UNITED STATES COURT OF APPEALS FOR THE D.C. CIRCUIT


SIERRA CLUB

v.

EPA


97-1686a

D.C. Cir. 1999


*	*	*


Williams, Circuit Judge: Section 129 of the Clean Air Act,  added by
the 1990 amendments, directs EPA to establish  performance standards
for new and existing medical waste  incinerators ("MWIs"), including
"emissions limitations and  other requirements" for new units and
"guidelines ... and  other requirements" for existing units. 42 U.S.C.
 s 7429(a)(1). In general, the standards1 are to


reflect the maximum degree of reduction in emissions of  air pollutants
... that the Administrator, taking into  consideration the cost of
achieving such emission reduc- tion, and any non-air quality health
and environmental 




__________

n 1 The EPA explains that the rules for existing units are "guide-
lines," while those for newly constructed units are "standards."  The
difference between the two appears to be that standards are  federal
requirements that apply directly to newly built MWIs, while 
guidelines do not directly govern MWIs, but are given effect  through
a requirement that states adopt rules that are at least as  strict as
the guidelines. See 62 Fed. Reg. 48,348, 48,351/1 (1997)  [J.A.
976/1]. Since the distinction is not important for purposes of  this
case, for convenience we refer to both sets as "standards."


impacts and energy requirements, determines is achiev- able for new or
existing units in each category.


42 U.S.C. s 7429(a)(2). The EPA explains that this level of  control is
commonly referred to as "maximum achievable  control technology," or
"MACT." See 62 Fed. Reg. 48,348,  48,351/3 (1997).


The statute supplements this general directive with specific 
requirements, detailed below, that dictate minimum levels of 
stringency below which EPA may not go (using the phrase  "shall not be
less stringent than"). See 42 U.S.C.  s 7429(a)(2). The parties refer
to these requirements as  "floor" provisions. (The nomenclature can be
confusing be- cause these sentences in fact establish maximums on the 
emissions that EPA's standards may permit.) The statute of  course
authorizes EPA to establish still stricter standards if it  finds them
"achievable." In its rulemaking, the EPA first  established each floor
(i.e., addressed the "not less stringent  than" provisions) and then
considered whether to set the  standard at a stricter level. See 62
Fed. Reg. 48,348, 48,353/2  (1997). In some cases EPA found greater
stringency achiev- able, in others not.


The petitioners, the Sierra Club and the Natural Resources  Defense
Council (both referred to here simply as the Sierra  Club), challenge
EPA's rule establishing MWI standards,  complaining principally that
EPA failed to comply with the  specifications of s 7429(a)(2) for the
floors. Although we  reject the Sierra Club's statutory construction
challenge, we  conclude that there are serious doubts about the
reasonable- ness of EPA's treatment of the floor requirements, and 
remand the rule for further explanation. The Sierra Club  also claims
that EPA should have required MWIs to use  pollution prevention
measures, such as programs to reduce  waste streams, and that it
unlawfully failed to consider cer- tain "non-air quality" effects of
MWI pollution on health and  the environment. We reject both these


I.Floors for Existing Units


The Clean Air Act contains the following floor requirement  for
existing MWIs:


Emissions standards for existing units in a category may  be less
stringent than standards for new units in the  same category but shall
not be less stringent than the  average emissions limitation achieved
by the best per- forming 12 percent of units in the category. The
Admin- istrator may distinguish among classes, types, ... and  sizes
of units within a category in establishing such  standards.


42 U.S.C. s 7429(a)(2).


The EPA's first step was to divide the MWI population into  three
subcategories, based on waste-burning capacity: small,  medium, and
large. 61 Fed. Reg. 31,736, 31,740/2 (1996).  Setting standards for
nine pollutants in each of these three  subcategories, EPA went on to
make 27 separate floor deter- minations.


To do so, it surveyed the emissions limits imposed by state 
regulations and permit requirements, reasoning that each  such limit
was an "emissions limitation" within the meaning of  the Clean Air
Act. Then, for each of the nine pollutants  covered by the standards,
EPA ranked the incinerators by  the stringency of the control
provisions to which they were  subject, from strictest to laxest.
Finally, it selected the 12  percent of the incinerator population
subject to the strictest  controls and set the floor level for the
subcategory by averag- ing the emissions limitations governing those
incinerators.  See 61 Fed. Reg. 31,736, 31,744-45 (1996).


