 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued September 15, 2017             Decided March 16, 2018

                        No. 16-1021

                    SIERRA CLUB, ET AL.,
                        PETITIONERS

                              v.

   ENVIRONMENTAL PROTECTION AGENCY AND E. SCOTT
PRUITT, ADMINISTRATOR, U.S. ENVIRONMENTAL PROTECTION
                       AGENCY,
                    RESPONDENTS

          AMERICAN CHEMISTRY COUNCIL, ET AL.,
                    INTERVENORS


                 Consolidated with 13-1256


         On Petitions for Review of Final Action of
     the United States Environmental Protection Agency


    James S. Pew argued the cause for petitioners. With him
on the briefs were Neil Gormley, Patton Dycus, and Eric
Schaeffer. Sanjay Narayan entered an appearance.

     Norman L. Rave Jr., Attorney, U.S. Department of
Justice, argued the cause and filed the brief for respondents.
Perry M. Rosen, Attorney, entered an appearance.
                               2

     Lauren E. Freeman argued the cause for industry
intervenor-respondents. With her on the brief were Makram B.
Jaber, William L. Wehrum Jr., Felicia H. Barnes, Douglas A.
McWilliams, Allen A. Kacenjar, Katy M. Franz, Robert D.
Cheren, William F. Lane, and Alan H. McConnell. David M.
Friedland, Lisa M. Jaeger, Quentin Riegel, Ronald A. Shipley,
Shannon S. Broome, and Charles H. Knauss entered
appearances.

    Before: ROGERS, SRINIVASAN and PILLARD, Circuit
Judges.

    Opinion for the Court filed by Circuit Judge PILLARD.

    PILLARD, Circuit Judge: Industrial boilers are heavy-duty
furnaces used to generate steam and other useful heat for a wide
range of applications, such as milling paper and manufacturing
car parts. These boilers reach and sustain extremely high
temperatures, relying on varying combinations of fuels and
combustion techniques to do so. But all share a common
environmental risk: Without adequate controls in place, they
send into the air large quantities of toxic pollutants that
endanger public health.

     To mitigate such dangers, the Environmental Protection
Agency (EPA or Agency) issued rules under the Clean Air Act
to govern emissions of those pollutants. See Final Rule on
Reconsideration, National Emission Standards for Hazardous
Air Pollutants for Major Sources: Industrial, Commercial, and
Institutional Boilers and Process Heaters, 78 Fed. Reg. 7138,
7144 (Jan. 31, 2013). A slew of legal challenges followed. We
have already considered and resolved most of them in United
States Sugar Corp. v. EPA (U.S. Sugar), 830 F.3d 579 (D.C.
Cir. 2016).      But because EPA granted petitions for
                                3
reconsideration on two issues, we agreed to sever those issues
from U.S. Sugar. Upon additional consideration, EPA made
some changes to its rules. Several environmental groups,
which we refer to collectively as Sierra Club, challenge the
reconsidered rules, and we now take up their petitions.1 See
Final Rule on Reconsideration II, National Emission Standards
for Hazardous Air Pollutants for Major Sources: Industrial,
Commercial, and Institutional Boilers and Process Heaters, 80
Fed. Reg. 72,790 (Nov. 20, 2015).

     The first challenge concerns EPA regulations that
indirectly control a group of organic pollutants by limiting
carbon monoxide emissions as a proxy for the targeted
pollutants. After calculating emissions limits for the organic
pollutants by reference to the amount of carbon monoxide
emitted by the best performing boilers in each subcategory,
EPA concluded that the lowest of the carbon monoxide limits
were too low, so it substituted a single, higher limit that it
deemed sufficient to control the pollutants. Sierra Club
contends that the EPA’s about-face was unjustified and
contrary to the Clean Air Act.

     The second challenge concerns rules governing how
boilers operate while starting up and shutting down. Given the
high temperatures involved, startup and shutdown can take
hours, during which conditions inside a boiler are in flux. EPA
found it infeasible to set numeric limits on pollutants during
startup and shutdown, so instead set qualitative “work practice”
standards. Sierra Club contends that those work practice
standards give boiler operators unlawful leeway to pollute.


1
  Six organizations jointly petitioned along with Sierra Club:
Chesapeake Climate Action Network, Clean Air Council,
EarthJustice, Environmental Integrity Project, Louisiana
Environmental Action Network, and Partnership for Policy Integrity.
                               4
      For the reasons that follow, we conclude that Sierra Club
is right on the first score but wrong on the second. EPA did not
adequately justify its change of direction on the carbon
monoxide limits because it failed to explain how the revised
limits would minimize the targeted pollutants to the extent the
Clean Air Act requires. But its startup and shutdown work
practice standards are permissible because, consistent with the
Clean Air Act, they reasonably approximate what the best-
performing boilers can achieve.

                               I.

      As amended in 1990, the Clean Air Act (Act) specifies a
list of nearly two hundred hazardous air pollutants (HAPs) for
which the EPA must set national emissions standards. See 42
U.S.C. § 7412(b), (d); U.S. Sugar, 830 F.3d at 593. EPA is first
required to categorize and, where appropriate, sub-categorize
potential sources of each HAP. See 42 U.S.C. § 7412(c), (d).
The Agency must categorize polluters by volume of emissions.
Id. § 7412(c). The most voluminous polluters, dubbed “major
sources,” id. § 7412(a)(1), must be regulated with particular
care, see id. § 7412(d)(1). The Agency must also distinguish
between new sources and existing ones. U.S. Sugar, 830 F.3d
at 593-94 (citing 42 U.S.C. § 7412(d)(3)). EPA also may
further “differentiate ‘among classes, types, and sizes of
sources.’” Id. (quoting 42 U.S.C. § 7412(d)(1)).

    Here, the relevant category is major-source industrial,
commercial, and institutional boilers and process heaters—
which EPA refers to, for short, as industrial boilers. This
category runs the gamut of heavy-duty boilers used by
industries and large institutions, but excludes similar,
separately regulated equipment that burns solid waste or
generates electricity. See Proposed Rule, National Emission
Standards for Hazardous Air Pollutants for Major Sources:
                                5
Industrial, Commercial, and Institutional Boilers and Process
Heaters, 75 Fed. Reg. 32,006, 32,009, 32,016 (June 4, 2010).
A single set of rules governs the industrial boilers at issue here
during startup and shutdown. See 80 Fed. Reg. at 72,824.
Because EPA identified “significant design and operational
differences” among these industrial boilers based on their
primary fuels and (for certain HAPs) the combustion
technology used to burn those fuels, however, EPA imposed
separate operating-state emissions limits on subcategories
identified by those criteria. Id. at 32,017; see also 78 Fed. Reg.
at 7144.

     After categorizing sources, EPA prescribes standards for
sources in each category or subcategory. The basic approach
is technology-forcing: For major sources like those at issue
here, EPA must identify the “maximum degree of reduction in
emissions” that is “achievable” using current technology. 42
U.S.C. § 7412(d)(2). It must then use that maximum
achievable degree of reduction as an emissions cap for all
similar sources. See U.S. Sugar, 830 F.3d at 594; Mexichem
Specialty Resins, Inc. v. EPA, 787 F.3d 544, 549-50 (D.C. Cir.
2015).

