 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued September 18, 2012          Decided November 9, 2012

                        No. 11-1113

  DESERT CITIZENS AGAINST POLLUTION AND SIERRA CLUB,
                     PETITIONERS

                              v.

   ENVIRONMENTAL PROTECTION AGENCY AND LISA PEREZ
     JACKSON, ADMINISTRATOR, U.S. ENVIRONMENTAL
                PROTECTION AGENCY,
                    RESPONDENTS

               NEVADA MINING ASSOCIATION,
                      INTERVENOR


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


    Seth L. Johnson argued the cause for petitioners. With
him on the briefs was James S. Pew.

    Justin Hayes, pro se, was on the brief as amicus curiae in
support of petitioners.

    Jon M. Lipshultz, Attorney, U.S. Department of Justice,
argued the cause and filed the brief for respondents.
                              2

     Denise W. Kennedy, John A. Bryson, Emily C. Schilling,
Michael A. Zody, and Jacob A. Santini were on the brief for
intervenor Nevada Mining Association in support of
respondents. Elizabeth A. Schulte entered an appearance.

    Before: SENTELLE, Chief Judge, GARLAND, Circuit
Judge, and WILLIAMS, Senior Circuit Judge.

   Opinion for the Court filed by Senior Circuit Judge
WILLIAMS.

     WILLIAMS, Senior Circuit Judge: Section 112(c)(6) of
the Clean Air Act requires action by the Environmental
Protection Agency on seven bioaccumulative hazardous air
pollutants (“HAPs”), each named specifically by Congress.
EPA is to list each pollutant’s sources and to “assur[e] that
sources accounting for not less than 90 per centum of the
aggregate emissions of each such pollutant are subject to
standards under subsection (d)(2) or (d)(4)” of § 112. Clean
Air Act § 112(c)(6), 42 U.S.C. § 7412(c)(6). In a rulemaking
effective February 17, 2011, EPA identified gold mine ore
processing and production as a source for purposes of
emissions of mercury, one of the seven HAPs named in
§ 112(c)(6). 76 Fed. Reg. 9450/1 (the “Gold Mine Rule”).

    In its response to comments, EPA took two positions
contested here by petitioners Desert Citizens Against
Pollution and Sierra Club. First, EPA rejected the claim that
§ 112(c)(6)’s cross-reference to § 112(d)(2) (in the instances
where (d)(2) rather than (d)(4) applies) requires that EPA
subject all HAPs emitted by a § 112(c)(6) source—even those
not enumerated in § 112(c)(6)—to standards at the stringency
level specified by § 112(d)(2). See 76 Fed. Reg. at 9457.
Second, EPA made clear that, despite language in the Gold
Mine Rule arguably suggesting that it covered “fugitive
emissions”—namely emissions from certain sources such as
                               3

“tailings ponds, leach fields, and waste rock piles”—in fact
the rule did not address such emissions. Id. at 9457/3-58/1.

     Petitioners timely challenged the rulemaking on both
issues. We address these claims in the above order, rejecting
both.


                            * * *

     Does § 112(c)(6) require EPA to impose the same
stringency levels in standards for non-§ 112(c)(6) HAPs
occurring at § 112(c)(6) sources that it does for § 112(c)(6)
HAPs? We start with a brief review of the statutory context.
In the early years of the Act, Congress left the choice of which
HAPs to regulate largely to EPA’s discretion. See New Jersey
v. EPA, 517 F.3d 574, 578 (D.C. Cir. 2008). But in 1990
Congress amended the Act to list 189 specific HAPs,
including mercury compounds, 42 U.S.C. § 7412(b)(1), and
then prescribed a two-step process whereby EPA would
regulate their emission. Under the first step, EPA lists
“major” and “area” sources of the HAPs, a distinction we
have discussed at length elsewhere. See, e.g., Nat’l Mining
Ass’n v. EPA, 59 F.3d 1351, 1353-54 (D.C. Cir. 1995).
(Briefly, “major sources” are those that emit 10 or more tons
of a specific HAP annually, or 25 or more tons of any
combination of HAPs, 42 U.S.C. § 7412(a)(1), and are
generally “subject to stricter regulatory control than are ‘area
sources,’” Nat’l Mining Ass’n, 59 F.3d at 1353. An “area
source” is “any stationary source of [HAPs] that is not a major
source,” 42 U.S.C. § 7412(a)(2); their listing and regulation is
more discretionary and context-dependent than is the case for
major sources. For example, under § 112(c)(3), “area sources
representing 90 percent of the area source emissions of the 30
[HAPs] that present the greatest threat to public health in the
largest number of urban areas” are subject to nondiscretionary
                              4

listing, whereas EPA “does not have to establish emission
standards for unlisted area sources.” Nat’l Mining Ass’n, 59
F.3d at 1353.) In the second step, EPA promulgates emission
standards pursuant to the procedures and criteria outlined in
various paragraphs of § 112(d), 42 U.S.C. § 7412(d).

