ENVIRONMENTAL DEFENSE et al. v. DUKE ENERGY CORP. et al.

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561 OCTOBER TERM, 2006
Syllabus
ENVIRONMENTAL DEFENSE et al. v. DUKE
ENERGY CORP. et al.
certiorari to the united states court of appeals for
the fourth circuit
No. 05–848. Argued November 1, 2006—Decided April 2, 2007
In the 1970s, Congress added two air pollution control schemes to the
Clean Air Act (Act): New Source Performance Standards (NSPS) and
Prevention of Significant Deterioration (PSD), each of which covers
modified, as well as new, stationary sources of air pollution. The NSPS
provisions define “modification” of such a source as a physical change to
it, or a change in the method of its operation, that increases the amount
of a pollutant discharged or emits a new one. 42 U. S. C. § 7411(a)(4).
The PSD provisions require a permit before a “major emitting facility”
can be “constructed,” § 7475(a), and define such “construction” to include
a “modification (as defined in [NSPS]),” § 7479(2)(C). Despite this defi
nitional identity, the Environmental Protection Agency’s (EPA) regula
tions interpret “modification” one way for NSPS but differently for PSD.
The NSPS regulations require a source to use the best available
pollution-limiting technology, see Chevron U. S. A. Inc. v. Natural Re
sources Defense Council, Inc., 467 U. S. 837, 846, when a modification
would increase the discharge of pollutants measured in kilograms per
hour, 40 CFR § 60.14(a), but the 1980 PSD regulations require a permit
for a modification only when it is a “major” one, § 51.166(b)(2)(i), and
only when it would increase the actual annual emission of a pollutant
above the actual average for the two prior years, § 51.166(b)(21)(ii).
After respondent Duke Energy Corporation replaced or redesigned
the workings of some of its coal-fired electric generating units, the
United States filed this enforcement action, claiming, among other
things, that Duke violated the PSD provisions by doing the work with
out permits. Petitioner environmental groups intervened as plaintiffs
and filed a complaint charging similar violations. Duke moved for sum
mary judgment, asserting, inter alia, that none of its projects was a
“major modification” requiring a PSD permit because none increased
hourly emissions rates. Agreeing, the District Court entered summary
judgment for Duke on all PSD claims. The Fourth Circuit affirmed,
reasoning that Congress’s decision to create identical statutory defini
tions of “modification” in the Act’s NSPS and PSD provisions affirma
tively mandated that this term be interpreted identically in the regula
tions promulgated under those provisions. When the court sua sponte

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requested supplemental briefing on the relevance of this Court’s decision
in Rowan Cos. v. United States, 452 U. S. 247, 250, that the Government
could not adopt different interpretations of the word “wages” in differ
ent statutory provisions, plaintiffs injected a new issue into the case,
arguing that a claim that the 1980 PSD regulation exceeded statutory
authority would be an attack on the regulation’s validity that could
not be raised in an enforcement proceeding, see 42 U. S. C. § 7607(b)(2),
since judicial review for validity can be obtained only by a petition to
the District of Columbia Circuit, generally within 60 days of EPA’s
rulemaking, § 7607(b)(1). The Fourth Circuit rejected this argument,
ruling that its interpretation did not invalidate the PSD regulations
because they can be interpreted to require an increase in the hourly
emissions rate as an element of a major “modification.”
Held: The Fourth Circuit’s reading of the PSD regulations in an effort
to conform them with their NSPS counterparts on “modification”
amounted to the invalidation of the PSD regulations, which must com
port with the Act’s limits on judicial review of EPA regulations for va
lidity. Pp. 573–582.
(a) Principles of statutory interpretation do not rigidly mandate iden
tical regulation here. Because “[m]ost words have different shades of
meaning and consequently may be variously construed, [even] when
[they are] used more than once in the same statute or . . . section,” the
“natural presumption that identical words used in different parts of the
same act are intended to have the same meaning . . . is not rigid and
readily yields whenever there is such variation in the connection in
which the words are used as reasonably to warrant the conclusion that
they were employed in different parts of the act with different intent.”
Atlantic Cleaners & Dyers, Inc. v. United States, 286 U. S. 427, 433. A
given term in the same statute may take on distinct characters from
association with distinct statutory objects calling for different ways of
implementation. The point is the same even when the terms share a
common statutory definition, if it is general enough. See Robinson v.
Shell Oil Co., 519 U. S. 337, 343–344. Robinson is not inconsistent with
Rowan, where the Court’s invalidation of the differing interpretations
of “wages,” 452 U. S., at 252, turned not on the fact that a “substantially
identical” definition of that word appeared in each of the statutory pro
visions at issue, but on the failure of the regulations in question to serve
Congress’s manifest “concern for the interest of simplicity and ease of
administration,” id., at 255. In fact, in a case close to Rowan’s facts,
the Court recently declined to follow a categorical rule of resolving am
biguities in identical statutory terms identically regardless of their sur
roundings, United States v. Cleveland Indians Baseball Co., 532 U. S.

