It is well
established under our precedent that the Act requires each
source category’s emission standard to address every
recognized hazardous pollutant that the source category is
known to emit. See Nat’l Lime Ass’n v. EPA, 
233 F.3d 625
,
634 (D.C. Cir. 2000). No party seriously disputes that
interpretation, nor could it. The problem here is that in
promulgating the emission standard for pulp mill combustion
sources in 2001 EPA addressed some but not all the hazardous
air pollutants they are known to emit. See Final Rule: National
Emission Standards for Hazardous Air Pollutants for Chemical
                                3
Recovery Combustion Sources at Kraft, Soda, Sulfite, and
Stand-Alone Semichemical Pulp Mills, 66 Fed. Reg. 3,180
(Jan. 12, 2001) (2001 Rule) (codified at 40 C.F.R. pt. 63, subpt.
MM).

     In the past when EPA left out requisite limits from a source
category’s emission standard, it has acted during its
congressionally mandated periodic review of that standard
under section 112(d)(6) to revise it to include the necessary
limits. The Act requires EPA every eight years to “review, and
revise as necessary” each of the “emission standards” it has
promulgated under section 112—and to do so “taking into
account developments in practices, processes, and control
technologies.” 42 U.S.C. § 7412(d)(6). In 2017, EPA
(belatedly) conducted its first section 112(d)(6) review and
revision of the 2001 pulp mill combustion source emission
standard, but this time it decided to review only the standard’s
limits on emissions of the toxics the standard already
controlled, leaving unlimited several other hazardous toxics
that the sources are known to emit but that were left out of the
2001 Rule. See Final Rule: National Emission Standards for
Hazardous Air Pollutants for Chemical Recovery Combustion
Sources at Kraft, Soda, Sulfite, and Stand-Alone Semichemical
Pulp Mills, 82 Fed. Reg. 47,238 (Oct. 11, 2017) (2017 Rule).

     The parties here dispute whether it was “necessary” under
section 112(d)(6) for EPA to calculate the missing emission
limits and “revise” the incomplete “emission standard”
promulgated in 2001 to include them.                Environmental
petitioners challenge as both contrary to law and arbitrary the
2017 Rule’s failure to correct the standard’s acknowledged
under-inclusiveness during the section 112(d)(6) review. EPA
agrees that the Act requires it to set controls in accordance with
the method described in section 112(d)(2)-(3) for every air
toxic a source category emits. As counsel for EPA put it at oral
                               4
argument, “[t]his case is not about whether EPA has an
obligation to set emission standards for each and every [air
toxic] from a source category: It does. National Lime in 2000
established that.” Oral Arg. Rec. at 17:53-18:08. But EPA
contends that section 112(d)(6) is unambiguous in confining
the requisite periodic review to whatever pollutants an existing
standard already controls, and “creates no obligation for EPA”
to consider any “pollutants not previously addressed.” EPA
Br. 2. EPA thinks Congress left the Agency to decide when to
finish the job: In EPA’s view, it may but is not required to
address during section 112(d)(6) review any listed but still
uncontrolled hazardous air pollutants the source category
emits. And the Agency says it decided not to do so here
because of time constraints that, it claims, left it with
inadequate information to calibrate the limits. Intervenor
American Forest and Paper Association takes a more restrictive
view of EPA’s authority, insisting that, to protect industry’s
entitlement to repose, any challenge to the standard’s under-
inclusiveness of listed air toxics should have been brought
within 60 days after the 2001 Rule. See 42 U.S.C.
§ 7607(b)(1). Petitioners respond that they have timely raised
a meritorious challenge to EPA’s 2017 Rule. Petitioners also
challenge EPA’s denial of their petition for reconsideration,
contending that EPA’s central rationale—that it lacked time to
promulgate the missing limits, and had no legal obligation to
do so during its section 112(d)(6) review—appeared for the
first time in the Final Rule, depriving the public of an
opportunity to comment.

