 United States Court of Appeals
        FOR THE DISTRICT OF COLUMBIA CIRCUIT



Argued November 16, 2015             Decided July 5, 2016

                       No. 14-1285

 ROSEBUD MINING COMPANY AND PARKWOOD RESOURCES,
                         INC.,
                     PETITIONERS
                          v.
    MINE SAFETY AND HEALTH ADMINISTRATION AND
  JOSEPH A. MAIN, ASSISTANT SECRETARY OF LABOR FOR
              MINE SAFETY AND HEALTH,
                    RESPONDENTS

                       No. 14-1286

         CANYON FUEL COMPANY, LLC, ET AL.,
                     PETITIONERS
                          v.
     MINE SAFETY AND HEALTH ADMINISTRATION AND
    JOSEPH A. MAIN, ASSISTANT SECRETARY OF LABOR
            FOR MINE SAFETY AND HEALTH,
                     RESPONDENTS


        On Petitions for Review of Decisions of the
  Assistant Secretary of Labor for Mine Safety and Health


        Ralph Henry Moore II argued the cause for the
petitioners. Patrick W. Dennison was with him on brief.
                              2
       Lynne B. Dunbar, Attorney, United States Department
of Labor, argued the cause for the respondents. W. Christian
Schumann, Counsel, was with her on brief.

        Before: HENDERSON, ROGERS and KAVANAUGH,
Circuit Judges.

    Opinion for the Court filed by Circuit Judge
HENDERSON.

     KAREN LECRAFT HENDERSON, Circuit Judge: Several
coal mine operators—Rosebud Mining Company, Parkwood
Resources, Inc., Canyon Fuel Company, LLC, Mountain Coal
Company, LLC, Bowie Resources, LLC and Peabody Sage
Creek Mining, LLC (collectively, petitioners)—seek review
of two orders of the United States Department of Labor
(Labor)—per its Mine Safety and Health Administration
(MSHA)—modifying the application of mandatory mine
safety standards to their respective mines. The petitioners
contend that the orders contain three conditions unnecessary
to ensure adequate mine safety, thus making them arbitrary
and capricious. For the reasons set forth below, we deny the
petitions for review.
                      I. BACKGROUND

    Under section 101(a) of the Federal Mine Safety and
Health Act of 1977 (Mine Act), 30 U.S.C. §§ 801 et seq., the
Labor Secretary must promulgate “mandatory health or safety
standards for the protection of life and prevention of injuries
in coal or other mines.” 30 U.S.C. § 811(a). The Assistant
Secretary of Labor for Mine Safety and Health (Assistant
Secretary)1 may grant mine-specific modifications of the

    1
       For MSHA matters, the Labor Secretary acts through the
Assistant Secretary. 29 U.S.C. § 557a.
                                  3
standards if he finds that “an alternative method of achieving
the result of such standard exists which will at all times
guarantee no less than the same measure of protection
afforded the miners of such mine by such standard, or that the
application of such standard to such mine will result in a
diminution of safety to the miners in such mine.” Id.
§ 811(c). Thus, the statute permits modification if an equally
effective alternative exists or if the standard itself negatively
affects mine safety.2 To satisfy either option, MSHA
conducts a two-step inquiry which asks, first, whether the
proposed alternative “promote[s] the same safety goals as the
original standard with no less than the same degree of
success” and, second, whether the “modification would
achieve a net gain, or at least equivalence, in overall mine
safety.” United Mine Workers of Am., Int’l Union v. MSHA,
928 F.2d 1200, 1202 (D.C. Cir. 1991) (S. Ohio Coal Co.)
(emphasis added).3 At the second step, “both advantages and

     2
       The latter scenario seems counter-intuitive—MSHA plainly
does not intend to harm miners—but can be conceptualized as
follows: assume arguendo that MSHA requires all elevator shafts
to be manually operated, reasoning that elevators with electrical
components could spark and start a mine fire. An operator with an
especially deep shaft might argue that the requirement nonetheless
results in a diminution in mine safety because a manual elevator is
relatively slow and, in a mine disaster, could prevent miners from
surfacing quickly. For another example, see Int’l Union, United
Mine Workers of Am. v. Fed. Mine Safety & Health Admin., 924
F.2d 340 (D.C. Cir. 1991) (Quarto Mining).
     3
       We have concluded that the “diminution of safety” clause
requires only that the Assistant Secretary determine “whether
application of a particular mandatory safety regulation would be
unsafe” and that “the Assistant Secretary need not balance the
efficacy of the existing rule against the net benefits produced by the
proposed modification,” Quarto Mining, 924 F.2d at 343, basing
our interpretation on the Assistant Secretary’s practice at the time.
                                4
disadvantages of the alternative method” are weighed,
including those that are unrelated to the original standard. Id.
The party seeking modification has the burden of proof to
establish that the proposed modification complies with
section 811(c). 30 C.F.R. § 44.30(b).