    Congress prescribed how EPA must define those
“maximum achievable control technology” (MACT)
standards. EPA must at least set a so-called “MACT Floor”
with respect to each pollutant—the minimum that sources must
do to control emissions of the pollutant.          42 U.S.C.
§ 7412(d)(3); see U.S. Sugar, 830 F.3d at 594. The “floor”
terminology can be confusing, because MACT Floors—
baseline emissions standards—are upper limits, or caps, on
emissions. MACT Floors allow emission of each pollutant
only up to the level achieved either by the “best controlled
similar source” in the relevant subcategory (for new sources),
or by the lowest-emitting twelve percent of sources (for
                               6
existing sources). 42 U.S.C. § 7412(d)(3); see U.S. Sugar, 830
F.3d at 594.

     EPA may be required to set a “beyond-the-floor” standard
as well—a more-stringent-still emissions cap calling on
sources to perform even better than the current best performers.
See U.S. Sugar, 830 F.3d at 594-95. EPA must set a beyond-
the-floor standard if it determines that additional emissions
reduction would be achievable “taking into account costs,
certain health and environmental effects, and energy
requirements.” Nat’l Ass’n for Surface Finishing v. EPA, 795
F.3d 1, 4-5 (D.C. Cir. 2015); see 42 U.S.C. § 7412(d)(2).
Ordinarily, MACT Floors and beyond-the-floor standards that
EPA crafts must apply “continuously” whenever sources are at
risk of emitting pollutants—even when the sources are
operating outside of normal parameters because they are
starting up, shutting down, or malfunctioning. See Sierra Club
v. EPA, 551 F.3d 1019, 1027-28 (D.C. Cir. 2008).

     The Act gives EPA certain kinds of carefully
circumscribed flexibility, two of which figure centrally in this
case. First, EPA may sometimes regulate a HAP indirectly, by
controlling a proxy, or “surrogate,” instead of the pollutant
itself. See Sierra Club v. EPA, 863 F.3d 834, 838 (D.C. Cir.
2017). The Act nowhere expressly contemplates regulation by
surrogate, but we have held it permissible in some
circumstances, so long as the resulting rules are reasonably
calculated to control the relevant HAPs to the extent the statute
demands. See U.S. Sugar, 830 F.3d at 628-29. Second, EPA
may sometimes set qualitative “work practice” standards,
requiring sources to use certain protocols designed to minimize
emissions in lieu of numeric limits measuring pollutants
actually emitted. See 42 U.S.C. § 7412(h). Work practice
standards can be thought of as a statutory Plan B; EPA may
resort to them only when using numeric limits is “not feasible.”
                               7
Id. § 7412(h)(1). The statute defines when EPA may conclude
that numeric limits are infeasible, including—as relevant
here—when “the application of measurement methodology to
a particular class of sources is not practicable due to
technological or economic limitations.” Id. § 7412(h)(2)(B).
When EPA sets work practice standards, those standards must
be, in EPA’s judgment, “consistent with” the Act’s MACT
requirements. Id. § 7412(h)(1); see U.S. Sugar, 830 F.3d at
663.

     The 1990 amendments to the Act called on EPA to
promulgate national standards for every source category by the
year 2000. See 42 U.S.C. § 7412(e)(1)(E). That deadline has
long since passed, and—after an earlier iteration of this rule
was vacated in its entirety and rewritten, see Nat. Res. Def.
Council v. EPA, 489 F.3d 1250, 1261-62 (D.C. Cir. 2007)—
these revised standards are among the last to be finalized. See
Leslie Sue Ritts & Ben Snowden, The Regulation of Hazardous
Air Pollutants, in Clean Air Act Handbook 249, 265-67 (Julie
R. Domike & Alec C. Zacaroli, eds., 4th ed. 2016). For current
purposes, we can pick up the tale with EPA’s 2011 iteration of
these rules. See Final Rule, National Emission Standards for
Hazardous Air Pollutants for Major Sources: Industrial,
Commercial, and Institutional Boilers and Process Heaters, 76
Fed. Reg. 15,608 (Mar. 21, 2011). Once it promulgated the
2011 version, EPA chose to reconsider that entire set of
regulations almost immediately, re-finalized them in 2013 with
significant changes, see 78 Fed. Reg. at 7138; Proposed Rule
on Reconsideration, National Emission Standards for
Hazardous Air Pollutants for Major Sources: Industrial,
Commercial, and Institutional Boilers and Process Heaters, 76
Fed. Reg. 80,598 (Dec. 23, 2011), then further amended and
reissued them as the 2015 final rule at issue here, 80 Fed. Reg.
72,790.
                               8
     Regulated industries and environmental groups mounted
various legal challenges to the 2013 final rule, most of which
we have already adjudicated. See generally U.S. Sugar, 830
F.3d 579. But, between 2011 and 2013, EPA had so
significantly changed certain aspects of the rule, including the
two challenged here, that EPA decided to allow more time for
public comment and to reconsider them yet again. See
Proposed Rule on Reconsideration II, National Emission
Standards for Hazardous Air Pollutants for Major Sources:
Industrial, Commercial, and Institutional Boilers and Process
Heaters, 80 Fed. Reg. 3090 (Jan. 21, 2015). Accordingly, at
EPA’s request, we severed challenges to these two aspects of
the 2013 final rule from the U.S. Sugar proceedings and held
them in abeyance pending the Agency’s reconsideration. See
Order, U.S. Sugar, No. 11-1108 (Oct. 16, 2013). The Agency’s
reconsideration of these aspects of the rules is now complete,
and Sierra Club’s remaining two challenges are now before us.

     The first challenge targets certain limits on carbon
monoxide (CO), which EPA controls as a surrogate for a group
of listed pollutants known as “organic HAPs” (a term which,
as used in this opinion, excludes dioxin and furan, two organic
HAPs that EPA decided to regulate directly). Unlike organic
HAPs, CO is not among the pollutants that EPA regulates
under Section 7412, see 42 U.S.C. § 7412(b); CO is regulated
under a different part of the Act, see Util. Air Regulatory Grp.
v. EPA, 134 S. Ct. 2427, 2435 (2014). In the 2011 rulemaking,
EPA determined as a factual matter, supported by its analysis
of the chemistry of combustion, that “minimizing CO
emissions will result in minimizing . . . organic HAP.” 75 Fed.
Reg. at 32,018. It therefore identified CO as an effective proxy
for those HAPs. Id. Then, using the data it had on boilers’ CO
emissions, EPA crunched the numbers to set a MACT Floor for
CO as it normally would for a HAP. See id. at 32,019-23,
32,027-29.
                              9
     The best achievable results varied widely by boiler
subcategory: For example, had EPA set MACT Floors
according to those results, new “[s]tokers designed to burn
pulverized coal/solid fossil fuel” would have been required to
emit no more than 6 parts per million (ppm) CO, while new
“[h]ybrid suspension/grate units designed to burn biomass/bio-
based solids” would have been permitted to emit up to 1,500
ppm. 76 Fed. Reg. 15,687. But when reconsidering the 2011
rule, EPA decided in the 2013 rule to scrap the lowermost of
its CO floors—those for which the calculated CO limit came in
below 130 ppm. See 78 Fed. Reg. at 7144-45. While EPA’s
data confirmed a close correlation between reduced CO
emissions and reduced emissions of formaldehyde (a prevalent
organic HAP) down to roughly that level, the same data
puzzlingly appeared to show, not further reduction, but a spike
in formaldehyde emissions at even lower CO levels. Id. at
7145. In EPA’s view, the data were therefore “not . . .
sufficiently reliable to use as a basis for establishing an
emissions limit” lower than 130 ppm. Id. EPA declined to
require any boiler to drive CO emissions lower, revising
MACT Floor standards for several boiler subcategories upward
to a new 130 ppm “threshold.” Id. On further reconsideration
in 2015, EPA stood by that upward-revised limit. 80 Fed. Reg.
at 3096.