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200, 213, but instead accorded “substantial judicial deference” to an
agency’s “longstanding,” “reasonable,” and differing interpretations of
the statutory term at issue, id., at 218–220. It makes no difference
here that the Act does not merely repeat the same definition in its NSPS
and PSD provisions, but that the PSD provisions refer back to the sec
tion defining “modification” for NSPS purposes. Nothing in the text or
legislative history of the statutory amendment that added the NSPS
cross-reference suggests that Congress meant to eliminate customary
agency discretion to resolve questions about a statutory definition by
looking to the surroundings in which the defined term appears. EPA’s
construction need do no more than fall within the outer limits of what
is reasonable, as set by the Act’s common definition. Pp. 573–576.
(b) The Fourth Circuit’s construction of the 1980 PSD regulations to
conform them to their NSPS counterparts was not a permissible reading
of their terms. The PSD regulations clearly do not define a “major
modification” in terms of an increase in the “hourly emissions rate.”
On its face, the definitional section specifies no rate at all, hourly or
annual, merely requiring a “physical change in or change in the method
of operation of a major stationary source that would result in a sig
nificant net emissions increase of any” regulated pollutant. 40 CFR
§ 51.166(b)(2)(i). But even when the regulations mention a rate, it is
annual, not hourly. See, e. g., § 51.166(b)(23)(i). Further at odds with
the idea that hourly rate is relevant is the mandate that “[a]ctual emis
sions shall be calculated using the unit’s actual operating hours,”
§ 51.166(b)(21)(ii), since “actual emissions” must be measured in a man
ner looking to the number of hours the unit is or probably will be actu
ally running. The Court of Appeals’s reasons for its different view are
no match for these textual differences. Consequently, the Court of Ap
peals’s construction of the 1980 PSD regulations must be seen as an
implicit invalidation of those regulations, a form of judicial review impli
cating the provisions of 42 U. S. C. § 7607(b), which limit challenges to
the validity of a regulation during enforcement proceedings when such
review “could have been obtained” in the Court of Appeals for the Dis
trict of Columbia Circuit within 60 days of EPA rulemaking. Because
the Court of Appeals did not believe that its analysis reached validity,
it did not consider the applicability or effect of that limitation here.
The Court has no occasion itself at this point to consider the significance
of § 7607(b). Pp. 577–581.
(c) Duke’s claim that, even assuming the Act and the 1980 regulations
authorize EPA to construe a PSD “modification” as it has done, EPA
has been inconsistent in its positions and is now retroactively targeting
20 years of accepted practice was not addressed below. To the extent

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564 ENVIRONMENTAL DEFENSE v. DUKE ENERGY CORP.
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the claim is not procedurally foreclosed, Duke may press it on remand.
Pp. 581–582.
411 F. 3d 539, vacated and remanded.
Souter, J., delivered the opinion of the Court, in which Roberts, C. J.,
and Stevens, Scalia, Kennedy, Ginsburg, Breyer, and Alito, JJ.,
joined, and in which Thomas, J., joined as to all but Part III–A. Thomas,
J., filed an opinion concurring in part, post, p. 582.
Sean H. Donahue argued the cause for petitioners. With
him on the briefs were David T. Goldberg, Jeffrey M. Glea
son, J. Blanding Holman IV, and Caleb Jaffe.
Deputy Solicitor General Hungar argued the cause for
the United States as amicus curiae urging reversal. With
him on the briefs were Solicitor General Clement, Assistant
Attorney General Wooldridge, James A. Feldman, Kather
ine J. Barton, Ann R. Klee, Chet M. Thompson, Granta Y.
Nakayama, Thomas W. Swegle, Carol S. Holmes, David W.
Schnare, and Alan Dion.
Carter G. Phillips argued the cause for respondent. With
him on the brief were Mark D. Hopson, Kathryn B. Thom
son, Stephen M. Nickelsburg, Henry V. Nickel, F. William
Brownell, Makram Jaber, Marc E. Manly, Catherine S.
Stempien, Garry S. Rice, T. Thomas Cottingham III, and
Nash E. Long III.*
*Briefs of amici curiae urging reversal were filed for the State of New
Jersey et al. by Zulima V. Farber, former Attorney General of New Jer
sey, Patrick DeAlmeida, Assistant Attorney General, and Kevin P. Auer
bacher and Jung W. Kim, Deputy Attorneys General, and by the Attorneys
General and other officials for their respective jurisdictions as follows:
Terry Goddard, Attorney General of Arizona, Joseph P. Mikitish, Assist
ant Attorney General, Robert J. Spagnoletti, former Attorney General of
the District of Columbia, Edward E. Schwab, Deputy Solicitor General,
and Donna M. Murasky, Senior Assistant Attorney General, Gregory D.
Stumbo, Attorney General of Kentucky, Douglas Scott Porter, Assistant
Attorney General, Michael A. Cox, Attorney General of Michigan, Thomas
L. Casey, Solicitor General, and Alan F. Hoffman and Neil D. Gordon,
Assistant Attorneys General, Rob McKenna, Attorney General of Wash
ington, and Leslie R. Seffern, Assistant Attorney General; for the State

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Opinion of the Court
Justice Souter delivered the opinion of the Court.
In the 1970s, Congress added two air pollution control
schemes to the Clean Air Act: New Source Performance
of New York et al. by Eliot Spitzer, former Attorney General of New
York, Caitlin J. Halligan, Solicitor General, Andrew Bing and Daniel J.
Chepaitis, Assistant Solicitors General, Peter H. Lehner, Robert Rosen
thal, J. Jared Snyder, and Michael J. Myers, Assistant Attorneys General,
by Susan Shinkman and Robert A. Reiley, and by the Attorneys General
and former Attorneys General for their respective States as follows: Bill
Lockyer of California, Richard Blumenthal of Connecticut, Carl C. Dan
berg of Delaware, Lisa Madigan of Illinois, Thomas J. Miller of Iowa, G.
Steven Rowe of Maine, J. Joseph Curran, Jr., of Maryland, Thomas F.
Reilly of Massachusetts, Mike Hatch of Minnesota, Kelly Ayotte of New
Hampshire, Patricia A. Madrid of New Mexico, Hardy Myers of Oregon,
Patrick Lynch of Rhode Island, and William H. Sorrell of Vermont; for
the American Lung Association et al. by Hope M. Babcock; for the Chesa
peake Bay Foundation et al. by Michael D. Goodstein and Julie Kaplan;
for Law Professors by Jared A. Goldstein; for the National Parks Conser
vation Association et al. by George E. Hays and Michael A. Costa; for
STAPPA et al. by Richard E. Ayres; for Current and Former Members of
Congress by Stephanie Tai; and for Former Administrator of the United
States Environmental Protection Agency Carol M. Browner et al. by
Holly D. Gordon and Deborah A. Sivas.
Briefs of amici curiae urging affirmance were filed for the State of
Alabama et al. by Troy King, Attorney General of Alabama, Kevin C.
Newsom, Solicitor General, and Robert D. Tambling, Assistant Attorney
General, and by the Attorneys General and former Attorneys General for
their respective States as follows: David W. Ma´ rquez of Alaska, John W.
Suthers of Colorado, Steve Carter of Indiana, Phill Kline of Kansas, Jon
Bruning of Nebraska, Henry D. McMaster of South Carolina, Lawrence
E. Long of South Dakota, Robert F. McDonnell of Virginia, and Patrick
J. Crank of Wyoming; for APA Watch by Lawrence J. Joseph; for the
American Public Power Association et al. by Janet Pitterle Holt, Rae E.
Cronmiller, and Richard H. Robinson; for the Electric Utility Industry
by Steven G. McKinney, Michael D. Freeman, and P. Stephen Gidiere III;
for Law Professors by David B. Rivkin, Jr., and Lee A. Casey; for the
Manufacturers Association Work Group by Charles H. Knauss, Robert V.
Zener, Julie C. Becker, Richard S. Wasserstrom, Kevin B. Belford,
M. Elizabeth Cox, Jan S. Amundson, Quentin Riegel, Robin S. Conrad,
and Amar D. Sarwal; for the National Environmental Development Asso
ciation’s Clean Air Project by Leslie Sue Ritts and Lorane F. Hebert; and

