 United States Court of Appeals
         FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued May 5, 2015                    Decided July 31, 2015

                       No. 14-1206

              THE AMERICAN COAL COMPANY,
                      PETITIONER

                              v.

 FEDERAL MINE SAFETY AND HEALTH REVIEW COMMISSION
             AND DEPARTMENT OF LABOR,
                   RESPONDENTS


        On Petition for Review of a Decision of the
     Federal Mine Safety & Health Review Commission


    Jason W. Hardin argued the cause for petitioner. On the
briefs was Kevin N. Anderson.

     Jerald S. Feingold, Attorney, Mine Safety & Health
Administration, argued the cause for respondent. With him on
the brief was W. Christian Schumann, Counsel. John T.
Sullivan, Attorney, entered an appearance.

   Before: BROWN, GRIFFITH, and PILLARD, Circuit Judges.

   GRIFFITH, Circuit Judge:

    The American Coal Company was cited and fined for a
“fire” on one of its coal stockpiles when safety inspectors
                               2
from the Mine Safety and Health Administration observed
patches of smoldering, smoking coal without visible flames.
The Federal Mine Safety and Health Act of 1977 allows the
issuance of safety orders and the imposition of citations and
fines when a mine operator permits an “accident” to occur in
its facility, including a “mine fire.” American Coal argues that
the citation and fine should be vacated because a “fire,” for
purposes of the Mine Act, exists only when there are visible
flames. The company also contends that even if a fire could
exist without visible flames, there was insufficient proof here
to show a fire of any kind. We disagree on both points and
deny American Coal’s petition for review. The statutory term
“fire” is ambiguous, the Secretary of Labor reasonably
determined that the term does not require the presence of
flames, and substantial evidence supports the conclusion that
the smoldering patches on American Coal’s stockpile satisfied
the Secretary’s interpretation of a “fire.”

                               I

                               A

    Congress passed the Federal Mine Safety and Health Act
of 1977 (the Mine Act), Pub. L. No. 95-164, 91 Stat. 1290
(1977) (codified as amended at 30 U.S.C. § 801 et seq.), “to
provide more effective means and measures for improving the
working conditions and practices” in American mines “in
order to prevent death and serious physical harm” to miners.
30 U.S.C. § 801(c). The Mine Act assigned enforcement and
other powers to the Secretary of Labor and created within the
Department of Labor a new agency, the Mine Safety and
Health Administration (MSHA), to administer its provisions.
Meredith v. Fed. Mine Safety & Health Review Comm’n, 177
F.3d 1042, 1054 & n.12 (D.C. Cir. 1999).
                               3
    Mine inspectors from MSHA perform frequent,
unannounced inspections to ensure that mine operators
comply with the Mine Act and related safety standards. 30
U.S.C. § 813(a). An inspector who discovers that a mine
operator has violated a provision of the Mine Act or any
related safety standard must issue a citation. Id. § 814(a). The
Secretary is also required to assess civil penalties for each
violation. Id. § 820(a).

    In addition to citations, the Mine Act authorizes safety
inspectors to issue “safety orders” to ensure onsite safety “[i]n
the event of any accident occurring in a coal or other mine.”
Section 103(k), 30 U.S.C. § 813(k) (emphasis added). Safety
orders allow inspectors to wield broad authority as they deem
necessary. Under the Mine Act, the term “accident” is defined
to include “a mine explosion, mine ignition, mine fire, or
mine inundation, or injury to, or death of, any person.”
Section 3(k), id. § 802(k) (emphasis added). In other words, a
safety order under section 103(k) can only issue in the face of
an active, ongoing accident, of which a mine fire is but one
example. In this case, the safety inspectors justified the safety
orders based on their conclusion that the smoldering patches
they observed on the coal stockpile were a “fire.”

    The Mine Act provides a different type of authority to
inspectors when they discover an “imminent danger.”
“Withdrawal orders” require the mine operator to evacuate the
area in which the imminent danger exists. Section 107(a), 30
U.S.C. § 817(a). The Mine Act defines an “imminent danger”
as “any condition or practice in a coal or other mine which
could reasonably be expected to cause death or serious
physical harm before such condition or practice can be
abated.” Id. § 802(j).
                                4
    A mine operator may contest any citation, order, or
penalty before the Federal Mine Safety and Health Review
Commission (the Commission), a five-member body also
established by the Mine Act. The Commission appoints
administrative law judges (ALJs) to hear and decide the
dispute in the first instance. Either party to a dispute can
appeal any decision of an ALJ to the Commission.