    The second challenge targets EPA’s “work practice”
standards that govern boilers during startup and shutdown.
Because conditions inside a boiler are in flux while heating up
and cooling down, EPA determined that it would not be
“feasible” to apply numeric emissions-testing methodologies,
which are generally calibrated to steady-state operations. See
76 Fed. Reg. at 15,642; 2015 Response to Reconsideration
Comments at II-3 (Oct. 2015) (Response to Comments), Joint
App’x (J.A.) 351. But it also proved hard to identify what work
practices EPA might feasibly require of boilers during startup
                               10
and shutdown, and at what point in the process boilers could be
treated as fully on line and thus meaningfully subject to
numeric emissions limits. After the 2013 final rule, EPA
reconsidered the startup and shutdown provisions a second
time and made several significant refinements. See 80 Fed.
Reg. at 3093-96. Those provisions, too, were finalized in 2015.
See 80 Fed. Reg. 72,790. Now that Sierra Club has timely
petitioned for review of that final rule, both aspects are ready
for our consideration.

                               II.

     We first consider whether EPA acted arbitrarily and
capriciously or violated the Act by revising certain CO limits
upward to 130 ppm. This challenge turns on whether EPA
supported the conclusion that no further reduction in organic
HAP emissions occurs once CO emissions fall below 130 ppm.
Because EPA did not, we hold that it acted arbitrarily and
capriciously. To explain our conclusion, we first describe in
more detail the process by which EPA formulated the
challenged limits and review our treatment of closely related
issues in U.S. Sugar. We then turn to explaining how EPA
failed to adequately justify its decision to revise these limits.

                               A.

     We have long recognized that regulation by surrogate is a
tool available to EPA, so long as it establishes that controlling
emissions of the surrogate is a “reasonable” way to achieve the
Act’s objective of limiting emissions of corresponding HAPs.
See Sierra Club, 863 F.3d at 838; U.S. Sugar, 830 F.3d at 628.
The determination is context-specific, but demanding; we ask
whether reducing surrogate emissions would “invariably” and
“indiscriminately” reduce the corresponding HAP. See Sierra
Club, 863 F.3d at 838. For example, we have twice affirmed
rules that limited hazardous metallic air pollutants by
                                11
controlling overall emissions of particulate matter (PM), of
which the targeted HAP metals were a small but ever-present
component. See Sierra Club v. EPA, 353 F.3d 976, 984-85
(D.C. Cir. 2004); Nat’l Lime Ass’n v. EPA, 233 F.3d 625, 639
(D.C. Cir. 2000). In those cases, we relied on EPA’s reasoned
conclusions that “each unit of PM emissions avoided ‘carries’
within it some quantum of HAP metals,” Nat’l Lime Ass’n, 233
F.3d at 639, so that PM controls “inevitably removed HAPs,”
Sierra Club, 353 F.3d at 984. In other words, even if the
precise concentration of the pollutant was unknown and might
have fluctuated somewhat, “strong direct correlations” linked
the targeted substance with the proxy used to measure it. Id. at
985 (alterations omitted) (quoting National Emission
Standards for Hazardous Air Pollutants for Source Categories,
65 Fed. Reg. 39,326, 39,329 (June 26, 2000)). Those direct
correlations—which, in both cases, EPA explained as
grounded in a straightforward, natural relationship between the
surrogate and the HAP—provided the crucial assurance that a
reviewing court requires: A standard based on the performance
of the sources that best control emissions of the surrogate will
“reflect what the best source or . . . sources . . . in the relevant
subcategory achieved with regard to the HAP.” U.S. Sugar,
830 F.3d at 628 (emphasis added).

     Against that backdrop, EPA in the 2011 rulemaking
determined that CO was a suitable surrogate for organic HAPs.
Noting that CO emissions would be easier to monitor and
control than a host of hard-to-measure individual organic
HAPs, EPA reasoned that CO limits were an appropriate
substitute for the target HAPs because “organic HAP are
products of incomplete combustion” and “CO is a good
indicator of incomplete combustion.” 75 Fed. Reg. at 32,018.
Accordingly, “minimizing CO emissions will result in
minimizing non-dioxin organic HAP.” Id.
                              12
     A natural chemical relationship among the relevant
molecules undergirded EPA’s approach. The record teaches
that the basics of that relationship are, roughly, as follows:
Combustion occurs when a boiler’s fuel—including carbon-
containing molecules (hydrocarbons)—is exposed to heat and
oxygen, triggering oxidation. See 78 Fed. Reg. at 7145; 75 Fed.
Reg. at 32,025. The resulting chemical transformations break
down the bulkiest hydrocarbons into smaller ones, then into
CO. See 78 Fed. Reg. at 7145. Further oxidation yields carbon
dioxide (CO2) in place of CO—the last step in the combustion
process. Id. (Water (H2O) is another byproduct of the
combustion process, id., but its presence is not relevant to
EPA’s analysis.) Because CO results from incomplete
oxidation, more complete combustion leaves less CO (and
more CO2) in the resulting emissions stream. By the same
token, the more complete the combustion, the lower the
emission of organic HAPs—carbon-based molecules that have
not been fully oxidized. See 78 Fed. Reg. at 7145; 75 Fed. Reg.
at 32,025. Hence the scientific conclusion giving rise to EPA’s
surrogacy determination: Both CO (not yet replaced by CO2)
and organic HAPs (not yet fully broken down) appear in an
emissions stream when combustion is not “complete,” while
driving combustion nearer to “completeness” reduces
emissions of both. See 76 Fed. Reg. at 15,654.

     Unpersuaded that this correlation was as robust as EPA
claimed, Sierra Club protested EPA’s decision to use CO as a
surrogate for organic HAPs. See U.S. Sugar, 830 F.3d at 630.
Industry, meanwhile, expressed concern about the most
stringent CO limits and suggested loosening them. In the 2013
reconsideration, EPA rejected Sierra Club’s arguments and
accepted industry’s, raising the lowest CO limits to the new
130 ppm common threshold. See id. at 628; 78 Fed. Reg. at
7144-45.     At that point, while EPA held for further
reconsideration that new threshold on the CO limits,
                              13
environmental petitioners brought to this court their general
challenge to use of CO as a surrogate in the first place, and we
denied that general challenge in U.S. Sugar.

     Sierra Club’s position in U.S. Sugar involved two
contentions. First, Sierra Club argued that EPA failed to
establish a sufficiently tight correlation between reduced CO
and reduced organic HAP emissions, “because record evidence
demonstrated a breakdown in th[at] correlation” when CO
levels dropped below 130 ppm. U.S. Sugar, 830 F.3d at 630.
Second, it argued that EPA failed to consider whether organic
HAP emissions could be even further limited if sources used
certain post-combustion controls—such as technologies that
extract pollutants from exhaust—instead of or in addition to
CO limits. Id. at 629. In other words, it argued that EPA failed
to establish that reliance on CO-based emission limits is both a
valid and, when exclusive, sufficient way to achieve the
requisite maximum control of organic HAPs.

