In the
 United States Court of Appeals
              For the Seventh Circuit
                        ____________

No. 06-3907
SIERRA CLUB, et al.,
                                                      Petitioners,
                               v.

U.S. ENVIRONMENTAL PROTECTION AGENCY,
                                                      Respondent,
                              and


PRAIRIE STATE GENERATING COMPANY, LLC,
                                        Intervenor-Respondent.
                        ____________
                Petition to Review an Order of the
               Environmental Appeals Board of the
              U.S. Environmental Protection Agency.
                             No. 03-05
                        ____________
      ARGUED MAY 31, 2007—DECIDED AUGUST 24, 2007
                        ____________


  Before POSNER, KANNE, and WILLIAMS, Circuit Judges.
  POSNER, Circuit Judge. The federal Environmental
Protection Agency (actually, Illinois’s counterpart to the
EPA, exercising authority that the federal EPA had dele-
gated to it, but we can ignore that detail) issued a permit
2                                                 No. 06-3907

to Prairie State Generating Company to build a 1,500-
megawatt coal-fired electrical generating plant in south-
ern Illinois, near St. Louis. Environmentalists asked the
EPA’s Environmental Appeals Board to reverse the issu-
ance of the permit, and, the Board having refused, In re
Prairie State Generating Co., No. 05-05 (EAB Aug. 24, 2006),
they renew the quarrel in this court. They claim that the
EPA violated two provisions of the Clean Air Act. One
requires as a condition of receiving a permit that a plant or
other source of air pollution be designed to have the “best
available control technology” for minimizing pollution
emitted by the plant. 42 U.S.C. § 7475(a)(4). The other
attaches the further condition that the plant’s emissions
not exceed the limits imposed by the Act’s national ambi-
ent air quality standards. § 7475(a)(3). The petitioners’ first
claim relates to the sulfur dioxide that will be produced as
a byproduct of the production of electricity by Prairie
State’s plant, the second to the ozone that it will produce.
  The plant is to be what is called a “mine-mouth” plant
because it has been sited at the location of a coal seam. The
seam is believed to contain 240 million tons of recoverable
coal—enough to supply the plant’s fuel needs for 30 years.
The siting of the plant will enable the coal to be brought by
a conveyor belt, more than half a mile long, from the mine
to the plant. Unfortunately, this coal has a high sulfur
content. To burn low-sulfur coal Prairie State would have
to arrange for it to be transported from mines more than
a thousand miles away and would have to make changes
in the design of the plant—specifically, the design of the
plant’s facilities for receiving coal. The petitioners argue
that the EPA must decide whether hauling low-sulfur
coal from afar would be the best available means of
controlling air pollution from the plant.
No. 06-3907                                                    3

   The Clean Air Act defines “best available control tech-
nology” as the “emission limitation” achievable by “appli-
cation of production processes and available methods,
systems, and techniques, including fuel cleaning, clean
fuels, or treatment of innovative fuel combustion tech-
niques.” 42 U.S.C. § 7479(3). A “proposed facility” that
would if built be a “major emitting facility,” as the pro-
posed Prairie State plant would be, must have “the best
available control technology for each pollutant subject to
regulation,” § 7475(4), including sulfur dioxide. The EPA’s
position is that “best available control technology” does not
include redesigning the plant proposed by the permit
applicant (“traditionally, EPA does not require a . . .
[permit] applicant to change the fundamental scope of its
project,” In re Old Dominion Electric Cooperative, 3 E.A.D.
779, 793 n. 38 (EPA Adm’r 1992); Environmental Protection
Agency, “New Source Review Workshop Manual: Preven-
tion of Significant Deterioration and Nonattainment
Permitting” B.13 (Draft, Oct. 1990)), unless the applicant
intentionally designs the plant in a way calculated to make
measures for limiting the emission of pollutants ineffectual.
In re Prairie State Generating 