For 17 out of the 27 floors to be established, however, EPA  found that
the share of the MWI population covered by any  regulatory requirement
was less than 12 percent. See Pat- rick Chang, Letter to Jim Pew, June
4, 1998, at 1. (Questions  about the validity of this finding are
discussed below.) So for  these 17 EPA supplemented the regulatory
data with "uncon- trolled" data--data from its test program recording
the per- formance of incinerators with no pollution controls. See 61 
Fed. Reg. 31,736, 31,745/2 (1996); Suzanne Shoraka Blair, 
"Determination of the Maximum Achievable Control Technol- ogy (MACT)
Floor for Existing Medical Waste Incinerators,"  Jan. 31, 1996, at 2
("Blair Mem."). For instance, EPA  estimated the total population of
small MWIs at 1,118, so that 


12 percent amounted to 135 units. Id. But it found that  state
limitations covered 135 or more MWIs only with regard  to two of the
nine pollutants. Its solution for the other seven  pollutants is
illustrated by its treatment of hydrogen chloride  (HCl). Estimating
that only 91 small MWIs were actually  subject to state HCl limits,
EPA assumed that the last 44  units in the top 12 percent were not
subject to emissions  control at all. To calculate the HCl standard it
averaged the  state ceilings (evidently weighted for the number of
units  covered), together with the highest (i.e., worst) of the
results  from its own testing of uncontrolled small MWIs, weighted 44 


A.Challenge to Statutory Construction


The Sierra Club argues that EPA's use of regulatory  permit data rather
than performance data violated the stat- ute's requirement to base the
floors on "emissions limita- tion[s] achieved." s 7429(a)(2). The EPA
defends itself  principally with a tortured argument that 42 U.S.C.  s
7602(k), which defines an "emission limitation" solely as a  type of
regulatory requirement, applies here in the sense of  allowing the use
of regulatory data, but not in the sense of  requiring the use of such
data exclusively. The Sierra Club's  arguments to the contrary lead
off with the claim that  s 7602(k) cannot apply here because it
defines an "emission  limitation," while s 7429(a)(2), the provision
calling for these  standards, refers to an "emissions limitation."


The parties beckon us into a labyrinth, but in this case,  unlike the
hapless Athenian youths and maidens given in  tribute to King Minos,
we are not compelled to enter. The  permissibility of EPA's approach
does not turn on the appli- cability of s 7602(k), but on whether
using the state regulato- ry data is a reasonable means of estimating
the performance  of the top 12 percent of MWIs in each subcategory. If
using  the state data is reasonable for this purpose, EPA does not 
need s 7602(k); if using the state data is unreasonable, then  EPA has
conceded that s 7602(k) will not save its position.2




__________

n 2 The EPA found in its response to comments that reasonable- ness
requires the use of data that allow the agency to conclude 


We first reject the Sierra Club's claim that EPA's decision  to base
the floors on regulatory data fails the first step of the  Chevron
test. None of the Sierra Club's arguments establish  that Congress has
"directly addressed" and rejected the use  of regulatory data. See
Chevron v. NRDC, 467 U.S. 837, 843,  845 (1984).


The Sierra Club argues that the plain meaning of  s 7429(a)'s words,
"average emissions limitation achieved by  the best performing 12
percent of units," precludes the use of  regulatory data. But this
phrase on its own says nothing  about how the performance of the best
units is to be calculat- ed. And the Sierra Club has disavowed any
interpretation  that would require measuring the performance of every
last  unit--it stated in its brief and confirmed at oral argument 
that the statutory language "does not preclude EPA from  relying on a
representative sample of the units in each  category." The phrase does
not by its plain meaning exclude  estimation, either by sampling or by
some other reliable  means.


The Sierra Club also claims that the legislative history of  s
7429(a)(2) reflects Congressional intent to prohibit EPA  from relying
on regulatory data. The Sierra Club cites an  earlier version of the
1990 Clean Air Act Amendments that  would have required emissions
standards to "reflect the  greatest degree of emission reduction
achievable ... which  ... (A) has been achieved in practice ..., or
(B) is contained  in a State or local regulation or any permit ...,
whichever is  more stringent." S. 1630, 101st Cong., 2d Sess. s 306
(1990)  ("Senate Bill"). The Sierra Club argues that the disparity 
between the language of the Senate Bill and that of the  enacted
amendments establishes Congress's intent to prohibit  the use of
regulatory data. Obviously Congress was deliber- ate in dropping the
Senate Bill's mandate that EPA use state  or local regulatory limits
whenever they were more stringent  than the results achieved in




__________

n "what the best performing 12 percent of existing HMIWI were able  to
achieve," EPA Response to Comments ("RTC"), July 1997, at 3- 28. [J.A.
736].


quite a stretch to infer that in thus reducing the mandated  degree of
stringency Congress expressed an intent to ban use  of regulatory data
as a proxy for what firms have achieved.