     We hold that, because the Act necessitates section 112-
compliant emission standards for each source category, and
section 112(d)(6) requires EPA at least every eight years to
review and revise emission standards “as necessary,” EPA’s
section 112(d)(6) review of a source category’s emission
standard must address all listed air toxics the source category
                                5
emits. The 2017 Rule failed to do so. We accordingly grant
the petition and remand the 2017 Rule without vacatur, and
direct EPA to set limits on the listed air toxics that pulp mill
combustion sources are known to emit but that EPA has yet to
control. In light of the remand, we dismiss as moot the denial
of the petition for reconsideration.

I.   BACKGROUND

     A. The Clean Air Act Amendments

     When Congress enacted the Clean Air Amendments of
1970, it directed EPA to identify and regulate hazardous air
pollutants. Pub. L. No. 91-604, § 4(a), 84 Stat. 1676, 1678-80
(codified as amended at 42 U.S.C. §§ 7407-09). Two decades
on, “[d]issatisfied with EPA’s progress in identifying
hazardous air pollutants, Congress amended the Act in 1990 to
name nearly 200 such pollutants” and “charged EPA with
identifying sources of those pollutants and setting emission
standards for them.” Nat’l Ass’n for Surface Finishing v. EPA,
795 F.3d 1
, 4 (D.C. Cir. 2015); see Nat’l Lime 
Ass’n, 233 F.3d
at 634 
(“Congress added the list of pollutants to be regulated,
regulation deadlines, and minimum stringency requirements to
the Clean Air Act precisely because it believed EPA had failed
to regulate enough HAPs [Hazardous Air Pollutants] under
previous air toxics provisions.”). The initial list of air toxics
that Congress compiled to make up for EPA’s slow start
appears at section 112(b)(1). 42 U.S.C. § 7412(b)(1).

     Section 112 of the Act, added as part of the 1990
Amendments, mandates EPA’s regulation of those listed air
toxics. See Clean Air Act Amendments, Pub. L. No. 101-549,
§ 301, 104 Stat. 2399, 2531-74 (1990) (1990 Amendments).
Section 112(b) calls on EPA to build on the initial list Congress
created to maintain an up-to-date list of air toxics. To that end,
the Act requires EPA periodically to review and revise the list
                                6
by rule.
 Id. § 7412(b)(2). 
In addition, “any person” may
petition EPA to add a hazardous air pollutant to the list or delete
one from it, and Congress directed that EPA grant or deny such
petition within 18 months.
 Id. § 7412(b)(3)(A). 
The listed air
toxics include known carcinogens as well as substances
causing serious non-cancer health effects to various bodily
organs and systems—including nerves, heart, lungs, liver, skin,
and reproductive systems—and to fetal development. Many of
these toxics affect people’s health through multiple pathways
(water, soil, food, air), are persistent (meaning that, once
emitted, they linger in the environment), and bio-accumulative
(such that small amounts inhaled or otherwise absorbed by
bodily tissues build up over time, thereby intensifying
associated health risks).

     To control emissions of the listed air toxics, EPA must
“promulgate regulations establishing emission standards for
each category or subcategory of major sources.”
              Id.
§ 7412(d)(1). 
The Clean Air Act currently lists 190 hazardous
pollutants. More than a hundred pulp mill chemical recovery
combustion sources operating in the United States collectively
emit more than 23 million pounds of those air toxics annually.
See Proposed Rule: National Emission Standards for
Hazardous Air Pollutants for Chemical Recovery Combustion
Sources at Kraft, Soda, Sulfite, and Stand-Alone Semichemical
Pulp Mills, 81 Fed. Reg. 97,046, 97,082 (Dec. 30, 2016) (2016
Proposed Rule). Those chemical recovery combustion sources
are an identified subset of the sources of hazardous emissions
at pulp mills. Pulp mills work to mush, grind, or dissolve wood
and other materials into pulp, typically used to make paper.
The pulping process creates a chemical liquor byproduct that
contains some of the original pulping chemicals. Pulp mills
can recover these chemicals for reuse through a variety of
chemical processes as well as generate energy through
incinerating other residual organic matter. These “chemical
                               7
recovery combustion sources” are the only sources of
emissions regulated by the 2001 and 2017 Rules.
 Id. at 97,051-
52.

