of leaded gasoline.2 We must decide whether the Administrator properly interpreted the meaning of Section 211(c)(1)(A) and the scope of his power thereunder, and, if so, whether the evidence adduced at the rule-making proceeding supports his final determination. Finding in favor of the Administrator on both grounds, and on all other grounds raised by petitioners, we affirm his determination.

I. THE FACTS, THE STATUTE, THE PROCEEDINGS, AND THE REGULATIONS

Hard on the introduction of the first gasoline-powered automobiles came the discovery that lead "antiknock" compounds, when added to gasoline, dramatically increase the fuel's octane rating. Increased octane allows for higher compression engines, which operate with greater efficiency. Since 1923 antiknocks have been regularly added to gasoline, and a large industry has developed to supply those compounds. Today, approximately 90 percent of motor gasoline manufactured in the United States contains lead additives, even though most 1975 and 1976 model automobiles are equipped with catalytic converters, which require lead-free gasoline. From the beginning, however, scientists have questioned whether the addition of lead to gasoline, and its consequent diffusion into the atmosphere from the automobile emission, poses a danger to the public health.3 As use of automobiles, and emission of lead particulates, has accelerated in the last quarter century, this concern has mounted. The reasons for concern are obvious (and essentially undisputed by petitioners): (1) lead in high concentrations in the body is toxic; (2) lead can be absorbed into the body from the ambient air; and (3) lead particulate emissions from gasoline engines account for approximately 90 percent of the lead in our air. Despite these apparent reasons for concern, hard proof of any danger caused by lead automotive emissions has been hard to come by. Part of the reason for this lies in the multiple sources of human exposure to lead.

Lead is an ubiquitous element. It is found in the land, in the sea, in plants, in animals, and, ultimately, in humans. Traces of lead ranging from 10 to 40 micrograms per 100 grams of blood (10-40 ug/100g)4 are found in everyone, including those living in environments with almost no atmospheric lead. National Academy of Sciences Committee on Biologic Effects of Atmospheric Pollutants, Airborne Lead in Perspective 118 (1972) (hereinafter NAS Report). Despite its universal presence, however, lead serves no known purpose in the human body, and at higher concentrations is toxic, causing anemia, severe intestinal cramps, paralysis of nerves, fatigue, and even death. Clinical symptoms of lead poisoning appear at blood lead levels of 80-100 ug or higher, and symptomatic lead poisoning may appear at levels of 50-60 ug, particularly in the presence of anemia. EPA's Position on the Health Implications of Airborne Lead (hereinafter Third Health Document) at III-1, Joint Appendix (hereinafter JA) 54-55.

Human body lead comes from three major sources. In most people, the largest source is the diet. EPA estimates daily dietary lead intake for adults to average 200-300 ug per day, with a range of 100-500 ug a day. Third Health Document at V-2, JA 82. Absorption of dietary lead into the bloodstream is estimated at about 10 percent, although in children absorption may be as high as 50 percent. Thus the average adult adds 20-30 ug of lead to his bloodstream daily from his diet alone. This daily intake, which may be highly variable depending on individual diets, NAS Report at 50, is generally regarded as, for all practical purposes, uncontrollable.5

A second major source of the body's lead burden, at least among urban children, is regarded as controllable, although effective control may be both difficult and expensive to achieve. Ingestion of lead paint by children with pica (the abnormal ingestion of non-food substances, a relatively common trait in pre-school children, particular ages 1-3) is generally regarded as "the principal environmental source in cases of severe acute lead poisoning in young children." NAS Report at 140. Lead-based paint was widely used in pre-1940 housing, for both interiors and exteriors, so children living in older housing, particularly in urban ghettos where such paint is both present and peeling, are most susceptible to this form of lead poisoning. Limited control has been achieved in that lead paints are now rarely used, and are frequently banned by statute, for interior surfaces. But while some local laws require removal of existing peeling lead paints, and there is federal legislation to aid states and municipalities in such efforts, Lead-Based Paint Poisoning Prevention Act, 42 U.S.C. §§ 4801 et seq. (1970), there is no concentrated national effort at removal, and the danger to children living in dilapidated housing will remain for some time.6

The last remaining major source of lead exposure for humans is the ambient air. This source is easily the most controllable, since approximately 90 percent of lead in the air comes from automobile emissions,7 and can be simply eliminated by removing lead from gasoline.8 While the extent to which such lead actually enters the body is vigorously contested by petitioners and lies at the heart of this appeal, all parties agree that, to some extent at least, airborne lead can be absorbed through the lungs as a person breathes lead-contaminated air and that it can be eaten by children with pica after larger lead particles fall to the ground and mix with dust. Once the lead is in the body, however, its source becomes irrelevant; all lead in the bloodstream, from whatever source, is essentially fungible. Thus so long as there are multiple sources of lead exposure it is virtually impossible to isolate one source and determine its particular effect on the body. The effect of any one source is meaningful only in cumulative terms.

The multiple sources of human exposure to lead explain in part why it has been difficult to pinpoint automobile lead emissions as a danger to public health. Obviously, any danger is caused only by the additive effect of lead emissions on the other, largely uncontrollable, sources of lead. For years the lead antiknock industry has refused to accept the developing evidence that lead emissions contribute significantly to the total human lead body burden. In the Clean Air Act Amendments of 1970, Pub.L. 91-604, December 31, 1970, 84 Stat. 1698-1700, however, Congress finally set up a legal mechanism by which that evidence could be weighed in a more objective tribunal. It gave the newly-created EPA authority to control or prohibit the sale or manufacture of any fuel additive whose emission products "will endanger the public health or welfare * * *." 42 U.S.C. § 1857f-6c(c)(1)(A) (1970). It is beyond question that the fuel additive Congress had in mind was lead.9

Given this mandate, EPA published on January 31, 1971 advance notice of proposed rule-making. The Administrator announced he was considering possible controls on lead additives in gasolines, both because of their possible danger to health and because of their incompatibility with the newly-developed catalytic converter emission control system.10 36 Fed.Reg. 1486 (1971). Proposed regulations were issued a year later, February 23, 1972, supported by a document Health Hazards of Lead11 (hereinafter First Health Document), prepared by the EPA scientific staff. Comments were invited for a 90-day period, later reopened for an additional 30 days. 37 Fed.Reg. 11786-11787 (1972). At the same time public hearings were held in Washington, D. C., Dallas, and Los Angeles.

On January 10, 1973 the Administrator, while issuing final regulations requiring availability of some lead-free gasoline to allow implementation of the catalytic converter system, 38 Fed.Reg. 1254; approved in Amoco Oil Co. v. EPA, 163 U.S.App.D.C. 162, 501 F.2d 722 (1974), reproposed the health-based regulations now at issue. 38 Fed.Reg. 1258. The reproposal was supported by a second health document, EPA's Position on the Health Effects of Airborne Lead (hereinafter Second Health Document), JA 158, and was necessitated by a modification of EPA's analysis of the health effects of lead emissions. The Agency concluded, after considering the comments received, that it was virtually impossible to identify the precise amount of airborne lead that will endanger public health. Instead, the control strategy would concentrate on evaluating the cumulative effect of airborne lead on total human lead exposure and the significance of that contribution. 38 Fed.Reg. 1258. The reproposed regulations themselves were similar to the original proposal, requiring a phased cutback to 1.25 grams of lead per gallon of leaded gasoline, but, in recognition of the industry's lead-time problems, pushing the timetable for reduction back one year. The Agency again invited public comment, this time for a 60-day period.