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Standards (NSPS) and Prevention of Significant Deteriora
tion (PSD), each of them covering modified, as well as new,
stationary sources of air pollution. The NSPS provisions
define the term “modification,” 42 U. S. C. § 7411(a)(4), while
the PSD provisions use that word “as defined in” NSPS,
§ 7479(2)(C). The Court of Appeals concluded that the stat
ute requires the Environmental Protection Agency (EPA) to
conform its PSD regulations on “modification” to their NSPS
counterparts, and that EPA’s 1980 PSD regulations can be
given this conforming construction. We hold that the Court
of Appeals’s reading of the 1980 PSD regulations, intended
to align them with NSPS, was inconsistent with their terms
and effectively invalidated them; any such result must be
shown to comport with the Act’s restrictions on judicial re
view of EPA regulations for validity.
I
The Clean Air Amendments of 1970, 84 Stat. 1676, broad
ened federal authority to combat air pollution, see Chevron
U. S. A. Inc. v. Natural Resources Defense Council, Inc., 467
U. S. 837, 845–846 (1984), and directed EPA to devise Na
tional Ambient Air Quality Standards (NAAQS) limiting
various pollutants, which the States were obliged to imple
ment and enforce, 42 U. S. C. §§ 7409, 7410. The amend
ments dealing with NSPS authorized EPA to require opera
tors of stationary sources of air pollutants to use the best
technology for limiting pollution, Chevron, supra, at 846; see
also 1 F. Grad, Environmental Law § 2.03 [14], p. 2–356
for the Washington Legal Foundation by Daniel J. Popeo and Paul D.
Kamenar.
Briefs of amici curiae were filed for the International Brotherhood of
Boilermakers, Iron Ship Builders, Blacksmiths, Forgers and Helpers,
AFL–CIO, et al. by Scott H. Segal and Jason B. Hutt; for Walter C. Bar
ber by Robert L. Brubaker; and for U. S. Representative Joe L. Barton by
George C. Landrith and Christopher C. Horner.

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(2006), both in newly constructed sources and those undergo
ing “modification,” 42 U. S. C. § 7411(a)(2). Section 111(a) of
the 1970 amendments defined this term within the NSPS
scheme as “any physical change in, or change in the method
of operation of, a stationary source which increases the
amount of any air pollutant emitted by such source or which
results in the emission of any air pollutant not previously
emitted,” 42 U. S. C. § 7411(a)(4).
EPA’s 1975 regulations implementing NSPS provided gen
erally that “any physical or operational change to an existing
facility which results in an increase in the emission rate to
the atmosphere of any pollutant to which a standard applies
shall be considered a modification within the meaning of
section 111.” 40 CFR § 60.14(a) (1976). Especially signifi
cant here is the identification of an NSPS “modification” as
a change that “increase[s] . . . the emission rate,” which
“shall be expressed as kg/hr of any pollutant discharged into
the atmosphere.” § 60.14(b).1
NSPS, however, did too little to “achiev[e] the ambitious
goals of the 1970 Amendments,” R. Belden, Clean Air Act 7
(2001) (hereinafter Belden), and the Clean Air Act Amend
ments of 1977, 91 Stat. 685, included the PSD provisions,
which aimed at giving added protection to air quality in cer
tain parts of the country “notwithstanding attainment and
1 EPA’s 1975 NSPS regulations did not specify that the “rate” means the
maximum rate possible for the technology, see 40 CFR §§ 60.14(a)–(b)
(1977), but the parties all read the regulations this way. See Brief for
Petitioners 2; Brief for United States 7; Brief for Respondent Duke 32.
At another point in the NSPS regulations, a different definition of “modi
fication” appeared: “ ‘Modification’ means any physical change in, or change
in the method of operation of, an existing facility which increases the
amount of any air pollutant (to which a standard applies) emitted into the
atmosphere by that facility,” § 60.2(h); see also New York v. EPA, 413 F.
3d 3, 11–12 (CADC 2005) (per curiam) (“[N]either the 1975 regulation nor
its preamble explained why EPA found it necessary to offer these two
separate glosses on ‘modification’ ”).