                                B

    American Coal, a subsidiary of Murray Energy, operates a
coal mine complex in Galatia, Illinois, composed of two
underground mines: the New Millennium mine and the New
Future mine. Each mine maintains various surface operations,
including coal stockpiles where raw coal is stacked once it is
extracted from the mines.

    On January 19, 2010, two mine inspectors visited the
Galatia complex and found what they determined were signs
of “fire” at the New Future stockpile. As the inspectors later
testified, they observed five spots on the stockpile that emitted
smoke, radiated heat waves, and were covered in whitish ash
produced by heated coal. One inspector also testified that he
smelled an odor like sulfur. Neither inspector, however,
observed any visible flames, glowing coals, or any other kind
of illumination. The American Coal safety officer who
accompanied the inspectors later testified that he did not
believe the spots were smoldering, and characterized what the
inspectors called white ash as nothing more than gray rock
pulled from the mine.

   Relying on their observations, the inspectors issued safety
orders under section 103(k) of the Mine Act for the New
Future stockpile, giving them broad authority over the
operation until the “fire . . . presently burning in the coal pile”
                              5
was brought under control. J.A. 24. The inspectors also issued
a citation to American Coal for failing to report the accident,
and the Secretary of Labor later assessed a civil penalty in
connection with that citation.

    American Coal contested the orders, citation, and penalty.
American Coal and the Secretary agreed that the dispute
turned exclusively on the meaning of the word “fire” in the
Mine Act. American Coal argued that the inspectors were not
authorized to issue safety orders under section 103(k) because
mere smoldering combustion is not a “fire.” The ALJ agreed
with American Coal, ruling that the term “fire”
unambiguously required the existence of visible flame.
Because all agreed that there were no visible flames on the
New Future stockpile, the ALJ concluded that the safety
orders were unjustified.

    The Secretary appealed his decision to the Commission.
Before the Commission, the Secretary explained that he
interpreted “fire” to include both “events marked by flaming
combustion” and “events marked by smoldering combustion
that reasonably has the potential to burst into flames.” The
Secretary insisted that the spots the inspectors had observed
on the surface of the stockpile satisfied his interpretation of
“fire” because they were instances of smoldering combustion
that could have ignited at any time. Thus the question before
the Commission was whether the term “fire” in the Mine Act
was ambiguous and, if so, whether the Secretary’s
interpretation of it was reasonable.

    The Commission resolved that question in the Secretary’s
favor. The Commission pointed out that the term “fire” in the
statute was inextricable from the preceding term “mine,” as
the only fires at issue under the Mine Act were necessarily
those associated with mining. The Commission therefore
                               6
analyzed the Secretary’s interpretation within the overall
meaning of the statute instead of standing in isolation. The
Commission concluded that the term “fire” was ambiguous
and that the Secretary was free to interpret it to include both
fires involving visible flames and smoldering fires that had
the reasonable possibility of bursting into flames.

    One member of the panel dissented. Though he agreed
that the statute did not require the presence of visible flames
to constitute “fire,” he found the Secretary’s definition
impermissibly vague because it did not provide adequate
guidance regarding when a given patch of smoldering
combustion would present a reasonable risk of bursting into
flame. The dissent feared that the term “reasonably” included
in the Secretary’s definition was too “open to subjective
interpretations” and would prove “ultimately useless to
operators.” J.A. 327.

    On remand, a new ALJ (the previous ALJ having retired),
applied the Commission’s ruling and upheld the safety order.
The ALJ concluded that the spots the inspectors had observed
met the Secretary’s interpretation. American Coal appealed
this decision, but this time the Commission declined to review
the ALJ’s determination.

    American Coal timely petitioned for review, arguing that
the term “fire” is not ambiguous, that the Secretary’s
interpretation of the term is not reasonable, and that there was
not even sufficient evidence to support the Commission’s
conclusion that the Secretary’s interpretation was satisfied
here. We have jurisdiction over a final order of the
Commission under 30 U.S.C. § 816(a)(1). We review the
Commission’s legal conclusions de novo. Sec’y of Labor v.
Twentymile Coal Co., 456 F.3d 151, 156 (D.C. Cir. 2006). We
review the Commission’s findings of fact for substantial
                               7
evidence, meaning that we “determine whether there is such
relevant evidence as a reasonable mind might accept as
adequate to support the judge’s conclusion.” Jim Walter Res.,
Inc. v. Sec’y of Labor, 103 F.3d 1020, 1023-24 (D.C. Cir.
1997).