On October 28, 1973, as a result of a motion filed in Natural Resources Defense Council, Inc. v. EPA, D.C.Cir. No. 72-2233, this court ordered EPA to reach within 30 days a final decision on whether lead additives should be regulated for health reasons. EPA published its final health document, entitled EPA's Position on the Health Implications of Airborne Lead, on November 28, 1973. JA 27. This document, the Third Health Document, extensively details and reviews the state of knowledge of the health effects of airborne lead. It candidly discusses the various scientific studies, both pro and con, underlying this information, and ultimately concludes that lead from automobile emissions will endanger the public health. The same day, based largely on the conclusions of the Third Health Document, EPA promulgated its final regulations, accompanied by a thorough discussion of its health conclusions, the impact of the regulations, and the alternative courses of action considered and rejected. 38 Fed.Reg. 33734. The final regulations require the same step-wise reduction of lead additives but, in response to the comments of a majority of refiners, calculate the reduction in a slightly different manner. Whereas the original and reproposed regulations set standards for permissible lead use by each refiner on the basis of grams of lead per gallon of leaded gasoline produced (leaded pool averaging), the final regulations base the standards on grams of lead per gallon of all gasoline produced (total pool averaging). The quantity of lead emitted into the atmosphere is the same under both systems; EPA simply converted its leaded pool figures into total pool figures. Under the final regulations, lead in all gasoline would be reduced over a five-year period to an average of 0.5 grams per gallon.12

Petitioners, various manufacturers of lead additives and refiners of gasoline, appealed the promulgation of low-lead regulations to this court under Section 307 of the Clean Air Act, 42 U.S.C. § 1857h-5. The appeal was heard by a division of the court on September 9, 1974. On December 20, 1974, the division, one judge dissenting, ordered the regulations set aside. The majority and dissenting opinions were published on January 28, 1975.13 Because of the importance of the issues presented, we granted EPA's petition for rehearing en banc on March 17, 1975, vacating the judgment and opinions of the division and setting the case for reargument on May 30, 1975. All parties were invited to submit supplementary briefs addressing the issues raised by the division opinions.

The regulations are challenged by petitioners on a variety of grounds, all of which will be addressed below. Their primary claims, and the ones on which the division majority based its reversal, are that the Administrator misinterpreted the statutory standard of "will endanger" and that his application of that standard is without support in the evidence and arbitrary and capricious.

Under Section 211(c)(1)(A) the Administrator may, on the basis of all the information available to him, promulgate regulations that

control or prohibit the manufacture, introduction into commerce, offering for sale, or sale of any fuel or fuel additive for use in a motor vehicle or motor vehicle engine (A) if any emission products of such fuel or fuel additive will endanger the public health or welfare * * *.

42 U.S.C. § 1857f-6c(c)(1)(A). The Administrator cannot act under Section 211(c)(1)(A), however, until after "consideration of all relevant medical and scientific evidence available to him, including consideration of other technologically or economically feasible means of achieving emission standards under (Section 202)." Section 211(c)(2)(A), 42 U.S.C. § 1857f-6c(c)(2)(A). Section 202 of the Act, 42 U.S.C. § 1857f-1, allows the Administrator to set standards for emission of pollutants from automobiles (as opposed to standards for the composition of the gasoline that produces the emissions), and is thus the preferred although not the mandatory alternative under the statutory scheme, presumably because it minimizes Agency interference with manufacturer prerogatives.14

The Administrator is also required, before prohibiting a fuel or fuel additive under Section 211(c)(1)(A), to find, and publish the finding, that in his judgment any fuel or fuel additive likely to replace the prohibited one will not "endanger the public health or welfare to the same or greater degree * * *." Section 211(c)(2)(C), 42 U.S.C. § 1857f-6c(c)(2)(C). It is significant that this is the only conclusion the Administrator is expressly required to "find" before regulating a fuel or fuel additive for health reasons.

In making his threshold determination that lead particulate emissions from motor vehicles "will endanger the public health or welfare," the Administrator provided his interpretation of the statutory language by couching his conclusion in these words: such emissions "present a significant risk of harm to the health of urban populations, particularly to the health of city children." 38 Fed.Reg. 33734. By way of further interpretation, he added that it was his view

that the statutory language * * * does not require a determination that automobile emissions alone create the endangerment on which controls may be based. Rather, the Administrator believes that in providing this authority, the Congress was aware that the public's exposure to harmful substances results from a number of sources which may have varying degrees of susceptibility to control.

Id. It is petitioners' first claim of error that the Administrator has erroneously interpreted Section 211(c)(1)(A) by not sufficiently appreciating the rigor demanded by Congress in establishing the "will endanger" standard. Therefore, petitioners argue, the Administrator's action is "short of statutory right," in violation of Section 10(e)(2)(C) of the Administrative Procedure Act (APA), 5 U.S.C. § 706(2)(C) (1970).

Petitioners argue that the "will endanger" standard requires a high quantum of factual proof, proof of actual harm rather than of a "significant risk of harm." See supplemental brief of petitioner Ethyl Corporation (hereinafter Ethyl Supp.Br.) at 20. Since, according to petitioners, regulation under Section 211(c)(1)(A) must be premised upon factual proof of actual harm, the Administrator has, in their view, no power to assess risks or make policy judgments in deciding to regulate lead additives. Moreover, petitioners argue, regulation must be based on the danger presented by lead additives "in and of themselves," so it is improper to consider, as the Administrator did, the cumulative impact of lead additives on all other sources of human exposure to lead. We have considered these arguments with care and find them to be without merit.15 It is our view that the Administrator's interpretation of the standard is the correct one.16

1. The Precautionary Nature of "Will Endanger." Simply as a matter of plain meaning, we have difficulty crediting petitioners' reading of the "will endanger" standard. The meaning of "endanger" is not disputed. Case law and dictionary definition agree that endanger means something less than actual harm.17 When one is endangered, harm is threatened ; no actual injury need ever occur. Thus, for example, a town may be "endangered" by a threatening plague or hurricane and yet emerge from the danger completely unscathed.18 A statute allowing for regulation in the face of danger is, necessarily, a precautionary statute. Regulatory action may be taken before the threatened harm occurs; indeed, the very existence of such precautionary legislation would seem to demand that regulatory action precede, and, optimally, prevent, the perceived threat. As should be apparent, the "will endanger" language of Section 211(c)(1)(A) makes it such a precautionary statute.

The Administrator read it as such, interpreting "will endanger" to mean "presents a significant risk of harm." 38 Fed.Reg. 33734. We agree with the Administrator's interpretation. This conclusion is reached not only by reference to the plain meaning of the statute, but by juxtaposition of Section 211(c)(1)(A) with other sections of the Clean Air Act and by analysis of pertinent precedent.19

Petitioners support their view of the rigorous nature of the "will endanger" standard by relying on two other sections of the Clean Air Act that also allow for regulation of air pollutants for health reasons. Ethyl Supp.Br. at 18-23; supplemental brief of petitioner Nalco Chemical Company (hereinafter Nalco Supp.Br.) at 20-25; NPRA Supp.Br. at 31-46. However, we find in the same sections relied upon by petitioners firm support for our view of the precautionary nature of Section 211(c)(1)(A). The provisions identified by petitioners are Sections 108 and 202 of the Act. Section 108 directs the Administrator to list, for the purpose of establishing national primary and secondary ambient air quality standards pursuant to Section 109, each air pollutant "which in his judgment has an adverse effect on public health or welfare * * *." Section 108(a)(1)(A), 42 U.S.C. § 1857c-3(a)(1)(A). Section 202 authorizes the Administrator to set standards for each automobile emission "which in his judgment causes or contributes to, or is likely to cause or to contribute to, air pollution which endangers the public health or welfare." Section 202(a)(1), 42 U.S.C. § 1857f-1(a)(1).

Petitioners suggest that the threshold decision to regulate under both Section 108 and Section 202 can be based on less evidence than required under Section 211 and that, therefore, the proof necessary for action under Section 211 must be particularly firm. Petitioners misread both sections. Section 108 requires an actual "adverse effect" on health before an air pollutant may be listed for regulation; actual harm must result.20 This firm threshold finding is reasonable in light of the considerable disruption caused by action under Sections 108-110. After a pollutant is listed under Section 108, the Administrator must issue national ambient air quality standards under Section 109 within a year. Promulgation of standards begins the implementation plan process of Section 110 under which the states must control, on a mandatory timetable, the listed pollutants to the extent necessary to achieve the federal standards. Section 110, 42 U.S.C. § 1857c-5. Sections 108-110 are "technology forcing" provisions; the attainment of the primary, health-based standards takes precedence over the cost and present technological feasibility of achieving the requisite control. Union Electric Co. v. EPA, 515 F.2d 206, 215-16 (8th Cir.), cert. granted, 423 U.S. 821, 96 S.Ct. 35, 46 L.Ed.2d 38, 44 U.S.L.Week 3200 (1975). Cf. Train v. Natural Resources Defense Council, 421 U.S. 60, 90-91, 95 S.Ct. 1470, 1487-1488, 43 L.Ed.2d 731, 752-753 (1975). See also S.Rep.No.91-1196, 91st Cong., 2d Sess. 1-3 (1970).