     In U.S. Sugar, we rejected the first line of argument but
agreed with the second. On the first, we deferred to EPA’s
“scientific judgment” that any “apparent breakdown” in the
otherwise-strong correlation between CO and organic HAPs
was “most likely caused by the difficulty of measuring the
regulated HAP at such extremely low emission levels, rather
than by a flaw in the correlation.” Id. at 630. On the second,
we identified a gap in the record and remanded for EPA to
consider “whether the best performing boilers might be using
alternative control technologies and methods that reduce
organic HAP emissions beyond what they achieve by
regulating CO alone,” id. at 629—though we anticipated that
EPA would likely be able to justify its purely CO-based
approach once it provided the missing information, id. at 630.
                               14
     Sierra Club continues to protest both EPA’s decision to use
CO as a surrogate and the adequacy of the Agency’s
consideration of post-combustion controls. We addressed
those broad contentions in U.S. Sugar. In that case, we
remanded to EPA for further consideration of the rule’s
reliance on CO as a surrogate as a general matter, to the
exclusion of alternative control methods. Id. The results of
that consideration are not before us, and we do not revisit those
arguments here.

     But U.S. Sugar did not address EPA’s decision, in light of
its general reliance on CO as a surrogate for a group of organic
HAPs, to establish the 130 ppm lower bound. Our U.S. Sugar
remand left all of EPA’s CO-based limits intact pending their
further consideration, and did not address the levels at which
any particular limits were set, only the decision to measure the
limits on organic HAP emissions in terms of CO levels. See id.
at 630. We therefore have yet to consider Sierra Club’s more
specific challenges to the 130 ppm limits, and we do so here.
Treating CO as generally a suitable surrogate for organic
HAPs, per U.S. Sugar, it remains for us to determine whether
EPA’s decision in 2013 (reaffirmed in 2015) to loosen the 2011
rule’s most stringent CO floors was reasonable and consistent
with the Act.

                               B.

     Sierra Club argues that EPA violated the Act and made an
arbitrary and capricious decision because the 130 ppm CO
threshold in the 2013 final rule weakened standards the agency
had earlier promulgated as MACT Floors for thirteen
subcategories. EPA responds that its revised CO standards are
just as effective as the original ones, assuring us that organic
HAP destruction is “complete,” or at least “essentially”
complete, once CO emissions fall to 130 ppm. Resp’t’s Br. 18-
                               15
19. We take EPA to mean that organic HAP emissions are
effectively nonexistent—or, in any event, cannot be further
reduced—whenever a boiler’s CO emissions are below 130
ppm. If articulated and adequately supported in the record,
such a position could well satisfy the Act. See 42 U.S.C.
§ 7412(d)(2) (EPA “shall require the maximum degree of
reduction in emissions of the [HAPs] subject to this section
(including a prohibition on such emissions, where
achievable)”). That conclusion would follow from the Act’s
focus on controlling specifically enumerated HAPs: So long
as a surrogate is not itself a regulated HAP—as CO is not—its
emissions need not be controlled beyond the point where EPA
can be confident that the targeted HAP emissions are reduced
as far as possible or, indeed, “eliminat[ed] . . . entirely.” U.S.
Sugar, 830 F.3d at 629.

     But the record does not support any such conclusion here.
When settling on the revised 130 ppm floors in 2013, EPA
explained that it had set out to determine “whether there is a
minimum CO level for boilers and process heaters below which
there is no further benefit in organic HAP
reduction/destruction.” 78 Fed. Reg. at 7144-45. To make that
assessment, the agency looked to data showing the relationship
between varying levels of CO emissions and corresponding
emissions of formaldehyde—the only organic HAP for which
it had such data. Id. at 7144. On their face, however, those
data did not show complete destruction of formaldehyde (or a
leveling-off of emissions) as CO dropped below 130 ppm. Id.
Nor did the data show continuation at those low levels of the
correlation on which EPA’s use of CO as a surrogate was
based. Instead, “[a]t levels lower than 150 ppm, the mean
levels of formaldehyde appear[ed] to increase, as d[id] the
overall maximum value and variability in formaldehyde
emissions.” Id.
                               16
     EPA was “aware of no reason why” the otherwise strong
correlation between lower CO emissions and lower
formaldehyde emissions would suddenly invert. Id. The
Agency accordingly determined the data were untrustworthy
and that they did not reflect an actual increase in formaldehyde
emissions. EPA explained: “[W]e do not believe that such
measurements are sufficiently reliable to use as a basis for
establishing an emissions limit.” Id. We deferred to EPA’s
scientific judgment on this exact point in U.S. Sugar, rejecting
Sierra Club’s argument that the imperfect formaldehyde data
disproved the general validity of CO as a surrogate and noting
EPA’s assurances that the “apparent breakdown” of the
relationship between formaldehyde and CO below 130 ppm
“was most likely caused by the difficulty of measuring the
regulated HAP at such extremely low emission levels.” U.S.
Sugar, 830 F.3d at 630.

     In separately attempting to justify its conclusion that CO
limits would not yield further reduction in organic HAPs if set
below the level where the formaldehyde data became
unreliable, however, EPA relied on the same data it had
elsewhere decisively characterized as untrustworthy. EPA
asserted in support of its decision to reject any limit more
stringent than 130 ppm that, “[a]t CO levels less than [130
ppm], our data indicate that there is no apparent relationship
between CO and organic HAP (i.e., formaldehyde).” 78 Fed.
Reg. at 7145 (emphasis added). In other words, EPA’s only
support for its upward-revised floors was the very data it had
just dismissed as inaccurate, now cited as reliable evidence that
reducing CO below 130 ppm does not in fact reduce organic
HAP emissions.

     That mismatch—treating data EPA had viewed as not
reliable at low emission levels as if it were affirmative support
for a breakdown of the correlation at those levels—makes
                              17
EPA’s decision arbitrary and capricious. EPA concluded that
the otherwise well-documented general correlation between
CO and organic HAPs does not persist below 130 ppm without
providing a reasoned basis for its conclusion. Importantly,
EPA was regulating against the backdrop of its own prior,
general determination that CO was a surrogate for organic
HAPs; it had concluded “that minimizing CO emissions will
result in minimizing . . . organic HAP.” 75 Fed. Reg. 32,018.
“EPA proposed using CO as a surrogate because . . . the lowest
possible CO emissions resulted in the lowest possible HAP
emissions . . . .” U.S. Sugar, 830 F.3d at 629.

     In U.S. Sugar, we relied on EPA’s conclusion that there
was tight correlation between reduced CO and reduced organic
HAP emissions to affirm EPA’s rule in part. See id. at 630.
We treated that conclusion as supported by both the
formaldehyde emissions data and the scientific principle
underlying them: For reasons EPA explained, we accepted that
incomplete combustion yields levels of CO and organic HAP
emissions that correlate very closely to one another. See id. at
628, 630. EPA’s refusal to extend that same logic to CO levels
below 130 ppm requires a reasoned justification. The Agency
failed to provide one.

     EPA came closest to a reasoned determination that the
surrogacy relationship broke down below 130 ppm in its
assertion that CO is a “conservative” surrogate for organic
HAPs because it is “a difficult to destroy refractory
compound.” 78 Fed. Reg. at 7145. Although EPA did not
define the term, one way to understand EPA’s characterization
of CO as a “conservative surrogate” is that organic HAPs might
all burn up in the combustion process at a level of completeness
where some CO emissions remained, because “oxidation of CO
to carbon dioxide is the slowest and last step of oxidation of
hydrocarbons.” Id. If that is true, there could theoretically be
                               18
some nonzero level of CO emissions below which no further
reduction in organic HAP emission occurs, because the HAPs
would be all gone (or perhaps still present in low amounts, yet
impervious to combustion) before CO emissions ceased.