     The modification process begins with an operator’s filing
a petition for modification with MSHA. See id. § 44.10.
After an investigation, the MSHA Administrator issues a
proposed order. Id. § 44.13. The operator may request a
hearing before an administrative law judge (ALJ), id.
§§ 44.14, 44.15, 44.20, who, after investigation/hearing,
issues his decision, id. § 44.32. Any party—including
MSHA—may then appeal to the Assistant Secretary. Id.
§ 44.33. The Assistant Secretary’s order may contain “special
terms and conditions” which “shall have the same effect as a
mandatory safety standard.” Id. § 44.4(c). “Only a decision
by the Assistant Secretary [is] final agency action for
purposes of judicial review.” Id. § 44.51.

     These six petitions for review involve MSHA’s
“permissibility” requirements, which, in general, mandate that
certain equipment located in certain mine areas be approved
by MSHA (i.e., that they be permissible). The focus of the
permissibility requirements is to “assure that [electrically
operated] equipment will not cause a mine explosion or mine

See id. at 344. The record reveals some confusion, however, about
whether MSHA now applies the Southern Ohio Coal Co. test to
both statutory options or to the first only. Compare ALJ’s Decision
and Order at 14, Rosebud Mining Co., Case Nos. 2010-MSA-1,
2011-MSA-2, -11, -12 (Dep’t of Labor Apr. 11, 2013) (Rosebud
ALJ Order I) with Assistant Secretary’s Decision and Order at 13,
Case Nos. 2010-MSA-1, 2011-MSA-2, -11, -12 (Dep’t of Labor
Nov. 14, 2013) (Rosebud Order I). Because the petitioners do not
raise this issue, we do not reach it.
                                 5
fire . . . . ” 30 C.F.R. § 75.2; see also Administrator’s
Proposed Decision and Order at 5, Canyon Fuel Co., Docket
No. M-2009-025-C (Dep’t of Labor May 6, 2011) (“MSHA
requirements for permissible . . . equipment are intended to
prevent mine explosions from unpredicted methane
accumulations, methane outbursts, or float coal dust in
suspension by removing a possible ignition source.”). MSHA
does not define “non-permissible” but its definition of
“permissible” substantially illuminates the former.
Permissible equipment includes, as relevant here, “completely
assembled electrical machine[ry]” for which MSHA has
issued “a formal approval.” 30 C.F.R. § 18.2. Thus,
electrical equipment without this approval is non-permissible
and, accordingly, unauthorized in certain mine areas.4 Not all
mine equipment is subject to the permissibility scheme—for
example, “[m]echanical surveying equipment,” which “poses
no risk of ignition,” requires no modification order for use.
Rosebud ALJ Order I at 5.5 For our review, the permissibility
scheme breaks down into three categories:           (1) non-
permissible equipment to which the non-use in certain mine
areas restriction applies; (2) non-permissible equipment with
a MSHA modification which removes the non-use restriction
and (3) equipment (like mechanical surveying equipment) for

    4
      The parties stipulated that “[t]he concern with any electrical
equipment is that if used in an explosive atmosphere it will produce
a spark, fire or heating with enough energy that an ignition of
methane and/or coal dust may result, possibly leading to a fire or
explosion.” Stipulations ¶ 19, In re Rosebud Mining Co., Docket
Nos. 2010 MSA-1, 2011 MSA-2, 2011-MSA-12, 2011 MSA-11
(Dep’t of Labor Mar. 28, 2013) (hereinafter Stipulations).
    5
      Relatedly—although not relevant for our review—MSHA
deems permissible “intrinsically safe” equipment, that is,
equipment “incapable of releasing enough electrical or thermal
energy . . . to cause ignition.” 30 C.F.R. § 18.2.
                                  6
which no modification order is needed to authorize its use in
certain mine areas.6

     The petitioners sought to use non-permissible equipment
and petitioned for modification of the following MSHA safety
standards: (1) 30 C.F.R. § 75.500, the standard requiring,
inter alia, that electrical equipment used in or inby the last
crosscut7 constitute permissible equipment, (2) Id. § 75.507-1,
the standard requiring that electrical equipment used in return