     Each source category’s “emission standard” must specify
the source’s maximum allowable emission “of hazardous air
pollutants listed for regulation.” 42 U.S.C. § 7412(d)(1). In
other words, each “emission standard” includes limits on
emissions of air toxics from a particular kind of air polluter.
An emission standard must contain limits for each listed air
toxic the relevant category of source emits.
 Id. § 7412(d)(1)-
(3); see also, e.g., Sierra Club v. EPA, 
479 F.3d 875
, 878, 883
(D.C. Cir. 2007) (reiterating EPA’s clear statutory obligation
to set limits on all air toxics a source emits, and invalidating
“no control” emission floors for brick and ceramics kilns);
Mossville Envtl. Action Now v. EPA, 
370 F.3d 1232
, 1236,
1242 (D.C. Cir. 2004) (reading section 112(d)(1) clearly to
require limits “for every HAP emitted from each category or
subcategory of major sources” and invalidating standard
governing polyvinyl chloride (PVC) manufacturers that
controlled only vinyl chloride); Nat’l Lime 
Ass’n, 233 F.3d at
634 
(recognizing EPA’s “clear statutory obligation to set
emission standards for each listed HAP” and invalidating
standard governing brick and ceramics kilns that placed “no
control” floors on their emissions of hydrogen chloride,
mercury, and hydrocarbons). The existing standard governing
pulp mill combustion sources limits a handful of the listed air
toxics those sources emit but sets no limit on many others.

    Section 112(d)(2)-(3) prescribes the method by which
EPA, in promulgating an emission standard, must calibrate
source-specific limits on emission of each air toxic.
Specifically, for existing major sources, EPA must “require the
maximum degree of reduction in emissions” by the particular
source category that the Agency “determines is achievable.”
                               8
42 U.S.C. § 7412(d)(2). To that end, the Act directs EPA to
calculate the average level of emissions of each air toxic
achieved by the best-performing 12 percent of facilities in a
given source category—those that emit the toxic at the lowest
levels.
 Id. § 7412(d)(3)(A). 
That baseline emissions limit is
referred to as the “maximum achievable control technology”
floor or “MACT floor.” Sierra Club v. EPA, 
895 F.3d 1
, 7-8
(D.C. Cir. 2018). EPA must then determine, considering cost,
health, and environmental effects, whether a more stringent
limit is “achievable.”
 Id. § 7412(d)(2). 
If so, EPA must
promulgate a “beyond-the-floor” limit at that more stringent
level. Surface 
Finishing, 795 F.3d at 5
.

      When Congress amended the Act in 1990 to jumpstart
implementation, it set a stringent timeline for EPA’s hazardous
air pollutant regulation. Congress required EPA to promulgate
standards for every area source and major source category and
subcategory in the United States “as expeditiously as
practicable.” 42 U.S.C. § 7412(e)(1). It set an overall deadline
for EPA to regulate all identified air toxics emitted by any
covered source within 10 years of the Act’s effective date—
i.e., by November 15, 2000.
 Id. § 7412(e)(1)(E). 
To propel
EPA to act, Congress set interim milestones. It directed EPA
to finalize standards for at least forty categories of sources
within two years of the effective date,
 id. § 7412(e)(1)(A), 
each
of which had to address all the listed pollutants the source
category emits,
 id. § 7412(b)(1). 
Congress further specified
that EPA must finalize standards for 25 percent of source
categories and subcategories within four years,
 id.
§ 7412(e)(1)(C), 
and at least another 25 percent within seven
years of the Act’s effective date,
 id. § 7412(e)(1)(D).
    The provision at issue here, section 112(d)(6), requires
EPA, on an ongoing periodic basis, to revisit and update
emission standards that it has already set for each source. No
                               9
less than every eight years, EPA must “review, and revise as
necessary (taking into account developments in practices,
processes, and control technologies), emission standards
promulgated under this section.”
 Id. § 7412(d)(6). 
That review
ensures that, over time, EPA maintains source standards
compliant with the law and on pace with emerging
developments that create opportunities to do even better.