                               II

                               A

    As a threshold matter, American Coal insists that we
should reverse and remand without considering the merits of
this dispute because the Commission exceeded its authority
under the statute. The Mine Act forbids the Commission from
considering any question that was not first presented to the
ALJ. 30 U.S.C. § 823(d)(2)(A)(iii), (d)(2)(B). In the hearing
before the ALJ, the parties stipulated that their dispute turned
on whether there was a “fire” on the stockpile in the sense
used in section 3(k) of the Mine Act. In its decision, the
Commission focused on the larger statutory term “mine fire,”
concluding that “mine” provided indispensable context for
“fire” and that a “mine fire” could exist even without visible
flames. Because the Commission considered the meaning of
the term “mine fire,” as opposed to the meaning of the term
“fire” standing alone, American Coal believes that the
Commission improperly considered an argument the parties
had not briefed.

    We find this argument unpersuasive. “[A] reviewing court
should not confine itself to examining a particular statutory
provision in isolation. The meaning—or ambiguity—of
certain words or phrases may only become evident when
placed in context. It is a ‘fundamental canon of statutory
construction that the words of a statute must be read in their
context and with a view to their place in the overall statutory
                               8
scheme.’” FDA v. Brown & Williamson Tobacco Corp., 529
U.S. 120, 132-33 (2000) (internal citation omitted) (quoting
Davis v. Michigan Dept. of Treasury, 489 U.S. 803, 809
(1989)). See also Deal v. United States, 508 U.S. 129, 132
(1993) (observing that it is a “fundamental principle of
statutory construction (and, indeed, of language itself) that the
meaning of a word cannot be determined in isolation, but
must be drawn from the context in which it is used”). In other
words, the Commission did not consider a different question
than the meaning of “fire” in section 3(k). It considered the
question that the parties agreed was at issue and employed
standard interpretive tools to answer it, including looking to
the statutory context of the disputed term. The Commission
was well within its authority to do so.

                               B

     The Secretary’s interpretation of the Mine Act must “‘be
given weight by both the Commission and the courts’” under
the familiar two-step Chevron standard. Sec’y of Labor v.
Excel Mining, LLC, 334 F.3d 1, 5-6 (D.C. Cir. 2003) (quoting
Sec’y of Labor v. Cannelton Indus., Inc., 867 F.2d 1432, 1435
(D.C. Cir. 1989)). Under the first step of Chevron we consider
whether Congress has unambiguously addressed the question.
See Cannelton Indus., 867 F.2d at 1435. If not, we ask
whether the Secretary’s interpretation is reasonable. Id.
Especially in the context of a remedial health-and-safety act
like the Mine Act whose “primary purpose . . . [is] to protect
mining’s most valuable resource—the miner,” Int’l Union,
United Mine Workers v. Mine Safety & Health Admin., 823
F.2d 608, 617 (D.C. Cir. 1987) (internal quotation marks
omitted), we must “‘liberally construe[]’” the Act’s terms,
meaning that we are all the more “obliged to defer to the
Secretary’s miner-protective construction of the Mine Act so
                              9
long as it is reasonable.” Cannelton Indus., 867 F.2d at 1437
(internal quotation marks omitted).

     In the statutory scheme of the Mine Act, “the Secretary’s
litigating position before [the Commission] is as much an
exercise of delegated lawmaking powers as is the Secretary’s
promulgation of a . . . health and safety standard,” and so is
also deserving of deference. Excel Mining, 334 F.3d at 6
(alterations in original) (quoting RAG Cumberland Res. LP v.
Fed. Mine Safety & Health Review Comm’n, 272 F.3d 590,
596 n.9 (D.C. Cir. 2001)); cf. Martin v. Occupational Safety
& Health Review Comm’n, 499 U.S. 144, 156-57 (1991)
(explaining that the OSH Act’s analogous allocation of
responsibilities requires according Chevron deference to the
Secretary’s litigating positions).

                              1

    We conclude that “fire” as used in the Mine Act is
ambiguous because there are competing, plausible ways to
read the term.