    When justifying its rule, however, EPA did not say that
organic HAP emissions are eliminated completely (or not
susceptible of any further reduction) below 130 ppm, nor has it
explained how any such theory follows from the only available
record evidence—the formaldehyde data on which EPA
otherwise exclusively relied. We cannot sustain an agency’s
decision on grounds it did not invoke. See SEC v. Chenery,
332 U.S. 194, 196 (1947).

      Three points highlight the lack of basis to sustain the rule
on a novel, “conservative surrogacy” ground. First, during the
rulemaking process, EPA never took the position that organic
HAP emissions fall to zero, nor gave any reason why they
could not be further reduced, once CO emissions reach 130
ppm. It said only that, where CO is emitted at or below 130
ppm, organic HAP emissions are “extremely low.” 78 Fed.
Reg. at 7145; see also U.S. Sugar, 830 F.3d at 630. But
describing HAP levels as “low,” even “extremely low,” or
saying that their combustion is “essentially” complete, implies
that HAPs have not been entirely eliminated. So EPA’s
observation that HAP emissions are “extremely low” when CO
is at 130 ppm is not a reasoned basis for concluding that organic
HAP emissions cannot be reduced still further. There is no
“close enough” exception to the requirement that EPA’s
MACT floors limit emissions to the full extent shown to be
achievable by the best-performing sources; to the contrary, the
Act’s MACT provisions instruct EPA to “maximize” the
reduction in emissions, up to and including “a prohibition on
such emissions, where achievable.” 42 U.S.C. § 7412(d)(2).
                               19
     Second, the formaldehyde data on which EPA generally
relied are the only data EPA offered for its decision not to
require that CO emissions be reduced below 130 ppm, and EPA
staked its “conservative surrogate” theory on those data. See
78 Fed. Reg. at 7145. But, in virtually the same breath, EPA
said those data were not a reliable indicator of what happens to
organic HAP emissions at the low levels in question. Again,
that contradiction leaves us unable to discern any reasoned
basis for determining that organic HAPs disappear from the
emission stream before CO does, or to otherwise conclude that
organic HAP emissions cannot be further reduced.

     Third, even if EPA had grounds to conclude that there is
some nonzero level of CO emissions that marks a point below
which organic HAP emissions cannot be further reduced, it
offered no basis for identifying 130 ppm as that level. As just
noted, EPA cites only the unreliable formaldehyde data—
which, on average, show HAP emissions increasing below 150
ppm of CO, not leveling off or zeroing out. See id. Accepting
that boomerang as a data flaw, and not as an accurate
representation of a shift in the physical correlation between CO
and HAP combustion, it is not evident how those unreliable
data could support a conclusion that emissions in fact plateau
at their lowest achievable level, rather than either increasing or
continuing to decrease, at an inflection point of 130 ppm. EPA
has not explained how the data could suffice.

    Industry intervenors’ brief (but not EPA’s) seeks to bolster
the evidence in the record by reference to two prior rules in
which EPA set CO limits at a level equivalent to what EPA
defends here. The conclusions reached in those other
rulemakings are irrelevant under our precedent, which takes
“every tub on its own bottom” when setting emissions
standards under the Act; EPA must justify its conclusions in
each proceeding. U.S. Sugar, 830 F.3d at 623 (quoting Sierra
                              20
Club, 353 F.3d at 986). It is not enough to have reached the
same (unreviewed) conclusion elsewhere.

     EPA did not in the rulemaking here rely on either of the
prior rules to which intervenors cite, nor on the records
supporting them. That makes sense because in neither prior
rulemaking did EPA reach, much less justify, the specific
conclusion that EPA has failed to support here: that a 130 ppm
CO level suffices to eliminate organic HAP emissions, or that
further reductions are not possible beyond that point. The first
rule, promulgated in 1991 under the Resource Conservation
and Recovery Act (RCRA), 42 U.S.C. § 6901 et seq., limited
organic matter emissions only to a level that would not “pose a
significant risk,” as that statute required; it did not conclude
that 130 ppm was the maximum achievable reduction. Burning
of Hazardous Waste in Boilers and Industrial Furnaces, 56
Fed. Reg. 7134, 7151 (Feb. 21, 1991). And, in finalizing the
second cited rule, a 2005 restriction on hazardous waste
combustors, EPA concluded only that CO levels below 130
ppm “may not provide significant reductions in organic HAP
emissions” because such emissions are “extremely low” when
CO levels are “in the range of zero to 100 ppm[]” (corrected to
seven percent oxygen, which is equivalent to 130 ppm when
corrected to three percent oxygen). See National Emission
Standards for Hazardous Air Pollutants: Final Standards for
Hazardous Air Pollutants for Hazardous Waste Combustors
(Phase I Final Replacement Standards and Phase II), 70 Fed.
Reg. 59,402, 59,462 (Oct. 12, 2005). In neither case did EPA
conclude that no below-130 ppm CO emissions limit would
improve the control of HAPs.

     EPA alternatively suggests that this court in U.S. Sugar
already decided this issue in its favor, but we did not. We
rejected the environmental petitioners’ argument in that case
that “record evidence demonstrated a breakdown in the
                               21
correlation between CO and organic HAP emissions below 130
ppm” such that EPA acted arbitrarily in relying on CO as a
surrogate. U.S. Sugar, 830 F.3d at 630. In accepting the
relationship between CO and HAP combustion as a general
matter, we deferred to EPA’s conclusion that there was only an
“apparent” breakdown in that relationship—a breakdown
“most likely caused by the difficulty of measuring the regulated
HAP” at those levels, rather than by variability in the
underlying relationship. Id. We did not endorse the conclusion
that EPA now advances—that the data affirmatively prove an
absence of further reductions.

    Given these deficiencies in EPA’s reasoning, we cannot
discern the “reasonable connection to the facts in the record”
necessary to defer to EPA’s decision to revise these CO floors.
U.S. Sugar, 830 F.3d at 829; see also Motor Vehicle Mfrs.
Ass’n of the U.S. v. State Farm Mut. Auto. Ins. Co., 463 U.S.
29, 43 (1983). EPA may have a hunch that setting CO limits
below a certain level would be ineffectual to control HAP
emissions. But the record we have before us does not
substantiate any such conclusion, much less provide a basis for
pinpointing that level at 130 ppm.

     It would be particularly inappropriate to give EPA a pass
on backing up its apparent hunch here, where EPA was
operating against the backdrop of its own prior reasoned
judgment that “minimizing CO emissions will result in
minimizing non-dioxin organic HAP,” 75 Fed. Reg. 32,018,
and where its conclusion appears to be counter to the only
empirical evidence EPA had before it. See State Farm, 463
U.S. at 43. If EPA concludes that the relationship it previously
identified between CO and organic HAP is actually valid only
to a point—a conclusion the likes of which our prior regulation-
by-surrogate cases have not endorsed—it must explain how the
limiting point it specifies reflects the emission control actually
                              22
achieved by the best performing sources and, further, that it is
the lowest emission level achievable with existing technology.

     We therefore remand to EPA to reconsider its decision to
adopt the 130 ppm CO limits. We do not vacate those limits,
because Sierra Club has asked us not to do so and because
“vacatur would cause substantial disruptive effects by
removing emissions limits for the regulated HAPs.” U.S.
Sugar, 830 F.3d at 630. EPA may, if it finds it feasible to do
so, undertake this reconsideration in conjunction with the
broader task we gave EPA when remanding in U.S. Sugar: To
further consider “the portion of the Major Boilers Rule
providing for CO’s use as a surrogate for non-dioxin/furan
organic HAPs.” Id.