The Sierra Club offers two additional arguments that the  use of
regulatory data fails the first step of Chevron. Nei- ther, it turns
out, has any bite so long as EPA used the  regulatory data merely to
generate a reasonable estimate of  the actual performance of the top
12 percent of units. First,  the Sierra Club says that using
regulatory data is impossible  because such data exists for fewer than
12 percent of units.  But if the regulatory data provide a good proxy
for the  performance of the units they do cover, then it is irrelevant
 that the coverage is incomplete. (The issue of how well the  units
work as proxies is addressed below.) Second, the  Sierra Club argues
that using regulatory data would imper- missibly "import an
achievability requirement" into the unit  floor computation. A premise
of the argument is the counter- intuitive proposition that an
"achieved" level may not be  "achievable," or, as Sierra Club puts it,
may be better than  "EPA's notions about what is 'achievable.' " Again
we need  not enter the thicket. The distinction is irrelevant if (as
here)  the permit data are used only to approximate what actually is 


Addressing the second step of Chevron, we find nothing  inherently
impermissible about construing the statute to per- mit the use of
regulatory data--if they allow EPA to make a  reasonable estimate of
the performance of the top 12 percent  of units. Indeed, the Sierra
Club conceded at oral argument  that "a reasonable sample" may be used
"to find out what the  best 12 percent are doing." Oral Arg. Tr. at
11. To be sure,  the Sierra Club did not concede that permit data may
be  used. But neither has it provided any basis for believing that 
state and local limitations are inherently such weak indicators  of
performance that using them is necessarily an impermissi- ble stretch
of the statutory terms.


EPA typically has wide latitude in determining the extent  of
data-gathering necessary to solve a problem. We general- ly defer to
an agency's decision to proceed on the basis of 


imperfect scientific information, rather than to "invest the 
resources to conduct the perfect study." See American Iron  & Steel
Inst. v. EPA, 115 F.3d 979, 1004 (D.C. Cir. 1997) (per  curiam).
Although the agency's choice of model will be  rejected if it "bears
no rational relationship to the reality it  purports to represent,"
Columbia Falls Aluminum Co. v.  EPA, 139 F.3d 914, 923 (D.C. Cir.
1998), the necessary  relationship seems quite possible here. Indeed,
it seems  likely that any jurisdiction bothering to impose limits
would  not knowingly set them below what it found firms to be 
achieving in practice. And there seems no reason to think  that
underachieving firms would be overrepresented in juris- dictions
making this effort. Of course those jurisdictions may  have proceeded
in error, may have outdated limits, and may  have contained
unrepresentatively high shares of bad actors,  but we address the
evidence of possible infirmities below in  our discussion of the
Sierra Club's challenge to EPA's meth- odology as arbitrary and
capricious. We note that since EPA  had data on only one percent of
about 3000 MWIs, RTC at 3- 28, the data-gathering costs of any


The Sierra Club complains that EPA never used the rea-
sonable-estimation rationale in explaining its decision to use 
regulatory data, but instead rested on its interpretation of  s
7602(k). Although much of EPA's explanation during the  rulemaking was
indeed devoted to s 7602(k), the agency did  state that use of the
regulatory data was permissible because  the agency "could conclude
from these data what the best  performing 12 percent of existing
[MWIs] were able to  achieve." RTC at 3-28. Similarly, it
characterized the  MACT floor as "a measure of the level of air
pollution control  currently used by a relatively small fraction of
the MWI."  RTC at 3-30. In its 1995 rulemaking proposal, EPA ex-
plained that it examined the state regulatory data "to deter- mine the
emission limitations achieved by the best-performing  12 percent of
units in each subcategory," and further defend- ed its decision by
observing that "[i]t was assumed that all  MWI's are ... achieving
their [regulatory] limits." 60 Fed.  Reg. 10,654, 10,674/2-3 (1995).


tory requirements and actual performance undergirded  EPA's decision to
use regulatory data when it first proposed  MWI standards in 1995, and
the agency certainly did not  disavow that rationale when it adopted
the standards in the  present rulemaking.