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maintenance of ” the NAAQS. 42 U. S. C. § 7470(1).2 The
1977 amendments required a PSD permit before a “major
emitting facility” could be “constructed” in an area covered
by the scheme. § 7475(a). As originally enacted, PSD ap
plied only to newly constructed sources, but soon a technical
amendment added the following subparagraph: “The term
‘construction’ when used in connection with any source or
facility, includes the modification (as defined in section
111(a)) of any source or facility.” § 14(a)(54), 91 Stat. 1402,
42 U. S. C. § 7479(2)(C); see also New York v. EPA, 413 F. 3d
3, 13 (CADC 2005) (per curiam). In other words, the “con
struction” requiring a PSD permit under the statute was
made to include (though it was not limited to) a “modifica
tion” as defined in the statutory NSPS provisions.
In 1980, EPA issued PSD regulations,3 which “limited the
application of [PSD] review” of modified sources to instances
of “ ‘major’ modificatio[n],” Belden 46, defined as “any physi
cal change in or change in the method of operation of a major
stationary source that would result in a significant net emis
sions increase of any pollutant subject to regulation under
the Act.” 40 CFR § 51.166(b)(2)(i) (1987). Further regula
tions in turn addressed various elements of this definition,
three of which are to the point here. First, the regulations
specified that an operational change consisting merely of
“[a]n increase in the hours of operation or in the production
rate” would not generally constitute a “physical change in or
change in the method of operation.” § 51.166(b)(2)(iii)( f ).
For purposes of a PSD permit, that is, such an operational
2 Statutory PSD superseded a regulatory PSD scheme established by
EPA in 1974. See 39 Fed. Reg. 42510. Under the regulations, the term
“modification” was defined as “any physical change in, or change in the
method of operation of, a stationary source which increases the emission
rate of any pollutant for which a national standard has been promulgated.”
Id., at 42514.
3 Although EPA had promulgated an earlier set of PSD regulations in
1978, 43 Fed. Reg. 26380, none of the parties argues that they govern the
conduct at issue in this case.

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change would not amount to a “modification” as the Act de
fines it. Second, the PSD regulations defined a “net emis
sions increase” as “[a]ny increase in actual emissions from a
particular physical change or change in the method of opera
tion,” net of other contemporaneous “increases and de
creases in actual emissions at the source.” § 51.166(b)(3)(i).
“Actual emissions” were defined to “equal the average rate,
in tons per year, at which the unit actually emitted the pol
lutant during a two-year period which precedes the particu
lar date and which is representative of normal source opera
tion.” § 51.166(b)(21)(ii). “[A]ctual emissions” were to be
“calculated using the unit’s actual operating hours [and] pro
duction rates.” Ibid. Third, the term “significant” was de
fined as “a rate of emissions that would equal or exceed” one
or another enumerated threshold, each expressed in “tons
per year.” § 51.166(b)(23)(i).
It would be bold to try to synthesize these statutory and
regulatory provisions in a concise paragraph, but three
points are relatively clear about the regime that covers this
case:
(a) The Act defines modification of a stationary source
of a pollutant as a physical change to it, or a change in
the method of its operation, that increases the amount
of a pollutant discharged or emits a new one.
(b) EPA’s NSPS regulations require a source to use the
best available pollution-limiting technology only when a
modification would increase the rate of discharge of pol
lutants measured in kilograms per hour.
(c) EPA’s 1980 PSD regulations require a permit for a
modification (with the same statutory definition) only
when it is a major one and only when it would increase
the actual annual emission of a pollutant above the ac
tual average for the two prior years.
The Court of Appeals held that Congress’s provision de
fining a PSD modification by reference to an NSPS modifica

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tion caught not only the statutory NSPS definition, but also
whatever regulatory gloss EPA puts on that definition at any
given time (for the purposes of the best technology require
ment). When, therefore, EPA’s PSD regulations specify the
“change” that amounts to a “major modification” requiring a
PSD permit, they must measure an increase in “the amount
of any air pollutant emitted,” 42 U. S. C. § 7411(a)(4), in terms
of the hourly rate of discharge, just the way NSPS regula
tions do. Petitioners and the United States say, on the con
trary, that when EPA addresses the object of the PSD
scheme it is free to put a different regulatory interpretation
on the common statutory core of “modification,” by measur
ing increased emission not in terms of hourly rate but by the
actual, annual discharge of a pollutant that will follow the
modification, regardless of rate per hour. This disagree
ment is the nub of the case.
II
Respondent Duke Energy Corporation runs 30 coal-fired
electric generating units at eight plants in North and South
Carolina. United States v. Duke Energy Corp., 411 F. 3d
539, 544 (CA4 2005). The units were placed in service be
tween 1940 and 1975, and each includes a boiler containing
thousands of steel tubes arranged in sets. Ibid. Between
1988 and 2000,4 Duke replaced or redesigned 29 tube assem
blies in order to extend the life of the units and allow them
to run longer each day. Ibid.
4 The United States argues that some of Duke’s projects were governed
by EPA’s PSD regulations promulgated in 1992 rather than the 1980 PSD
regulations. Brief for United States 20, n. 4. Duke disputes this. Brief
for Respondent Duke 14, n. 4. Because the United States acknowledges
that the two sets of regulations “did not materially differ with respect to
the legal question at issue here,” Brief for United States 20, n. 4, we will
assume, as did the Court of Appeals and the District Court, that the 1980
PSD regulations control. 411 F. 3d, at 543, n. 1; United States v. Duke
Energy Corp., 278 F. Supp. 2d 619, 629 (MDNC 2003).

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The United States filed this action in 2000, claiming,
among other things, that Duke violated the PSD provisions
by doing this work without permits. Environmental De
fense, North Carolina Sierra Club, and North Carolina Public
Interest Research Group Citizen Lobby/Education Fund in
tervened as plaintiffs and filed a complaint charging similar
violations.
Duke moved for summary judgment, one of its positions
being that none of the projects was a “major modification”
requiring a PSD permit because none increased hourly rates
of emissions. The District Court agreed with Duke’s read
ing of the 1980 PSD regulations. It reasoned that their ex
press exclusion of “ ‘[a]n increase in the hours of operation’ ”
from the definition of a “ ‘physical change or change in the
method of operation’ ” implied that “post-project emissions
levels must be calculated assuming” preproject hours of op
eration. 278 F. Supp. 2d 619, 640–641 (MDNC 2003). Con
sequently, the District Court said, a PSD “major modifica
tion” can occur “only if the project increases the hourly rate
of emissions.” Id., at 641. The District Court found fur
ther support for its construction of the 1980 PSD regulations
in one letter and one memorandum written in 1981 by EPA’s
Director of the Division of Stationary Source Enforcement,
Edward E. Reich. Id., at 641–642.
The United States and intervenor-plaintiffs (collectively,
plaintiffs) subsequently stipulated “that they do not contend
that the projects at issue in this case caused an increase in
the maximum hourly rate of emissions at any of Duke Ener
gy’s units.” App. 504. Rather, their claim “is based solely
on their contention that the projects would have been pro
jected to result in an increased utilization of the units at
issue.” Ibid. Duke, for its part, stipulated to plaintiffs’
right to appeal the District Court’s determination that proj
ects resulting in greater operating hours are not “major
modifications” triggering the PSD permit requirement, ab
sent an increase in the hourly rate of emissions. The Dis