, slip op. at 30, 33-34.
But that is not contended in this case. Another provision
of the Act, distinct from the one requiring adoption of the
best available control technology, directs the EPA to
consider “alternatives” suggested by interested persons
(such as the Sierra Club) to a proposed facility. 42 U.S.C.
§ 7475(a)(2); see, e.g., In re NE Hub Partners, L.P., 7 E.A.D.
561, 583 (EAB 1998). But that provision has not been
invoked by the petitioners. Only compliance with the
“BACT” (best available control technology) requirement
is in issue.
  The Act is explicit that “clean fuels” is one of the con-
trol methods that the EPA has to consider. Well, nuclear
4                                                 No. 06-3907

fuel is clean, and so the implication, one might think, is that
the agency could order Prairie State to redesign its plant as
a nuclear plant rather than a coal-fired one, or could
order it to explore the possibility of damming the Missis-
sippi to generate hydroelectric power, or to replace coal-
fired boilers with wind turbines. That approach would
invite a litigation strategy that would make seeking a
permit for a new power plant a Sisyphean labor, for
there would always be one more option to consider. The
petitioners to their credit shy away from embracing
the extreme implications of such a strategy, which
would stretch the term “control technology” beyond the
breaking point and collide with the “alternatives” provi-
sion of the statute. But they do not suggest another stop-
ping point.
  Now it is true that a difference between this case and our
nuclear hypothetical is that a plant designed to burn coal
cannot run on nuclear fuel without being redesigned from
the ground up, whereas Prairie State’s proposed plant
could burn coal transported to the plant from afar. But to
convert the design from that of a mine-mouth plant to one
that burned coal obtained from a distance would require
that the plant undergo significant modifications—con-
cretely, the half-mile-long conveyor belt, and its interface
with the mine and the plant, would be superfluous and
instead there would have to be a rail spur and facilities for
unloading coal from rail cars and feeding it into the plant.
See Kathryn Heidrich, “Mine-Mouth Power Plants: Conve-
nient Coal Not Always a Simple Solution,” Coal Age, June
2003, pp. 28, 30; Richard H. McCartney, “Bringing Coal
Yards Into the 21st Century,” Power Engineering, July
2005, p. 36.
 So it is no surprise that the EPA, consistent with our
nuclear hypothetical and the petitioners’ concession
No. 06-3907                                                 5

regarding it, distinguishes between “control technology” as
a means of reducing emissions from a power plant or other
source of pollution and redesigning the “proposed facility”
(the plant or other source)—changing its “fundamental
scope.” The agency consigns the latter possibility to the
“alternatives” section of the Clean Air Act, which as we
said is not involved in this case. Refining the statutory
definition of “control technology”—“production processes
and available methods, systems, and techniques, includ-
ing fuel cleaning, clean fuels, or treatment of innovative
fuel combustion techniques”—to exclude redesign is the
kind of judgment by an administrative agency to which a
reviewing court should defer. Environmental Defense v. Duke
Energy Corp, 

, 1434 (2007); New York v. EPA,

, 19-20 (D.C. Cir. 2005); Alabama Power Co. v.
Costle, 

, 397-98 (D.C. Cir. 1979).
  But this opens the further and crucial question where
control technology ends and a redesign of the “proposed
facility” begins. As it is not obvious where to draw that line
either, it makes sense to let the EPA, the author of the
underlying distinction, draw it, within reason.
  Suppose this were not to be a mine-mouth plant but
Prairie State had a contract to buy high-sulfur coal from a
remote mine yet could burn low-sulfur coal as the fuel
source instead. Some adjustment in the design of the
plant would be necessary in order to change the fuel
source from high-sulfur to low-sulfur coal, Brian
Schimmoller, “Western Coal Pushes East,” Power En-
gineering, Aug. 1999, http://pepei.pennnet.com/articles/
article_display.cfm?article_id=36230 (visited Aug. 21,
2007), but if it were no more than would be necessary
whenever a plant switched from a dirtier to a cleaner fuel
the change would be the adoption of a “control technol-
6                                                 No. 06-3907