     In addition to its section 112(d)(6) review, EPA under
section 112(f)(2) must conduct a one-time review within 8
years of promulgating an emission standard to, among other
things, evaluate the residual risk to the public from each source
category’s emissions and promulgate more stringent limits as
necessary “to provide an ample margin of safety to protect
public health.”
 Id. § 7412(f)(2)(A). 
Petitioners do not here
challenge EPA’s section 112(f)(2) risk assessment.

   B.    History of the 2017 Rule

     After notice and comment, EPA in 2001 promulgated
national emission standards to control hazardous air pollutants
emitted by “new and existing sources used in chemical
recovery processes at kraft, soda, sulfite, and stand-alone
semichemical pulp mills.” 2001 Rule, 66 Fed. Reg. at 3,180.
We refer to this regulated source, which includes recovery
furnaces, smelt dissolving tanks, and lime kilns at pulp mills,
as “pulp mill combustion sources.” See
 id. at 3,181-82. 
The
2001 Rule failed to include any limit on some of the hazardous
air pollutants that pulp mill combustion sources emitted, but
nobody challenged it at the time.

    In 2009, Sierra Club and other environmental nonprofit
organizations petitioned EPA to conduct a rulemaking to
control the air toxics the 2001 Rule overlooked. As relevant
here, they urged the Agency to set limits for dozens of
additional hazardous air pollutants that pulp mill combustion
                               10
sources were known to emit. More than a decade later, EPA
has not responded to that petition, nor have petitioners sued on
grounds of unreasonable delay to enjoin the Agency to do so.

     In February 2011, EPA issued an Information Collection
Request asking pulp and paper manufacturers for information
needed to conduct its section 112(d)(6) review, as well as its
section 112(f)(2) “risk review.” Its request sought updated
inventory data for all pulp and paper emission sources, and
available information on chemical recovery combustion
equipment and control devices currently in use. See 2016
Proposed Rule, 81 Fed. Reg. at 97,052. Despite a 100 percent
response rate from manufacturers providing the requested
information,
 id., EPA did 
not take needed action to revise the
emission standard for pulp mill combustion sources to limit any
of the listed but as-yet uncontrolled air toxics.

     Sierra Club and California Communities Against Toxics
sued EPA in 2015 in the Northern District of California under
the citizen-suit provision of the Clean Air Act, 42 U.S.C.
§ 7604, challenging the Agency’s failure to conduct the
statutorily required follow-up reviews for pulp mill combustion
sources. See Sierra Club v. McCarthy, No. 15 Civ. 1165
(HSG), 
2016 WL 1055120 
(N.D. Cal. Mar. 15, 2016). EPA
agreed that it had failed to fulfill its mandatory rulemaking
duties under sections 112(d)(6) and (f)(2) of the Act, and asked
only for sufficient time within which to do so.
 Id. at *1. 
After
carefully examining EPA’s representations about what it had
done so far against what it had yet to do, the court in early 2016
ordered EPA to issue final determinations on both reviews by
October 1, 2017.
 Id. at *7. 
EPA did not object to that timeline,
nor did it at any time seek to extend it.