    In the paragraph at issue here, § 112(c)(6), Congress
additionally singled out seven specific persistent,
bioaccumulative HAPs—some of them separately listed in
§ 112(b)(1)—and required EPA to list their sources and
promulgate emissions standards. In doing so, Congress did
not employ the distinction between “major” and “area
sources”:

    With respect to [the seven named HAPs] the
    Administrator shall, not later than 5 years after
    November 15, 1990, list categories and
    subcategories of sources assuring that sources
    accounting for not less than 90 per centum of the
    aggregate emissions of each such pollutant are
    subject to standards under subsection (d)(2) or
    (d)(4) of this section.

42 U.S.C. § 7412(c)(6).        Although, like § 112(c)(3),
§ 112(c)(6) imposes a special deadline for listing sources that
account for 90% of specified emissions (in the case of
§ 112(c)(3), emissions of the 30 most hazardous HAPs), it is
unique in denying EPA any choice in the selection of HAPs
chosen for special treatment.
   Section 112(d)(2), in turn, sets out a level of stringency
known as “maximum achievable control technology” or
“MACT”:
    Emissions standards promulgated under this
    subsection and applicable to new or existing sources
    of hazardous air pollutants shall require the
                               5

    maximum degree of reduction in emissions of the
    hazardous air pollutants subject to this section
    (including a prohibition on such emissions, where
    achievable) that the Administrator, taking into
    consideration the cost of achieving such emission
    reduction, and any non-air quality health and
    environmental impacts and energy requirements,
    determines is achievable for new or existing sources
    in the category or subcategory to which such
    emission standard applies. . . .
42 U.S.C. § 7412(d)(2) (emphasis added). “[M]ajor sources
must comply with . . . MACT standards.” Nat’l Mining Ass’n,
59 F.3d at 1353. “For listed area sources, EPA may choose to
promulgate emission standards requiring only ‘generally
available control technologies or management practices,’” or
GACT. Id.

     We review the competing statutory constructions under
the familiar standards of Chevron, U.S.A., Inc. v. NRDC, Inc.,
467 U.S. 837 (1984), first determining whether there is a
relevant textual ambiguity in the statute, and then, if there is,
deciding whether the implementing agency’s construction is
reasonable. Nat’l Cable & Telecomm. Ass’n v. Brand X
Internet Servs., 545 U.S. 967, 980 (2005) (citing Chevron, 467
U.S. at 843–44 & n. 11).

     Petitioners’ claim turns entirely on § 112(c)(6)’s cross-
reference to §§ 112(d)(2) and (d)(4). (More on § 112(d)(4)
momentarily.) Looking to the language of (d)(2) emphasized
above, petitioners argue that its phrase “emissions of the
hazardous air pollutants subject to this section” means that
whenever EPA creates MACT standards for § 112(c)(6) HAPs
for a source, it must similarly impose MACT standards for
emissions from that source of any HAP listed anywhere in
                               6