ogy.” Otherwise “clean fuels” would be read out of the
definition of such technology. At the other end of the
spectrum is our nuclear hypothetical. The plant proposed
in this case falls between that hypothetical example and
the example of a plant that has alternative off-site
sources of high- and low-sulfur coal respectively.
  We hesitate in a borderline case, such as this, to pro-
nounce the EPA’s decision arbitrary, the applicable stan-
dard for judicial review of its granting the permit. Alaska
Department of Environmental Conservation v. EPA, 

, 496-97 (2004). The decision required an expert judg-
ment. The petitioners’ brief, though long, contains nothing
about mine-mouth power stations. The petitioners pitch
their case on the naked proposition that if a plant is
capable—with redesign—of burning a clean fuel, it must
undergo a “best available control technology” analysis. But
they flinch by carving an exception for the nuclear case
without explaining the principle that distinguishes it
from this case. Of course there is a distinction, but it is
one of degree and the treatment of differences of degree
in a technically complex field with limited statutory
guidance is entrusted to the judgment of the agency that
administers the regulatory scheme rather than to courts of
generalist judges. Chevron U.S.A. Inc. v. Natural Resources
Defense Council, Inc., 

, 842-43 (1984); Sierra Club
v. EPA, 

, 539-40 (7th Cir. 2004).
  What must give us pause, however, is the scantiness of
the Environmental Appeals Board’s discussion of the
difference between, on the one hand, adopting a control
technology, and, on the other hand, redesigning the
proposed plant, in the specific setting of this case. Here are
the critical passages: “’With respect to alternate sources of
coal, e.g., low-sulfur western coal from Wyoming or
No. 06-3907                                                   7

Montana, the proposed plant is being designed and
developed to burn high-sulfur Illinois coal, the locally
available coal. It would be inconsistent with the scope of
the project to use coal from other regions of the country.
Rather, the BACT [best available control technology]
determination addresses the appropriate control technol-
ogy for SO2 [sulfur dioxide] emissions associated with use
of this coal at the proposed plant . . . . The project that must
be addressed when evaluating BACT is the project for
which an application has been submitted, i.e., a proposed
mine-mouth power plant. The source of coal for which the
plant would be developed is a specific reserve of 240
million tons of recoverable coal, which would meet the
needs of the proposed plant for more than 30 years.
Accordingly, the use of a particular coal supply is an
inherent aspect of the proposed project. To require an
evaluation of an alternative coal supply . . . would consti-
tute a fundamental change to the project.’ ” In re Prairie
State Generating 

, slip op. at 20-21. Alternative coal
supplies would be “ ‘beyond the scope of the project, a
power plant fueled from coal delivered by a conveyor belt
from an adjacent dedicated mine.’ ” 

“ ‘The devel-
opment of a mine-mouth power plant is an intrinsic aspect
of the proposed plant, which would be developed to use a
specific reserve of fuel, which is adequate for the expected
life of the plant.’ . . . [C]onsideration of low-sulfur coal,
because it necessarily involves a fuel source other than the co-
located mine, would require Prairie State to redefine the
fundamental purpose or basic design of its proposed
Facility.” 

36 (emphasis added).
  These passages might be read as merging two separate
issues: the difference between low-sulfur (clean) and high-
sulfur (dirty) coal as a fuel source for a power plant, and
8                                               No. 06-3907