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trict Court then entered summary judgment for Duke on all
PSD claims.
The Court of Appeals for the Fourth Circuit affirmed, “al
beit for somewhat different reasons.” 411 F. 3d, at 542.
“[T]he language and various interpretations of the PSD reg
ulations . . . are largely irrelevant to the proper analysis of
this case,” reasoned the Court of Appeals, “because Con
gress’ decision to create identical statutory definitions of the
term ‘modification’ ” in the NSPS and PSD provisions of the
Clean Air Act “has affirmatively mandated that this term
be interpreted identically” in the regulations promulgated
under those provisions. Id., at 547, n. 3, 550. The Court of
Appeals relied principally on the authority of Rowan Cos. v.
United States, 452 U. S. 247, 250 (1981), where we held
against the Government’s differing interpretations of the
word “wages” in different tax provisions. 411 F. 3d, at 550.
As the Court of Appeals saw it, Rowan establishes an “effec
tively irrebuttable” presumption that PSD regulations must
contain the same conditions for a “modification” as the NSPS
regulations, including an increase in the hourly rate of emis
sions.5 411 F. 3d, at 550.
As the Court of Appeals said, Duke had not initially relied
on Rowan, see 411 F. 3d, at 547, n. 4, and when the Court
sua sponte requested supplemental briefing on Rowan’s rele
vance, plaintiffs injected a new issue into the case. They
argued that a claim that the 1980 PSD regulation exceeded
statutory authority would be an attack on the validity of the
regulation that could not be raised in an enforcement pro
ceeding. See 42 U. S. C. § 7607(b)(2). Under § 307(b) of the
5 The Court of Appeals noted that EPA was free to abandon the require
ment that a “modification” be accompanied by an increase in the hourly
rate of emissions, provided it did so for both the NSPS and PSD programs.
411 F. 3d, at 550–551. In other words, the Court of Appeals raised no
question about the reasonableness of the definition of “modification” in the
1980 PSD regulations, apart from its deviation from the definition con
tained in NSPS regulations.

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Act, they said, judicial review for validity can be obtained
only by a petition to the Court of Appeals for the District of
Columbia Circuit, generally within 60 days of EPA’s rule
making. 42 U. S. C. § 7607(b).
The Court of Appeals re jected this argument. “Our
choice of this interpretation of the PSD regulations . . . is
not an invalidation of those regulations,” it said, because
“the PSD regulations can be interpreted” to require an in
crease in the hourly emissions rate as an element of a major
“modification” triggering the permit requirement. 411
F. 3d, at 549, n. 7. To show that the 1980 PSD regulations
are open to this construction, the Court of Appeals cited the
conclusions of the District Court and the Reich opinions.
We granted the petition for certiorari brought by
intervenor-plaintiffs, 547 U. S. 1127 (2006), and now vacate.
III
The Court of Appeals understood that it was simply con
struing EPA’s 1980 PSD regulations in a permissible way
that left them in harmony with their NSPS counterpart and,
hence, the Act’s single definition of “modification.” The
plaintiffs say that the Court of Appeals was rewriting the
PSD regulations in a way neither required by the Act nor
consistent with their own text.
It is true that no precise line runs between a purposeful
but permissible reading of the regulation adopted to bring it
into harmony with the Court of Appeals’s view of the stat
ute, and a determination that the regulation as written is
invalid. But the latter occurred here, for the Court of Ap
peals’s efforts to trim the PSD regulations to match their
different NSPS counterparts can only be seen as an implicit
declaration that the PSD regulations were invalid as written.
A
In applying the 1980 PSD regulations to Duke’s conduct,
the Court of Appeals thought that, by defining the term

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“modification” identically in its NSPS and PSD provisions,
the Act required EPA to conform its PSD interpretation of
that definition to any such interpretation it reasonably ad
hered to under NSPS. But principles of statutory construc
tion are not so rigid. Although we presume that the same
term has the same meaning when it occurs here and there in
a single statute, the Court of Appeals mischaracterized that
presumption as “effectively irrebuttable.” 411 F. 3d, at 550.
We also understand that “[m]ost words have different shades
of meaning and consequently may be variously construed,
not only when they occur in different statutes, but when used
more than once in the same statute or even in the same sec
tion.” Atlantic Cleaners & Dyers, Inc. v. United States, 286
U. S. 427, 433 (1932). Thus, the “natural presumption that
identical words used in different parts of the same act are
intended to have the same meaning . . . is not rigid and
readily yields whenever there is such variation in the connec
tion in which the words are used as reasonably to warrant
the conclusion that they were employed in different parts of
the act with different intent.” Ibid. A given term in the
same statute may take on distinct characters from asso
ciation with distinct statutory objects calling for different
implementation strategies.
The point is the same even when the terms share a com
mon statutory definition, if it is general enough, as we recog
nized in Robinson v. Shell Oil Co., 519 U. S. 337 (1997).
There the question was whether the term “employees” in
§ 704(a) of Title VII of the Civil Rights Act of 1964 covered
former employees. Title VII expressly defined the term
“employee,” 42 U. S. C. § 2000e(f), but the definition was
“consistent with either current or past employment,” 519
U. S., at 342, and we held that “each section” of Title VII
“must be analyzed to determine whether the context gives
the term a further meaning that would resolve the issue in
dispute,” id., at 343–344.