Thus, before ordering this extensive chain of action to begin, Congress demanded a threshold determination that the pollutant causes actual harm.21 In this sense Section 108 is not a precautionary statute at all, and so differs sharply from Section 211. However, the effects of such after-the-fact regulation are somewhat ameliorated by the Act; Congress did provide a precautionary element in standard-setting under Sections 108-110. Section 109 expressly requires that the ambient air standards ultimately issued provide for "an adequate margin of safety." 42 U.S.C. § 1857c-4(b)(1). Thus, while the threshold decision to regulate under Sections 108-110 is not precautionary but rather requires proof of demonstrable harm caused by the suspect pollutant, once the decision is made the standards promulgated must be preventive in nature. Congress' choice of this scheme is in direct contrast to the procedures it established under Section 211. Under that section the decision to regulate is based on perceived danger. Unless we are to assume Congress chose its language carelessly, regulation in the face of "danger" rather than in the face of "adverse effects" must mean that the threshold decision to regulate under Section 211 is precautionary.22 The contrast between the standards of Sections 108-110 and of Section 211 supports our view of the precautionary nature of the "will endanger" standard.23

Petitioners also rely on Section 202 to support their strict reading of Section 211. Ethyl suggests that Section 202 is more lenient than Section 211 in that it allows regulation of "likely" dangers. Ethyl Supp.Br. at 18-19. See also Nalco Supp.Br. at 20-21; NPRA Supp.Br. at 31-36. Section 202 provides that the Administrator may regulate

The emission of any air pollutant (from any new motor vehicle) which in his judgment causes or contributes to, or is likely to cause or to contribute to, air pollution which endangers the public health or welfare.

42 U.S.C. § 1857f-1(a)(1) (emphasis added). While this language may be unnecessarily opaque, we think a fair reading disproves petitioners' suggestion. The italicized language upon which petitioners rely refers not to the causal relationship between air pollution and health, but to the relationship between automobile emissions and air pollution.24 Thus regulation may not be premised on a threshold determination of likely danger; rather regulation must be premised on a determination of danger, a finding that "air pollution which endangers the public health" is the end product of the emission to be regulated. This is essentially the same finding of endangerment as under Section 211. "Likely" enters the equation only in determining whether the emitted air pollutant, which would be regulated, contributes to the air pollution which is found dangerous. Here the statute allows for a somewhat attenuated chain of causation. Regulation may be premised on a determination that an air pollutant emitted from a new automobile is likely to contribute to air pollution which endangers the public health. In establishing this chain of causation Section 202 is more lenient than Section 211,25 but in making the threshold determination of danger both sections are the same:26 air pollution must endanger the public health before regulation is justified.27

Thus the two sections of the Clean Air Act presented by petitioners in support of their stringent reading of the threshold requirement for action under Section 211 turn out, upon analysis, to be of no support at all. Section 108 only bolsters our reading of Section 211 as a precautionary statute while Section 202 includes the same standard as Section 211 and thus is of no guidance one way or the other. While petitioners have little more to offer to prove that the "will endanger" standard demands proof of actual harm and is not precautionary in nature, we may turn, in support of our interpretation, to the relevant case law. While cases interpreting the meaning of "endanger" are few in number,28 at least one recent case is directly on point and fully in accord with our view.

In Reserve Mining Co. v. EPA, 514 F.2d 492 (8th Cir. 1975) (en banc ), the Eighth Circuit addressed, among other issues, the meaning of the phrase "endangering the health or welfare of persons" under Section 112 of the Federal Water Pollution Control Act of 1970 (FWPCA), 33 U.S.C. § 1160. FWPCA and the Clean Air Act together constitute the bulk of this nation's substantive environmental protection legislation.29 As such, and because of their contemporaneous enactment, interpretations of provisions of one Act have frequently been applied to comparable provisions of the other. See, e. g., Natural Resources Defense Council, Inc. v. Train, 166 U.S.App.D.C. 312, 321-322, 510 F.2d 692, 701-702 (1975). Thus Reserve Mining's interpretation of "endangering" is relevant to the meaning of the term "endanger" in the Clean Air Act. Indeed, it is particularly relevant because in construing the language before it the Eighth Circuit borrowed extensively from the interpretation of the "will endanger" language of Section 211 expressed in the dissent from the division opinion in this case, the same interpretation we adopt here. See Reserve Mining Co. v. EPA, supra, 514 F.2d at 528-529. After analysis of the plain meaning of the FWPCA provision, comparison with other sections of that Act, and reference to our division's dissent, the Eighth Circuit's unanimous conclusion fully supports our view of the "will endanger" standard:

In the context of this environmental legislation, we believe that Congress used the term "endangering" in a precautionary or preventive sense, and, therefore, evidence of potential harm as well as actual harm comes within the purview of that term.

Id. at 528.

In sum, based on the plain meaning of the statute, the juxtaposition of Section 211 with Sections 108 and 202, and the Reserve Mining precedent, we conclude that the "will endanger" standard is precautionary in nature and does not require proof of actual harm before regulation is appropriate.30

Perhaps because it realized that the above interpretation was the only possible reading of the statutory language, petitioner Ethyl addresses this interpretation and argues that even if actual harm is not required for action under Section 211(c)(1)(A), the occurrence of the threatened harm must be "probable" before regulation is justified. Ethyl Supp.Br. 12. While the dictionary admittedly settles on "probable" as its measure of danger, we believe a more sophisticated case-by-case analysis is appropriate.31 Danger, the Administrator recognized, is not set by a fixed probability of harm, but rather is composed of reciprocal elements of risk and harm, or probability and severity. Cf. Carolina Environmental Study Group v. United States, 166 U.S.App.D.C. 416, 419, 510 F.2d 796, 799 (1975); Reserve Mining Co. v. EPA, supra, 514 F.2d at 519-520. That is to say, the public health may properly be found endangered both by a lesser risk of a greater harm and by a greater risk of a lesser harm.32 Danger depends upon the relation between the risk and harm presented by each case, and cannot legitimately be pegged to "probable" harm, regardless of whether that harm be great or small. As the Eighth Circuit found in Reserve Mining, these concepts "necessarily must apply in a determination of whether any relief should be given in cases of this kind in which proof with certainty is impossible."33 514 F.2d at 520.

In Reserve Mining the issue was whether asbestiform wastes flushed into Lake Superior by the Reserve Mining Company endangered health.34 The polluted lake waters formed the drinking supply of several surrounding communities, while a medical theory, bolstered only by inconclusive evidence, suggested that ingestion of the wastes caused cancer. See pages --- - --- of --- U.S.App.D.C., pages 45-46 of 541 F.2d infra. Applying the "endangering the health or welfare of persons" standard of the FWPCA, the court found the wastes to be a danger cognizable under the Act. The court did not find that the danger was probable; rather it found the wastes to be "potentially harmful," 514 F.2d at 528, and potential harm to be embraced by the "endangering" standard, id. See page --- of --- U.S.App.D.C., page 17 of 541 F.2d supra. The court concluded:

The record shows that Reserve is discharging a substance into Lake Superior waters which under an acceptable but unproved medical theory may be considered as carcinogenic. As previously discussed, this discharge gives rise to a reasonable medical concern over the public health. We sustain the district court's determination that Reserve's discharge into Lake Superior constitutes pollution of waters "endangering the health or welfare of persons" within the terms of §§ 1160(c)(5) and (g)(1) of the Federal Water Pollution Control Act and is subject to abatement.

514 F.2d at 529 (footnote omitted; emphasis added). The court thus allowed regulation of the effluent on only a "reasonable" or "potential" showing of danger, hardly the "probable" finding urged by Ethyl as the proper reading of the "endanger" language in Section 211. The reason this relatively slight showing of probability of risk justified regulation is clear: the harm to be avoided, cancer, was particularly great. However, because the risk was somewhat remote, the court did not order the immediate cessation of asbestiform dumping, but rather ordered such cessation within "a reasonable time." Id. at 538.