We therefore reject the Sierra Club's argument that the  Clean Air Act
forbids the use of permit and regulatory data,  and hold that the use
of such information is permissible as  long as it allows a reasonable
inference as to the performance  of the top 12 percent of units.
Similarly, as long as there is a  reasonable basis for believing that
some of the best perform- ing 12 percent of units are uncontrolled,
EPA may include  data points giving a reasonable representation of the
perfor- mance of those units in its averaging.


B.Challenge as Arbitrary and Capricious


We now examine whether EPA was justified in using its  combination of
regulatory data and uncontrolled values to  approximate the
performance of the top 12 percent of MWIs.  The Sierra Club argues
that EPA has not pointed to evidence  supporting the reasonableness of
the approximation, and we  agree that at a minimum further explanation
is needed. We  outline the problems that need to be resolved on


Although EPA said that it believed the combination of  regulatory and
uncontrolled data gave an accurate picture of  the relevant MWIs'
performance, it never adequately said  why it believed this. We refer
to two specific areas to  illustrate the deficiencies in EPA's
explanation.


First, EPA has said nothing about the possibility that  MWIs might be
substantially overachieving the permit lim- its.3 If this were the
case, the permit limits would be of little  value in estimating the
top 12 percent of MWIs' performance.




__________

n 3 Although the agency conceded in its response to comments  that
"actual emission data routinely fall below the State and permit 
emission limits," RTC at 3-27, the context makes reasonably clear 
that the EPA was referring to data on "actual emissions" during 
tests; EPA implied that "these levels are not routinely achieved in 


Data in the record suggest that the regulatory limits are in  fact much
higher than the emissions that units achieve in  practice. For 13 of
the 27 cases EPA considered, the floor,  which is the weighted average
of the regulatory limits and the  uncontrolled data, is higher than
the value used for the  uncontrolled data.4 For instance, in the case
of the HCl floor  for small MWIs, the value for uncontrolled emissions
was  2,770 parts per million volume, and the floor (the weighted 
average of the regulatory and uncontrolled data) was 4,426  ppmv.
Blair Mem. at 2. Thus, unless EPA made a mathe- matical error (or we
have), the average of the regulatory data  must have been 5,227 ppmv5,
or 89% higher than the uncon- trolled emissions.


Even under the most deferential standard, it is difficult to  accept a
method under which the emissions of the best- performing 12% of units
are hypothesized to pollute nearly  twice as badly as the worst of
test units that lacked any  emissions controls. Our observations are
based on our own  analysis of EPA's data, and we may have omitted some
 crucial step in the process, but the exercise highlights the  need
for additional explanation even if our calculation is  wrong.


Second, EPA never gave any reason for its apparent belief  that MWIs
that were not subject to permit requirements did  not deploy emission
controls of any sort. Unless there is  some finding to this effect, it
is difficult to see the rationality  in using the "uncontrolled" data
for the units that were not  subject to regulatory requirements.


Furthermore, data on which EPA relied strongly suggest  that it was
irrational to suppose that any of the incinerators 




__________

n 4 Such at least is our reading of the Blair Memorandum. It  lists
what appear to be the test results used (evidently with no  breakdown
for size category), and the floor emission levels selected.  See id.
at 3.


5 We know the data for the uncontrolled 44 and the average of  all 135,
with only the figure for the permit data unknown (X):  ((2,770 * 44) +
91X)/135 = 4,426. Solving for X we get 5,227.


in the top 12 percent were uncontrolled--at least for the six 
pollutants that wet scrubbers control.6 Data submitted by  the
American Hospital Association in 1995 indicate that over  55% of MWIs
in each category were controlled by wet  scrubbers.7 See Comments and
Recommendations of the  American Hospital Association, April 28, 1995,
Exhibit 3.  Particularly since the AHA data were the starting point
for  EPA's estimate of the number of MWIs,8 see 61 Fed. Reg.  31,736,
31,739/3, it is difficult to see how it was rational to  include any
uncontrolled units in the top 12 percent, at least  with respect to
pollutants that wet scrubbing controls.