     6
       As the parties stipulated, there is a difference between
category two equipment and “permissible” equipment. The parties
refer to category two equipment as “permitted” equipment—
meaning it is “non-permissible equipment allowed to be used at a
particular mine pursuant to the granting of a petition for
modification.” Stipulations ¶ 29. By contrast, permissible
equipment has “a formal approval [without conditions] . . . issued
by MSHA[].” Id.
     7
        Throughout the record, this area of the mine is referred to as
“in or inby the last open crosscut.” See, e.g., Rosebud Order I at 17
(emphasis added); see also FMC Wyoming Corp., v. MSHA, 16
FMSHRC 1787, 1994 WL 445344, at *4 (Aug. 16, 1994) (“the
term ‘last open crosscut’ is interchangeable with ‘last crosscut’ ”).
MSHA defines “[t]he area of a coal mine inby the last open
crosscut” as the “working place.” 30 C.F.R. § 75.2. The parties
stipulated that the “the ‘last open crosscut’ is the last crosscut
without a permanent stopping in a line of pillars containing the
permanent stoppings that separate the intake air courses and the
return air courses. This area includes the most advanced mining
area in the mine, where the ventilating air reaches the areas of
active coal removal and deepest penetration and starts its course
back out of the coal mine.” Stipulations ¶ 10. “ ‘Inby’ refers to
something facing the direction of the coal face. Conversely,
‘outby’ refers to the direction facing the mine entrance (the
surface).” Andalex Res., Inc. v. MSHA, 792 F.3d 1252, 1254 n.2
(10th Cir. 2015).
                                  7
airways8 constitute permissible equipment and (3) Id.
§ 75.1002, the standard requiring that electrical equipment
used within 150 feet of pillar workings or longwall faces9
constitute permissible equipment. In short, the modification
petitions sought authorization to use non-permissible
equipment in three mine locations where use of the equipment
was otherwise off-limits. Each of the three described
locations is “more likely to have an explosive environment”
than other mine areas, thus triggering the applicable standard.
Assistant Secretary’s Decision and Order at 27, Canyon Fuel
Co., Case Nos. 2011-MSA-00006 to 00009, 2011-MSA-
00014 to 00021, 2013-MSA-00012, -00024, -00025, -00037
(Dep’t of Labor Nov. 24, 2014) (Canyon Fuel Order).

     A. MSHA PROCEEDINGS REGARDING ROSEBUD AND
                    PARKWOOD

     Petitioners Parkwood Resources and Rosebud Mining
filed identical modification petitions in December 2008 and
January 2009.10 Each operator sought to use non-permissible
     8
       Return air is “[a]ir that has ventilated the last working place
on any split of any working section or any worked-out area whether
pillared or nonpillared. If air mixes with air that has ventilated the
last working place on any split of any working section or any
worked-out area, whether pillared or nonpillared, it is considered
return air. For purposes of § 75.507–1, air that has been used to
ventilate any working place in a coal producing section or pillared
area, or air that has been used to ventilate any working face if such
air is directed away from the immediate return is return air.” 30
C.F.R. § 75.301.
     9
      Pillar workings and longwall faces are simply “areas in
which miners extract coal.” Andalex Res., 792 F.3d at 1254.
     10
       The Parkwood and Rosebud petitions were subsequently
consolidated at the administrative level and we follow suit,
hereinafter referring to them as the Rosebud petitioners.
                                  8
equipment—specifically, battery-powered (i.e., electrical)
surveying instruments—11in or inby the last open crosscut and
in return airways. See 30 C.F.R. §§ 75.500; 75.507-1. They
maintained that the two applicable safety standards hampered
both their ability to accurately and quickly map the mines—
resulting in a “diminution of safety” to miners, see 30 U.S.C.
§ 811(c)—12as well as their compliance with other MSHA
regulations, see 30 C.F.R. § 75.372 (requiring “up-to-date
map of the mine drawn to a scale of not less than 100 nor
more than 500 feet to the inch”), id. § 75.1200 (requiring
mine operator to maintain “accurate and up-to-date map” of
mine “in a fireproof repository located in an area on the
surface of the mine”), and state law, see 52 PA. STAT. ANN.
§ 690-224 (requiring “professional quality map of the mine on
a scale of not less than 200 feet to the inch”), that require
current and accurate mine maps. To obtain the modification,
the Rosebud petitioners proposed seven conditions on their
use of the NPESE, see generally S. Ohio Coal Co., 928 F.2d
at 1202 (alternative must “promote the same safety goals as
the original standard with no less than the same degree of




     11
       This equipment is hereinafter referred to as non-permissible
electronic surveying equipment (NPESE).
     12
       According to all six petitioners, accurate surveying is critical
because it “prevents intersection of the mine with abandoned
working of other mines which may contain water in large
quantities, explosive gas or the absence of oxygen.” Pet’rs’ Br. at
12. Surveying is also necessary to avoid “sealed areas,” id. at 13,
which areas MSHA subjects to regular “monitoring.” See 30
C.F.R. § 75.336.
                                 9
success”), one of which—no use when float coal dust13 is in
suspension14—is of particular relevance to our review.15

                1. ADMINISTRATOR’S DECISION

     The Rosebud petitioners’ “diminution of safety”
argument pressed that the NPESE was needed in order to
accurately map mines because of its ability to obtain
measurements superior to non-electric (mechanical) surveying
equipment.       The Administrator rejected the Rosebud
petitioners’ arguments for two reasons. First, he determined
that “when using [NPESE] the equipment need not be taken
into return air or inby the last open crosscut if the surveying is
carefully coordinated with the mining activity.”
Administrator’s Proposed Decision and Order at 5, Parkwood
Res. Inc., Docket No. M-2008-054-C (Dep’t of Labor Jan.
29, 2010). In other words, he found that the Rosebud
petitioners could use their preferred surveying tools—the
NPESE—without violating the permissibility regulations
because they did not need to use the equipment in the areas to
which the permissibility regulations apply.             Thus the
regulations restricting the areas into which the operators could