    Mindful of its court-ordered timeline, EPA in December
2016 solicited notice and comment on a proposed revision of
                               11
the emission standard for pulp mill combustion sources. See
2016 Proposed Rule, 81 Fed. Reg. at 97,046. EPA announced
it was conducting the requisite residual-risk review under
section 112(f)(2) and a “technology review” under section
112(d)(6).
 Id. at 97,048. 
It proposed some changes to the 2001
Rule, but none to address the lack of limits on many of the
hazardous air pollutants the pulp mill combustion sources emit.
Petitioner Sierra Club commented on the 2016 Proposed Rule
and emphasized that section 112(d)(6) requires EPA to set
limits on previously uncontrolled hazardous air pollutants.
Intervenor American Forest and Paper Association also
commented, insisting for its part that “EPA has no obligation
to expand the scope of the existing standards and does not in
fact have statutory authority to do so.” Joint Appendix (J.A.)
374.

    EPA, in its September 2017 response to comments,
acknowledged that “standards for certain combinations of
pollutants and processes in the . . . [pulp mill combustion]
source category have not been promulgated.” J.A. 378. Only
then did EPA announce that, although it has the authority “to
develop standards under 112(d)(2) and (3) for previously
unregulated pollutants at the same time the Agency completes
the section 112(d)(6) review, nothing in section 112(d)(6)
expressly requires the EPA to do so as part of that review.”
 Id.

     In October 
2017, EPA promulgated its Final Rule without
setting any limits for those hazardous air pollutants it
acknowledges it must regulate but that it had omitted from the
2001 standard for pulp mill combustion sources. See 2017
Rule, 82 Fed. Reg. at 47,335. Apart from its disclaimer of any
obligation to do so under section 112(d)(6), EPA’s sole
explanation for not including the requisite limits was that “[t]he
compressed schedule for this rulemaking, due to the court-
ordered deadline, did not make it reasonable to appropriately
                              12
evaluate new standards for unregulated pollutants and
processes.”
 Id.

    C. Procedural 
History

     On December 11, 2017, Petitioners Louisiana
Environmental Network, PT Air Watchers, and Sierra Club
sought review of the 2017 Rule in this court and also petitioned
EPA for reconsideration. EPA denied the petition for
reconsideration on July 9, 2018. See National Emission
Standards for Hazardous Air Pollutants for Chemical Recovery
Combustion Sources at Kraft, Soda, Sulfite, and Stand-Alone
Semichemical Pulp Mills, 83 Fed. Reg. 32,213 (July 12, 2018).
Petitioners sought our review of that denial on September 10,
2018, and we consolidated the petition challenging the denial
of reconsideration with their earlier petition for review of the
2017 Rule.

II. DISCUSSION

    A. Jurisdiction

     We have jurisdiction under section 307(b)(1) of the Act to
review EPA’s Final Rule. 42 U.S.C. § 7607(b)(1). EPA does
not contest Petitioners’ standing, but Intervenor argues that
Petitioners lack an injury in fact and, in any event, have not
shown any redressable injury. Petitioners submitted six
member declarations in support of their organizational
standing, two from each of the three Petitioner organizations.
All of the members allege that they experience various
symptoms that they attribute to emissions from neighboring
pulp mills, and each alleges having curtailed favored activities
accordingly. “[E]nvironmental plaintiffs adequately allege
injury in fact when they aver that they used the affected area
and are persons ‘for whom the aesthetic and recreational values
of the area will be lessened’ by the challenged activity.”
                               13
Friends of the Earth, Inc. v. Laidlaw Envtl. Servs. (TOC), Inc.,
528 U.S. 167
, 183 (2000) (quoting Sierra Club v. Morton, 
405
U.S. 727
, 735 (1972)). The alleged injuries readily meet that
standard.