    American Coal insists that there was widespread
agreement at the time the Mine Act was passed in 1977 that a
fire existed only when there were visible flames. To support
this proposition, American Coal cites several general-usage
dictionary definitions and a number of fire insurance cases
ranging from 1905 to 1969, which arguably identify fire
exclusively with the presence of flames. See, e.g., W. Woolen
Mill v. N. Assurance Co. of London, 139 F. 637, 639 (8th Cir.
1905) (“No definition of fire can be found that does not
include the idea of visible heat or light, and this is also the
popular meaning given to the word.”).
                               10
    The Secretary responds principally in two ways. First, he
cites a number of cases of similar vintage that explicitly
distinguish between “smoldering fires” and “flaming fires,” to
show that contemporary usage employed the term “fire” in
different ways. See, e.g., Triple A Machine Shop, Inc. v.
Waterman Steamship Co., 221 F.2d 916, 917 (9th Cir. 1955)
(“[A] fire so started in the [ship’s hold] would smoulder many
hours before it burst into flame.”); Ravenscroft v. United
States, 88 F.2d 418, 419 (2d Cir. 1937) (noting “the danger of
admitting air to a smoldering fire in cotton”); Petition of
United States, 105 F. Supp. 353, 359 (S.D.N.Y. 1952)
(“[Opening the hatch] created a strong circulation of air,
which fanned the smoldering fire into flame.” (internal
quotation marks omitted)). Second, the Secretary points to a
number of technical references focusing on mining and fire
prevention that distinguish smoldering fire from flaming fire.
See, e.g., NATIONAL FIRE PROTECTION ASSOCIATION, FIRE
PROTECTION HANDBOOK 2-18 (Gordon P. McKinnon & Keith
Tower eds., 14th ed. 1976) (“The observer can be sure there is
fire where flame can be seen. Flame is rarely separated from
the burning materials by any appreciable distance. However,
in certain types of smoldering fires without evidence of flame,
heat, smoke, and gas can develop.”); DICTIONARY OF MINING,
MINERAL, AND RELATED TERMS 246, 436 (1st ed. 1968)
(defining “fire” as “[f]uel in a state of combustion” and
defining “combustion” as “[t]he action or operation of
burning” that can be but is not necessarily “accompanied by
the generation of light and heat”).

    We agree with the Secretary. The parties have both
presented contemporary readings of the term “fire” that
support their position. For that reason, Chevron step one is
relatively straightforward here. “Confronting diverse readings
of the statutory text, we are obliged to defer to the Secretary’s
miner-protective construction of the Mine Act so long as it is
                               11
reasonable.” Cannelton Indus., 867 F.2d at 1437. The parties
have both provided competing uses of the term in
contemporary judicial decisions, showing that lawyers and
judges of the time sometimes understood “fire” to require
visible flames and sometimes understood that a “fire” could
exist even when there was only smoldering combustion. And
American Coal cannot successfully distinguish the
contemporary cases cited by the Secretary that clearly
differentiate between smoldering fires and flaming fires. True,
each of those cases involved a smoldering fire that was
succeeded by a flaming fire, and the damage that provoked
the dispute in each circumstance was caused by the later
flaming stage. But that merely underscores that in each case
the court considered the fire to have begun once smoldering
commenced, even though flames had not yet broken out. This
alone is reason enough to conclude that “fire” is ambiguous:
Congress may have meant to include only flaming fires in the
illustrative list of accidents in the Mine Act, but it may also
have intended that list to include smoldering fires as well.

    We also note that the term “fire” is identified in the statute
merely as one item in an inclusive list designed to illustrate,
not comprehensively enumerate, the various forms of
“accident” that can justify issuing a safety order under section
103(k). See 30 U.S.C. § 802(k) (stating that an “accident,” for
purposes of the Mine Act, “includes a mine explosion, mine
ignition, mine fire, or mine inundation, or injury to, or death
of, any person” (emphasis added)); see also Burgess v. United
States, 553 U.S. 124, 131 n.3 (2008) (“[T]he word ‘includes’
is usually a term of enlargement, and not of limitation.”
(internal quotation marks and citation omitted)). In other
words, Congress enacted the Mine Act to create a
comprehensive scheme empowering the Secretary and his
mine inspectors to respond rapidly and flexibly to risks to
miner safety. And as we have already pointed out, the Mine
                              12
Act is a remedial health-and-safety statute, meaning that its
terms are to be read broadly to offer maximum protection for
miner safety. It would be senseless, in this context, to read a
single term in the statute’s inclusive, illustrative list of
possible accidents in the narrowest possible way, based on a
cherry-picked selection of contemporary decisional law, so as
to preclude the Secretary from adopting a reasonable
construction that increased the safety of miners. On this basis,
we are satisfied that there are competing, plausible
interpretations of the term “fire,” and so find it ambiguous.
See Cannelton Indus., 867 F.2d at 1437.