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If Robinson were inconsistent with Rowan (on which the
Court of Appeals relied), it would be significant that Robin
son is the later case, but we read the two as compatible. In
Rowan, the question was whether the value of meals and
lodging given to employees by an employer for its own con
venience should be counted in computing “wages” under the
Federal Insurance Contributions Act (FICA), 26 U. S. C.
§ 3101 et seq. (2000 ed. and Supp. IV), and the Federal Unem
ployment Tax Act (FUTA), 26 U. S. C. § 3301 et seq. (2000
ed. and Supp. IV). Treasury Regulations made this value
“includable in ‘wages’ as defined in FICA and FUTA, even
though excludable from ‘wages’ under the substantially iden
tical” statutory definition of “wages” for income-tax with
holding purposes. 452 U. S., at 252. Although we ulti
mately held that the income-tax treatment was the proper
one across the board, we did not see it this way simply be
cause a “substantially identical” definition of “wages” ap
peared in each of the different statutory provisions. In
stead, we relied on a manifest “congressional concern for the
interest of simplicity and ease of administration.” Id., at
255 (internal quotation marks omitted). The FICA and
FUTA regulations fell for failing to “serve that interest,” id.,
at 257, not for defying definitional identity.
In fact, in a setting much like Rowan, we recently declined
to require uniformity when resolving ambiguities in identical
statutory terms. In United States v. Cleveland Indians
Baseball Co., 532 U. S. 200 (2001), we rejected the notion that
using the phrase “wages paid” in both “the discrete taxation
and benefits eligibility contexts” can, standing alone, “compel
symmetrical construction,” id., at 213; we gave “substantial
judicial deference” to the “longstanding,” “reasonable,” and
differing interpretations adopted by the Internal Revenue
Service in its regulations and Revenue Rulings. Id., at 218–
220. There is, then, no “effectively irrebuttable” presump
tion that the same defined term in different provisions of the

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same statute must “be interpreted identically.” 411 F. 3d,
at 550. Context counts.
It is true that the Clean Air Act did not merely repeat the
term “modification” or the same definition of that word in its
NSPS and PSD sections; the PSD language referred back to
the section defining “modification” for NSPS purposes. 42
U. S. C. § 7479(2)(C). But that did not matter in Robinson,
and we do not see the distinction as making any difference
here. Nothing in the text or the legislative history of the
technical amendments that added the cross-reference to
NSPS suggests that Congress had details of regulatory im
plementation in mind when it imposed PSD requirements on
modified sources; the cross-reference alone is certainly no
unambiguous congressional code for eliminating the custom
ary agency discretion to resolve questions about a statutory
definition by looking to the surroundings of the defined term,
where it occurs. See New York, 413 F. 3d, at 19 (“So far as
appears, . . . [this] incorporatio[n] by reference [is] the equiv
alent of Congress’s having simply repeated in the [PSD]
context the definitional language used before in the NSPS
context”); cf. 91 Stat. 745 (expressly incorporating in an
unrelated provision of the 1977 amendments “the interpreta
tive regulation of the [EPA] Administrator . . . published in
41 Federal Register 55524–30” with specified exceptions);
New York, supra, at 19 (“Congress’s failure to use such an
express incorporation of prior regulations for ‘modification’
cuts against” any suggestion that “Congress intended to in
corporate” into the Act the “preexisting regulatory defini
tion” of “modification”). Absent any iron rule to ignore the
reasons for regulating PSD and NSPS “modifications” differ
ently, EPA’s construction need do no more than fall within
the limits of what is reasonable, as set by the Act’s com
mon 6 definition.
6 Duke argues that the 1977 amendments intended to incorporate EPA’s
definition of “modification” under the 1974 regulatory PSD program.
Brief for Respondent Duke 44; see also n. 2, supra. We find no support

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Opinion of the Court
B
The Court of Appeals’s reasoning that the PSD regulations
must conform to their NSPS counterparts led the court to
read those PSD regulations in a way that seems to us too
far a stretch for the language used. The 1980 PSD regula
tions on “modification” simply cannot be taken to track the
Agency’s regulatory definition under the NSPS.
True, the 1980 PSD regulations may be no seamless nar
rative, but they clearly do not define a “major modification”
in terms of an increase in the “hourly emissions rate.” On
its face, the definition in the PSD regulations specifies no
rate at all, hourly or annual, merely requiring a physical or
operational change “that would result in a significant net
emissions increase of any” regulated pollutant. 40 CFR
§ 51.166(b)(2)(i). But even when a rate is mentioned, as in
the regulatory definitions of the two terms, “significant” and
“net emissions increase,” the rate is annual, not hourly.
Each of the thresholds that quantify “significant” is de
scribed in “tons per year,” § 51.166(b)(23)(i), and a “net emis
sions increase” is an “increase in actual emissions” measured
against an “average” prior emissions rate of so many “tons
for this argument in the statutory text, which refers to the statutory
NSPS definition rather than the regulatory PSD definition. Although
Duke correctly points out that “Congress instructed that the bulk of the
pre-existing rules ‘shall remain in effect,’ ” Brief for Respondent Duke 44
(quoting 42 U. S. C. § 7478(a)), this instruction was a temporary measure
“[u]ntil such time as an applicable implementation plan is in effect,”
§ 7478(a). We therefore do not read this language as a restriction on
EPA’s authority to interpret the statutory PSD provisions reasonably in
a manner that departs from the 1974 regulations. Duke also invokes
Bragdon v. Abbott, 524 U. S. 624, 631 (1998), for the proposition that “use
of the pre-existing term ‘modification’ ‘carries the implication that Con
gress intended the term to be construed in accordance with pre-existing
regulatory interpretations.’ ” Brief for Respondent Duke 44. But this
reasoning is unavailing here, given the existence of at least three distinct
regulatory definitions of “modification” at the time of the 1977 amend
ments. See supra, at 567–568, and nn. 1, 2.