With these numbers, EPA's method looks hopelessly irra- tional.
Moreover, assuming the regulatory data was a good  proxy for the
better controlled units and that there were  shortfalls in reaching
the necessary 12 percent, EPA has  never explained why it made sense
to use the highest of its  test run data to make up the gap.
Nonetheless, we do not  vacate the standard. It is possible that EPA
may be able to  explain it, and the Sierra Club has expressly
requested that  we leave the current regulations in place during any
remand,  rather than eliminate any federal control at all. We
therefore  remand the floor determinations for existing units for




__________

n 6 These pollutants include particulate matter, dioxin precursors, 
HCl, lead, cadmium, and mercury. See Brian Strong, Memoran- dum to
Rick Copland, March 20, 1996, at 2.


7 The exact figures are 690 of 1,214 small units (56.8%), 365 of  589
medium units (62.0%), and 281 of 430 large units (65.3%). AHA 
Comments, Exhibit 3.


8 The EPA added about 400 MWIs to the AHA data and  deleted about 200
from that total. See 61 Fed. Reg. at 31,739/3.  Even in the unlikely
event that all the added MWIs lacked scrub- bers and all the deleted
ones had them, more than 12 percent of  MWIs in these subcategories
would as a matter of mathematical  necessity have to be controlled.
Yet in each subcategory the EPA  used uncontrolled data points for at
least some of the pollutants  that scrubbers control. See Chang Letter


II.Floors for New Units


The Clean Air Act prescribes that standards for newly  constructed MWIs
"shall not be less stringent than the  emissions control that is
achieved in practice by the best  controlled similar unit, as
determined by the Administrator."  42 U.S.C. s 7429(a)(2).


To implement this provision, EPA examined each subcate- gory and
identified the most effective technology in use by an  incinerator in
that subcategory. That technology became the  basis for the new unit
standard for incinerators in the subca- tegory. For instance, the most
effective technology that it  identified as in use by a small MWI was
a so-called moderate- efficiency wet scrubber, so the floor for new
small MWIs is  based on the performance capability of such a scrubber.
If  EPA had identified any small MWIs employing high- efficiency wet
scrubbing, the new small MWI floor would  presumably reflect the
performance of that technology. See  61 Fed. Reg. 31,745-46 (1996).


To determine the performance of a given technology, EPA  consulted the
data from its own testing program and data  provided by private
parties and identified the highest level of  emissions recorded in any
test of an incinerator using the  technology in question. It then
increased that value by 10  percent and rounded up to "an appropriate
round number" to  arrive at the emissions performance figure it
ultimately used  for that technology. See Mark B. Turner & Katie
Hanks,  Memorandum to Richard A. Copland, May 20, 1996, at 10 


The Sierra Club, pointing to the statutory reference to the  "best
controlled similar unit," 42 U.S.C. s 7429(a)(2), pur- ports to find
two deficiencies in EPA's approach. First, it  argues that EPA should
have identified the single best- performing unit in each subcategory
and based the new unit  floor for that subcategory on that particular
unit's perfor- mance, rather than considering the performance of other
 units employing the same technology. Next, it argues that  EPA
compounded its error by basing the floor on the emis- sions of the
worst-performing unit employing the technology 


in question. We address the Sierra Club's claims in the  reverse of
the order of presentation.


First, EPA would be justified in setting the floors at a level  that is
a reasonable estimate of the performance of the "best  controlled
similar unit" under the worst reasonably foresee- able circumstances
(we use the subjunctive because it is not  clear from the record
whether the agency was doing this). It  is reasonable to suppose that
if an emissions standard is as  stringent as "the emissions control
that is achieved in prac- tice" by a particular unit, then that
particular unit will not  violate the standard. This only results if
"achieved in prac- tice" is interpreted to mean "achieved under the
worst fore- seeable circumstances." In National Lime Ass'n v. EPA, 627
 F.2d 416, 431 n.46 (D.C. Cir. 1980), we said that where a  statute
requires that a standard be "achievable," it must be  achievable
"under most adverse circumstances which can  reasonably be expected to
recur." The same principle should  apply when a standard is to be
derived from the operating  characteristics of a particular unit.
Although this potential  rationale for EPA's method was made clear in
the briefs for  the agency and the parties intervening on its behalf,
it does  not appear in the rulemaking record with enough clarity for 
us to say that the agency's "path may reasonably be dis- cerned."
Bowman Transportation, Inc. v. Arkansas-Best  Freight System, Inc.,


The Sierra Club also claims EPA erred in considering the  emissions of
units other than the best controlled unit. The  EPA simply has not
explained why the phrase "best con- trolled similar unit" encompasses
all units using the same  technology as the unit with the best
observed performance,  rather than just that unit itself, as the use
of the singular in  the statutory language suggests. We do not mean to
say that  EPA's interpretation is impossible. Perhaps considering all 
units with the same technology is justifiable because the best  way to
predict the worst reasonably foreseeable performance  of the best unit
with the available data is to look at other  units' performance. Or
perhaps EPA reasonably considered  all units with the same technology
equally "well-controlled,"  so that each unit with the best technology
is a "best-


controlled unit" even if such units vary widely in perfor- mance. But
we do not know what interpretation the agency  chose, and thus cannot
evaluate its choice.