Reserve Mining convincingly demonstrates that the magnitude of risk sufficient to justify regulation is inversely proportional to the harm to be avoided. Cf. Carolina Environmental Study Group v. United States, supra. It would be a bizarre exercise in balancing horrors to determine whether cancer or lead poisoning is a greater harm to be avoided,35 but fortunately such balancing is unnecessary in this case. Undoubtedly, the harm caused by lead poisoning is severe; nonetheless, the Administrator does not rely on a "potential" risk or a "reasonable medical concern" to justify the regulations before us. Instead, he finds a "significant" risk of harm to health. While this finding may be less than the "probable" standard urged by Ethyl, it is considerably more certain than the risk that justified regulation in Reserve Mining of a comparably "fright-laden" harm. Cf. Environmental Defense Fund, Inc. v. EPA, 150 U.S.App.D.C. 348, 358, 465 F.2d 528, 538 (1972). Moreover, like the Reserve Mining court, in the face of this still less than certain risk the Administrator did not order the cessation of use of lead additives, but rather directed a phased step-down to a plateau level. Thus we conclude that however far the parameters of risk and harm inherent in the "will endanger" standard might reach in an appropriate case, they certainly present a "danger" that can be regulated when the harm to be avoided is widespread lead poisoning and the risk of that occurrence is "significant."36

2. The Administrator's Power to Assess Risks. Petitioners argue that Section 211 requires the Administrator to make a "threshold factual determination" that automobile emissions "will endanger" the public health, Nalco Supp.Br. at 15-20; Ethyl Supp.Br. at 24-26, and dispute EPA's claim that the Administrator may make "an essentially legislative policy judgment, rather than a factual determination, concerning the relative risks of underprotection as compared to overprotection." Industrial Union Department, AFL-CIO v. Hodgson, 162 U.S.App.D.C. 331, 339, 499 F.2d 467, 475 (1974). We must reject petitioners' argument, since the power to assess risks, without relying solely on facts, flows inexorably from the nature of the "will danger" standard. We have already found that Section 211 allows the Administrator to regulate fuel content when he finds that emissions cause a significant risk of harm to the public health. Yet, how can the Administrator determine that a risk is a significant risk if he cannot assess risks? And how can he assess risks if he cannot make policy judgments? Surely reliance on "facts" as contemplated by petitioners will provide little guidance. However, sole reliance on facts was not demanded by Congress.37

Originally, it is true, it appeared that Congress would severely restrict the Administrator's ability to assess risks and make policy judgments to protect public health. The bill sent to the floor of the House, and eventually passed by the House, would have allowed EPA to control fuels or fuel additives only if the determination that their emission products would endanger the public health were established

on the basis of specific findings derived from relevant medical and scientific evidence, including * * * a finding that it is not otherwise technologically or economically feasible to achieve the emission standards established pursuant to section 202 of this Act.

H.R. 17255, 91st Cong., 2d Sess. § 210(g)(1) (1970) (emphasis added). The specific findings requirement had more than procedural significance. It limited the scope of evidence on which the Administrator could proceed, made alternative action under Section 202 mandatory if possible, and was widely interpreted in the House, as the extracts of debate relied upon by petitioners show, Ethyl Supp.Br. at 24; NPRA Supp.Br. at 30; Nalco Supp.Br. at 15, as requiring that any EPA action be based solely on facts.38 See also H.R.Rep. No. 91-1146, 91st Cong., 2d Sess., at 13 (1970), U.S.Code Cong. & Admin.News 1970, p. 5356.

But the House bill did not become law. The Senate's preference for less restriction of EPA freedom in regulating fuel additives for health reasons39 was adopted by the conference committee and ultimately enacted into law.40 Although the legislative history does not expressly discuss the reasons for the change,41 the contrast in language is stark. As Section 211 now reads, the only "finding" the Administrator is required to make is that any fuel additive that might replace one that is prohibited does not cause emissions that will pose the same or greater danger to the public health.42 The substantive impact of the change is clear. All of the requirements for specific findings quoted above are replaced by requirements that the Administrator "consider" the specified evidence. The mandatory deference to Section 202 is removed.43 And the Administrator may act based on all information available to him.44 As we recognized in Amoco, construing a similar change in parallel Section 211(c)(1)(B), the conference committee's decision "was a deliberated one and was meant to have significance." Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 173, 501 F.2d at 733.

In this case the decision has even more significance than it did in Amoco, for under Section 211(c)(1)(B), at issue in Amoco, the Administrator was still required to make "findings." All the conferees dropped was the adjective "specific." For regulation under Section 211(c)(1)(A), at issue here, however, the entire requirement was dropped, and no greater restriction was placed on the Administrator than that required by the "basis and purpose" statement of Section 4(b) of the APA, 5 U.S.C. § 553(c). We interpreted the meaning of the limited findings requirement for acting under Section 211(c)(1) (B) in Amoco :

(W)e read Section 211(c)(2)(B) as incorporating the commonsense approach which the courts have developed in applying Section 4(b)45 of the APA. Where EPA's regulations turn crucially on factual issues, we will demand sufficient attention to these in the statement to allow the fundamental rationality of the regulations to be ascertained. Where, by contrast, the regulations turn on choices of policy, on an assessment of risks, or on predictions dealing with matters on the frontiers of scientific knowledge, we will demand adequate reasons and explanations, but not "findings" of the sort familiar from the world of adjudication.

Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 180-181, 501 F.2d at 740-741 (emphasis added). Thus the Amoco court read the limited findings necessary for action under Section 211(c)(1)(B) as a flexible requirement that demanded actual findings for questions of fact and something less "adequate reasons and explanations" for questions of policy.

As petitioners correctly point out, the Amoco court applied this approach by holding that the threshold determination whether to regulate at all under Section 211(c)(1)(B) was essentially a question of fact, for which a factual finding must be offered. In suggesting that such a finding is necessary for action under Section 211(c)(1)(A) as well, however, petitioners completely ignore the fact that a finding is required for action under Section 211(c)(1) (B) and is not required under Section 211(c)(1)(A). By so glossing over the language of the statute, petitioners miss a vital difference between the threshold determinations under Sections 211(c)(1)(A) and (B). Recall what the Administrator must find in order to act under Section 211(c)(1)(B): that the emission products of the fuel or additive to be regulated

will impair to a significant degree the performance of any emission control device or system which is in general use, or which the Administrator finds has been developed to a point where in a reasonable time it would be in general use were such regulation to be promulgated.

42 U.S.C. § 1857f-6c(c)(1)(B). At its core, this is a peculiarly factual finding. Will the emission impair the performance of a control device? Is the device in general use, or has it the potential to be? These were the questions, highly suitable to factual proof, that the Amoco court construed. Contrary to petitioners' implication, the court did not hold that any threshold determination to regulate must be based on factual proof, but that this threshold determination must be factually based. The Section 211(c)(1)(A) threshold determination, however, is inherently unlike that of its sister section. "Endanger," as we have suggested above, is not a standard prone to factual proof alone. Danger is a risk, and so must be decided by assessment of risks as well as by proof of facts.

Thus a reason emerges for the deletion of the findings requirement for action under the "will endanger" standard. The Amoco court held the findings requirement of Section 211(c)(1)(B) no more than reiterated the minimal demands of the basis and purpose statement of the APA. Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 179, 501 F.2d at 739. What Congress was doing, then, was using the findings requirement to indicate which section demanded a factual threshold determination and which section did not. While inclusion of the phrase was unnecessary in that the APA would have demanded a factual threshold determination under Section 211(c)(1)(B) in any case, the selective use of the findings requirement served to emphasize to the Administrator the demands of the APA and the intent of the Congress. While Congress did not discuss the extent of the Administrator's power under the "will endanger" standard, its actions the statute it enacted and the one it rejected make the legislative intent clear. We find that deletion of the findings requirement for action under Section 211(c)(1)(A) was a recognition by Congress that a determination of endangerment to public health is necessarily a question of policy that is to be based on an assessment of risks and that should not be bound by either the procedural or the substantive rigor proper for questions of fact.

This conclusion follows not only from the language of Section 211(c)(1)(A) and its legislative history, but from the nature of the Administrator's charge: to protect the public from danger. Regulators such as the Administrator must be accorded flexibility, a flexibility that recognizes the special judicial interest in favor of protection of the health and welfare of people, even in areas where certainty does not exist. Environmental Defense Fund, Inc. v. Ruckelshaus, 142 U.S.App.D.C. 74, 88, 439 F.2d 584, 598 (1971).