     It is true, as American Coal points out, that the general-
usage dictionaries from the period when Congress passed the
Mine Act, define fire only as flaming combustion. But these
general-usage dictionaries do not change our view that the
term “fire” is ambiguous in the Act. General-usage
dictionaries cannot invariably control our consideration of
statutory language, especially when the “dictionary definition
of . . . isolated words[] does not account for the governing
statutory context.” Bloate v. United States, 559 U.S. 196, 205
n.9 (2010). After all, “‘[t]he plainness or ambiguity of
statutory language is determined [not only] by reference to the
language itself, [but as well by] the specific context in which
that language is used, and the broader context of the statute as
a whole.’” Yates v. United States, 135 S. Ct. 1074, 1081-82
(2015) (plurality opinion) (quoting Robinson v. Shell Oil Co.,
519 U.S. 337, 341 (1997) (alterations in Yates)); see also id.
at 1092 (“[W]e interpret particular words in their context and
with a view to their place in the overall statutory scheme. And
sometimes that means . . . that the dictionary definition of a
disputed term cannot control.” (Kagan, J., dissenting)
(internal quotation marks and citation omitted)). Though our
assessment of the ambiguity of statutory text sometimes
begins and ends with the definitions provided in
                              13
contemporary general-usage dictionaries, on other occasions
it is useful and important to consult more technical sources
where, as here, the statute focuses on a specific technical
context. The Mine Act is designed to secure and enhance the
safety of miners in and around mines. The characteristics of
fire that matter for the purposes of this statute are those
relevant in the context of mining and industrial safety. And
the general-usage dictionaries American Coal cites cannot and
do not account for these particular characteristics. The
Secretary, on the other hand, has provided technical
resources, also from the period when Congress passed the
Act, that define fire in the specific context of mining and
industrial safety. For example, the Fire Protection Handbook
that the Secretary has produced unmistakably supports the
Secretary’s position by identifying the distinct risks
associated with “smoldering fires.” NATIONAL FIRE
PROTECTION ASSOCIATION, FIRE PROTECTION HANDBOOK 4-
34 (George H. Tryon & Gordon P. McKinnon eds., 13th ed.
1969); see also NATIONAL FIRE PROTECTION ASSOCIATION,
FIRE PROTECTION HANDBOOK 2-18 (Gordon P. McKinnon &
Keith Tower eds., 14th ed. 1976) (same). And the Dictionary
of Mining, Mineral, and Related Terms identifies “the
generation of light and heat” merely as an “example” of the
attributes of fire, not as a necessary precondition for fire to
exist. DICTIONARY OF MINING, MINERAL, AND RELATED
TERMS 246, 436 (1st ed. 1968); see also DICTIONARY OF
MINING, MINERAL, AND RELATED TERMS 114 (2d ed. 1997)
(same). In other words, paying attention to the context of the
Mine Act, the exchange of contrasting definitions from
various dictionary sources provides further basis to conclude
that the term “fire” is ambiguous.

   American Coal argues that the structure of the Act as a
whole unambiguously requires that the term “fire” cover only
combustion that displays visible flames. Specifically,
                              14
American Coal claims that the Secretary’s interpretation is
clearly foreclosed by the interaction between the two types of
orders inspectors can issue to deal with mine accidents: safety
orders under section 103(k) and withdrawal orders under
section 107(a). American Coal argues that withdrawal orders
under section 107(a) are designed to deal with conditions that
pose a future risk of danger, while safety orders under section
103(k) are designed to deal only with ongoing or completed
accidents, not their prevention. In American Coal’s view, a
smoldering fire is a pre-accident condition, dangerous only
because it poses the risk that it will ignite and become a
flaming fire. Thus, American Coal insists that section 103(k)
should not permit inspectors to issue safety orders to control
smoldering fires because 103(k) orders are authorized only to
confront actively occurring accidents, not to prevent future
accidents. Instead, inspectors should be authorized to deal
with smoldering fires only by issuing withdrawal orders under
section 107(a) because that section offers appropriate
authority for prophylactic measures.

    We disagree with American Coal’s view of the statute.
For one thing, we cannot agree that smoldering combustion is
nothing more than a pre-fire state, important only because it
might burst into flames in the future. Just the opposite. As the
Secretary has explained, the self-heating properties of coal
mean that coal stockpiles can begin smoldering and reach
high temperature points without igniting, generating
substantial heat and smoke that can imperil miners in the
vicinity even without bursting into flames. Worse, smoldering
combustion consumes coal just as surely as does flaming
combustion. Patches of smoldering combustion can thus eat
away at the stockpile from within, creating a hidden cavity
that can destabilize the stockpile as a whole or into which a
miner can fall. A smoldering fire poses active risks and can
                              15
constitute an “accident occurring” on a coal stockpile in its
own right. 30 U.S.C. § 813(k).