    13
        “Float coal dust” is defined as “coal dust consisting of
particles of coal that can pass a No. 200 sieve.” 30 C.F.R.
§ 75.400–1(b).
    14
       MSHA regulations do not define the term “in suspension”
but the parties stipulated that it means dust “suspended in the air
during mining.” Stipulations ¶ 33.
    15
        The other conditions included: (1) regular examination of
the NPESE, (2) continuous monitoring for methane during use of
NPESE, (3) mandatory shutdown if methane concentration reaches
a certain level, (4) changing and charging of batteries in fresh air,
(5) proper training of personnel using NPESE and (6) use of
NPESE after MSHA inspection only.
                               10
use NPESE did not impair the miners’ ability to map the
mines to the desired accuracy level and likewise did not
(because of inaccurate mapping) result in a diminution of
safety. Second, the Administrator determined that “levels of
accuracy fully capable of protecting miners can be achieved
using optical non-electric surveying equipment”—i.e.,
mechanical equipment—and “can achieve even higher levels
of accuracy . . . through repetition of measurements and
statistical applications.” Id. Thus, to him, use of NPESE was
not necessary.

     In addition, the Administrator found the proposed
conditions duplicative because many of them simply tracked
MSHA regulations; those that did not were found insufficient
because they failed to ensure an adequate level of safety.
Thus, the Rosebud petitioners’ proposed conditions did not
“promote the same safety goals as the original standard with
no less than the same degree of success.” See S. Ohio Coal
Co., 928 F.2d at 1202. Regarding the proposed float coal dust
ban, the Administrator found that its implementation was
impossible unless mining were to cease during surveying.

                      2. ALJ’S DECISION

    The Rosebud petitioners sought ALJ review. The ALJ
held two separate hearings on the consolidated petitions,
made findings of fact and issued his decision on April 11,
2013.

     The ALJ first explained how methane and coal dust can
result in a mine fire. First, he observed that methane is
explosive at an aerial concentration between five and fifteen
per cent. According to him, coal dust can also result in a
mine fire but that, in order to ignite, the dust must be “in
suspension . . . [and] sufficiently thick that you couldn’t see a
light bulb that was turned on about four feet in front of you.”
                              11
Rosebud ALJ Order I at 6 (alterations and quotations
omitted). He next recognized that mechanical surveying
equipment “poses no risk of ignition” and that, although
NPESE does present such a risk, nonetheless “it has a low
potential for ignition.” Id. at 5. For support on the latter
point, the ALJ relied on the testimony of MSHA electrical
engineer Chad Huntley and fire-and-explosion expert Noah
Ryder. Huntley estimated “the possibility that both the
methane detector would fail and the electronic surveying
equipment would ignite at the same time is one in ten
thousand.” Id. at 4. Ryder testified that the potential for a
coal dust ignition “inside one of the[] [NPESE]” was
“nonexistent” because, through water immersion and dust
swab tests, he found that dust would “settle
on . . . component[s]” in the devices and, “if it settled there,
it’s not in suspension and won’t ignite.” Id. at 6 & n.9
(emphasis added). Ryder also testified that NPESE was less
dangerous than other equipment MSHA has approved via
modification petitions.

     Some findings were in apparent tension with others. For
example, Rosebud surveying manager Michael Groff testified
that NPESE “does not get hot when it’s running” and that he
had “never seen a spark or arc when removing the battery.”
Id. at 5 n.6. But Huntley and Ryder both testified that
sparking could occur when “the battery was physically
disconnected” or if “an inside component broke.” Id. at 6.
Huntley testified that NPESE could “overheat . . . and ignite
methane” but also noted that it had “a thermal breaker for de-
energizing the battery pack at a temperature below the
ignition temperature for methane.” Id. at 5 n.6 (emphasis
added).    Some NPESE equipment also came with a
manufacturer safety warning indicating that it should not be
used in an underground coal mine and that an explosion could
                                12
result if so used.16 Because the manufacturer was unable to
testify as to the basis of the warning, however, the ALJ gave it
no weight. The ALJ also recognized that Rosebud had been
using NPESE “in all areas of [its] mine[s]” for over 20 years
and that MSHA, by not issuing any citation during that time,
had “tacitly approved [its] use.” Id. at 13.

     The ALJ, concluding that mechanical surveying
equipment was “obsolete, far less accurate than electronic
surveying equipment, and above all, not realistically available
on the commercial market except in used condition,” id. at 2,
approved the petitions. He anticipated that the conditions he
set out in his order “promote[d] the same safety goals as the
original standard with no less than the same degree of
success.” Id. at 14 (quoting S. Ohio Coal Co., 928 F.2d at
1202). The ALJ’s conditions were substantially similar to
those contained in the petitions, including the prohibition on
surveying in the presence of float coal dust. He added a
requirement that the Rosebud petitioners gradually phase out
old equipment so that, within five years, the NPESE in use
would be no more than five years old. The ALJ thought this
condition would “prevent the degradation of [NPESE] seals”
through which float coal dust could enter and cause ignition.
Id. at 17. He observed that his conditions closely replicated
those included in an earlier MSHA consent decree allowing
NPESE. Id. at 4 n.5; see Initial Decision Approving
Settlement and Order of Dismissal at 2–4, Twentymile Coal
Co., Case No. 2007-MSA-00002 (Dep’t of Labor Dec. 5,
2007) (Twenty Mile Consent Order).