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per year,” §§ 51.166(b)(3)(i) and (21)(ii). And what is fur
ther at odds with the idea that hourly rate is relevant is the
mandate that “[a]ctual emissions shall be calculated using the
unit’s actual operating hours,” § 51.166(b)(21)(ii), since “ac
tual emissions” must be measured in a manner that looks to
the number of hours the unit is or probably will be actually
running. What these provisions are getting at is a measure
of actual operations averaged over time, and the regulatory
language simply cannot be squared with a regime under
which “hourly rate of emissions,” 411 F. 3d, at 550 (emphasis
deleted), is dispositive.
The reasons invoked by the Court of Appeals for its differ
ent view are no match for these textual differences. The
appellate court cited two authorities ostensibly demonstrat
ing that the 1980 PSD regulations “can be interpreted con
sistently” with the hourly emissions test, the first being the
analysis of the District Court in this case. Id., at 549, n. 7.
The District Court thought that an increase in the hourly
emissions rate was necessarily a prerequisite to a PSD
“major modification” because a provision of the 1980 PSD
regulations excluded an “ ‘increase in the hours of operation
or in the production rate’ ” from the scope of “ ‘[a] physical
change or change in the method of operation.’ ” 278 F. Supp.
2d, at 640–641 (quoting 40 CFR §§ 51.166(b)(2)(iii)( f ) and
(3)(i)(a) (1987)). The District Court read this exclusion to
require, in effect, that a source’s hours of operation “be held
constant” when preproject emissions are being compared
with postproject emissions for the purpose of calculating the
“net emissions increase.” 278 F. Supp. 2d, at 640.
We think this understanding of the 1980 PSD regulations
makes the mistake of overlooking the difference between the
two separate components of the regulatory definition of
“major modification”: “[1] any physical change in or change
in the method of operation of a major stationary source
that [2] would result in a significant net emissions increase

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of any pollutant subject to regulation under the Act.”
§ 51.166(b)(2)(i); cf. New York, 413 F. 3d, at 11 (“[The statu
tory] definition requires both a change—whether physical or
operational—and a resulting increase in emissions of a pol
lutant” (emphasis in original)); Wisconsin Elec. Power Co. v.
Reilly, 893 F. 2d 901, 907 (CA7 1990) (same). The exclusion
of “increase in . . . hours . . . or . . . production rate,”
§ 51.166(b)(2)(iii)( f ), speaks to the first of these compo
nents (“physical change . . . or change in . . . method,”
§ 51.166(b)(2)(i)), but not to the second (“significant net emis
sions increase,” ibid.). As the preamble to the 1980 PSD
regulations explains, forcing companies to obtain a PSD per
mit before they could simply adjust operating hours “would
severely and unduly hamper the ability of any company to
take advantage of favorable market conditions.” 45 Fed.
Reg. 52704. In other words, a mere increase in the hours of
operation, standing alone, is not a “physical change or change
in the method of operation.” 40 CFR § 51.166(b)(2)(iii).
But the District Court took this language a step further.
It assumed that increases in operating hours (resulting in
emissions increases at the old rate per hour) must be ignored
even if caused or enabled by an independent “physical
change . . . or change in the method of operation.”
§ 51.166(b)(2)(i). That reading, however, turns an exception
to the first component of the definition into a mandate to
ignore the very facts that would count under the second,
which defines “net emissions increase” in terms of “actual
emissions,” § 51.166(b)(3)(i), during “the unit’s actual operat
ing hours,” § 51.166(b)(21)(ii); see also 57 Fed. Reg. 32328
(1992) (“[A]n increase in emissions attributable to an increase
in hours of operation or production rate which is the result
of a construction-related activity is not excluded from [PSD]
review . . . ”).7
7 Two Courts of Appeals agree. See United States v. Cinergy Corp.,
458 F. 3d 705, 708 (CA7 2006) (“[M]erely running the plant closer to its

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580 ENVIRONMENTAL DEFENSE v. DUKE ENERGY CORP.
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The Court of Appeals invoked one other source of support,
the suggestion in the Reich opinions that a physical or opera
tional change increasing a source’s hours of operation, with
out an increase in the hourly emissions rate, cannot be a
PSD “major modification.” Duke continues to rely on those
opinions here, asserting that “there are no contrary Agency
pronouncements.” Brief for Respondent Duke 28. The
Reich letters are not, however, heavy ammunition. Their
persuasiveness is elusive, neither of them containing more
than one brief and conclusory statement supporting Duke’s
position. Nor, it seems, are they unembarrassed by any
“contrary Agency pronouncements.” See, e. g., App. 258
(Memorandum of Don R. Clay, Acting Assistant EPA Admin
istrator for Air and Radiation (Sept. 9, 1988) (when “plans to
increase production rate or hours of operation are inextrica
bly intertwined with the physical changes planned,” they are
“precisely the type of change in hours or rate o[f] operation
that would disturb a prior assessment of a source’s environ
mental impact and should have to undergo PSD review scru
tiny” (internal quotation marks and alterations omitted)));
see also 57 Fed. Reg. 32328. In any event, it answers the cita
tion of the Reich letters to realize that an isolated opinion of
maximum capacity is not a major modification because it does not involve
either a physical change or a change in the method of operation. If, how
ever, a physical change enables the plant to increase its output, then, ac
cording to the EPA’s interpretation, the exclusion for merely operating
the plant for longer hours is inapplicable” (emphasis in original)); Wiscon
sin Elec. Power Co. v. Reilly, 893 F. 2d 901, 916, n. 11 (CA7 1990) (the
regulatory exclusion for increases in the hours of operation “was provided
to allow facilities to take advantage of fluctuating market conditions, not
construction or modification activity”); Puerto Rican Cement Co. v. EPA,
889 F. 2d 292, 298 (CA1 1989) (“[T]here is no logical contradiction in rules
that, on the one hand, permit firms using existing capacity simply to in
crease their output and, on the other, use the potential output of new
capacity as a basis for calculating an increase in emissions levels” (empha
sis in original)).

