Full opinion text
Opinion for the Court filed by Senior Circuit Judge McGOWAN. McGOWAN, Senior Circuit Judge: This case concerns the amount of credit electric power plants and other major sources of air pollution may receive for the height of their emissions stacks in calculating limitations on their emission of pollutants. Under the Clean Air Act as amended (“the Act”) and its regulations, emissions limitations for each such source are fixed on the basis of local, ground-level concentrations of pollutants, which cannot exceed certain national standards or incremental increase limitations. Since taller stacks tend to disperse pollutants over a greater area, a utility or other source can lower the ambient pollution concentrations not only by reducing the amount of pollutants it emits into the air, but also by raising the height of its stack. After the basic provisions of the Act were passed in 1970, many chose the latter route. In 1977 amendments to the Act, Congress declared that such tall stacks and other dispersion techniques were not to be taken into account in calculating the limitations on emissions imposed by the Act. 42 U.S.C. § 7423 (Supp. V 1981). Rather, pollution standards were to be achieved by direct limitations on emissions. The present case brings before us final regulations issued by the Environmental Protection Agency (EPA) to implement this provision. The regulations at issue are detailed and somewhat complex. Generally speaking, under the 1977 amendments credit for stack height in calculating emissions limitations is limited to the height dictated by “good engineering practice” (GEP). Id. § 7423(a)(1). This height was defined by Congress to be that necessary to ensure against certain kinds of localized atmospheric disturbance created by the source itself or nearby obstacles, and resulting in excessive concentrations of pollutants in the immediate vicinity of the source. Id. § 7423(c). The regulations under review define a number of the statutory terms, such as “nearby” and “excessive,” provide various methods for determining GEP height and determine when each may be used, implement a statutory bar on credit for use of “dispersion techniques” other than stack height, define a statutory “grandfather” clause for pre-1970 stacks, and provide a timetable for implementation of the regulations by the states, which are the primary enforcers of the Act. We have reviewed carefully the specific provisions challenged here. Among them we find certain aspects of the regulatory scheme to be contrary to the terms of the statute and others to be arbitrary and capricious exercises of the discretion conferred on the EPA by the Act. These provisions must therefore be overturned. We remand certain other provisions for further consideration by the agency in light of our discussion here. The remainder of the challenged regulations we uphold. I The events leading up to the enactment of the section of the Clean Air Act Amendments of 1977 involved in this case have been described in our opinion in Alabama Power Co. v. Costle, 636 F.2d 323, 388-91 (D.C.Cir.1979), and in the House committee report accompanying those amendments, H.R.Rep. No. 294, 95th Cong., 1st Sess. 81-92 (1977), [U.S.Code Cong. & Admin.News 1977, p. 1077] [hereinafter cited as House Report]. Briefly, under the drastic overhaul of the Clean Air Act undertaken in 1970, EPA was directed to prescribe national ambient air quality standards for various pollutants. 42 U.S.C. § 1857c-4 (1976). Upon promulgation of an air quality standard, each state was required to adopt and submit to EPA a state implementation plan providing for attainment and enforcement of the standard. Id. § 7410(a). Initially, EPA approved state plans that authorized, in place of direct limitations on emissions, the use of tall stacks to meet air quality standards. EPA also allowed the use of other dispersion techniques called supplemental or intermittent control systems, which are programs that vary the release of pollutants over time depending on whether meteorological conditions favor dispersion. EPA’s policy was overturned by the courts, which, led by the Fifth Circuit, ruled that the Act allowed reliance on dispersion techniques only after implementation of “the maximum degree of emission limitation achievable.” NRDC v. EPA, 489 F.2d 390, 410 (5th Cir.1974), rev’d on other issues sub nom. Train v. NRDC, 421 U.S. 60, 95 S.Ct. 1470, 43 L.Ed.2d 731 (1975); see Kennecott Copper Corp. v. Train, 526 F.2d 1149, 1151-60 (9th Cir.1975); cert. denied, 425 U.S. 935, 96 S.Ct. 1665, 48 L.Ed.2d 176 (1976); Big Rivers Electric Corp. v. EPA, 523 F.2d 16, 20-22 (6th Cir.1975), cert. denied, 425 U.S. 934, 96 S.Ct. 1663, 48 L.Ed.2d 175 (1976). “Informed as well as chastened by these judicial decisions,” Alabama Power, 636 F.2d at 390, EPA in 1976 issued guidelines that, while placing primary emphasis on emissions reductions, allowed the use of tall stacks to meet ambient standards in two situations: (1) where the source was already using “the best available emission control technology,” or (2) where use of such technology would be “economically unreasonable or technologically unsound.” Stack Height Increase Guideline, 41 Fed. Reg. 7450, 7451-52 (1976) [hereinafter cited as 1976 Guideline]. Congress emphatically rejected this approach in the 1977 amendment of the Act that is the subject of the present controversy. In introducing the bill on the Senate floor, Senator Muskie criticized the 1976 guidelines for allowing any use of tall stacks whatsoever in meeting ambient standards: “Far from prohibiting the construction of tall stacks or the use of intermittent controls, the guidelines provide that once minimal emission control requirements are met, polluters are encouraged to substitute unlimited stack height for any further control of emissions.” 123 Cong.Rec. 18,027 (1977). The mood in the House was the same, id. at 16,203 (remarks of Rep. Wax-man, a sponsor of the House bill) (“The committee has unequivocally rejected the use of tall stacks and intermittent controls as a final means of compliance with the Clean Air Act’s requirements.”), and in section 123 of the amended Act Congress banned virtually all reliance on tall stacks or “any other dispersion technique” in achieving compliance with ambient air quality standards, 42 U.S.C. § 7423(a) (Supp.V 1981). Congress did not, however, actually prohibit tall stacks or limit their height; in fact, section 123 specifically enjoins the EPA Administrator (“the Administrator”) from prohibiting any increase in stack height or restricting the height of any stack in any manner. Id. § 7423(c). Rather, the law limits the credit that may be obtained for such stack height in determining whether the plant will cause ambient air standards to be violated or increase pollution by too large an increment. The credit system is based on techniques of modeling whereby, via mathematical or small-scale physical demonstrations, a plant’s emissions can be assumed to emerge from a certain stack height and then mapped as they fall to earth in order to see their effect on ambient pollution. Thus, section 123 is intended to eliminate any credit a plant might receive for the dispersive effects of a tall stack in the calculation of its emissions limitations, although the stack itself remains in place. There were essentially three reasons for Congress’s refusal to allow reliance on tall stacks and intermittent control