A similar analysis applies to the agency's choices to add 10  percent
to the observed emission levels and to further round  up the result,
often in ways that seem contrary to ordinary  principles of rounding.
See, e.g., Turner/Hanks Memo at 11  (rounding from 0.0198 to 0.03).
Each of these may be  justifiable as a means of reasonably estimating
the upper  bound of the best-controlled unit's performance, but in the
 absence of agency explanation of both the decision to increase  the
levels and the choice of method for determining the  increases, we are
in no position to decide.


III.Other Sierra Club Claims


The Sierra Club's remaining claims are directed not to the  floors EPA
established for the various types of facilities, but  to the emissions
standards themselves. The floor provision  require only a minimum
level of stringency, and the emissions  standards themselves are to
"reflect the maximum degree of  reduction in emissions of air
pollutants ... that the Adminis- trator, taking into consideration the
cost of achieving such  emission reduction, and any non-air quality
health and envi- ronmental impacts and energy requirements, determines
is  achievable." 42 U.S.C. s 7429(a)(2). The Sierra Club argues  that
EPA failed to consider two separate factors in moving  from the floors
to the standards themselves: the effectiveness  of pollution
prevention measures and non-air quality health  and environmental
effects arising from water, soil, and food  contamination by MWI
pollutants. We reject both of these  claims.


A.Pollution Prevention Measures


The Sierra Club starts its argument with the observation  that
emissions standards are to be based on "methods and  technologies for
removal or destruction of pollutants before,  during, or after
combustion," 42 U.S.C. s 7429(a)(3), and  focuses on the "before" in
that requirement. It claims that  EPA wrongly failed to require MWIs
to undertake programs 


to reduce the mercury and chlorinated plastics in their waste 
streams.


The EPA does not deny that the waste stream reductions  the Sierra Club
calls for would reduce pollution. The less  mercury in, the less
mercury out, and the less chlorinated  plastic in, the less HCl out.
But the EPA has consistently  argued in its response to comments and
here that it does not  have evidence that allows quantification of the
relevant output  reduction. For mercury, the only quantitative
evidence be- fore EPA was that a pollution prevention program aimed at
 mercury could reduce mercury emissions from very high  levels to
typical levels. See RTC at 7-14 to 7-15. For  chlorinated plastics,
there was no quantitative evidence before  the agency. See RTC at
7-16, 7-18. The Sierra Club does  not contest the adequacy of EPA's
data-gathering with re- spect to these measures.


There also doesn't appear to be any evidence in the record  about the
costs of the pollution prevention measures the  Sierra Club advocates.
In the absence of any type of quanti- fication of benefits or costs,
the Administrator had no basis  for finding that, "taking into account
the cost," emissions  reductions from pollution prevention programs
were "achiev- able" as the statute uses the word.


B.Non-Air Quality Health and Environmental Effects


Finally, the Sierra Club observes that EPA failed to consid- er the
fact that dioxin and mercury from MWIs can contami- nate water,
sediment, and soil, and can bioaccumulate in food.  The Sierra Club
argues in a paragraph that this omission was  improper because the
Clean Air Act directs EPA to consider  "any non-air quality health and
environmental impacts" in  setting the MWI emissions standards. But
the Sierra Club  has made no serious effort, either in its briefs or
in its  comments to the agency, to show that the problems about  which
it complains are actually "non-air quality" effects within  the
meaning of s 7429(a)(2). Because this threshold step is  unbriefed,
and because the Sierra Club's argument is present- ed in such a
conclusory manner, we decline to consider the 


challenge. See Texas Rural Legal Aid, Inc. v. Legal Servs.  Corp., 940
F.2d 685, 697 (D.C. Cir. 1991).


* * *


The case is remanded to EPA for further explanation of its  reasoning
in determining the "floors" for new and existing  MWIs. Petitioners'
claims are otherwise rejected.


So ordered.