1
(Slip Opinion)
OCTOBER TERM, 2006
Syllabus
NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
being done in connection with this case, at the time the opinion is issued.
The syllabus constitutes no part of the opinion of the Court but has been
prepared by the Reporter of Decisions for the convenience of the reader.
See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.
SUPREME COURT OF THE UNITED STATES
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 in-
creases 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 definitional identity, the Environmental
Protection Agency’s (EPA) regulations 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 Resources 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 in-
crease the actual annual emission of a pollutant above the actual av-
erage for t