measures. First, dispersion techniques do not reduce the amount of pollution in the air, but merely spread it around, exporting it to other areas where it is too late to control the problem, and exposing previously pristine areas to contamination. See, e.g., House Report, supra p. 440, at 84-85. Second, the long-range transport of certain pollutants was also linked to the formation of “acid rain,” which is precipitation containing acidic derivations of sulfur oxide and nitrogen oxide emissions. Acid rain was thought responsible for reduced soil and water productivity in certain areas, particularly the Northeast and Canada. See, e.g., id. at 83-84, 85-86; 123 Cong.Rec. 18,026 (1977) (remarks of Sen. Muskie). Third, intermittent control systems, which are dependent on synchronizing plant operation with weather conditions, were thought to be unreliable and virtually impossible to enforce. See, e.g., id.; House Report, supra p. 440, at 82-83, 87. In rejecting the limited permission to use dispersion techniques contained in the 1976 guidelines, however, Congress largely adopted the distinction drawn in those guidelines, and in a 1973 stack height proposal, between stack heights that would be allowed without question and those that would be regarded as a dispersion technique. See id. at 93 (statute “affirm[s]” the standard used by the Administrator). This distinction is the main battleground of the present litigation. The guidelines had based the distinction on “good engineering practice,” which the preamble to the 1973 proposal defined as follows: [A] stack which conforms to good engineering practice is sufficiently tall that emissions from the stack are not significantly affected by the atmospheric down-wash, eddies, or wakes created by the facility or nearby structures and terrain. Emissions from stacks which are shorter than required by good engineering practice often can cause excessively high ground level concentrations and nuisances within, and in the immediate vicinity of, the facility. Use of Supplementary Control Systems and Implementation of Secondary Standards, 38 Fed.Reg. 25,697, 25,700 (1973) (proposed rules) [hereinafter cited as 1973 Guidelines]. Congress appears to have taken the main elements of this statement in its definition of good engineering practice height. Section 123 defines that height as the height necessary to insure that emissions from the stack do not result in excessive concentrations of any air pollutant in the immediate vicinity of the source as a result of atmospheric down-wash, eddies and wakes which may be created by the source itself, nearby structures or nearby terrain obstacles. 42 U.S.C. § 7423(c) (Supp. V 1981). As the quoted language from the 1973 proposal suggests, downwash, eddies, and wakes are caused by the wind hitting structures or other obstacles near the stack. The turbulence created in the wake of the obstacles tends to suck a plume of emissions down to the earth before it has had a chance to disperse, resulting in inordinately high pollution concentrations near the plant. See also House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News 1977, p. 1171] (“Without some provision for stack height, a plume released downwind of such a structure might become engulfed by turbulent eddys [sic] within the wake of the structure.”); 122 Cong.Rec. 34,384 (1976) (remarks of Sen. Muskie) (“This is necessary in order to allow good plume rise without downwashing the plume onto the local area.”). While the statute generally left the determination of GEP stack height to regulations to be promulgated by the EPA Administrator, it set an upper limit of two- and-one-half times the height of the stack’s source. 42 U.S.C. § 7423(c) (Supp. V 1981). This, too, was taken from the 1973 and 1976 guidelines. See 1976 Guideline, supra p. 440, at 7451-52; 1973 Guidelines, supra p. 441, at 25,700, 25,701. The formula was seen as codifying “the stack height-nearby structure relationship that has been looked to historically as a responsible way of dealing with the problem of aerodynamic down-wash.” House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News 1977, p. 1171]. While the guidelines used the two- and-one-half-times standard as the standard GEP stack height, however, Congress was clear in its conference report that the statutory formula was generally intended to be an upper limit, and that if EPA found that the problem of downwash, eddies, and wakes could be prevented by stacks of less than two-and-one-half times facility height, it was to give credit only for the lower height. 123 Cong.Rec. 27,071 (1977) (Clean Air Conference Report (1977): Statement of Intent; Clarification of Select Provisions). Nevertheless, like the guidelines, the statute provides that a plant operator can get credit for a greater-than-formula height by demonstrating to the satisfaction of the Administrator that a greater height is needed to prevent the downwash problem described in the Act. 42 U.S.C. § 7423(c) (Supp. V 1981). In barring any credit for tall stacks and other dispersion techniques, Congress believed it was merely reaffirming a command it had given in the 1970 Clean Air Act amendments. It “intended to ratify the general thrust, if not the specific holdings, of the three U.S. courts of appeals” that had interpreted the earlier Act to bar primary reliance on such techniques. House Report, supra p. 440, at 91, [U.S.Code Cong. & Admin.News 1977, p. 1170]. As a result, the section’s grandfather clause provides an exemption only for stacks “in existence” or dispersion techniques “implemented” before the date of the 1970 amendments, and not for those built between 1970 and the enactment of the 1977 amendments. 42 U.S.C. § 7423(a) (Supp. Y 1981); see House Report, supra p. 440, at 93. Section 123 directs EPA to issue regulations implementing these provisions by February 7, 1978, 42 U.S.C. § 7423(c) (Supp. V 1980) (“[n]ot later than six months after August 7, 1977”), and the states are directed to revise their applicable implementation plans, as necessary, within nine months of the promulgation of EPA’s regulations, id. § 7401 note (the later of one year after enactment of the Act or nine months after promulgation of EPA regulations). Proposed regulations were not issued until January 12, 1979, however. Stack Height Regulations, 44 Fed.Reg. 2608 (1979). Under a court-ordered timetable, Sierra Club v. Gorsuch, No. 81-0094 (D.D.C. June 22, 1981, modified Aug. 20, 1981, and Feb. 17, 1982), EPA then issued a revised set of proposed regulations on October 6, 1981, 46 Fed.Reg. 49,814 (1981), and finally issued final regulations on February 8, 1982, 47 Fed.Reg. 5864 (1982) (to be codified in 40 C.F.R. §§ 51.1, 51.12, 51.18). Petitioners NRDC and Sierra Club filed motions for reconsideration on various grounds, all of which were denied. Petitions for review were then filed in this court under the Act’s sixty-day review provision. 42 U.S.C. § 7607(b) (Supp. V 1981). The core of the new regulations is the determination of GEP stack height. The rules provide three methods; a source operator may use whichever of the methods yields the greatest GEP height. First, a “de minimis height” of sixty-five meters is permitted for all sources regardless of the size or location of any structures or terrain features. 