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Opinion of the Court
an agency official does not authorize a court to read a regula
tion inconsistently with its language.8
In sum, the text of the 1980 PSD regulations on “modifica
tion” doomed the Court of Appeals’s attempt to equate those
regulations with their NSPS counterpart. As a conse
quence, we have to see the Court of Appeals’s construction
of the 1980 PSD regulations as an implicit invalidation of
those regulations, a form of judicial review implicating the
provisions of § 307(b) of the Act, which limit challenges to
the validity of a regulation during enforcement proceedings
when such review “could have been obtained” in the Court
of Appeals for the District of Columbia Circuit within 60
days of EPA rulemaking. See 42 U. S. C. § 7607(b); see also
United States v. Cinergy Corp., 458 F. 3d 705, 707–708 (CA7
2006); Wisconsin Elec. Power Co., 893 F. 2d, at 914, n. 6.
Because the Court of Appeals did not believe that its analy
sis reached validity, it did not consider the applicability or
effect of that limitation here. We have no occasion at this
point to consider the significance of § 307(b) ourselves.
IV
Finally, Duke assumes for argument that the Act and the
1980 regulations may authorize EPA to construe a PSD
“modification” as it has done, but it charges that the agency
has taken inconsistent positions and is now “retroactively
8 Duke now offers an alternative argument for applying the hourly emis
sions test for the PSD program: before a project can become a “major
modification” under the PSD regulations, 40 CFR § 51.166(b)(2)(i) (1987),
it must meet the definition of “modification” under the NSPS regulations,
§ 60.14(a). That sounds right, but the language of the regulations does
not support it. For example, it would be superfluous for PSD regulations
to require a “major modification” to be a “physical change in or change in
the method of operation,” § 51.166(b)(2)(i), if they presupposed that the
NSPS definition of “modification,” which contains the same prerequisite,
§ 60.14(a), had already been satisfied. The NSPS and PSD regulations are
complementary and not related as set to subset.

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Thomas, J., concurring in part
targeting twenty years of accepted practice.” Brief for Re
spondent Duke 37; see also Brief for State of Alabama et al.
as Amici Curiae. This claim, too, has not been tackled by
the District Court or the Court of Appeals; to the extent it
is not procedurally foreclosed, Duke may press it on remand.
* * *
The judgment of the Court of Appeals is vacated, and the
case is remanded for further proceedings consistent with
this opinion.
It is so ordered.
Justice Thomas, concurring in part.
I join all but Part III–A of the Court’s opinion. I write
separately to note my disagreement with the dicta in that
portion of the opinion, which states that the statutory
cross-reference does not mandate a singular regulatory
construction.
The Prevention of Significant Deterioration (PSD) statute
explicitly links the definition of the term “modification” to
that term’s definition in the New Source Performance Stand
ard (NSPS) statute:
“The term ‘construction’ when used in connection with
any source or facility, includes the modification (as de
fined in section 7411(a) of this title) of any source or
facility.” 42 U. S. C. § 7479(2)(C).
Section 7411(a) contains the NSPS definition of “modifica
tion,” which the parties agree is the relevant statutory defi
nition of the term for both PSD and NSPS. Because of
the cross-reference, the definitions of “modification” in PSD
and NSPS are one and the same. The term “modification”
therefore has the same meaning despite contextual varia
tions in the two admittedly different statutory schemes.
Congress’ explicit linkage of PSD’s definition of “modifi
cation” to NSPS’ prevents the Environmental Protection

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583 Cite as: 549 U. S. 561 (2007)
Thomas, J., concurring in part
Agency (EPA) from adopting differing regulatory definitions
of “modification” for PSD and NSPS. Cf. IBP, Inc. v. Al
varez, 546 U. S. 21, 34 (2005) (concluding that an “explicit
reference” to a previous statutory definition prohibits inter
preting the same word differently).
Section 7479(2)(C)’s cross-reference carries more meaning
than the mere repetition of the same word in a different stat
utory context. When Congress repeats the same word in a
different statutory context, it is possible that Congress might
have intended the context to alter the meaning of the word.
See Atlantic Cleaners & Dyers, Inc. v. United States, 286
U. S. 427, 433 (1932). No such possibility exists with
§ 7479(2)(C). By incorporating NSPS’ definition of “modifi
cation,” Congress demonstrated that it did not intend for
PSD’s definition of “modification” to hinge on contextual fac
tors unique to the PSD statutory scheme. Thus, United
States v. Cleveland Indians Baseball Co., 532 U. S. 200
(2001), which analyzes the mere repetition of the same word
in a different statutory context, carries little weight in this
situation.
Likewise, this case differs from the circumstance we faced
in Robinson v. Shell Oil Co., 519 U. S. 337 (1997). In Robin
son, we considered whether “employee,” as used in § 704(a)
of Title VII of the Civil Rights Act of 1964, included former
employees. We determined that under the clear language
of the statute, certain statutory provisions using the term
“employee” made sense only with respect to former employ
ees or current employees, but not both. Id., at 342–343.
Accordingly, upon analyzing the context of § 704(a), we were
compelled to conclude that the term “employee” included for
mer employees. This case does not present a similar situa
tion. The statute here includes a statutory cross-reference,
which conveys a clear congressional intent to provide a com
mon definition for the term “modification.” And the con
textual differences between PSD and NSPS do not compel

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Thomas, J., concurring in part
different meanings for the term “modification.” Robinson
is, therefore, inapplicable.
Even if the cross-reference were merely the equivalent of
repeating the words of the definition, we must still apply our
usual presumption that the same words repeated in different
parts of the same statute have the same meaning. See At
lantic Cleaners, supra, at 433; ante, at 574. That presump
tion has not been overcome here. While the broadly stated
regulatory goals of PSD and NSPS differ, these contextual
differences do not compel different definitions of “modifica
tion.” That is, unlike in Robinson, reading the statutory
definition in the separate contexts of PSD and NSPS does
not require different interpretations of the term “modifica
tion.” EPA demonstrated as much when it recently pro
posed regulations that would unify the regulatory definitions
of “modification.” See 70 Fed. Reg. 61083, n. 3 (2005) (term
ing the proposal “an appropriate exercise of our discretion”
and stating that the unified definition better serves PSD’s
goals).
The majority opinion does little to overcome the presump
tion that the same words, when repeated, carry the same
meaning. Instead, it explains that this Court’s cases do not
compel identical language to be interpreted identically in all
situations. Granting that point, the majority still has the
burden of stating why our general presumption does not con
trol the outcome here. It has not done so.

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Reporter’s Note
The next page is purposely numbered 801. The numbers between 584
and 801 were intentionally omitted, in order to make it possible to publish
the orders with permanent page numbers, thus making the official cita
tions available upon publication of the preliminary prints of the United
States Reports.