Questions involving the environment are particularly prone to uncertainty. Technological man has altered his world in ways never before experienced or anticipated. The health effects of such alterations are often unknown, sometimes unknowable. While a concerned Congress has passed legislation providing for protection of the public health against gross environmental modifications,46 the regulators entrusted with the enforcement of such laws have not thereby been endowed with a prescience that removes all doubt from their decisionmaking. Rather, speculation, conflicts in evidence, and theoretical extrapolation typify their every action. How else can they act, given a mandate to protect the public health but only a slight or nonexistent data base upon which to draw? Never before have massive quantities of asbestiform tailings been spewed into the water we drink.47 Never before have our industrial workers been occupationally exposed to vinyl chloride48 or to asbestos dust.49 Never before has the food we eat been permeated with DDT50 or the pesticides aldrin and dieldrin.51 And never before have hundreds of thousands of tons of lead emissions been disgorged annually into the air we breathe. Sometimes, of course, relatively certain proof of danger or harm from such modifications can be readily found. But, more commonly, "reasonable medical concerns" and theory long precede certainty. Yet the statutes and common sense demand regulatory action to prevent harm, even if the regulator is less than certain that harm is otherwise inevitable.

Undoubtedly, certainty is the scientific ideal to the extent that even science can be certain of its truth.52 But certainty in the complexities of environmental medicine may be achievable only after the fact, when scientists have the opportunity for leisurely and isolated scrutiny of an entire mechanism. Awaiting certainty will often allow for only reactive, not preventive, regulation.53 Petitioners suggest that anything less than certainty, that any speculation, is irresponsible. But when statutes seek to avoid environmental catastrophe, can preventive, albeit uncertain, decisions legitimately be so labeled?

The problems faced by EPA in deciding whether lead automotive emissions pose a threat to the public health highlight the limitations of awaiting certainty. First, lead concentrations are, even to date, essentially low-level, so that the feared adverse effects would not materialize until after a lifetime of exposure. Contrary to petitioners' suggestion, however, we have not yet suffered a lifetime of exposure to lead emissions. At best, emissions at present levels have been with us for no more than 15-20 years.54 Second, lead exposure from the ambient air is pervasive, so that valid control groups cannot be found against which the effects of lead on our population can be measured. Third, the sources of human exposure to lead are multiple, so that it is difficult to isolate the effect of automobile emissions. Lastly, significant exposure to lead is toxic, so that considerations of decency and morality limit the flexibility of experiments on humans that would otherwise accelerate lead exposure from years to months, and measure those results.55 Cf. Environmental Defense Fund, Inc. v. EPA (Shell), 167 U.S.App.D.C. 71, 78, 510 F.2d 1292, 1299 (1975).

The scientific techniques for attempting to overcome these limitations are several: toxicology can study the distribution and effect of lead in animals; epidemiological techniques can analyze the effects of lead emissions on entire populations; clinical studies can reproduce in laboratories atmospheric conditions and measure under controlled circumstances the effects on humans. All of these studies are of limited usefulness, however. Dr. J. H. Knelson, Director of EPA's Human Studies Laboratory, has described, in the context of setting ambient air standards, the limitations of these various investigative tools:

Each of these investigative approaches classic toxicology, epidemiology, and clinical research has its advantages and disadvantages. The toxicologist can control the dose and use invasive or destructive techniques in measuring response in the animal, but is always faced with the problem of extrapolating results to humans. Epidemiology is most relevant because it studies phenomena actually occurring in humans under "natural" conditions, but can only draw inference from observed correlations rather than prove cause and effect relationships. Clinical research can provide the most accurate dose-response relationships in the species of interest. Precisely because the study subjects are humans, however, many experimental design problems are encountered in assuring their safety. Although the dose of an atmospheric pollutant can be carefully controlled and measured in the clinical laboratory, qualitative comparability to the multiplex variable of atmospheric pollution cannot always be assured.

The best scientific criteria for establishing air quality standards result from interactions between these disciplines. Clinical studies must be preceded by exhaustive toxicological assessment in other species; observations from population studies should play an important role in the experimental design of clinical research. Biomedical data from all these sources, taken in their entirety, should be used for the prudent definition of air pollution control needs.

JA 582-583. The best biomedical evidence will be derived from relating all three research approaches. This EPA did. That petitioners, and their scientists, find a basis to disagree is hardly surprising, since the results are still uncertain, and will be for some time. But if the statute accords the regulator flexibility to assess risks and make essentially legislative policy judgments, as we believe it does, preventive regulation based on conflicting and inconclusive evidence may be sustained. Recent cases have recognized this flexibility in similar situations.

In Industrial Union Department, AFL-CIO v. Hodgson, supra, this court considered the Secretary of Labor's delegated power under the Occupational Safety and Health Act (OSHA), 29 U.S.C. § 651 et seq., to protect the health of industrial workers by setting standards for exposure to industrial pollutants. Under review was a standard for exposure to asbestos dust, thought to be carcinogenic. Judge McGowan, writing for the division, laid down the rule:

From extensive and often conflicting evidence, the Secretary in this case made numerous factual determinations. With respect to some of those questions, the evidence was such that the task consisted primarily of evaluating the data and drawing conclusions from it. The court can review that data in the record and determine whether it reflects substantial support for the Secretary's findings. But some of the questions involved in the promulgation of these standards are on the frontiers of scientific knowledge, and consequently as to them insufficient data is presently available to make a fully informed factual determination. Decision making must in that circumstance depend to a greater extent upon policy judgments and less upon purely factual analysis. 18

18. Where existing methodology or research in a new area of regulation is deficient, the agency necessarily enjoys broad discretion to attempt to formulate a solution to the best of its ability on the basis of available information, Permian Basin Area Rate Cases, 390 U.S. 747, 811, 88 S.Ct. 1344, 20 L.Ed.2d 312 (1968).

162 U.S.App.D.C. at 338, 499 F.2d at 474.

This rule was likewise applied in Society of the Plastics Industry, Inc. v. OSHA, 509 F.2d 1301 (2d Cir.), cert. denied, 421 U.S. 992, 95 S.Ct. 1998, 44 L.Ed.2d 482 (1975), where the Second Circuit reviewed regulations limiting industrial exposure to vinyl chloride, also considered a carcinogen:

As in Industrial Union Department, AFL-CIO v. Hodgson, supra, the ultimate facts here in dispute are "on the frontiers of scientific knowledge," and, though the factual finger points, it does not conclude. Under the command of OSHA, it remains the duty of the Secretary to act to protect the workingman, and to act even in circumstances where existing methodology or research is deficient. The Secretary, in extrapolating the MCA study's finding from mouse to man, has chosen to reduce the permissible level to the lowest detectable one. We find no error in this respect.

Id. at 1308. And in Reserve Mining the Eighth Circuit agreed. Although reviewing the determination of a District Court, not an expert Administrator, the court recognized that it must nonetheless apply the test of the preventive statute before it:

(W)e note that many of the issues in this case do not involve "historical" facts subject to the ordinary means of judicial resolution. Indeed, a number of the disputes involve conflicting theories and experimental results, about which it would be judicially presumptuous to offer conclusive findings (quoting Amoco Oil Co. v. EPA, 163 U.S.App.D.C. 181, 501 F.2d at 741, which relied on Industrial Union ). In such circumstances, the finder of fact must accept certain areas of uncertainty, and the findings themselves cannot extend further than attempting to assess or characterize the strengths and weaknesses of the opposing arguments.

514 F.2d at 507 n.20. See also id. at 529.

These cases, recognizing as they do the developing nature of environmental medicine, fortify our analysis of the "will endanger" language of Section 211.56 Where a statute is precautionary in nature,57 the evidence difficult to come by, uncertain, or conflicting because it is on the frontiers of scientific knowledge, the regulations designed to protect the public health, and the decision that of an expert administrator, we will not demand rigorous step-by-step proof of cause and effect. Such proof may be impossible to obtain if the precautionary purpose of the statute is to be served. Of course, we are not suggesting that the Administrator has the power to act on hunches or wild guesses. Amoco makes it quite clear that his conclusions must be rationally justified. Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 180-181, 501 F.2d at 740-741. However, we do hold that in such cases the Administrator may assess risks. He must take account of available facts, of course, but his inquiry does not end there. The Administrator may apply his expertise to draw conclusions from suspected, but not completely substantiated, relationships between facts, from trends among facts, from theoretical projections from imperfect data, from probative preliminary data not yet certifiable as "fact," and the like. We believe that a conclusion so drawn a risk assessment may, if rational, form the basis for health-related regulations under the "will endanger" language of Section 211.58

All of this is not to say that Congress left the Administrator free to set policy on his own terms. To the contrary, the policy guidelines are largely set, both in the statutory term "will endanger" and in the relationship of that term to other sections of the Clean Air Act. These prescriptions direct the Administrator's actions. Operating within the prescribed guidelines, he must consider all the information available to him. Some of the information will be factual, but much of it will be more speculative scientific estimates and "guesstimates" of probable harm, hypotheses based on still-developing data, etc. Ultimately he must act, in part on "factual issues," but largely "on choices of policy, on an assessment of risks, (and) on predictions dealing with matters on the frontiers of scientific knowledge * * *." Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 181, 501 F.2d at 741. A standard of danger fear of uncertain or unknown harm contemplates no more.