47 Fed.Reg. at 5868 (to be codified at 40 C.F.R. § 51.1(h)(1)). The Administrator stated that this height represented “a reasonable estimate of the height needed to insure that emissions will not be affected by common ground-level meteorological phenomena which may produce excessive pollutant concentrations.” Id. at 5865. The de minimis feature is not challenged in this case. Second, the regulations provide two mathematical formulas, one to be used by stacks in existence on January 12,1979, the date of publication of EPA’s original proposed rules, and the other for stacks whose construction commenced after that date. The formula to be used by the earlier stacks is what the Administrator termed “the traditional engineering formula of two and one-half times the height of the nearby structure” (hereinafter called the 2.5 Rule). Id. The second formula is a refinement of the first that is intended to reflect the reduced height needed to surmount the less severe downwash effects produced by tall, thin structures. See 46 Fed.Reg. at 49,815. The refined formula gives credit for the height of the nearby structure plus one- and-one-half times the lesser of the height or width of the structure (hereinafter called the 1 + 1.5 Rule). 47 Fed.Reg. at 5868 (to be codified at 40 C.F.R. § 51.1(ii)(2)(ii)). Under either formula, only “nearby” structures may be used. “Nearby” is defined as a distance from the stack of five times the lesser of the height or width of the structure itself, up to one-half mile. Id. at 5869 (to be codified at 40 C.F.R. § 51.1(jj)). The third method for calculating GEP stack height is by a physical demonstration, either a fluid model or a field study. This method must be used to obtain credit for downwash produced by terrain features or by any obstacle that is not “nearby.” The demonstration must show that a greater-than-formula height is needed to ensure that emissions from the stack do not result in excessive concentrations of any air pollutant. Id. at 5868-69 (to be codified at 40 C.F.R. § 51.1(ii)(3)). “Excessive concentrations” is defined as maximum concentrations of a pollutant at least forty percent in excess of the maximum concentrations of the pollutant in the absence of the down-wash, eddy, or wake effects. Id. at 5869 (to be codified at 40 C.F.R. § 51.1(kk)). We will describe other details of the challenged regulations as needed in dealing with each of petitioners’ objections to the new rules. We will discuss first those objections going roughly to the formulation and operation of the three methods, next those going to when the various methods are applied, then objections to certain exemptions from the regulations, then the various grandfather clauses provided by the regulations, and finally an objection to EPA’s timetable for state implementation of the regulations. II A. Definition of “Nearby” As noted, the new regulations define “nearby” for purposes of application of the formulas as five times the height or width of the structure, up to one-half mile. There is no specific limit on the distance that structures and terrain obstacles may be from the stack in order to be taken into account in a demonstration. NRDC and Sierra Club argue that Congress intended to limit the structures and terrain obstacles that may be said to cause downwash to those within one-quarter mile of the stack, so that the regulations are contrary to law with regard to both the formulas and demonstrations. The source of petitioners’ argument is certain language in the House Report that they say indicates Congress’s understanding that “nearby” meant no more than one-quarter mile away. The Report’s discussion of the term is as follows: In affirming the 2Vi times standard used by the Administrator, the committee referred to downwash problems created by both manmade structures and to terrain features located “nearby” the source. The committee intends that the term “nearby” be strictly construed, in keeping with the general policy of statutory interpretation favoring strict construction of exceptions and variances. If this term were construed too broadly (that is, to apply to manmade structures or terrain features one-fourth to one-half mile away from the source or more), the result could be an open invitation to raise stack heights to unreasonably high elevations and to defeat the basic underlying committee intent. House Report, supra p. 440, at 93, [U.S. Code Cong. & Admin.News 1977, p. 1171] (emphasis added). EPA apparently selected the one-half-mile limitation solely in response to this expression of congressional intent; it itself believed that downwash effects occur at greater distances from the obstacles and that the five-times-height-or-width rule was a better approximation of the longevity of those effects than is the one-half-mile limit. See 44 Fed.Reg. at 2610; 46 Fed.Reg. at 49,819. We agree that the one-half-mile limitation is a sufficient response to the congressional intent. The statute specifically gives the EPA Administrator discretion to promulgate regulations to determine GEP height and the House report clearly indicates that that discretion extends to defining terms' such as “nearby” as necessary, presumably in light of the Administrator’s expertise. The report standing alone is ambiguous on whether it was trying to impose a specific limitation on the definition of “nearby,” but when read in light of the statute’s broad conferral of discretion it is most readily interpreted as an attempt only to suggest the scale of magnitude that the committee had in mind, and not to pick a specific figure. The one-half-mile limitation that EPA chose is at or near the outer edge of the range Congress was thinking of, but it does not go beyond it. Thus, we find the Administrator’s choice to be consistent with both the legislative history and the statute. The refusal to give any content to the statutory term “nearby” when applied to demonstrations is quite a different matter. The rationales offered were that (1) some obstacles create downwash effects at distances of more than one-half mile, (2) the fluid modeling methods would accurately tell precisely when such effects were occurring, and (3) the boundaries of many terrain features are not always distinct and thus a specific distance limitation would be difficult to apply. See 46 Fed.Reg. at 49,819; see also id. at 49,821 (accuracy of fluid modeling). Thus, the Administrator believed that the statute was intended to allow credit for the height needed to avoid the effects of any downwash that could cause excessive concentrations of pollutants. While such an approach might make a good deal of sense, we do not think it is the approach commanded by the statute. The primary support for the Administrator’s reading is that the language from the House Report quoted above, which places great emphasis on the word “nearby” as a carefully imposed limitation on the determination of GEP height, discusses the term only in connection with the formula method, not the demonstration technique. The Report discusses demonstrations in a different paragraph and does not mention the word “nearby” there at all. House Report, supra p. 440, at 93. Moreover, applying the “nearby” limitation only to the formula method and not to demonstrations would certainly be rational because, unlike demonstrations, the formulas do not otherwise select the obstacles to be taken into account. Without some limitation, the formulas could conceivably be used to give credit for the height of any obstacle upwind of the stack, even though the turbulence created in the wake of those obstacles could not possibly disrupt the plume. Demonstrations, however, do select the obstacles that will be taken into account, because they more accurately tell which will actually cause downwash. Nevertheless, the legislative history is not explicit enough to refute the clear thrust of the statutory language. In describing the demonstrations that are permitted, the statute states that the operator may show “that a greater height [than two-and-one-half times the height of the source] is necessary as provided under the preceding sentence.” 