    16
        Specifically, the warning stated: “Safety Cautions; Warning;
May ignite explosively. Never use an instrument near flammable
gas, liquid matter, and do not use in a coal mine.” Rosebud ALJ
Order I at 3.
                                13
      The ALJ also concluded that “granting [the] petitions for
modification would engender a net gain in miner safety.”
Rosebud ALJ Order I at 15 (emphasis in original); see also S.
Ohio Coal Co., 928 F.2d at 1202 (asking whether
“modification would achieve a net gain, or at least
equivalence, in overall mine safety”), because, although
“mechanical surveying equipment can meet . . . accuracy
requirement[s],” “the use of mechanical equipment may
require multiple set ups, increasing the length of surveyors’
exposure to hazardous conditions.” Rosebud ALJ Order at
15. Moreover, “mechanical parts cannot be reliably calibrated
or repaired . . . [and] surveyors are not currently trained in
their use. . . . Therefore, application of the [permissibility]
standard[s] is less safe than application of the modification, as
it is unsafe to use equipment that is not calibrated or repaired
properly, or that surveyors have not been trained to use.” Id.
Finally, he reasoned that NPESE “is 8-10 times more accurate
than mechanical equipment” and “greater accuracy leads to
increased safety in the mines.” Id.17

              3. ASSISTANT SECRETARY’S DECISION

     The Administrator appealed the ALJ’s order to the
Assistant Secretary who, applying a de novo standard of
review, conducted an independent analysis of the evidence
and rejected many of the ALJ’s factual findings. For

    17
        The ALJ made no finding regarding diminution of safety,
treating the case as one arising under the first prong of 30 U.S.C.
§ 811(c) (asking whether “an alternative method of achieving the
result of such standard exists which will at all times guarantee no
less than the same measure of protection afforded the miners of
such mine by such standard”). But, as noted, see supra n.3, the
Rosebud petitioners do not challenge MSHA’s application of both
section 101(c)’s “alternative method” option and its “diminution of
safety” option to their petitions.
                               14
example, although MSHA never sanctioned Rosebud for its
20-year use of NPESE, the Assistant Secretary declined to
conclude that MSHA had thus tacitly approved thereof in
view of the fact that Rosebud produced no evidence that
MSHA knew of the use; moreover, MSHA had sanctioned
other operators for similar use. The Assistant Secretary also
disputed Ryder’s opinion that the Rosebud petitioners’
NPESE was “well-sealed against [methane] gas and [coal]
dust” ingress because Ryder had tested “none of . . . the
specific instruments that [the Rosebud petitioners] identified
in [the] petitions.” Rosebud Order I at 28–29. Moreover, the
Assistant Secretary found Ryder’s assertion that he tested
substantially similar equipment “suspect” given Ryder’s
failure to “take apart any of the specific instruments identified
in the petitions” to determine their similarity vel non. Id. at
29. In addition, the Assistant Secretary credited Huntley’s
testimony that tended to discredit Ryder’s tests—specifically,
that, according to International Electrotechnical Commission
standards, “ingress protection tests” using “dust and moisture”
were not proper surrogates for gas. Id. at 30. And, even
assuming Ryder’s tests were fair proxies, “moisture was
detected inside all of the pieces of used equipment that Ryder
tested.” Id.

     The Assistant Secretary also rejected the ALJ’s
characterization of some of Huntley’s testimony.             For
instance, the “one-in-ten-thousand probability” of both the
“methane detector failing and the electronic surveying
equipment igniting” was based on a premise with which
Huntley explicitly disagreed. Id. at 28–29 n.12. The
Assistant Secretary also rejected the ALJ’s Ryder-supported
conclusion that coal dust did not present an ignition concern.
Although “Ryder testified that coal dust . . . would settle on a
component and not remain in suspension”—thus, not
igniting—Huntley testified that coal dust can “enter non-
                               15
permissible electronic equipment, layer itself on internal
components, and cause the equipment to overheat and ignite
methane.” Id. at 32. The Assistant Secretary also disagreed
with the ALJ’s conclusion that, “because the equipment has
internal thermal breakers that are designed to de-energize the
battery pack at a temperature below the ignition temperature
of methane, coal dust layering on the internal
components . . . is not a concern,” id. at 32–33, because, the
Assistant Secretary opined, “thermal breakers can fail, and
there [wa]s no evidence concerning their reliability,” id. at 33.
Moreover, he noted the likelihood of a coal dust-based
explosion even in the absence of the required aerial
concentration because coal dust can “be rapidly placed in
suspension, [and] even a vigilant surveyor may not have the
time to de-energize his instrument before it encounters an
explosive concentration of coal dust.” Id.