3. Propriety of the Cumulative Impact Approach. In addition to demanding that the Administrator act solely on facts, petitioner Ethyl insists that those facts convince him that the emission product of the additive to be regulated "in and of itself," i. e., considered in isolation, endangers health. The Administrator contends that the impact of lead emissions is properly considered together with all other human exposure to lead. See page --- of --- U.S.App.D.C., page 12 of 541 F.2d supra. We agree.

First, Ethyl points to the language of Section 202 allowing regulation of any automobile emission that "causes or contributes to, or is likely to cause or to contribute to, air pollution which endangers the public health or welfare." Section 202(a)(1), 42 U.S.C. § 1857f-1(a)(1) (emphasis Ethyl's in its brief at 18). This is contrasted with Section 211's language allowing regulation of fuels or fuel additives whose "emission products * * * will endanger the public health or welfare." By italicizing the "contribute to" language of Section 202 Ethyl presumably suggests that Section 202 is more lenient, allowing use of the cumulative impact theory while Section 211 does not. This argument is but a variant on Ethyl's already rejected claim that Section 202 allows for regulation of "likely" dangers while Section 211 does not. As with the earlier argument, this reading of Section 202 must be rejected. While it is possible that Section 202's inclusion of emissions that "cause or contribute to air pollution" is more encompassing than Section 211's term "emission products," it is clear that whatever leniency Section 202 suggests only concerns the makeup of air pollution. This is irrelevant to the cumulative impact theory. However they differ, both Section 202 and Section 211 allow regulation of lead additives only on a determination that the resultant polluted air, however composed, endangers the public health.59 Both provisions leave open the question whether emissions or air pollution can be found to endanger the public health when the endangerment is not caused by that pollution alone.60

While Ethyl's comparison suggests no answer, the question is directly raised here. The Administrator found endangerment, but recognized that the national lead exposure problem is caused, not by air pollution alone, but by an aggregate of sources, including food, water, leaded paint, and dust. He believed that regulation was justified because the aggregate was dangerous, and because leaded gasoline was a significant source that was particularly suited to ready reduction. 38 Fed.Reg. 33734. To the question whether the Administrator was correct in his belief, comparison with the "contribute to" language of Section 202 provides no guidance.

The only other evidence relied upon by Ethyl for its "in and of itself" theory is a quotation, taken out of context, from Senator Baker. When the context of Ethyl's partial quotation is revealed the irrelevance of his statement to Ethyl's theory becomes clear. Senator Baker reviewed the Committee bill for the Senate:

The (Administrator) is authorized to either control or prohibit the sale of any given fuel when he finds one of two things:

First. That the combustion or evaporation of such fuel produces emissions that, in and of themselves, endanger the public health or welfare; or

Second. That such emissions prevent the operation of a system that is necessary to reduce automobile emissions to the levels required by standards issued by the (Administrator) under section 202 of the act.

116 Cong.Rec. 32920 (1970) (emphasis added). Ethyl relies on the Senator's subsequent repetition of the words "in and of themselves," italicized above. Ethyl brief at 16. But the second paragraph makes clear that Senator Baker's meaning is contrary to Ethyl's implication. He is simply emphasizing the different nature of the two provisions and thus uses the words "in and of themselves" to show that under Section 211(c)(1)(A) the Administrator may act only because of the direct effects of lead additives on a legislative goal, protection of health, while under Section 211(c)(1)(B) he may act because of their indirect effects on another goal, implementation of emission control systems.61

Beyond these two points, Ethyl has nothing more to offer in support of its "in and of itself" reading. Thus it has made out no case at all, particularly in light of the realities of human exposure to lead and what Congress knew about those realities. Such consideration demonstrates both that, under Ethyl's approach, EPA regulation of lead on health grounds would be impossible and that Congress could not possibly have intended the restrictive "by itself" reading. As has been discussed more extensively above, see pages --- - --- of --- U.S.App.D.C., pages 8-9 of 541 F.2d supra, lead enters the human body from multiple sources, so that the effect of any one source is meaningful only in cumulative terms. If, for example, airborne lead were the only source of the lead body burden, and it caused, by itself, a blood lead level of 30 ug, there would be no danger to the public health. But if that hypothetical 30 ug is added to a possible 30 ug attributable to dietary ingestion, the blood lead level would be 60 ug, a definite threat to health. Under Ethyl's approach, despite obvious endangerment such a cumulative finding is insufficient to justify regulation. Airborne, lead, in and of itself, may not be a threat. But the realities of human lead exposure show that no one source in and of itself (except possibly leaded paint) is a threat. Thus, under Ethyl's tunnel-like reasoning, even if parallel legislation permitted regulation of other sources of lead exposure, which it does not, no regulation could ever be justified.

Such cannot be the case. Congress understood that the body lead burden is caused by multiple sources. It understood that determining the effect of lead automobile emissions, by themselves, on human health is of no more practical value than finding the incremental effect on health of the fifteenth sleeping pill swallowed by a would-be suicide.62 It did not mean for "endanger" to be measured only in incremental terms.63 This the Administrator also understood. He determined that absorption of lead automobile emissions, when added to all other human exposure to lead, raises the body lead burden to a level that will endanger health. He realized that lead automobile emissions were, far and away, the most readily reduced significant source of environmental lead. And he determined that the statute authorized him to reduce those emissions on such a finding. We find no error in the Administrator's use of the cumulative impact approach.

4. Summary of the "Will Endanger" Determination. In sum, we must reject petitioners' cramped and unrealistic interpretation of Section 211(c)(1)(A). Their reading would render the statute largely useless as a basis for health-related regulation of lead emissions. Petitioners' arguments are rebuffed by the plain meaning of the statute and the Administrator's interpretation of it,64 by the legislative history and the implications that can drawn from other sections of the same statute, by the relevant precedents, and by the established maxim that health-related legislation is liberally construed to achieve its purpose.65

We believe the Administrator may regulate lead additives under Section 211(c) (1)(A) when he determines, based on his assessment of the risks as developed by consideration of all the information available to him, and as guided by the policy judgment inherent in the statute, that lead automobile emissions significantly increase the total human exposure to lead so as to cause a significant risk of harm to the public health. Before so regulating, he must consider the possibility of regulation under Section 202.66 This interpretation of Section 211 does not allow for baseless or purposeless regulation, but does grant the Administrator the flexibility needed to confront realistically the public health problem presented by massive diffusion of lead emissions from automobiles.

Even when the Administrator has determined that a fuel or fuel additive causes emissions which endanger the public health, he is not yet free to prohibit the substance under Section 211. He must first find, and publish his finding,

that in his judgment such prohibition will not cause the use of any other fuel or fuel additive which will produce emissions which will endanger the public health or welfare to the same or greater degree than the use of the fuel or fuel additive proposed to be prohibited.

Section 211(c)(2)(C), 42 U.S.C. § 1857f-6c(c)(2)(C).