42 U.S.C. § 7423(c) (Supp. V 1981) (emphasis added). The “preceding sentence” defines “good engineering practice” as the height necessary to ensure that excessive concentrations will not result from down-wash created by the source, “nearby structures or nearby terrain obstacles.” Id. (emphasis added). Thus, the statute explicitly applies the “nearby” limitation to demonstrations. If such a reading were utterly nonsensical, we might be tempted, as a matter of interpretation of likely intent, to strain the statutory language to arrive at a more rational result. See, e.g., American Tobacco Co. v. Patterson, 456 U.S. 63, 71, 102 S.Ct. 1534, 1538, 71 L.Ed.2d 748 (1982) (“Statutes should be interpreted to avoid ... unreasonable results whenever possible.”); 2A C. Sands, Statutes and Statutory Construction § 45.12 (4th ed. 1973) (same). But such an approach to interpretation of statutes must be used with utmost caution, for the line between irrationality and mere bad policy is a wavering and uncertain one. Here, sense can be made of a limitation on the stack-height credit operators may receive in addition to the requirement that the height be necessary to avoid excessive concentrations of pollutants in the immediate vicinity of the plant. When EPA originally proposed these regulations in 1979, for example, it apparently intended some version of the “nearby” limitation to apply to demonstrations; it explained that it interpreted “the Congressional guidance as a criterion to indirectly establish a reasonable upper limit on GEP stack heights.” 44 Fed. Reg. at 2610; see id. at 2611 (“As in the case of GEP determinations using the empirical equation, the definition of ‘nearby’ is integral to determine the extent to which structures or terrain features may reasonably influence the fluid modeling or field study based GEP determination.”). That is, Congress may merely have wanted to place an absolute cap on the credit a source could receive for a tall stack, perhaps out of a distrust of the political and scientific methods by which the agency’s determinations of stack height credit were to be made. In addition, as the Administrator’s rationale for eliminating the “nearby” limitation suggests, that limitation will primarily affect sources located in hilly terrain, since it is unlikely that a manmade obstacle will be large enough to create downwash problems at distances of greater than half a mile. Yet there are strong indications in the legislative history that Congress specifically sought to discourage utilities from locating in hilly terrain, because such locations tend to require very tall stacks, leading to greater dispersion of pollutants. For example, the House report states that it was “the expectation of this committee that persons responsible for siting new facilities will not locate them next to terrain features which will produce ... downwash.” House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News 1977, p. 1171]. See infra p. 455 (other indications of this intent). Applying the “nearby” limitation to all methods of deriving GEP heights may be further evidence of Congress’s lack of solicitude for utilities located next to mountains (as opposed to those located next to very local, and presumably smaller, terrain features that may be surmounted with less stack height). Finally, even if there is an element of arbitrariness in Congress’s applying the “nearby” limitation to demonstrations, we note that the entire GEP stack height allowance was already regarded as something of a concession from the strict command that dispersion not be used to meet air quality standards. Congress may simply have been unwilling to compromise further the predominant purpose of reducing emissions in order to take account of what it may have regarded as fairly attenuated claims of downwash. There is frequently some arbitrariness when a lawmaker says, “Thus far and no further,” but such lines frequently must be drawn. The statutory language must thus be interpreted to apply “nearby” to demonstrations as well as to the formulas as a limitation on the amount of downwash that will be taken into account in giving credit for stack height. We remand, therefore, for the EPA to include new regulations that apply the same “nearby” limitation to demonstrations as is applied to the formulas. B. Definition of “Excessive Concentrations” When a source owner seeks to obtain credit for stack height greater than provided by the formulas, it must demonstrate that downwash can be expected to cause “excessive concentrations” of pollutants in the vicinity of the plant. The regulations define “excessive concentrations” as a forty-percent increase over the levels in the absence of the downwash-creating obstacle. 47 Fed.Reg. at 5869 (to be codified at 40 C.F.R. § 51.1(kk)). Petitioners NRDC and Sierra Club argue that such a definition is arbitrary and capricious because it does not measure any absolute amount of pollutant that is a danger to health or welfare, but instead invokes a relative measure. They state that the forty-percent rule would permit a source located in a very clean area to raise its stack height credit, even if the downwash avoided would only increase pollutant concentrations by a very small amount that would be of no harm to anyone. They urge a return to a standard like the one EPA originally proposed in 1979. Under that standard a source would have had to show that down-wash would both increase pollutant concentrations by at least forty percent and cause a violation of a national ambient air quality standard or, in certain areas, an incremental increase limitation. 44 Fed.Reg. at 2611. EPA eliminated the second of these criteria — requiring a violation of an air quality standard — in 1981. It said that its air quality standards and incremental limitations are unable to measure the high pollutant concentrations of extremely short duration that are typical of downwash. 46 Fed.Reg. at 49,819. This is because the standards measure pollutants after they have dispersed in the air, not in the concentrated doses caused by downwash. Id. Also, the standards measure average concentrations over time periods ranging from one hour to one year, which is too long to measure accurately the peak concentrations of down-wash pollutant. Id. (In this court, EPA states that it is currently reviewing its S02 standard to determine whether a short-term standard is necessary to protect public health. Brief of Respondents at 35 n. 27.) Thus, only the forty-percent test was left. In this circumstance, the basis for, and derivation of, that test become especially important. The forty-percent figure was derived from a review of the scientific literature on stack heights, including reports of wind tunnel tests EPA itself conducted during the rulemaking. EPA discovered a consensus in the literature that “the well established 2.5 times rule” was the stack height necessary to avoid “significant effects” for most buildings. The 1 + 1.5 Rule was found to be the consensus for tall, thin buildings. See Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for Stack Height Regulations) (July 1981), Joint Appendix (J.A.) at 1086 [hereinafter cited as Technical Support Doc.]