     Finally, the Assistant Secretary disagreed with the ALJ
on the importance of the NPESE warning. Although the
manufacturer was unable to explain the reason for the
warning, “[the Rosebud petitioners], not the Administrator,
ha[d] the burden of proof in th[e] proceeding.” Id. at 34
(citing 30 C.F.R. § 44.30(b)).

    On November 14, 2013, the Assistant Secretary issued
his decision upholding the ALJ’s modification grant but
substantially modifying and tightening the conditions. In
addition to prohibiting NPESE use when float coal dust was
in suspension, the Assistant Secretary required that coal
production shut down while the equipment was used in or
inby the last open crosscut and in return air and that, if
“viable” mechanical equipment became available, use of
NPESE must cease. Rosebud Order I at 50. With these
conditions in place, the Assistant Secretary concluded that the
modification “promotes the same safety goals as [the
                                16
standards] with no less than the same degree of
safety. . . . [and] that the overall effect of the proposed
alternative method, including the modifications . . . will
achieve at least a net least [sic] equivalence in overall mine
safety.” Id. at 14 (applying S. Ohio Coal Co. test, 928 F.2d at
1202).

     The Assistant Secretary remanded to the ALJ to consider
two conditions for which the record contained insufficient
support (and which are not before us on appeal). The ALJ
subsequently approved a consent agreement applying four
new conditions (in lieu of the remanded pair) and the Rosebud
petitioners then appealed to the Assistant Secretary to renew
their objections to the originally disputed conditions and to
facilitate judicial review therefrom.18 See 30 C.F.R. § 44.51
(“Only a decision by the Assistant Secretary [is] final agency
action for purposes of judicial review.”). On November 24,
2014, the Assistant Secretary issued Rosebud Order II, once
again rejecting the Rosebud petitioners’ arguments.

    The Rosebud petitioners argued in the second round
before the Assistant Secretary that three of the unchanged
requirements “[we]re unnecessary to meet [the modification]
standard.” Rosebud Order II at 3. It was undisputed that,
with the Assistant Secretary’s conditions, the modification
grant “guarantee[d] no less than the same measure of
protection afforded the miners of such mine by” the
permissibility standards, see S. Ohio Coal Co., 928 F.2d at
    18
         The Administrator asserted that the Rosebud petitioners’
objections “essentially reargue[d] matters already unsuccessfully
litigated” and the Assistant Secretary accordingly treated them “in
the nature of a motion for reconsideration.” Assistant Secretary’s
Decision and Order at 3–4, Rosebud Mining Co., Case Nos. 2010-
MSA-1, 2011-MSA-2, -11, -12 (Dep’t of Labor Nov. 24, 2014)
(Rosebud Order II).
                                 17
1202. The Rosebud petitioners argued that cessation of coal
production while surveying took place was unnecessary
because (1) “surveying will not be conducted in an entry
where production is occurring,” Rosebud Order II at 4; (2)
“surveying will not be set up close to the face” of the mine,
id.; (3) “surveying generally will be upwind of the . . . mining
machine, and, even when it is downwind, methane and [coal]
dust will be removed by the ventilation system” and other
safeguards, id. at 4–5; (4) “surveyors spend minimal time in
or inby the last open crosscut or in the return,” id. at 7; (5)
“surveying equipment . . . does not [cut into or] liberate
methane or generate coal dust,” id.; and (6) the ALJ-imposed
condition that, “if one percent methane is detected,” use of
NPESE was to cease, was sufficient to protect against
methane explosions, id. at 8.

     The Assistant Secretary was not persuaded.              He
concluded that the first, second and fourth objections relied on
factual assertions rebutted by the record.19 He found the third
objection “d[id] not offset the decrease in safety from using”
NPESE because the ventilation system and other safety
features were “present whether surveyors use mechanical,
permissible, or non-permissible surveying equipment.” Id. at
5–6. Further, he reasoned that “ventilation systems do not
always work effectively and [that] operators do not always
comply with ventilation requirements.” Id. at 6. He rejected
the fifth objection because, although it “might mean that the
risk of using non-permissible surveying equipment is less than

    19
        See id. at 4 n.2 (Rosebud surveyors testified only that
“usually we coordinate ourselves in different entries”) (emphasis in
original); id. n.3 (“Rosebud Surveying Manager Groff testified that
he has taken shots as close as 50 feet from the face.”); id. at 7 n.4
(“Groff . . . acknowledged that he does not always set up in the
middle of the entry.”).
                              18
the risk of using other types of non-permissible equipment,” it
did not mean that NPESE was safe. Id. at 7–8. Finally, the
Assistant Secretary criticized the methane monitoring
condition because the “detectors may fail” and because there
“is a lag time in methane detectors and that if there were a
sudden inundation of methane, by the time the methane
detector registered one percent methane, and by the time the
surveyor reacted to shut the surveying equipment off, there
might already be an explosive amount of methane
surrounding the equipment.” Id. at 8.