     Because we remand, we need not pass on Sierra Club’s
additional contention that EPA failed to consider beyond-the-
floor standards under 42 U.S.C. § 7412(d)(2). In revisiting the
CO-based standards (in light of both this decision and U.S.
Sugar), however, EPA must consider both (1) whether the
standards it adopts are Section 7412(d)(3)-compliant MACT
Floors and (2) whether Section 7412(d)(2) beyond-the-floor
standards are called for here. See Nat’l Lime Ass’n, 233 F.3d
at 634-35.

                              III.

     Sierra Club also challenges EPA’s startup and shutdown
work practice standards as arbitrary and capricious and
contrary to the Act. It challenges the duration of the startup
period EPA allows, as well as the content of both the startup
and shutdown work practices EPA prescribed. Sierra Club
contends that EPA’s approach to the duration of startup
arbitrarily and unlawfully gives all sources four extra hours
before they must begin complying with numeric standards,
even though some sources admittedly can achieve stable
                              23
operations in less time. Sierra Club also claims that the work
practices EPA requires during startup are arbitrary and
unlawful because they do too little to reduce emissions—most
notably by allowing boiler operators latitude to activate many
pollution controls only when “possible.” See 80 Fed. Reg. at
72,824. Finally, Sierra Club contends that the shutdown work
practice provisions are too lenient and are internally
inconsistent.

     We first summarize the content of the challenged
standards and how EPA developed them. We then analyze
Sierra Club’s claims against the standards as finalized.

                              A.

     The startup and shutdown work practice standards EPA
finalized in 2015 were the product of considerable trial and
error. In its 2011 final rule, EPA concluded that a work
practice standard was called for during startup and shutdown
because it was “not technically feasible” for the regulated
boiler operators to conduct the emissions testing necessary to
enforce numeric limits. 76 Fed. Reg. at 15,613. Sierra Club
does not contest that finding, at least not as a general matter.
The content of the original 2011 rule was, however, notably
meager: It required boiler operators only to “follow[] the
[boiler] manufacturer’s recommended procedures for
minimizing periods of startup and shutdown.” Id.

     The 2013 iteration of the rule began to make its
requirements more specific. It gave startup a defined end point:
“when steam or heat is supplied for any purpose.” 78 Fed. Reg.
at 7146. It required boiler operators to use certain enumerated
clean fuels to initiate startup, and to “engage all of the
applicable [pollution] control devices” upon transitioning to
the boiler’s primary fuel, except that four specified devices
needed only to be engaged “as expeditiously as possible.” 78
                               24
Fed. Reg. at 7199. Shutdown practice was essentially the
inverse: EPA defined shutdown to begin when the boiler
stopped generating useful steam or heat, or “at the point of no
fuel being fired . . . , whichever is earlier.” Id. at 7147. As
long as primary fuel kept firing during shutdown, boiler
operators, again, had to “operate all applicable control devices,
except” the specified four. Id. at 7199. And, during both
periods, boiler operators had to collect and report monitoring
data. Id.

     EPA concluded that additional public comment could help
it further refine the startup and shutdown provisions, so it
initiated the reconsideration process that gave rise to the 2015
rule. 80 Fed. Reg. at 3092. The agency proceeded cautiously
through what it treated as a delicate balancing act. On the one
hand, EPA had determined numeric standards were infeasible
because boiler conditions were too variable while heating up
and cooling down, and the agency had scant data about those
volatile periods. 76 Fed. Reg. at 15,641-42. EPA had also
recognized serious risks of explosions and equipment damage
that might result if it required operators to engage pollution
controls too early, while boiler conditions remained in flux. 80
Fed. Reg. at 3094; see also Response to Comments at II-6, J.A.
354. EPA accordingly was attentive to industry concerns that
it not set the end of startup too early or impose otherwise
unrealistically demanding standards.

     At the same time, EPA’s work practice standards had to be
“consistent with” the Act’s MACT stringency provisions. See
42 U.S.C. § 7412(h)(1); U.S. Sugar, 830 F.3d at 663. EPA also
had some evidence that emissions might be elevated during
startup, Response to Comments at II-11, J.A. 359; id. at II-28,
J.A. 364, and was aware that, the sooner startup ended, the
sooner boiler emissions would be subject to numeric limits.
EPA thus aimed to transition boilers to numeric limits as soon
                               25
as the best-performing units could achieve stability. See 80
Fed. Reg. at 3094.

      The 2015 rule, as proposed and finalized, balanced those
considerations in two ways. First, faced with evidence that
many boilers could not achieve stable operations as soon as
they began supplying useful steam or heat, EPA set its primary
definition of the startup period to end four hours after a boiler
first supplies “useful thermal energy”—i.e., provides the steam
or heat that is its raison d’être. 80 Fed. Reg. at 72,824.
Although EPA had scant data about the boilers to be regulated,
it had a better dataset on technologically similar boilers whose
primary function is electricity generation. Those boilers are
subject to a different regulatory regime under which EPA
collects hourly operations data. Response to Comments at II-
5-6, J.A. 353-54. Using those data, EPA calculated that the
best-performing twelve percent of those electricity-generating
boilers achieved stable operations four hours after they began
supplying useful thermal energy. 80 Fed. Reg. at 72,795; 80
Fed. Reg. at 3094.

     Second, EPA further adjusted its work practices, making
all pollution control devices subject to the “as expeditiously as
possible” standard, with the exception of particulate matter
controls that EPA required operators to engage within one hour
of first using fuels other than the clean fuels specifically
mandated for use during startup (as opposed to the dirtier fuels
consumed during ordinary operation). 80 Fed. Reg. at 72,824.
Operators who can show that they are unable safely to meet the
one-hour timeframe and have a control device adequately
designed and sized to meet the filterable PM emission limit
may seek a case-specific time extension from the relevant
permitting authority. Id. EPA also added a requirement that
every source operator “develop and implement a written startup
and shutdown plan,” id., while retaining the requirement that it
                               26
monitor, record, and report data concerning fuel usage, boiler
conditions, and control device operations, 80 Fed. Reg. at
72,816, 72,824.

      At the same time, the 2015 rule also retained the shorter
2013 definition of startup as an alternative, letting boiler
operators opt into it if they can meet it. 80 Fed. Reg. at 72,824.
EPA was aware that at least some industrial boilers could
achieve stability more quickly than the average of the best-
performing electricity generators. See Response to Comments
at II-4-5, J.A. 352-53. Because it did not know precisely which
boilers could do so, however, it retained the faster startup
definition as an option, offering those earliest-to-stabilize
boilers an incentive to opt for the faster definition by pairing
that standard with leaner recordkeeping and reporting
obligations than EPA requires of operators starting up more
slowly. See 80 Fed. Reg. 72,816-17; 80 Fed. Reg. at 3094.
That approach was crafted with one eye to the future periodic
reviews the Act requires. See 42 U.S.C. § 7412(d)(6). Once
boiler operators either provide improved data to EPA or opt for
the shorter startup period and succeed in complying with it,
EPA assures us that it will consider further refining and
tightening these standards. Resp’t’s Br. 40.

    The changes coming out of EPA’s reconsideration focused
on the startup provisions; the 2015 shutdown provisions were,
for our purposes, essentially unchanged from 2013. See 80
Fed. Reg. at 72,824.

                               B.