. EPA then looked to the amount by which ground-level concentrations of pollutant were increased when a 2.5 or 1 + 1.5 stack was in place. It found that these formulas did not eliminate all effects of downwash but limited increased concentrations to roughly forty to eighty percent over the preexisting concentrations, with eighty percent representing an unusually high amount. See id. at 1096. EPA reasoned that if under traditional engineering practice the increase in concentrations was limited to about forty percent, then increases over forty percent could be regarded as excessive. See Draft Technical Support Document for Determination of Good Engineering Practice Stack Height (July 31, 1978), J.A. at 42 (Where range of increases recorded at GEP height was 20% to 40%, report concluded that “an increase in maximum concentrations less than 20% is less than expected for GEP stack height while an increase in maximum concentrations greater than 40% is excessive.”). The forty-percent figure is thus the lesson of history: it is what the engineering community has regarded as too much down-wash. NRDC and Sierra Club argue that this approach is all wrong, since Congress’s real concern in encouraging stack heights high enough to prevent “excessive concentrations” of downwash was the protection of human health. Therefore, they say, EPA must exercise its own independent judgment and define the term to allow enough height to prevent health-threatening downwash, and no more. Because the forty-percent standard is based on a relative value that varies with background concentrations, NRDC and Sierra Club assert, it does not measure the absolute levels of pollutant that are a danger to health. Our review of the statute and its legislative history discloses sharply conflicting signals on whether Congress intended to legislate the preexisting engineering practice or only so much stack height as would protect health or welfare. On the one hand, the primary statutory standard was “good engineering practice,” suggesting an intent to mandate whatever engineers had been doing. Moreover, the entire purpose of the statute was to remedy an abuse that had arisen whereby polluters were building stacks solely to evade Clean Air Act requirements; Congress may well have wanted to return to whatever engineers would do in the absence of the Act. In addition, the statute explicitly endorsed the standard — the 2.5 Rule — that Congress thought was the traditional engineering practice. Nevertheless, Congress did not stop there, for it provided a very precise definition of what it regarded as “good engineering practice,” suggesting that it did not intend to leave the question of stack heights entirely to professional standards. While the statutory term at the center of that definition, “excessive concentrations,” is ambiguous — it could be read as either in excess of the previously recognized standard or in excess of some absolute standard, such as safety — the legislative history points strongly in the direction of a meaning turning on danger to public health. The House committee report describes why downwash can be a problem in the following terms: “When this [downwash] occurs even the plume from a well-controlled source may cause air quality standards (or other requirements) to be violated.” House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News 1977, p. 1171]. We think it a strain to refer to the previously recognized standard as a “requirement”; it seems likely that the committee was referring to other legal requirements, such as state nuisance law or the Clean Air Act’s prevention of significant deterioration increments. EPA’s 1973 stack height guidelines, which, as we have said, appear to have been Congress’s main source for the statutory language, also strongly suggest that the evil sought to be avoided by good engineering practice was tied to some minimum level of danger or inconvenience to the community. They provided: Emissions from stacks which are shorter than required by good engineering practice often can cause excessively high ground level concentrations and nuisances within, and in the immediate vicinity of, the facility.... The use of stack height up to the level of good engineering practice is encouraged by EPA in order to avoid local nuisances. 1973 Guidelines, supra p. 441, at 25, 700; see also, e.g., Commonwealth v. South Covington & C. St. Ry., 181 Ky. 459, 463, 205 S.W. 581, 583 (1918) (“[A] common or public nuisance is the doing of or failure to do something that injuriously affects the safety, health, or morals of the public, or works some substantial annoyance, inconvenience, or injury to the public ....”); United States v. County Board, 487 F.Supp. 137, 143 (E.D.Va.1979) (“The term ‘nuisance’ . .. includes everything that endangers life or health, gives offense to the senses, violates the laws of decency, or obstructs the reasonable and comfortable use of property.”). What seems most likely is that Congress thought traditional engineering practice and protection of health were the same thing. If that is the case, however, what are we to do if it develops, as NRDC and Sierra Club argue, that traditional engineering practice in fact dictates a height that is in some cases much higher than necessary to protect human health? We are saved from the full rigors of this potential conundrum by the conference committee. Its report states: [I]f it should be determined that down-wash, eddies, and wakes can be prevented by stacks of less than 2V2 times facility height, the Administrator’s rule should give ‘credit’ only for the height needed to avoid these conditions. ... In other words, it was not our purpose to make a Congressional judgment about what stack height was needed to prevent downwash. We intend EPA to make this judgment, subject only to the Congressional prohibition on the excessively high stacks of over 2lh times building height. 123 Cong.Rec. 27,071 (1977). What this passage suggests is that Congress wanted the Administrator to determine the height necessary to avoid excessive concentrations of downwash-caused pollution. Since Congress believed that the 2.5 Rule was in fact the good engineering practice rule, see House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin. News 1977, p. 1171] (“A stack height value produced by reference to this historical relationship is referred to as ‘good engineering practice’ stack height and has been used by EPA in its regulations.”), the passage suggests that the conference committee saw the possibility of a distinction between its definition of the amount of downwash to be avoided and what engineers had been doing. If that turned out to be the case, the committee was clear that' the statute’s definition would govern. Thus, development of a standard governing the height of stacks by reference solely to what engineers had been doing, with no regard for some real life values, was contrary to the intent of Congress. Reading the House committee report as a whole confirms this view. It begins its discussion of the details of the bill with the observation that downwash is a problem because it causes air quality standards or other requirements to be violated, and only then discusses the “historically ... responsible way of dealing with the problem.” Id. This suggests that meeting air quality standards was primary in its mind and that good engineering practice was merely a way to do so. Finally, we are considerably bolstered in our view that Congress was thinking primarily of dangers to health because that was the position originally taken by the Administrator. In the 1979 proposed regulations, primary reliance was placed on the requirement that the downwash in question cause a violation of air quality standards. The forty-percent proviso was merely intended, EPA wrote, “to establish a reasonable upper bound for creditable stack heights.” 