Preliminarily it must be noted that the section requires a finding only before the Administrator "prohibits" a fuel or fuel additive under Section 211. Since the proposed regulations only "control" lead additives, the findings requirement, on its face, does not apply to the EPA action.67 If the requirement is read to apply, however, it plainly demands no more than the findings requirement of Section 211(c)(2)(B), as construed in Amoco, see pages --- - --- of --- U.S.App.D.C., page 23 of 541 F.2d supra. This conclusion is compelled by the identical genesis of the two provisions, plus the fact that the Section 211(c)(2)(C) finding is judgmental by its own terms. Thus where the judgment turns "on factual issues" we will "demand sufficient attention to these in the statement to allow the fundamental rationality * * * to be ascertained." Amoco Oil Co. v. EPA, supra, 163 U.S.App.D.C. at 180-181, 501 F.2d at 740-741. By contrast, where the judgment is necessarily more speculative, we will "demand adequate reasons and explanations, but not 'findings' of the sort familiar from the world of adjudication." Id., 163 U.S.App.D.C. at 181, 501 F.2d at 741.68

After making the "will endanger" determination and the "substitute additives" finding, EPA has complied with the statutory mandate and is free to regulate the fuel or fuel additive under Section 211.

In promulgating the low-lead regulations under Section 211, EPA engaged in informal rule-making. As such, since the statute does not indicate otherwise, its procedures are conducted pursuant to Section 4 of the APA,69 5 U.S.C. § 553, and must be reviewed under Section 10 of the Act,70 5 U.S.C. § 706(2) (A)-(D). Our review of the evidence is governed by Section 10(e)(2)(A), which requires us to strike "agency action, findings, and conclusions" that we find to be "arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law * * *."71 5 U.S.C. § 706(2)(A). This standard of review is a highly deferential one. It presumes agency action to be valid. Citizens to Preserve Overton Park v. Volpe, 401 U.S. 402, 415, 91 S.Ct. 814, 823, 28 L.Ed.2d 136, 152 (1971); Pacific States Box & Basket Co. v. White, 296 U.S. 176, 185-186, 56 S.Ct. 159, 163-164, 80 L.Ed. 138, 146-147 (1935); United States v. Chemical Foundation, 272 U.S. 1, 14-15, 47 S.Ct. 1, 6, 71 L.Ed. 131, 142-143 (1926).72 Moreover, it forbids the court's substituting its judgment for that of the agency, Citizens to Preserve Overton Park v. Volpe, supra, 401 U.S. at 416, 91 S.Ct. at 823, 28 L.Ed.2d at 153, and requires affirmance if a rational basis exists for the agency's decision.73 Bowman Transportation, Inc. v. Arkansas-Best Freight System, Inc., 419 U.S. 281, 290, 95 S.Ct. 438, 444, 42 L.Ed.2d 447, 458 (1974). Cf. United States v. Allegheny-Ludlum Steel Corp.,406 U.S. 742, 749, 92 S.Ct. 1941, 1946, 32 L.Ed.2d 453, 460 (1972).

This is not to say, however, that we must rubber-stamp the agency decision as correct. To do so would render the appellate process a superfluous (although time-consuming) ritual. Rather, the reviewing court must assure itself that the agency decision was "based on a consideration of the relevant factors * * *."74 Moreover, it must engage in a "substantial inquiry" into the facts, one that is "searching and careful." Citizens to Preserve Overton Park v. Volpe, supra, 401 U.S. at 415, 416, 91 S.Ct. at 823, 824, 28 L.Ed.2d at 152, 153. This is particularly true in highly technical cases such as this one.

A court does not depart from its proper function when it undertakes a study of the record, hopefully perceptive, even as to the evidence on technical and specialized matters, for this enables the court to penetrate to the underlying decisions of the agency, to satisfy itself that the agency has exercised a reasoned discretion, with reasons that do not deviate from or ignore the ascertainable legislative intent.

Greater Boston Television Corp. v. FCC, 143 U.S.App.D.C. 383, 392, 444 F.2d 841, 850 (1970), cert. denied, 403 U.S. 923, 91 S.Ct. 2229, 2233, 29 L.Ed.2d 701 (1971). See also Essex Chemical Corp. v. Ruckelshaus, 158 U.S.App.D.C. 360, 367, 486 F.2d 427, 434 (1973), cert. denied, 416 U.S. 969, 94 S.Ct. 1991, 40 L.Ed.2d 558 (1974); Portland Cement Assn v. Ruckelshaus, 158 U.S.App.D.C. 308, 335, 486 F.2d 375, 402 (1973), cert. denied, 417 U.S. 921, 94 S.Ct. 2628, 41 L.Ed.2d 226 (1974); International Harvester Co. v. Ruckelshaus, 155 U.S.App.D.C. 411, 444, 478 F.2d 615, 648 (1971).75

There is no inconsistency between the deferential standard of review and the requirement that the reviewing court involve itself in even the most complex evidentiary matters; rather, the two indicia of arbitrary and capricious review stand in careful balance. The close scrutiny of the evidence is intended to educate the court. It must understand enough about the problem confronting the agency to comprehend the meaning of the evidence relied upon and the evidence discarded; the questions addressed by the agency and those bypassed; the choices open to the agency and those made. The more technical the case, the more intensive must be the court's effort to understand the evidence, for without an appropriate understanding of the case before it the court cannot properly perform its appellate function. But that function must be performed with conscientious awareness of its limited nature. The enforced education into the intricacies of the problem before the agency is not designed to enable the court to become a superagency that can supplant the agency's expert decision-maker. To the contrary, the court must give due deference to the agency's ability to rely on its own developed expertise. Market Street Railway v. Railroad Commission, 324 U.S. 548, 559-561, 65 S.Ct. 770, 776-777, 89 L.Ed. 1171, 1180-1182 (1945). The immersion in the evidence is designed solely to enable the court to determine whether the agency decision was rational and based on consideration of the relevant factors. Citizens to Preserve Overton Park v. Volpe, supra, 401 U.S. at 416, 91 S.Ct. at 823, 28 L.Ed.2d at 153; Bowman Transportation, Inc. v. Arkansas-Best Freight System, Inc., supra, 419 U.S. at 285, 290, 95 S.Ct. at 441, 444, 42 L.Ed.2d at 455, 458. It is settled that we must affirm decisions with which we disagree so long as this test is met.76 Bowman Transportation, Inc. v. Arkansas-Best Freight System, Inc., supra, 419 U.S. at 290, 95 S.Ct. at 444, 42 L.Ed.2d at 458; United States v. Allegheny-Ludlum Steel Corp., supra, 406 U.S. at 749, 92 S.Ct. at 1946, 32 L.Ed.2d at 460.

Thus, after our careful study of the record, we must take a step back from the agency decision. We must look at the decision not as the chemist, biologist or statistician that we are qualified neither by training nor experience to be, but as a reviewing court exercising our narrowly defined duty of holding agencies to certain minimal standards of rationality.77 " Although (our) inquiry into the facts is to be searching and careful, the ultimate standard of review is a narrow one." Citizens to Preserve Overton Park v. Volpe, supra, 401 U.S. at 416, 91 S.Ct. at 824, 28 L.Ed.2d at 153. We must affirm unless the agency decision is arbitrary or capricious.78

With the "arbitrary and capricious" standard firmly in mind, we now turn to the evidence supporting the regulations before us.

Petitioners vigorously attack both the sufficiency and the validity of the many scientific studies relied upon by the Administrator, while advancing for consideration various studies allegedly supportive of their position. The record in this case is massive over 10,000 pages. Not surprisingly, evidence may be isolated that supports virtually any inference one might care to draw. Thus we might well have sustained a determination by the Administrator not to regulate lead additives on health grounds. That does not mean, however, that we cannot sustain his determination to so regulate. As we have indicated above, we need not decide whether his decision is supported by the preponderance of the evidence, nor, for that matter, whether it is supported by substantial evidence.79 To the contrary, we must sustain if it has a rational basis in the evidence. Keeping in mind the precautionary "will endanger" standard under which the Administrator acted, we have no difficulty in terming his decision rational.

A word about our approach to the evidence may be in order. Contrary to the apparent suggestion of some of the petitioners, we need not seek a single dispositive study that fully supports the Administrator's determination. Science does not work that way; nor, for that matter, does adjudicatory fact-finding. Rather, the Administrator's decision may be fully supportable if it is based, as it is, on the inconclusive but suggestive results of numerous studies. By its nature, scientific evidence is cumulative: the more supporting, albeit inconclusive, evidence available, the more likely the accuracy of the conclusion.80 If, as petitioners suggest, one single study or bit of evidence were sufficient independently to mandate a conclusion, there would, of course, be no need for any other studies. Only rarely, however, is such limited study sufficient. Thus, after considering the inferences that can be drawn from the studies supporting the Administrator, and those opposing him, we must decide whether the cumulative effect of all this evidence, and not the effect of any single bit of it, presents a rational basis for the low-lead regulations.