§ 112 (“this section”)—including the 189 HAPs listed in
§ 112(b)(1).
     Petitioners’ reading of the statute is linguistically
possible. After all, § 112(c)(6) directs EPA to assure that “the
emissions of each such pollutant [the seven § 112(c)(6) HAPs]
are subject to standards under subsection (d)(2) or (d)(4),” and
(d)(2) says that “[e]missions standards promulgated under this
subsection” must require MACT reductions “in emissions of
the hazardous air pollutants subject to this section,” which
would seem to mean all HAPs identified in § 112. Further,
we have read subparagraphs (1) and (3) of § 112(d) to require
the regulation of all HAPs listed in § 112(b)(1). See, e.g.,
Nat’l Lime Ass’n v. EPA, 233 F.3d 625, 633-34 (D.C. Cir.
2000), Sierra Club v. EPA, 479 F.3d 875, 883 (D.C. Cir.
2007). And, consistently with petitioners’ view of the phrase
“subject to this section” in § 112(d)(2), we have recently
recognized that Congress’s usual “hierarchical scheme in
subdividing statutory sections” refers to a section of the U.S.
Code, followed by subsections, paragraphs, subparagraphs,
and clauses. U.S. v. Hines, 694 F.3d 112, 118 (D.C. Cir.
2012) (citing Koons Buick Pontiac GMC, Inc. v. Nigh, 543
U.S. 50, 60–61 (2004)).
     But however linguistically possible petitioners’
interpretation, it is not unambiguously correct. The textual
ambiguity does not arise from § 112(d)(2), but from
§ 112(c)(6), and lies in the phrase “subject to standards under
subsection (d)(2) or (d)(4) of this section.”
      “Standards under subsection (d)(2)” could be given the
construction that petitioners advance, namely, that “standards
under” incorporates every word of (d)(2), thereby triggering
MACT standards for non-§ 112(c)(6) HAPs emitted by a
§ 112(c)(6) source. But alternatively Congress may have
plausibly intended simply to set MACT as the standard for the
seven § 112(c)(6) HAPs, as opposed to the less restrictive
                               7

GACT standard specified in § 112(d)(5). This reading makes
particular sense given that the usual criterion for selecting
MACT versus GACT standards—whether a source is “major”
or “area”—is missing from the framework established by
§ 112(c)(6).
     As EPA pointed out in the rulemaking, petitioners’
interpretation would have the anomalous effect of changing
the required stringency of non-§ 112(c)(6) HAPs at a given
area source—from the GACT level to the more demanding
MACT level—simply on the fortuity that the non-§ 112(c)(6)
HAPs in question shared a source with one or more
§ 112(c)(6) HAPs. 76 Fed. Reg. 9457/2. Thus a subsection
designed for seven HAPs that Congress thought deserved
special attention—a temporal priority and a demanding
stringency level—would, under petitioners’ view, require
EPA to apply those special rules to a broad array of HAPs
when they chanced to occur at a § 112(c)(6) source.
    A further curiosity of petitioners’ interpretation is that it
leaves the cross-reference to (d)(4) hanging. That subsection
reads, in full:
    With respect to pollutants for which a health
    threshold has been established, the Administrator
    may consider such threshold level, with an ample
    margin of safety, when establishing emission
    standards under this subsection.
42 U.S.C. § 7412(d)(4). Whereas the (d)(2) cross-reference
provides a linguistic hook for tightening the required
stringency of controls over non-§ 112(c)(6) HAPs at
§ 112(c)(6) sources, there is no comparable hook in (d)(4)—
no language equivalent to (d)(2)’s mandate to cover “the
hazardous air pollutants subject to this section.” Thus,
petitioners ask us to hold that Congress used (d)(2) to upshift
the required stringency for some non-§ 112(c)(6) HAPs, with
no similar upshift for § 112(c)(6) HAPs governed by (d)(4).
                                8

     Given that the language “standards under subsection
(d)(2)” might simply reflect Congress’s intention to set the
stringency level for § 112(c)(6) HAPs in a way the
architecture of the Act does not otherwise make obvious, and
that petitioners’ reading has the effect of tightening the
stringency of standards for non-§ 112(c)(6) HAPs from
sources that happen to emit § 112(c)(6) HAPs, the meaning of
§ 112(c)(6)’s “subject to standards under subsection (d)(2) or
(d)(4)” is ambiguous. EPA reasonably resolves the ambiguity
by reading the cross-references as simply supplying the level
of stringency for § 112(c)(6) standards—either MACT under
(d)(2) or “health threshold” under (d)(4).1 Congruently, it
sees the cross-references as saying nothing about the
standards governing non-§ 112(c)(6) HAPs when EPA sets
out to assure that the seven § 112(c)(6) HAPs “are subject to
standards” of the requisite type.
     Such an interpretation is not the only one available, as
EPA itself acknowledged. See 76 Fed. Reg. at 9457 (the
“language [of § 112(c)(6)] can reasonably be read to mean
standards . . . for all HAP emitted by the source.”) But our
duty is to accept the agency’s interpretation if it is “based on a
permissible construction of the statute.” Chevron, 467 U.S. at
843.
     We further note that petitioners’ view would seriously
risk undercutting the priority that Congress obviously
assigned the § 112(c)(6) HAPs. If the § 112(c)(6) cross-
references triggered a duty to impose more stringent standards
on non-§ 112(c)(6) HAPs at § 112(c)(6) sources, such a
triggering would almost certainly precipitate pushback from
the operators of such sources and slow the process of
imposing MACT standards on the § 112(c)(6) HAPs.