     The Rosebud petitioners also argued that the prohibition
on surveying when float coal dust existed was both unclear
and unnecessary and that the requirement to switch to
“viable” mechanical surveying equipment if it became
available was unreasonable. Regarding the first claim, the
Rosebud petitioners asserted that float coal dust in suspension
always exists. But, as the Assistant Secretary observed, the
condition could be implemented if production ceased.
Moreover, he clarified and interpreted the condition to allow
for a “visual determination of whether there is float coal dust
in suspension.” Id. at 11 n.7. As to the latter objection, the
Assistant Secretary explained that mechanical equipment
would be viable if “sufficiently accurate for use in
underground mines” and that MSHA’s resources should not
be spent on ensuring the NPESE’s compliance with
conditions if viable mechanical equipment—i.e., equipment
that can be used without conditions—exists. Id. at 15.

  B. MSHA PROCEEDINGS REGARDING CANYON FUEL AND
       MOUNTAIN COAL (CANYON FUEL PETITIONERS)

    On July 15, 2009 petitioners Canyon Fuel and Mountain
Coal filed nearly identical petitions for modification, seeking
to use NPESE in or inby the last crosscut, in return airways
                                 19
and within 150 feet of pillar workings and longwall faces.
See 30 C.F.R. §§ 75.500, 75.507-1, 75.1002. As did the
Rosebud petitioners, Canyon Fuel and Mountain Coal claimed
that the mandatory standards resulted in diminution in miner
safety and inability to meet mapping requirements and they
proposed comparable conditions, with one exception (the float
coal dust condition was omitted). The Administrator denied
the petitions for reasons substantially similar to his denial of
the Rosebud petitioners’ petitions.

      The MSHA ALJ held a hearing on the consolidated
Canyon Fuel and Mountain Coal petitions and released a
decision on April 3, 2014.20 In light of the intervening
Rosebud Order I, MSHA agreed that the petitions should be
granted if the Assistant Secretary’s conditions set forth in
Rosebud Order I were imposed. See ALJ’s Decision and
Order at 7, Canyon Fuel Co., Docket Nos. 2011-MSA-00006
to 00009, 00014 to 00021, 2013-MSA-00024, -00025, -00037
(Dep’t of Labor April 3, 2014) (“The issues have evolved
since the petitions were first filed. No longer is the issue . . .
whether       the    proposed      modification   should      be
granted . . . . The question now is simply what conditions are
necessary.”). The ALJ subsequently revised the Rosebud
Order I conditions—as applied to Canyon Fuel—in three
significant respects.

    First, he found that it was “not appropriate” to disallow
NPESE if and when “viable new mechanical surveying
equipment” became available. Id. at 13–14. To him, the

    20
        Petitioners Peabody Sage Creek and Bowie Resources had
similar petitions pending and filed a letter with the ALJ agreeing to
be bound by his decision in the Canyon Fuel case. Canyon Fuel
references hereinafter include not only Canyon Fuel and Mountain
Coal but also Peabody Sage Creek and Bowie Resources.
                                20
accuracy of mechanical surveying equipment—even,
apparently, “viable” mechanical surveying equipment—was
inferior and reduced miner safety. He also found the ban on
surveying when float coal dust was in suspension “vague and
ambiguous” because the condition did not include a
measurement of float coal dust and because surveying would
be “impossible”—due to “visibility restrictions”— long
before an explosive quantity was in suspension. Id. at 20.
Finally, he narrowed the restriction on surveying during coal
production, requiring only that surveying not occur at “the
longwall or a working face during production.” Id. at 23.
The Administrator appealed once more to the Assistant
Secretary who issued a final order simultaneously with the
Rosebud II Order with identical conditions based on
materially similar reasoning.

    Both sets of operators timely filed petitions for review.21
Our jurisdiction arises under section 101(d) of the Mine Act.
30 U.S.C. § 811(d).22

                        II.     ANALYSIS

    Our review of the Assistant Secretary’s two final orders
is pursuant to the Administrative Procedure Act, that is, we