     B. EPA’s Duty Periodically to Revise Major Sources’
        Emission Standards as Necessary Requires
        Adding Missing Limits on Air Toxics the Source
        Emits

     Section 112(d)(6) is the statutory mechanism for
reviewing and updating emission standards applicable to listed
sources. It requires that, no less often than every eight years,
EPA “shall review, and revise as necessary (taking into account
developments in practices, processes, and control
technologies), emission standards promulgated under this
section.” 42 U.S.C. § 7412(d)(6). The question here is
whether, when EPA undertakes those updates, it must add
limits, calculated consistent with section 112(d)(2)-(3), for any
air toxics the source emits that the existing standard does not
address. In past section 112(d)(6) reviews of other sources’
emission standards, EPA has updated the standards to include
limits for hazardous air pollutants that the sources emit but the
Agency had not previously limited. See, e.g., Final Rule:
National Emissions Standards for Hazardous Air Pollutants for
Mineral Wool Production and Wool Fiberglass Manufacturing,
80 Fed. Reg. 45,280, 45,311 (July 29, 2015); Final Rule:
National Emission Standards for Hazardous Air Pollutant
Emissions: Hard and Decorative Chromium Electroplating and
Chromium Anodizing Tanks, 77 Fed. Reg. 58,220, 58,238
(Sept. 19, 2012). At oral argument both Petitioners and EPA
represented that EPA had previously always updated its
standards to set limits on unregulated air toxics in the course of
its section 112(d)(6) review. Oral Arg. Rec. at 5:02-5:38
(Petitioners);
 id. at 18:29-19:20 
(EPA). EPA later identified
                               14
three section 112(d)(6) reviews during which it did not set
limits on previously unaddressed air toxics. See EPA 28(j)
Letter (Dec. 10, 2019) (citing 80 Fed. Reg. 75,178 (Dec. 1,
2015); 77 Fed. Reg. 55,698 (Sept. 11, 2012); 76 Fed. Reg.
70,834 (Nov. 15, 2011)); see also Pet’rs 28(j) Response (Dec.
17, 2019) (not disputing EPA’s characterization). EPA argued
here that its past practice of adding missing limits during
section 112(d)(6) review was optional, and that the Act grants
it discretion whether or not to bring underinclusive standards
into compliance with section 112(d)(2)-(3) when conducting its
periodic section 112(d)(6) review.

     There is no dispute that the Act requires EPA to have in
place emission standards to control all the listed pollutants that
a source category emits, and requires the Agency to revise
existing standards that are underinclusive to add section
112(d)(2)-(3) controls for listed but unaddressed pollutants.
The only question is whether EPA lawfully may complete a
section 112(d)(6) review and “revise” an existing,
underinclusive emission standard “as necessary” without
supplying the missing controls. Put differently, the issue is
whether section 112(d)(6)’s periodic, mandatory review and
revision “as necessary” is textually confined to those air toxics
already limited under the source’s existing emission standard,
or whether that provision compels consideration of the
adequacy of the emission standard to control all the air toxics
the source category emits.

     We read the statutory text to require EPA during its section
112(d)(6) review to establish any missing limits. Because we
conclude that the text of the statute unambiguously supports
Petitioners’ reading, we resolve the case without resort to any
deference to EPA under Chevron, USA, Inc. v. NRDC, Inc., 
467
U.S. 837 
(1984).
                                15
     Two key features of the statutory language make clear that
EPA’s duty periodically to review and revise existing standards
under section 112(d) encompasses a mandate to add missing
limits:

     First, the “standards” to which section 112(d)(6) refers—
the “emission standards promulgated under this subsection”—
are statutorily defined as comprehensive controls for each
source category that must include limits on each hazardous air
pollutant the category emits. See 42 U.S.C. § 7412(d)(1)-(3),
(6). The statutory definitions EPA points to do not support its
contention that an emission standard is a limit on an individual
pollutant, as opposed to a regulation governing the set of
pollutants emitted by a source category. The Act’s “generally
applicable definition,” EPA Br. 5, describes an emission
standard as governing “emissions” of “pollutants” plural,
specifically, “a requirement . . . which limits the quantity, rate,
or concentration of emissions of air pollutants,”
 id. § 7602(k).
The regulatory definition it says is “[m]ore particular to the
Act’s hazardous air pollutant program,” EPA Br. 5, merely
refers back to a “national standard . . . promulgated in a subpart
of this part pursuant to sections 112(d), 112(h), or 112(f) of the
Act,” 40 C.F.R. § 63.2. And, in calling for “emission standards
for each category or subcategory of major sources of hazardous
air pollutants listed for regulation,” section 112(d) defines air
pollution “emission standards” as source-specific, not toxic-
specific. 42 U.S.C. § 7412(d)(1). Accordingly, as used in
section 112(d), an emission standard includes as many limits as
needed to control all the emitted air toxics of a particular source
category.

    EPA also urges that what counts as an emission standard
“promulgated” under section 112 is frozen once EPA
completes its initial promulgation, meaning that section
112(d)(6)’s reference to an “emission standard[] promulgated”
                              16
under section 112 would reach only the subset of air toxics a
source’s existing standard addressed.           But “emission
standards” under section 112(d)(6) are not constrained by past,
potentially flawed and underinclusive agency action, as EPA
now suggests.       Congress’ requirement that EPA must
periodically review and revise as necessary its “emission
standards” is a mandate to address the adequacy of each
emission standard on the books against the statutory demand of
section 112(d)(2) for an “emission standard” for each source
category—one with the requisite degree of control of all of the
air toxics the source emits. The obligatory periodic review and
revision of “emission standards” thus must ensure that each
source category’s standard imposes appropriate limits—not
just on whatever subset of toxics the existing standard
addressed, but on all the toxics the source category emits.

     The second textual reason to read section 112(d)(6) to
require EPA to add missing controls is the subsection’s
mandate to “revise” emission standards “as necessary.” 42
U.S.C. § 7412(d)(6). Again, the core demand of section 112 is
that EPA promulgate emission standards for every source
category addressing all listed hazardous air pollutants. And
Congress established deadlines to make clear that time is of the
essence. We conclude that when EPA reviews an existing
standard that fails to address many of the listed air toxics the
source category emits, adding limits for those overlooked
toxics is a “necessary” revision under section 112(d)(6).

     EPA asks us to read the statutory phrase—“as necessary
(taking into account developments in practices, processes, and
control technologies),” id.—to require only changes related to
the practical or technological advances mentioned in the
parenthetical. But the language is to the contrary. The
operative standard is “revise as necessary,” with the
parenthetical pointing to a non-exhaustive list of
                                17
considerations. Unlike other of EPA’s statutory mandates,
section 112(d)(6) does not state only that EPA “shall take into
account” listed factors,
 id. § 11002(a)(4) 
(EPA’s authority to
revise lists of toxic chemicals), or that listed others shall not be
taken into account,
 id. § 7473(c)(1) 
(concentrations of
pollutants EPA must ignore). Instead, EPA must revise its
emission standards “as necessary,” while “taking into account”
certain factors.