    Nor do we think American Coal’s account of the structure
of the Mine Act is correct. Section 103(k) orders are broad,
flexible tools, authorizing inspectors to confront many
different circumstances that present immediate risks. For
example, once safety orders are issued under section 103(k),
inspectors often modify them to change the requirements
imposed on mine operators as the accident evolves. See
Performance Coal Co. v. Fed. Mine Safety & Health Review
Comm’n, 642 F.3d 234, 237 (D.C. Cir. 2011) (“It is
undisputed that [section] 103(k) orders undergo frequent
modifications.”). Inspectors sometimes issue section 103(k)
safety orders first, while trying to deal with an accident, and
only thereafter issue section 107(a) withdrawal orders to shut
the mine down completely. See, e.g., Clinchfield Coal Co. v.
Fed. Mine Safety & Health Review Comm’n, 895 F.2d 773,
774 (D.C. Cir. 1990). Most obviously, section 103(k) safety
orders allow inspectors to impose whatever restrictions or
requirements they judge appropriate to deal with the accident
in question, while section 107(a) withdrawal orders simply
close the mine. In short, section 107(a) is an emergency
blunderbuss, unsubtle and extreme, for circumstances in
which getting miners out and away is the only appropriate
response. Section 103(k), which the inspectors used here, is a
subtler instrument that can be tailored to any situation.

     There is no risk that finding the term “fire” ambiguous
will destabilize the statute. To the contrary, allowing the
Secretary to wield section 103(k) orders in a broader range of
circumstances accords with the statute’s structure by making a
flexible, nuanced tool available to handle accidents while they
happen but before they become critical.
                              16
    To sum up, the contemporary body of decisional law
shows competing definitions of the term “fire” in a variety of
contexts. General-usage and technical dictionary definitions
from the period when Congress passed the Mine Act offer
support for both sides of the debate. And finding ambiguity in
this statutory term does not pose a risk to the structure of the
statute but rather will conform to that structure by enabling
the Secretary to use the flexible tool of section 103(k) safety
orders in an appropriate range of circumstances. Because the
parties to this dispute have shown that the term “fire” is
susceptible to multiple plausible interpretations, we find it
ambiguous and move on to consider whether the Secretary’s
interpretation warrants deference.

                               2

    Under Chevron step two, we defer to the Secretary’s
interpretation of an ambiguous term unless it is unreasonable
or inconsistent with the statute. The Secretary’s interpretation
contains two elements. To qualify as “fire,” non-flaming
combustion must qualify as “smoldering” combustion, and it
must also present a reasonable chance of bursting into flame.
We find both elements of this interpretation perfectly
reasonable.

     First, nothing in the statute expressly prohibits the
Secretary from reading “fire” to cover smoldering
combustion. And as we explained in our analysis under
Chevron step one above, there is substantial authority in
judicial decisions and relevant technical references to support
the conclusion that smoldering combustion qualifies as a type
of “fire” whether it displays visible flames or not. It comports
with the goal of protecting miner safety not to ignore an entire
category of fire simply because it does not show flames.
                               17
     We also conclude that the Secretary was entitled to limit
his interpretation of “fire” to include only smoldering fires
that reasonably could ignite at any time. The Secretary has
explained that the risks posed by a smoldering fire cross a
critical line that warrants regulation when it reaches the point
at which it might burst into flame. At that point, the Secretary
has concluded, a smoldering fire’s danger to miners is
significant enough to require regulation. That conclusion is
reasonable. By limiting the scope of his authority to
smoldering fires that reasonably could burst into flame at any
time, as opposed to all smoldering combustion, the Secretary
allows “operators and inspectors [to] focus their attention
where it will do the most good” and avoids “unduly or
unnecessarily burden[ing] operators or keep[ing] inspectors
from attending to other important matters.” Resp. Br. 43. This
explanation adequately justifies the Secretary’s decision to
limit the scope of his oversight.