44 Fed.Reg. at 2611. When EPA dropped the air quality standards as one of its criteria for measuring “excessive,” it did not address whether the forty-percent test measured danger to health or welfare. Neither EPA nor the utilities that have intervened on its behalf have disputed the contention of Sierra Club and NRDC that the forty-percent rule, because it measures relative changes in “preexisting concentration,” cannot measure the absolute levels of pollutant that are a danger to health. See Brief of Petitioners at 39. We agree that the level of pollutant that is a danger to health is an absolute value: the Act clearly envisions that the national ambient air quality standards that are to protect health and welfare will be defined in terms of maximum concentrations of each pollutant, see, e.g., 42 U.S.C. § 7409(c) (1976) (requiring promulgation of standard for “NO2 concentrations over a period of not more than 3 hours”), and that is the way the Administrator has consistently implemented the Act, see, e.g., American Petroleum Institute v. Costle, 665 F.2d 1176 (D.C.Cir.1981) (approving ozone standard of 0.12 parts per million), cert. denied, 455 U.S. 1034, 102 S.Ct. 1737, 72 L.Ed.2d 152 (1982). We disagree, however, with the petitioners’ interpretation of the present regulation, and thus with their conclusion regarding the relation of the regulation to health. NRDC and Sierra Club appear to assume that the forty-percent increase in concentration is to be an increase over the preexisting levels in the area, including background concentrations of pollutants. We believe, however, that the increase is to be measured against the amount of the source’s own plume that falls to the ground even without downwash, regardless of preexisting or background concentrations of pollutant from other sources. Petitioners’ interpretation is arguably the most natural reading of the language of the regulation itself, which is as follows: “Excessive concentrations” for the purpose of determining good engineering practice stack height in a fluid model or field study means a maximum concentration due to downwash, wakes, or eddy effects produced by structures or terrain features which is at least 40 percent in excess of the maximum concentration experienced in the absence of such down-wash, wakes, or eddy effects. 47 Fed.Reg. at 5869 (to be codified at 40 C.F.R. § 51.1(kk)). The regulation does not specify whether the “maximum concentration experienced in the absence of such downwash” is to be measured with or without pollutants generated by other sources. The 1979 proposal was similarly ambiguous. See 44 Fed.Reg. at 2614 (maximum concentrations “due in part or whole to downwash, wakes, or eddy effects”). The preamble to the final regulations, however, is explicit that the forty-percent standard requires the source to demonstrate that “maximum concentrations caused by the source’s emissions from its proposed stack height, without consideration of nearby structures or terrain obstacles, will increase by at least 40 percent when the effects of the structures or terrain obstacles are considered.” 47 Fed.Reg. at 5865 (emphasis added). Even more important, we see no reason why, as a scientific matter, one would rationally include background concentrations in the calculation. Although this court’s knowledge of the operation of downwash is admittedly rudimentary, it seems most likely that the amount of additional pollutant that is brought to the earth by means of down-wash will somehow relate to the amount of the original fallout, either because fallout is a process similar to downwash and therefore produces proportionate amounts of grounded pollutants, or because both relate proportionately to the density of the plume. Presumably, therefore, the amount of a plume that falls to earth near the plant even in the absence of downwash is either an absolute number for all plumes emitted at a certain height or, more likely, varies with the density of the plume. Conversely, it seems unlikely that the amount of down-washed pollutant from a given stack height should vary with background concentrations of that pollutant in the area. That would mean that the eddies and whirlpools that bring the plume down to earth would operate more effectively the dirtier the surrounding air becomes, which, even if possible, seems odd. We conclude, therefore, that the forty-percent increase refers to an increase in the amount of pollutants from the plume that fall to the ground regardless of atmospheric conditions. We have not had any argument on whether the regulation, so interpreted, in fact estimates dangers to health and welfare. On the one hand, it seems likely that it was such dangers to health and welfare that traditional engineering practice, from which the forty-percent figure was derived, sought to prevent, at least in a very rough way. On the other hand, EPA has not made this argument, relying instead on congressional approval of good engineering practice per se. In addition, the EPA scientists who derived the forty-percent figure did not describe it as an accurate measure of danger to health or welfare. It was rather a measure of what scientists conducting wind tunnel experiments in the past had thought was “a significant concentration difference” as they observed the smoke pouring out of their model smokestacks. See Technical Support Doc., supra p. 446, at 1092. The EPA scientists cautioned: The visualized ... studies can be strongly biased by the observer’s eye and are extremely sensitive to the density of the smoke. The information from concentration profiles is influenced strongly by where the traverse through the plume is made [to determine the plume centerline] and the judgment in determining what constitutes a significant concentration difference.... ... Although the consensus opinion in the scientific literature strongly supports using [the 1 + 1.5 Rule] to determine GEP stack height, actual studies could show the need for a much taller or lower stack depending on one’s interpretation of what is a significant influence and on the effect of possible plume rise. Id. at 1089-92 (emphasis added). Moreover, we note that EPA chose the forty-percent figure from a range of increased concentrations produced by a stack 2.5 times the height of the obstacle, and forty percent was at the low end of that spectrum. The choice of forty percent as the definition of “excessive” is more consistent with an attempt to arrive at a reasonable upper limit on stack height — i.e., to place a floor on the amount of increased' concentration that would justify an increased stack height — rather than an attempt to discern the historical concept of safe levels of downwash. Reliance on the lower figure alone also does not appear to be consistent with the clearly expressed