     To complete a defined task while taking certain factors
into account means to be aware of or consider the factors, not
to treat them as the exclusive determinants. For example,
nobody would understand a school principal’s instructions to a
teacher revising lesson plans “as necessary (taking into account
available textbooks and online enrichment programs)” as
confining the teacher’s planning to those criteria, to the
exclusion of the school district’s or state’s comprehensive
grade-level curricular requirements. No reasonable reader of
the principal’s instruction would think that it would render
optional (let alone prohibited) the teacher’s revisions to adhere
to baseline curricular mandates. Dictionaries confirm that
commonsense understanding. The Oxford English Dictionary,
for example, defines “to take into account” as “to take into
consideration as an existing element, to notice.” Oxford
English Dictionary (2d ed. 1989). And Merriam-Webster’s
describes to “take into account” as “to make allowance for.”
Take,         Merriam-Webster          Dictionary         Online,
http://www.merriam-webster.com/dictionary/take (last visited
April 10, 2020). In its periodic reviews of emission standards,
EPA must consider practical and technological advances, but
section 112(d)(6)’s parenthetical specification does not
undercut EPA’s obligation to “revise” an emission standard “as
necessary” to bring it into compliance with the Act’s own
definition in section 112(d)(1)-(3) of a minimally adequate
“emission standard.”
                               18
     EPA claims that its reading of section 112(d)(6) review as
confined to consideration of “developments in practices,
processes, and control technologies” finds support from our
2015 decision in Surface Finishing, but EPA correctly
concedes that “Surface Finishing and the precedent it follows
are not directly on point.” EPA Br. 26. In Surface Finishing,
we examined whether EPA’s section 112(d)(6) review of a
standard that already imposed MACT limits must ignore those
limits and proceed as if EPA were promulgating new ones from
scratch. See
 id. at 7-9. 
We held that section 112(d)(6)
permitted EPA to respond to intervening developments by
adjusting existing limits on toxics already subject to emission
standards without recalculating MACT floors for those toxics.
See
 id. at 8. 
Here, we address not a recalculation of existing
limits within a source standard, but whether the Act
necessitates revising source standards to include for the first
time MACT limits for many of the hazardous air pollutants the
source category emits. The section 112(d)(6) requirement that
EPA, when it undertakes its eight-year review, revise emission
standards “as necessary” means that EPA must conform them
to the basic requisites of “emission standards” under section
112, including by setting controls on previously unaddressed
hazardous air pollutants.

     EPA and Intervenor’s readings not only contravene the
text of section 112(d)(6), but effectively would deprive of
practical effect the Act’s specified processes for adding to or
subtracting from the statutory list of hazardous air pollutants,
and its direction to EPA to act within 18 months on a petition
to modify the list. 42 U.S.C. § 7412(b)(3)(A). The point of
prompt action to update the air toxics list is to ensure that
emission standards timely reflect new information about
hazards. Under EPA and Intervenor’s readings of section
112(d)(6), however, after the requisite swift action to list a new
pollutant or to remove a substance that need not be controlled,
                               19
the Agency could choose to ignore it indefinitely, even as EPA
updates other features of standards governing the very source
categories known to emit it. Such a result would contravene
Congress’ intent.

     The statutory text alone suffices to resolve this petition,
but we note that Petitioners’ reading of section 112(d)(6) is also
the only reading that comports with the Act’s overall structure
and purpose. A principal focus of the 1990 Clean Air Act
Amendments was to hasten EPA’s regulation of hazardous air
pollutants. As already discussed, Congress in the 1990
Amendments specified a series of ambitious deadlines for EPA
to promulgate emission standards within a decade—and with a
series of intermediate deadlines—for all sources of hazardous
air pollutants. It also set a two-year deadline for EPA to
promulgate emission standards from scratch for any newly
listed source category.
 Id. § 7412(c)(5). 
And it required
compliance with emission standards “as expeditiously as
practicable, but in no event later than three years after the
effective date of such standard.”
 Id. § 7412(i)(3)(A). 
Section
112(d)(6) review is the sole periodic, ongoing review of
emission standards the Act requires. Out of line with
Congress’s temporal vigilance to compel EPA’s prompt
promulgation of standards for every source category to limit
each air toxic it emits, EPA’s reading of the Act to allow but
not require the Agency to address previously uncontrolled air
toxics during a scheduled section 112(d)(6) review implausibly
leaves no statutory prompt for the completion of statutorily
deficient controls.

     Intervenor, for its part, denies even EPA’s claimed
authority to opt, in its discretion, to use the occasion of a
section 112(d)(6) review to add unlawfully omitted controls on
listed air toxics to a source category’s existing emission
standard. That reading makes no sense. Intervenor does not
                               20
deny that EPA has authority separately to promulgate new rules
to set the missing emission limits for regulated sources. See