    American Coal raises three challenges to the
reasonableness of the Secretary’s interpretation. None
succeed. First, American Coal argues that the interpretation is
unconstitutionally vague, leaving mine operators unable to
comply and vulnerable to arbitrary and capricious
enforcement, which, it submits, is a very ineffective way to
promote miner safety. The Secretary rejoins that the
interpretation is adequately specific because it limits its scope
to smoldering combustion that “reasonably” might ignite. The
Secretary is confident that “reasonable” mine operators,
experienced in the industry and well-schooled in the
characteristics of coal and its propensity to self-heat and
ignite, will be able to comply.

    We agree. There is no doubt that the Secretary has
provided limited direction. But an interpretation need not be
prolix to avoid impermissible vagueness. It must merely
                               18
provide sufficient guidance so that reasonable regulated
parties, aware of the goal the regulation seeks to accomplish,
have “fair warning” of what the regulation requires. Freeman
United Coal Mining Co. v. Fed. Mine Safety & Health Review
Comm’n, 108 F.3d 358, 362 (D.C. Cir. 1997). This
interpretation passes that bar. After all, the opinions that will
matter in enforcing this standard are those of mine safety
inspectors and mine operators who see smoldering patches on
coal stockpiles with great regularity and have extensive
experience in recognizing those patches of smoldering
combustion that might soon ignite. We are confident that
reasonable mine operators and reasonable safety inspectors
will prove able to implement the Secretary’s standard in
practice.

    Second, American Coal argues that the Secretary failed to
provide a reasonable explanation for his decision to limit his
interpretation of “fire” to cover only smoldering combustion
that reasonably has the potential to burst into flames. We
disagree. Based on his experience, the Secretary concluded
that once smoldering fires have reached the point at which
ignition is imminent, they pose risks to the miner significant
enough to constitute an active accident. As we have already
explained above, this conclusion was reasonable. American
Coal seems to suggest that the Secretary could only
reasonably define “fire” to include all smoldering combustion
as well as all flaming combustion. But there is no basis for
that position. After all, an agency need not target every danger
in order to target any danger. See, e.g., Pers. Watercraft
Indus. Ass’n v. Dep’t of Commerce, 48 F.3d 540, 544 (D.C.
Cir. 1995) (“An agency does not have to make progress on
every front before it can make progress on any front.”
(internal quotation marks omitted)). And agencies may
marshal their limited resources by pursuing their goals “as
priorities demand.” Nat’l Cong. of Hispanic Am. Citizens (El
                               19
Congreso) v. Marshall, 626 F.2d 882, 888 (D.C. Cir. 1979).
In light of these principles, the Secretary has provided a
reasonable explanation for the scope of his interpretation of
“fire.”

    We acknowledge, as American Coal points out, that none
of the technical treatises the Secretary has cited define fire by
pointing to the reasonable possibility of ignition. But the
Secretary has adequately explained his reasoning in limiting
the scope of “fire” under the statute to cover only smoldering
combustion that reaches the point at which ignition is an
immediate risk. The fact that a dictionary or manual does not
make that distinction in no way invalidates the Secretary’s
otherwise reasonable explanation for adopting it.

    Finally, American Coal argues again that the Secretary’s
interpretation of the statute is foreclosed by the interaction
between section 103(k) and section 107(a) of the Mine Act.
We reject this argument for the reasons we have already
stated.

    The Secretary’s interpretation of “fire” is reasonable. It
furthers the statute’s purpose, provides adequate guidance for
its implementation, and conforms harmoniously to the
statute’s text and structure. We defer to the Secretary.

                               C

     Separately, American Coal argues that, even if the
Secretary’s interpretation of the term “fire” is reasonable, the
Commission erred in finding that interpretation satisfied here
because there was insufficient evidence either that the patches
on the stockpile were instances of smoldering combustion or
that those patches could reasonably burst into flame at any
time. Under the deferential standard of review we use to
                              20
evaluate the Commission’s factual determinations, we reject
both arguments. See Jim Walter Res., Inc., 103 F.3d at 1023-
24.

                              1

    Substantial evidence supported the Commission’s
determination that the safety inspectors observed patches of
smoldering combustion. One mine safety inspector, Wendell
Crick, testified that he observed smoking or smoldering areas
and whitish ash, smelled a sulfur-like odor, and observed heat
waves rising from the smoldering areas. The other inspector
also testified that he saw smoldering patches. American Coal
sought to counter this evidence with the competing testimony
of its supervisor, who claimed that what Crick saw was
nothing more than gray rock. We conclude that the
Commission was justified to rely on the testimony of the mine
safety inspectors over that of American Coal’s witness.