congressional expectation that credit for stacks in excess of the 2.5 Rule would “be highly infrequent and that the latitude given the Administrator to allow full credit for such stack height [would] be exercised with circumspection and utmost caution in those rare circumstances proven to justify its use,” House Report, supra p. 440, at 93, U.S.Code Cong. & Admin.News 1977, p. 1171. Finally, apart from the intention of EPA in developing the forty-percent rule, we think it unlikely that even the reinterpreted present rule will measure an absolute pollutant concentration that is dangerous to health, rather than a range of concentration increases varying with the density of the plume and other factors. Of course, it may be that, for all stacks large enough to be of concern, that range of increases will be entirely above the threshold of danger to health or welfare. EPA has not said so, however, and our examination of the derivation of the figure suggests strongly that that is not the case. We think our best course is to remand the definition of “excessive concentrations” to the Administrator with instructions to develop a standard directly responsive to the concern for health and welfare that motivated Congress to establish the downwash exception. We do not condemn the historical approach EPA has taken to deriving that standard, but we caution EPA to be aware that it is writing under substantially different conditions from those that faced the engineers who first developed the rules of thumb for stack height. The engineers could be satisfied with a conservative rule that was absolutely sure to eliminate health hazards, but only local ones; EPA must bé more stringent, since any extra height will mean increased emissions and longer transport of pollutants, both of which Congress has instructed the agency to minimize. Therefore, EPA must satisfy itself in some way independent of history that the standard it derives in fact fairly approximates the stack height level needed to protect local health and welfare; in doing so, moreover, it should err on the side of reducing stack height, in keeping with Congress’s command that credit for stack heights above the 2.5 Rule height be granted with “utmost caution.” These two precepts are the heart of our holding on this issue. C. Failure to Consider Plume Rise Plume rise refers to the tendency of exhaust gases to continue to rise after they leave the stack because of their momentum and heat. Petitioners NRDC and Sierra Club assert that in deriving the 1 + 1.5 and 2.5 Rules EPA has ignored plume rise. They argue that the failure to consider plume rise is arbitrary and capricious, because it will lead to the prediction of excessive concentrations where none will occur since the “effective” height of the stack will be much higher than the physical height. Thus, they say, the GEP formulas will allow higher GEP height than needed to ensure against excessive concentrations of pollutant caused by downwash. EPA admits that the formula does not take account of plume rise, but asserts that “[u]nder the very high wind conditions that cause downwash, no plume rise takes place near the source.” Summary of Comments and Responses on the October 7, 1981 Proposal of the Stack Height Regulations (Dec. 1981), J.A. at 1190 [hereinafter cited as 1981 Responses]; accord Technical Support Doc., supra p. 446, at 1102 (“[T]he critical conditions for determining GEP stack height for most sources are considered likely to be high winds associated with neutral atmospheric stability with little plume rise near the sources.”); see also 46 Fed.Reg. at 49,-820 (“[T]he comments correctly pointed out that the technical support document was based on studies which did not include plume rise.”). NRDC and Sierra Club dispute this factual conclusion, citing two pieces of evidence. First, they point out that EPA’s own regulations for running the demonstrations that operators may use to gain above-formula height require the inclusion of plume rise. See 46 Fed.Reg. at 49,820 (“Fluid models and field studies take into account gas flow rates and temperature, in addition to stack height. Accordingly, they will take into account some plume rise in establishing the GEP stack height.”); Draft Guideline for Use of Fluid Modeling to Determine Good Engineering Practice Stack Height (June 1979), J.A. at 677-78, 680 [hereinafter cited as Fluid Modeling Guideline]. Second, they cite a report submitted by the Tennessee Valley Authority in this rulemaking that estimates the plume rise above the top of the stack during strong winds to be 99, 84, and 145 meters at 76, 91, and 305 meter stacks, respectively. See An Analysis of Terrain-Induced Aerodynamic Disturbances Near the Kingston Steam Plant, Kingston, Tennessee, at 1-1, Record at 77 app. We view this as a factual dispute that we must review under the substantial evidence standard, see 5 U.S.C. § 706(2)(E) (1976). We side with EPA. Its conclusion that plume rise is not significant rests primarily on the wind tunnel studies cited in its technical support document, which did not provide for plume rise, yet derived approximately the same formula as apparently had arisen as “a practical formula” from years of empirical observation, presumably including any plume rise. Compare Technical Support Doc., supra p. 446, at 1080 (“This rule arose during the early part of this century as a practical formula. [A 1955 study] report[s] that the rule had been successfully used by the British electricity generating industry during the previous 20 years.”) with id. at 1086 (“A review and evaluation of the current literature ... reveals a consensus that [the 2.5 Rule is] the stack height necessary to avoid significant effects for buildings whose projected width is greater than its height. ... [Otherwise, the 1 + 1.5 Rule is appropriate.]”). Since studies that do not account for plume rise arrived at the same conclusion that was derived from observations including plume rise, it seems fair to infer that at least the usual amount of plume rise does not have a significant effect on downwash. In response to concerns about the possibility of artificially increasing plume rise beyond the normal amount, EPA’s regulations prohibit as a dispersion technique installation of fans and heaters intended to enhance plume rise. See 47 Fed.Reg. at 5868 (to be codified at 40 C.F.R. § 51.1(hh)); id. at 5867; 46 Fed.Reg. at 49,820; infra pp. 461-64. That EPA has allowed operators to account for plume rise in demonstrations does not persuade us that the 1 + 1.5 Rule is unprincipled, because we think it entirely rational for EPA to be more concerned that even small amounts of plume rise be accounted for when a source is seeking greater height than provided by the formulas. Congress specifically directed that such increased height be allowed only “with circumspection and utmost caution in those rare circumstances proven to justify its use.” House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News, 1977, p. 1171]. (The report was referring to heights in excess of the 2.5 Rule, but we think it clear that its thinking applies equally to the more accurate 1 + 1.5 Rule.) The formulas, on the other hand, are intended to be easy to apply and, of necessity, somewhat rough rules of thumb. We think such a dual approach was within the contemplation of Congress and is not a capricious use of EPA’s limited resourcés. Petitioners’ citation of the significant plume rise