       1
         We do not know and need not address how the
“may” in (d)(4) is to be construed.
                               9

    For the reasons noted above, we find EPA’s interpretation
eminently reasonable.
     Does the Gold Mines Rule embrace fugitive emissions?
As we observed at the outset, fugitive emissions are ones from
sources such as “tailings ponds, leach fields, and waste rock
piles.” 94 Fed. Reg. at 9458/1. In its response to comments
EPA made clear that the rule would not address such
emissions.

     Prior to that response, the rule could be said to have left
some obscurity as to its coverage. The regulation declares,
“You are subject to this subpart if you own or operate a gold
mine ore processing and production facility as defined in
§ 63.11651, that is an area source.” 40 C.F.R. § 63.11640(a).
And § 63.11651 in turn defines such a facility as “any
industrial facility engaged in the processing of gold mine ore
that uses any of [a number of specified production
processes].” Id. § 63.11651. These definitions paint rather
broadly. But in another section, EPA appeared to narrow the
rule’s scope, saying first that “[t]his subpart applies to each
new or existing affected source,” 76 Fed. Reg. at 9480,
codified at 40 C.F.R. § 63.11640(b) (emphasis added), and
then defining “affected sources” as
    each collection of “ore pretreatment processes” at a
    gold mine ore processing and production facility,
    each collection of “carbon processes with mercury
    retorts” at a gold mine ore processing and
    production facility, each collection of “carbon
    processes without mercury retorts” at a gold mine
    ore processing and production facility, and each
    collection of “non-carbon concentrate processes” at
    a gold mine ore processing and production facility,
    as defined in § 63.11651.
Id. Petitioners do not contend that any of the “affected
sources” listed encompasses fugitive emissions.
                              10

     In response to petitioners’ comments advocating the
broader definition, EPA resolved any resulting ambiguity in
favor of the narrower definition, making the exclusion of
“fugitive emissions” from “affected sources” express rather
than implicit. It characterized affected sources more generally
as consisting of “the thermal processes that occur after ore
crushing, including roasting operations (i.e., ore dry grinding,
ore preheating, roasting, and quenching), autoclaves, carbon
kilns, electrowinning, preg tanks, mercury retorts, and
furnaces,” and excluding “tailings ponds, leach fields and
waste rock piles.” 76 Fed. Reg. at 9458.
     We review EPA’s interpretation of its previous rules even
more deferentially than we review its interpretation of
statutory ambiguity. We must give “controlling weight” to
the agency’s interpretation “unless it is plainly erroneous or
inconsistent with the regulation.” Thomas Jefferson Univ. v.
Shalala, 512 U.S. 504, 512 (1994) (internal quotation marks
and citations omitted).
     EPA’s prose may be inelegant, even recognizing that
drafting rules for mercury emissions from gold mines seems
unlikely to inspire elegance. But the basic structure is
plausible: gold mines are a broad concept and those who own
or operate them are indeed “subject to” this subpart, as
§ 63.11640(a) says. Having set out that broad term, however,
EPA can logically state that the subpart applies only to
affected sources, and then define such sources as including
considerably less than all activities at a gold mine.
     Thus, even assuming that before the clarification a reader
might have supposed the rule to cover fugitive emissions, its
resolution of the possible linguistic confusion was not “plainly
erroneous or inconsistent with the regulation,” Auer v.
Robbins, 519 U.S. 452, 461 (1997) (citation omitted), and is
entitled to our deference.
                              11

    Petitioners assert further that, assuming EPA’s rule
actually excluded fugitive emissions (as we have just held), its
explanations for doing so were arbitrary and capricious. These
arguments are without merit. EPA reasonably concluded that
the record before it provided insufficient information about
the quantity of fugitive emissions or available methods of
controlling them.
                            * * *

    The petition for review is therefore

                                                  Denied