    Sierra Club contends, first, that EPA acted arbitrarily and
unlawfully by allowing boiler operators to define startup to
extend four hours beyond when a boiler begins supplying
useful energy. Specifically, Sierra Club contends that EPA
should not have subjected any boilers to a work practice
                               27
standard during that four-hour window without first making a
finding under 42 U.S.C. § 7412(h)(2) that it is “not practicable”
to impose numeric limits on the relevant “particular class of
sources” during that four-hour window. Sierra Club asserts
that EPA has not done so here because, by retaining the shorter
startup period as a compliance option, the agency implicitly
conceded that some sources can comply sooner.

     But EPA did determine that, for the class of industrial
boilers as a whole, four hours after beginning to supply useful
energy was a reasonable estimate of how long the best
performers’ operations would remain unstable. 80 Fed. Reg. at
3094. EPA had “very limited information” about the industrial
boilers under consideration. Id. And EPA’s efforts to obtain
more information through notice and comment in the double-
reconsideration process yielded only industry-provided survey
data that were of limited utility. Id. EPA accordingly estimated
time-to-stability for all of the boilers in this category based on
the closest analogue at hand: the best performing electricity-
generating boilers. Id. EPA’s authority to resort to a work
practice standard does not depend on its determining that
numerically gauging emissions would be impractical
throughout the entire startup period for every single source to
which a work practice applies; the Act requires only that EPA
determine that it is impractical to measure emissions for the
“particular class of sources” at issue.                42 U.S.C.
§ 7412(h)(2)(B).

    Though EPA was painting in broad strokes, its approach
was reasonable. EPA knew boilers had heterogeneous startup
processes, and it reasonably concluded that startup
performance (and associated variability) was not correlated
with any easily isolated boiler characteristics. This left EPA
with no basis on which to apply different definitions of startup
                               28
to different boilers by subcategorizing them into different
“classes” or “types.” 42 U.S.C. § 7412(d)(1).

     EPA was, as it acknowledged, working from “very limited
information specifically for industrial boilers.” 80 Fed. Reg. at
3094. But here we “defer to [EPA]’s decision to proceed on
the basis of imperfect scientific information, rather ‘to invest
the resources to conduct the perfect study.’” Sierra Club v.
EPA, 167 F.3d 658, 662 (D.C. Cir. 1999) (quoting Am. Iron &
Steel Inst. v. EPA, 115 F.3d 979, 1004 (D.C. Cir. 1997) (per
curiam)); see also Cement Kiln Recycling Coal. v. EPA, 255
F.3d 855, 867 (D.C. Cir. 2001) (quoting Sierra Club, 167 F.3d
at 662). EPA applied its expertise to determine that electricity-
generating units had sufficient technological similarity to
industrial boilers for data on the former to also inform
operations of the latter. 80 Fed. Reg. at 3094. Four hours after
supplying useful thermal energy was the time the best-
performing twelve percent of those analogous boilers took
before engaging controls, so their data gave EPA a reasonable
basis for concluding that its definition was consistent with
MACT. See id. EPA’s data, though admittedly scant, pass
muster in part because EPA’s reliance thereon is only a
stopgap; as noted, the data-collection and recordkeeping
requirements in EPA’s work practices standard are designed to
generate more directly relevant data that promise to provide
grounds to further revise the rule (or to confirm its
appropriateness).

     Sierra Club disputes whether data showing when
electricity-generating units engage controls may reasonably be
thought to reflect the earliest time at which they are capable of
doing so. A premise of Sierra Club’s argument is that
electricity-generating boiler operators might not engage
controls at the first opportunity “absent a regulatory
requirement.” Pet’rs’ Br. 50. But electricity-generating boilers
                               29
face such requirements; EPA has so much data on them
precisely because they are subject to—among various federal
and state regulatory regimes—the Clean Air Act’s Acid Rain
program, 80 Fed. Reg. at 3094, which follows a market-based
cap-and-trade approach that attaches costs to each unit of
uncontrolled emissions, see North Carolina v. EPA, 531 F.3d
896, 902 (D.C. Cir. 2008). EPA could reasonably assume that
operators of the best-performing electricity-generating boilers
engage controls at their earliest opportunity.

     Sierra Club also contends that EPA, in retaining the 2013
rule’s shorter startup definition as an alternative compliance
option, impermissibly delegated its impracticability
determination to the regulated boiler operators. Sierra Club’s
premise is that only the shorter definition may lawfully apply,
unless EPA makes boiler-specific impracticability
determinations justifying longer startup. As just discussed,
however, the longer startup period represented EPA’s reasoned
estimate of what the best-performing twelve percent of
industrial boilers could achieve. It is thus reasonable and
consistent with the statute. Sierra Club does not contend that
the longer definition would have been unlawful if EPA had
imposed it alone, without the shorter alternative. Sierra Club’s
claim thus reduces to little more than an objection to allowing
boiler operators to choose between two options.

     EPA’s approach was reasonable here. EPA concluded by
the time it issued the 2015 rule that the more stringent standard
it had imposed (without a longer allowance for startup) in the
2013 version of the rule was beyond what all the boilers in the
top twelve percent benchmark group could accomplish, but that
it might nonetheless be achievable for some. Because EPA did
not know precisely which boilers could meet the more stringent
timeframe, it encouraged those that could do so to identify
themselves and opt into complying with numeric emissions
                               30
limits sooner than they would otherwise have to. That creative
approach reasonably offered eased recordkeeping and
reporting as an incentive for a subset of industrial boilers to
reduce emissions further than EPA could otherwise require,
even as EPA recognized the need to collect additional data
from the rest of the field.

                               C.

     Sierra Club also challenges the remaining content of
EPA’s work practice standards as not sufficiently demanding.
We conclude that, despite imperfect data about industrial boiler
startup and shutdown, EPA reasonably accommodated what it
identified as legitimate safety concerns in deciding what work
practices were achievable. Evidence in the administrative
record shows that, while starting up and shutting down,
industrial boilers are prone to “overheating,” “[l]eaks,” and
“thermal stresses” if not carefully managed. Response to
Comments at II-5-6, J.A. 353-54. It also reveals that “startup
and low load operations” place boilers at heightened risk of
“furnace explosions.” Id. at II-6, J.A. 354. Engaging certain
control technologies too early, EPA learned, could be not just
dangerous but counterproductive: Running a given device
below a certain temperature or pressure “could permanently
destroy . . . its performance potential.” Id. At the same time,
EPA was told that startup procedure “varies widely” across
boilers. 80 Fed. Reg. at 3094. EPA therefore reasonably
fashioned relatively contextual work practice standards.

     Sierra Club first contends that EPA’s requirement that
boilers engage most pollution control devices “as expeditiously
as possible” is tantamount to the empty “general duty” standard
we invalidated in Sierra Club v. EPA, 551 F.3d 1019, 1026-28
(D.C. Cir. 2008). But the rule we rejected in that case was quite
different: It eschewed defining obligations altogether, relying
                               31
instead on the regulated community’s background “general
duty” to limit emissions in a manner “consistent with good air
pollution control practice for minimizing emissions.” Id. at
1022 (quoting 40 C.F.R. § 60.11(d)). We rejected that
approach because it neither set a numeric emissions limit nor
followed Section 112(h)’s requirements for setting work
practice standards during startup and shutdown. Id. at 1027-
28. Sierra Club thus held that, whenever HAP sources are in
operation, including during startup and shutdown, EPA must
continuously subject them to either numeric limits or
Section 112(h)-compliant work practice standards.