reported in the TVA study does not substantially undermine EPA’s assertion that plume rise is fairly insignificant under downwash conditions. The TVA report does not contradict EPA’s finding that, particularly as wind speed increases, plume rise near the stack, where downwash occurs, is generally small even though plume rise further downwind may be significant. Such a phenomenon is an expected consequence of high wind conditions, and it is consistent with EPA’s view that plume rise has little effect on downwash near the stack. See Fluid Modeling Guideline, supra p. 451, at 666 (“Under such conditions [high wind speed and neutral stability], plume rise near the source where its rise is dominated by momentum flux, will be small while its rise farther downwind may be largely due to buoyancy [heat] flux.”). We affirm the refusal to include plume rise in the derivation of the formulas. D. Inclusion of Plume Impaction Plume impaction occurs when a plume of exhaust gases emitted from a stack hits a higher hill or mountain downwind of the stack. Under stable atmospheric conditions, the plume can hit the mountain before it has dispersed, causing high concentrations on the mountainside. See 1981 Responses, supra p. 451, at 1166. In response to comments received after its 1979 proposed regulations, EPA decided to allow credit for stack heights needed to avoid violation of national air quality standards on the elevated terrain. See 46 Fed. Reg. at 49,815-16. The new regulations allow a source to receive credit for the amount of its stack necessary to ensure that violations will not occur on the mountain as a result of the amount of the mountain’s height that is above GEP height. 47 Fed. Reg. at 5869 (to be codified at 40 C.F.R. § 51.12(f)). Without plume impaction credit, the source would have to reduce its emissions to prevent modeled violations on the mountainside. Petitioners NRDC and Sierra Club argue that section 123 does not permit EPA to give credit for stack height necessary to avoid any phenomena other than “down-wash, eddies and wakes,” and that plume impaction is not one of these statutorily specified exemptions. Plume impaction is caused by obstacles downwind of the stack, rather than upwind of it, and generally occurs when there is little wind to disperse the plume, rather than when the wind is strong. EPA admits that plume impaction is not the same as downwash, eddies, or wakes, but argues that they are sufficiently similar that the same rationale should apply to it. 47 Fed.Reg. at 5866 (“In all of these events, structures or terrain features interfere with plume dispersion.”); 1981 Responses, supra p. 451, at 1167-68 (“These conditions are similar but independent of each other since they generally occur under different meteorological conditions.”). The agency relies on its general authority under section 301 of the Act to “prescribe such regulations as are necessary to carry out [its] functions under [the Act].” 42 U.S.C. § 7601(a)(1) (Supp. Y 1981). EPA’s construction of the statute is condemned by the general rule that when a statute lists several specific exceptions to the general purpose, others should not be implied. See, e.g., A.H. Phillips, Inc. v. Walling, 324 U.S. 490, 493, 65 S.Ct. 807, 808, 89 L.Ed. 1095 (1945) (“Any exemption from such humanitarian and remedial legislation must ... be narrowly construed ....”); Colorado Public Interest Research Group, Inc. v. Train, 507 F.2d 743, 747 (10th Cir. 1974) (“[W]here the legislature has acted to except certain categories from the operation of a particular law, it is to be presumed that the legislature in its exceptions intended to go only as far as it did, and that additional exceptions are not warranted.”). Had we any doubts that this rule should apply to this case, they are eliminated by the specific instruction in the House committee report that the term “nearby” should be “strictly construed, in keeping with the general policy of statutory interpretation favoring strict construction of exceptions and variances.” House Report, supra p. 440, at 93, [U.S.Code Cong. & Admin.News 1977, p. 1171]; see also supra pp. 443-46 (discussion of meaning of “nearby”). Not only the word “nearby,” but the entire permission to give credit for “good engineering practice” height constitute exceptions or variances. Congress sought to prohibit reliance on stack height to achieve air quality standards except in certain cases that it very specifically defined. The specified cases are where stack height is needed to prevent excessive concentrations resulting from “downwash, eddies and wakes.” We should be extremely chary of adding any others. EPA argues, however, that this rule should be softened when it appears that Congress was not informed of the problem and therefore did not deliberately omit it. EPA asserts that this is such a case. See 47 Fed.Reg. at 5866 (“Section 123 does not mention impaction. However, neither the language of the statute nor the legislative history show that this omission was deliberate.”). We agree that where there is evidence that Congress considered or was informed of other things of more or less the same species as the ones placed in the statute, the case is stronger for inferring that the others were deliberately excluded. See Lubrizol Corp. v. EPA, 562 F.2d 807, 817-18 (D.C.Cir.1977) (fact that hearing witness listed several factors, including fuel and fuel additives, that affect automobile emissions suggests that at least some members of Congress did not expect statutory term “fuel” and “fuel additives” to include other potential causes of pollution, such as motor oil). We conclude, however, that the obverse rule advanced by EPA — that if there is no evidence that Congress knew of or considered the other things, the presumption should be that they are included in the statutory exception — would be contrary to the strict construction of exceptions directed by judicial doctrine and by the legislative history of this law. Cf. Harrison v. PPG Industries, 446 U.S. 578, 592, 100 S.Ct. 1889, 1897, 64 L.Ed.2d 525 (1980) (“[I]t would be a strange canon of statutory construction that would require Congress to state in committee reports or elsewhere in its deliberations that which is obvious on the face of a statute.”). In any case, there is some evidence in the legislative history of the 1977 amendments that Congress was in fact made aware of the problem of plume impaction in hilly terrain. Representatives of electric utilities appeared before the congressional subcommittees considering the amendments and discussed the deleterious effects the new laws would have on utilities. They contended that several provisions in combination would limit the availability of plant sites in mountain areas. Among the provisions mentioned were those that sought to prevent the deterioration of clean-air areas, called prevention of significant deterioration (PSD) provisions, and the tall stacks section. Thus, a representative of the Edison Electric Institute, the principal national association of investor-owned electric companies, reported the following adverse effect of the amendments: (2) More coal-fired power plants would have to be built in the mid-west and in eastern coastal plains and less in the Appalachians and the West because of the substantial penalties imposed on plant size by [PSD] and tall stack limits in areas of hilly terrain. Clean Air Act Amendments of 1977: Hearing on S. 251, S. 252, and S. 253 Befor