While we have studied the record with great care, we do not discuss it all here; to do so would make this already lengthy opinion completely unwieldy. Instead, we shall briefly review the bases for the Administrator's conclusions that petitioners have singled out for special attack.81 Before we turn to those issues, however, we should note that some things appear to be uncontested. Thus petitioners seem to concede the following: that lead serves no known purpose in the human body; that lead in sufficiently high quantity is destructive to the body, causing anemia, severe intestinal cramps, paralysis, neurologic damage, and, in sufficient dosage, death, Third Health Document at III-1, 2, JA 54-55; that more than 250,000 tons of lead per year are used in production of lead additives, accounting, according to EPA, for approximately 90 percent of all airborne lead,82 id. at Table II-1, JA 46; that lead concentrations in the air over our largest cities are 2,000 times greater than lead concentrations in the air over the mid-Pacific, NAS Report at 205; that lead in the ambient air contributes to body blood lead levels, Nalco Supp.Br. at 37; supplemental brief of petitioners PPG Industries and E. I. duPont de Nemours & Company (hereinafter PPG/duPont Supp.Br.) at 23; and that blood lead levels are a reasonable indication of the body's lead burden. Stripped of their generalized and largely unsubstantiated claims of "bias" and "distortion of the evidence," petitioners principally challenge three EPA conclusions: (1) that, based on a preliminary determination that blood lead levels of 40 ug are indicative of danger to health, elevated blood lead levels "exist to a small but significant extent in the general adult population, and to a very great extent among children," Third Health Document at VII-3, JA at 144; (2) that airborne lead is directly absorbed in the body through respiration to a degree that constitutes a significant risk to public health; and (3) that airborne lead falls to the ground where it mixes with dust and poses a significant risk to the health of urban children.

1. Blood Lead Levels are Elevated Among the General Public

a. Blood lead levels of 40 ug are indicative of danger to health. Although recognizing that a blood lead level of 40 ug "does not represent a sharp demarcation between health and disease," the Administrator found it "prudent to regard blood lead levels over 40 ug/100g as indicators of lead intake that should be prevented." Third Health Document at III-11, JA 64. Petitioners contest this determination. Much of their argument, however, adds up to nothing more than that they are unhappy the Administrator chose to exercise his judgment and think the statute does not permit such discretion. Petitioners cite medical studies that show not that a blood lead level of 40 ug is unrelated to danger, but that scientists are simply uncertain about the effect of such blood levels. Ethyl brief at 37-38 and Supp.Br. at 41-42; PPG/duPont brief at 11-12; Nalco brief at 14. But, as we have seen, exercise of reasoned discretion based on the evidence is not only permitted, but mandated, by Section 211(c)(1)(A). The administrator recognized from the outset that the health effects of varying human blood lead levels were uncertain and that continuing research was necessary.83 His determination of the 40 ug level is, on its face, an assessment of risks based on the known facts and not improper as such.

In addition to their generalized attack on the Administrator's power, petitioners challenge the sufficiency of the evidence to support his determination. Under the "will endanger" standard, however, we find the evidentiary basis for the Administrator's determination to be more than adequate. Petitioners do not contest the recommendation of the United States Public Health Service that 80 ug be taken as the standard of unequivocal lead poisoning, or the Service's recommendation that blood lead levels of 50-79 ug justify immediate evaluation for possible lead poisoning. Medical Aspects of Childhood Lead Poisoning, HSMHA Health Reports, 86(2), 140-143 (1971), cited in Third Health Document at IV-3, JA 71. What draws petitioners' fire is only the last of the Service's recommendations, adopted by EPA, that for older children and adults "a blood lead concentration of 40 ug or more per 100 ml of whole blood * * * be considered evidence suggestive of undue absorption of lead, either past or present." Id. While the reasonableness of establishing as a danger point a lead exposure level somewhat lower than that at which actual damage may occur seems both logically unassailable and well within the intent of the "will endanger" standard, the Administrator went beyond that sufficient conclusion. He recognized, and warned of, possible dangers, particularly to children, from lead at the 40-60 ug level, and even below, thereby suggesting that the Public Health Service's recommended danger level, designed for adults, may be set too high. Because this latter evidence was only suggestive, however, the Administrator conservatively settled on the 40 ug standard.84 Third Health Document III-5, 6, 7, 8, JA 58-61. Such a "prudent" determination is well within the Administrator's discretion under the "will endanger" standard. We have examined the evidence relied upon carefully and, while it is not necessary to summarize it here,85 we find that it provides a rational basis for the Administrator's determination.

b. Blood lead levels are elevated among a small, but significant, number of adults and a considerable number of children. Again, petitioners challenge the Administrator's determination as unsupported by the evidence. The problem here is one of choosing among the items of evidence. Petitioners rely heavily on the results of the so-called Seven Cities Study,86 which found a very small percentage of adults with elevated (in excess of 40 ug) blood levels.87 PPG/duPont brief at 12-15; Nalco brief at 16. The Administrator, on the other hand, finds serious methodological flaws in the Seven Cities Study that limit its usefulness, 38 Fed.Reg. 33735, and relies instead on studies which concededly support his conclusion, Third Health Document at Tables VII-1, 2 & 3, JA 145-147, but which petitioners score as representative only of certain occupational groups. PPG/duPont brief at 15; Nalco brief at 15-16.

Having analyzed this evidence and the arguments of the parties, we would again defer to the Administrator's judgment. First we note that, while contesting the source of lead exposure, petitioners do not challenge at all the Agency's conclusion that blood lead levels are elevated in a large number of children, including a possible 25 percent of all preschool children living in substandard housing.88 Third Health Document at Table VII-3, JA 147.

Next, while the studies relied on by the Administrator are largely of various occupational groups, they are frequently occupations whose only exposure to lead is through the ambient air in which their workers policemen, mailmen, service station employees, parking lot attendants, and the like are forced to spend their working hours.89 Third Health Document at Table VII-1, JA 145. Contrary to the arguments of petitioners, studies of occupational groups are often particularly valuable in acting as an early warning system of possible effects on the public at large. The intensive exposure in particular occupations essentially accelerates the effects of long-term exposure of the general public and provides an identifiable, highly exposed, and readily accessible group of research subjects. See D. Clark & B. MacMahon, Preventive Medicine 26 (1967) (hereinafter Clark & MacMahon). Thus much of the evidence in Reserve Mining of the danger to the general public of absorption of asbestos particles came from epidemiological studies of asbestos workers. Reserve Mining Co. v. EPA, supra, 514 F.2d at 507.90 Because of this the occupational data are not only in themselves indicative of high blood lead levels among a significant portion of the adult population, but they suggest the future likelihood, absent controls, of increasing high blood lead levels among even more widely dispersed portions of the public. In any case, however, the evidence relied upon by the Administrator was not only of occupational groups. Some evidence of high blood lead level relates to such "specialized" groups as "Drivers of Cars," "Male Commuters," and "Urban" residents. Third Health Document at Tables VII-1 & 2, JA 145-146.

Petitioners argue that the negative conclusions of the Seven Cities Study and several other studies should outweigh the positive indications of the studies described above. PPG/duPont Supp.Br. at 16-23; NPRA Supp.Br. at 5-8. The Administrator disagreed, and we cannot fault his conclusion. Because of the many uncontrolled variables in epidemiological studies, valid conclusions may be obscured. But, while the possibility of false negative conclusions is a real one, the possibility of false positive findings is considerably less likely. Clark & MacMahon, supra, at 100. Thus the Administrator credited the occupational and other studies reporting a small, but significant, portion of the general population with elevated blood lead levels and discounted the negative studies on which petitioners rely. Third Health Document at VII-3, JA 144. We cannot say his conclusion was arbitrary or capricious.

2. Automobile Lead Emission Products are Directly Absorbed in the Body to a Significant Extent

Since it is apparent from the face of his decision and the Third Health Document that the Administrator considered all the evidence before him, the only issue is whether he treated that evidence in a rational manner. Petitioners have now conceded that lead emissions are directly absorbed in the body from the ambient air, and they challenge only whether the extent of absorption is significant enough to justify these regulations. Nalco Supp.Br. at 37; PPG/duPont Supp.Br