     Here, because EPA chose to regulate startup and shutdown
via work practice standards, the question before us is whether
those standards comport with Section 112(h). We conclude
that they do. Sierra Club’s contention that the work practice
standards here challenged are akin to the contentless “pollute
as little as you can” edict we rejected in Sierra Club in 2008
overlooks that the requirement at issue here has substantive
content that was missing from that rule. The general duty
requirement we deemed inadequate in Sierra Club was limited
to the admonition that “owners and operators shall, to the
extent practicable, maintain and operate any affected
facility . . . in a manner consistent with good air pollution
control practice for minimizing emissions.” Id. at 1022
(quoting 40 C.F.R. § 60.11(d)). Here, by contrast, the
requirement to start certain pollution control devices “as
expeditiously as possible” applies to specific devices and is just
one aspect of a multifaceted work practice standard. 80 Fed.
Reg. at 72,824. The standard also includes (most notably)
requirements to initiate startup with clean fuels, and to start
particulate matter controls at a specified time. Id. And EPA
built an implicit emissions limitation into the startup definition
itself, by pegging it to the production of useful energy. Boiler
operators lack incentives to combust fuel for no useful purpose,
                               32
simply as a means to avoid engaging pollution controls, so
presumably they do not tarry in heating their equipment to that
point. By requiring numeric-standard compliance as soon
thereafter as possible, the rule minimizes emissions by
ensuring startup is not needlessly drawn out. Cf. U.S. Sugar,
830 F.3d at 666-67 (approving a work practice standard that
required boiler operators to minimize the duration of startup
and shutdown). That reality reinforces EPA’s conclusion that
its work practice standard has constraining effect that a
general-duty standard lacks. EPA’s work practices are
admittedly less than exacting, but they are materially more
precise and demanding than the general duty standard we
disapproved in Sierra Club in 2008.

     Petitioners further contend that the work practice standards
impermissibly delegate to boiler operators decisions about
what is achievable with respect to many pollution control
devices, and, relatedly, that a standard that varies depending on
what is practicable for each individual boiler is contrary to the
technology-forcing design of § 7412(d). Despite the generality
of “as expeditiously as possible,” we accept EPA’s reasoning
as to why it is a meaningful constraint. The rule requires each
boiler operator to create a written startup and shutdown plan
and make it available for public inspection. 80 Fed. Reg. at
72,795; 80 Fed. Reg. at 3095. And, more generally, these
boilers are subject to enhanced permitting and recordkeeping
requirements applicable to all major sources—requirements
that enable EPA, state regulators, and interested third parties to
check the boiler operators’ homework. Those obligations
include periodic “compliance certifications” that describe how
the boiler is meeting each applicable requirement—including
the work practice standards. See 42 U.S.C. § 7414(a)(3); 40
C.F.R. § 70.6(c)(5)(iii); Nat. Res. Def. Council, Inc. v. EPA,
194 F.3d 130, 132-34 (D.C. Cir. 1999). Boiler operators thus
must at all times be able to explain why they cannot engage
                              33
controls sooner than they do, and EPA’s work practices
specifically require sources to monitor and record data about
conditions that guide their determinations, such as temperature
and pressure inside the boiler. 80 Fed. Reg. at 72,816-17.
Those obligations put in place meaningful avenues to double
check boiler operators’ assertions about what is possible.
Moreover, data EPA gathers while these rules are in effect
should inform the case-by-case determinations as well as future
refinements of these rules when they are periodically reviewed.

     In sum, having reasonably discerned that sound operation
of this heterogeneous class of boilers requires allowing
operators some discretion to determine the earliest time when
certain control devices can safely come online, EPA
permissibly concluded that its work practices were “consistent
with” the Act’s MACT approach. See U.S. Sugar, 830 F.3d at
663. The record corroborates EPA’s concerns about equipment
safety if controls were subject to across-the-board engagement
times, and its conclusion that boiler operating constraints
during startup and shutdown vary widely. Section 7412(h)—
the provision authorizing EPA to adopt a “design, equipment,
work practice, or operational standard, or combination thereof”
in lieu of an emissions standard—centrally relies on “the
judgment of the Administrator” regarding when an emissions
standard is not feasible, and “the Administrator’s judgment” as
to whether the standard is appropriately stringent to meet the
statute’s objectives. 42 U.S.C. § 7412(h)(1); see U.S. Sugar,
830 F.3d at 663. That judgment of course must be guided by
permissible factors. One such factor here was known
technological limitations on the use of control devices during
the volatile conditions that characterize startup. See U.S.
Sugar, 830 F.3d at 664-65. Tellingly, Sierra Club struggles to
identify what more EPA could realistically have required of
boiler operators.
                               34
                               D.

      Finally, we address Sierra Club’s contentions that the
shutdown provisions are too lax. Specifically, Sierra Club
argues that EPA should not exempt enumerated control
devices—dry scrubbers, fabric filters, selective catalytic
reduction, and (for fluidized bed boilers) limestone injection—
from operation during shutdown, and it asserts that EPA
“claimed to have required boilers to use clean fuels during
shutdown, when in fact it did not do so.” Pet’rs’ Br. 46. The
first argument fails for reasons similar to those that led us to
uphold the startup provisions. EPA requires boilers to “operate
all applicable control devices, except” the enumerated four
during shutdown, 80 Fed. Reg. at 72,824 (emphasis added),
having reasonably concluded that the exempted control devices
could not safely be operated under conditions encountered
during shutdown. See, e.g., 78 Fed. Reg. at 7147.

     And the second argument is a semantic quibble that
overlooks the reality of how shutdowns unfold. It is technically
true that, under EPA’s work practice standard, boilers may
sometimes use no clean fuels, and some fuels that are not clean,
during shutdown. That is because shutdown requires gradually
cooling a boiler, which involves phasing out the boiler’s
primary fuel. Secondary fuels may be burned during this
process to, for example, help stabilize cooling, but they are not
always needed. Accordingly, the shutdown work practices
require the use of clean fuels “[i]f, in addition to the fuel used
prior to initiation of shutdown, another fuel must be used”
during that process. 80 Fed. Reg. at 72,824. Whenever that
happens, the regulations—as EPA accurately notes—require
those fuels to be clean.

   Sierra Club is right that the rule contemplates that some
amount of “the fuel used prior to initiation of shutdown”—i.e.,
                                 35
the boiler’s primary, and thus relatively “dirty,” fuel—may
sometimes be burned during shutdown, which might seem
inconsistent with a requirement to use clean fuels. But, again,
any permissible use of fuels during shutdown necessarily must
unfold as a practical matter; it does not reflect any sleight of
hand by EPA. Shutdown primarily consists of gradually
phasing out the boiler’s primary fuel, and EPA’s shutdown
work practices apply beginning either at the point in that
process when useful energy is no longer supplied “or when no
fuel is being fed to the boiler . . . , whichever is earlier.” Id. at
72,818. So a boiler may sometimes burn its primary fuel while
shutdown work practices are in effect, but only after the boiler
has cooled beyond the point of supplying useful thermal
energy—in other words, during a brief window when some
dwindling amount of primary fuel is still being phased out.
Apart from that narrow window when useful energy production
has ceased and the boiler is still cooling, any fuel that is burned
during shutdown must be “clean.”                   EPA has not
mischaracterized its rule.

                               ***

     For the foregoing reasons we grant in part and deny in part
the petitions for review, and remand to EPA the CO limits for
which EPA adopted a revised limit of 130 ppm for further
proceedings consistent with our opinion.

                                                        So ordered.