the difference between a plant co-located with a coal mine
and a plant that obtains its coal from afar. The former is a
difference in control technology, the latter a difference in
design (or so the EPA can conclude). We think it is suffi-
ciently clear from the passages that we have quoted from
the Environmental Appeals Board’s opinion, and especially
from the clause that we italized, that the Board did
not confuse the two issues; that it granted the permit not
because it thinks that burning low-sulfur coal would require
the redesign of Prairie State’s plant (it would not), but
because receiving coal from a distant mine would require
Prairie State to reconfigure the plant as one that is not co-
located with a mine, and this reconfiguration would
constitute a redesign.
  So the Board’s ruling on the BACT issue must be upheld,
and we move on to the ozone issue. Measuring the contri-
bution of a power plant to atmospheric ozone is difficult
because the ozone is not emitted directly by the plant;
rather, it is produced by the interaction of some of the
chemicals that the plant emits with sunlight. Until 2003
the EPA determined that a power plant was violating
the limit on contributing to ozone in the area in which
Prairie State’s plant is to be located when on at least one
day there was an hour in which the average concentration
of ozone exceeded .12 parts per million. But that year it
decided to replace the “1 hour” standard as it was called
with an “8 hour” standard. The new standard looks at
whether the concentration of ozone during an average 8-
hour period (more precisely, a three-year average of the
fourth-highest daily maximum 8-hour concentration)
exceeds .08 parts per million. The agency explained that
“the 8-hour standard is more protective of public health
and more stringent than the 1-hour standard, and there are
No. 06-3907                                                9

more areas that do not meet the 8-hour standard than there
are areas that do not meet the 1-hour standard.” “Proposed
Rule to Implement the 8-Hour Ozone National Ambient
Air Quality Standard,” 68 Fed. Reg. 32,802, 32,804 (June 2,
2003) (to be codified at 40 C.F.R. Pt. 51).
  The concentrations measured over these intervals are
not actual measurements of ozone; they are estimates based
on the levels of contributing factors, the chemicals and
sunlight. The formula for estimating the average ozone
concentration in one hour is not necessarily applicable
to the 8-hour estimate, but the EPA has yet to adopt a
formula for the latter estimate. So it used the 1-hour
formula not only to show compliance with the 1-hour
standard but also to generate an 8-hour estimate, and it
used results from earlier studies of the St. Louis area to
reinforce its conclusion. From both the 1-hour formula
applied to 8-hour stretches and the earlier studies, the
agency concluded that Prairie State’s plant would not
increase the amount of ozone in the local atmosphere. As
best the agency could estimate, its 1-hour measurement
would turn out to be below the limit of .08 parts per million
that the EPA has set for the 8-hour limit.
  This was a plausible expectation because, as a matter
of arithmetic, the emissions in the highest hour of a mea-
surement period have to be at least as great as the emis-
sions averaged over the highest eight hours in that period.
Suppose the emissions in the highest hour are 10 parts per
million, in the next highest hour 9 parts per million, then
8 parts, 7, 6, 5, 4, and 3. The average would be 6.5, which
would have to be lower than the amount in the highest
hour (10) unless the emissions were the same in every
hour, in which event the 1-hour and the 8-hour averages
would be identical. Admittedly, the example oversimplifies
10                                              No. 06-3907

the case because different methods of averaging are used
for the different standards. But an emissions level that
satisfies the 1-hour standard is likely though not certain to
satisfy the new standard as well even though the agency
considers the latter to be more stringent.
   The petitioners argue that the EPA simply cannot be
permitted to rely on the 1-hour standard because it has
been superseded by the 8-hour standard. It has; but
pending adoption of a compliance measure tailored to the
new standard, the agency was entitled to use the measure
used for the older standard as a stopgap to demonstrate
that if the plant complied with that measure it would be
unlikely to violate the new standard. The petitioners do
not suggest an alternative except to criticize the inference
the agency drew from earlier studies. The criticisms have
some merit but not enough to enable us to conclude that
the agency was unreasonable in concluding that the plant
is unlikely to increase the ozone level.
  The petition for review is
                                                    DENIED.
A true Copy:
       Teste:

                          _____________________________
                          Clerk of the United States Court of
                            Appeals for the Seventh Circuit




                   USCA-02-C-0072—8-24-07