The Clean Air Act establishes three main regulatory pro-
grams to control air pollution from stationary sources such
as power plants. Clean Air Amendments of 1970, 84 Stat.
1676, 42 U. S. C. §7401 et seq. One program is the New
Source Performance Standards program of Section 111, at
issue here. The other two are the National Ambient Air
Quality Standards (NAAQS) program, set out in Sections
108 through 110 of the Act, 42 U. S. C. §§7408–7410, and
the Hazardous Air Pollutants (HAP) program, set out in
Section 112, §7412. To understand the place and function
of Section 111 in the statutory scheme, some background on
the other two programs is in order.
The NAAQS program addresses air pollutants that “may
reasonably be anticipated to endanger public health or wel-
fare,” and “the presence of which in the ambient air results
from numerous or diverse mobile or stationary sources.”
§7408(a)(1). After identifying such pollutants, EPA estab-
lishes a NAAQS for each. The NAAQS represents “the max-
imum airborne concentration of [the] pollutant that the
public health can tolerate.” Whitman v. American Trucking
Assns., Inc., 531 U. S. 457, 465 (2001); see §7409(b). EPA,
though, does not choose which sources must reduce their
pollution and by how much to meet the ambient pollution
target. Instead, Section 110 of the Act leaves that task in
the first instance to the States, requiring each “to submit to
[EPA] a plan designed to implement and maintain such
standards within its boundaries.” Train v. Natural Re-
sources Defense Council, Inc., 421 U. S. 60, 65 (1975); §7410.
The second major program governing stationary sources
is the HAP program. The HAP program primarily targets
pollutants, other than those already covered by a NAAQS,
that present “a threat of adverse human health effects,” in-
cluding substances known or anticipated to be “carcino -
genic, mutagenic, teratogenic, neurotoxic,” or otherwise
“acutely or chronically toxic.” §7412(b)(2).
EPA’s regulatory role with respect to these toxic pollu-
tants is different in kind from its role in administering the
NAAQS program. There, EPA is generally limited to deter-
mining the maximum safe amount of covered pollutants in
the air. As to each hazardous pollutant, by contrast, the
Agency must promulgate emissions standards for both new
and existing major sources. §7412(d)(1). Those standards
must “require the maximum degree of reduction in emis-
sions . . . that the [EPA] Administrator, taking into consid-
eration the cost of achieving such emission reduction, and
any non-air quality health and environmental impacts and
energy requirements, determines is achievable . . . through
application of measures, proc esses, methods, systems or
techniques” of emission reduction. §7412(d)(2). In other
words, EPA must directly require all covered sources to re-
duce their emissions to a certain level. And it chooses that
level by determining the “maximum degree of reduction” it
considers “achievable” in practice by using the best existing
technologies and methods. §7412(d)(3).
Thus, in the parlance of environmental law, Section 112
directs the Agency to impose “technology-based standard[s]
for hazardous emissions,” Alaska Dept. of Environmental
Conservation v. EPA, 540 U. S. 461, 485, n. 12 (2004) (em-
phasis added). This sort of “‘technology-based’ approach fo-
cuses upon the control technologies that are available to in-
dustrial entities and requires the agency to . . . ensur[e]
that regulated firms adopt the appropriate cleanup technol-
ogy.” T. McGarity, Media-Q uality, Technology, and Cost-
Benefit Balancing Strategies for Health and Environmen-
tal Regulation, 46 Law & Contemp. Prob. 159, 160 (Summer
1983) (McGarity). (Such “technologies” are not limited to
literal technology, such as scrubbers; “changes in the design
and operation” of the facility, or “in the way that employees
perform their tasks,” are also available options. Id., at 163,
n. 18.)
The third air pollution control scheme is the New Source
Performance Standards program of Section 111. §7411.
That section directs EPA to list “categories of stationary
sources” that it determines “cause, or contribute signifi-
cantly to, air pollution which may reasonably be anticipated
to endanger public health or welfare.” §7411(b)(1)(A)