    American Coal attacks Crick’s testimony by insisting that,
when he discussed visible heat waves rising from the patches
on the coal stockpile, he was merely speaking generically
about the phenomena generally associated with smoldering
coal, not any observations he actually made at the time. But
when pressed about what he saw rising from the stockpile,
Crick responded, “You can see the heat waves and . . . a
whitish coat of ash around the areas that smoke was rising
from.” J.A. 103. The Commission was entitled to determine
from this testimony, especially Crick’s reference to “the areas
that smoke was rising from,” that Crick was in fact reporting
his observations, not speaking in the abstract. Id. After all,
when reviewing an agency determination for substantial
evidence the question is not whether the challenger’s
construction is plausible but whether the record can support
the agency’s conclusion. See Fla. Gas Transmission Co. v.
                               21
FERC, 604 F.3d 636, 645 (D.C. Cir. 2010). The
Commission’s determination satisfies that standard.

    American Coal also argues that the inspectors’ testimony
was not reliable because the Secretary did not show that either
held advanced academic degrees or had completed scientific
studies relevant to the self-heating properties of coal. There is
no basis for this challenge. Crick had extensive experience in
coal mining as a foreman, safety analyst, and surface
operation instructor. He had been a mine safety inspector for
three years, and he had extensive volunteer firefighter training
and experience. We need not determine the minimum
credentials a mine inspector must have for his judgments to
provide evidence on which an ALJ can permissibly rely.
Crick’s credentials are adequate to justify the Commission’s
reliance on his observations.

                                2

    We also find that substantial evidence supports the
Commission’s determination that the smoldering patches
reasonably had the potential to burst into flame.

     In the original hearing, Crick testified that in his judgment
“if the oxygen or the wind blows or . . . an amount of air hit[]”
the smoldering patches “just right,” they could “burst into
flame spontaneously at any time.” J.A. 107-08. American
Coal presented no evidence controverting Crick’s assessment.
The Commission was entitled to credit Crick’s testimony and
rely on it to conclude that the smoldering patches satisfied the
Secretary’s interpretation.

     American Coal argues that Crick’s testimony contradicts
itself. Crick testified that the wind was blowing while he was
inspecting the New Future stockpile, yet the smoldering
                               22
patches had not ignited. Thus his claims about the ignitibility
of the smoldering patches must be false. But Crick did not say
that the smoldering patches he observed would necessarily
ignite whenever the wind blew. The fact that there was wind
does not mean that the patches were exposed to a sudden
increase in oxygen level sufficient to trigger their ignition.

    American Coal also argues that Crick’s testimony should
not have been credited because the first ALJ rejected his
testimony after seeing him testify in person, while the second
ALJ reversed and credited his testimony on remand based
only on the transcript of the hearing. Not so. The first ALJ
concluded only that Crick’s testimony did not warrant “great
weight,” not that it was incredible. And that determination
was a natural one given that he had already rejected the
Secretary’s interpretation of the term “fire.” Crick’s testimony
regarding the smoldering combustion he observed only makes
a difference if smoldering combustion can qualify as a “fire.”
Because the ALJ had rejected that construction, it made little
difference that Crick had testified that “a fire could start up at
any time.” After the Commission held that the ALJ was
wrong to reject the Secretary’s interpretation, Crick’s
testimony was of course far more valuable. Thus on remand
the second ALJ had a blank slate to evaluate the credibility of
Crick’s testimony. And an ALJ’s credibility determination is
“entitled to great deference.” Sec’y of Labor v. Keystone Coal
Mining Corp., 151 F.3d 1096, 1107 (D.C. Cir. 1998). We
cannot say that the second ALJ erred in any way when she
relied on Crick’s uncontradicted testimony, backed up by
Crick’s experience, that the patches on the stockpile that day
posed the risk of bursting into flame.

   We acknowledge that there was little evidence presented
on whether the smoldering piles might soon burst into flames.
But “[t]he substantial evidence inquiry turns not on how many
                              23
discrete pieces of evidence the Commission relies on, but on
whether that evidence adequately supports its ultimate
decision.” Fla. Gas Transmission, 604 F.3d at 645. Crick’s
testimony was enough to show that the Secretary’s
interpretation was satisfied. As we have already noted, the
question we ask when evaluating agency action under the
substantial evidence standard is not whether the petitioner can
reasonably read the evidence another way but only whether
the agency was reasonable to read the evidence the way it did.
Id. The Commission’s determination on this score satisfies
that standard.

                              III

     For the foregoing reasons, we deny the petition for
review.