    21
        The Rosebud petitioners, however, did not petition for
modification of 30 C.F.R. § 75.1002 (permissibility requirement for
“equipment . . . located within 150 feet of pillar workings or
longwall faces”). With this exception, both sets of petitioners
challenge the same conditions and are therefore hereinafter referred
to as the petitioners. Because Canyon Fuel made no discrete
argument regarding 30 C.F.R. § 75.1002, we reject its challenge
thereto without more.
    22
       Both sets of petitioners filed a consolidated brief and we
likewise consolidate the petitions for disposition.
                              21
determine “whether the granting of the petition for
modification was arbitrary, capricious, an abuse of discretion
or otherwise not in accordance with the law.” Int’l Union,
United Mine Workers of Am. v. MSHA, 830 F.2d 289, 292
(D.C. Cir. 1987) (Emerald Mine Corp.) (citing 5 U.S.C.
§ 706(2)(A)). This “[h]ighly deferential” standard, AT&T
Corp. v. FCC, 349 F.3d 692, 698 (D.C. Cir. 2003), is
especially applicable when we review “technical
determinations on matters to which the agency lays claim to
special expertise.” Bldg. and Const. Trades Dep’t v. Brock,
838 F.2d 1258, 1266 (D.C. Cir. 1988); see also Int’l Union,
United Mine Workers of Am. v. MSHA, 407 F.3d 1250, 1258
(D.C. Cir. 2005) (Jim Walter Res., Inc.) (equivalent safety
determination is within Assistant Secretary’s expertise). We
uphold the agency if it “considered the relevant factors and
articulated a rational connection between the facts found and
the choice made.” Nat’l Ass’n of Clean Air Agencies v. EPA,
489 F.3d 1221, 1228 (D.C. Cir. 2007) (internal quotation
marks omitted). Because the challenged orders involve “an
area within the [Assistant] Secretary’s expertise,” Jim Walter
Res., Inc., 407 F.3d at 1258, and because they are supported
by “substantial evidence and . . . a reasoned explanation,”
Bldg. and Const. Trades Dep’t., 838 F.2d at 1266, we deny
the petitions for review.

    The thrust of the petitioners’ argument is that the three
above-discussed conditions—the requirement that coal
production cease while surveying with NPESE occurs in or
inby the last open crosscut, in return air or within 150 feet of
longwall faces or pillar workings (high risk areas), the bar on
surveying with NPESE when float coal dust is in suspension
and the instruction to use viable mechanical surveying
equipment if it becomes available—are unnecessary and
                                 22
therefore arbitrary and capricious.23 But the Assistant
Secretary weighed the relevant factors—whether the
alternative “promote[s] the same safety goals as the original
standard with no less than the same degree of success” and
whether it improves “overall mine safety,” S. Ohio Coal Co.,
928 F.2d at 1202—and “articulated a rational connection
between the facts found and the choice made,” Nat’l Ass’n of
Clean Air Agencies, 489 F.3d at 1228 (internal quotation
marks omitted). In so concluding, we note that “the Mine Act
and its standards require redundant safety measures.”
Rosebud Order II at 6 (emphasis added).

                 A. CESSATION OF PRODUCTION

    It is uncontested that the condition requiring coal
production to stop while the NPESE is used in high risk areas
enhances mine safety. What is at issue is whether this

     23
       The petitioners also contend that the Assistant Secretary’s de
novo review of the ALJ orders and factual findings is ultra vires.
Section 101(c) of the Mine Act provides that a petition for a
modification hearing is subject to section 554 of the Administrative
Procedure Act (APA). 30 U.S.C. § 811(c). Section 554 of the
APA in turn cross-references section 557 which provides that “[o]n
appeal from or review of [an] initial decision, the agency has all the
powers which it would have in making the initial decision.” 5
U.S.C. § 557(b) (emphasis added); see also Kay v. FCC, 396 F.3d
1184, 1189 (D.C. Cir. 2005) (“The law is settled that an agency is
not required to adopt the credibility determinations of an
administrative law judge.”); id. (agency not in position analogous to
appellate court reviewing trial court). We have suggested that
findings dependent on “demeanor of witnesses” must be “given
special weight,” Mathew Enter., Inc. v. NLRB, 498 F. App’x. 45, 46
(D.C. Cir. 2012) (quoting 2 Richard J. Pierce, Jr., Administrative
Law Treatise § 11.2 (5th ed. 2010)) (emphasis added), but
demeanor is not at issue here.
                               23
condition is unnecessary and, indeed, whether it is so
unnecessary as to fail arbitrary and capricious review. The
objections of the petitioners break down into the following
groups: (1) surveying equipment is not used to mine coal, (2)
use of the NPESE must stop if the methane level approaches a
level well below its explosive threshold, (3) even while
production is ongoing, the NPESE will not come in contact
with methane and coal dust, (4) the NPESE has a slight
potential for ignition, (5) it is unlikely methane or coal dust
will enter the NPESE compartments that contain electrical
components, (6) previously approved modification petitions
manifest that this condition is unnecessary and (7) the
manufacturer’s warning about use of NPESE in coal mines
was “not probative,” Pet’rs’ Br. at 61. We address the
objections in seriatim.

1. Surveying equipment is not used to mine coal

     The petitioners argue that the Assistant Secretary failed
to appreciate the differences between NPESE and other—
riskier—mine equipment. For example, they claim that he
failed to account for the fact that the NPESE does not cut
coal, that it is peripheral in the mining process and that it does
not cause methane to disperse or coal dust to be in suspension.
But the Assistant Secretary addressed this argument. He
reasoned that “[a]lthough these circumstances . . . might mean
that the risk of using non-permissible surveying equipment is
less than the risk of using other types of non-permissible
equipment, nothing in the record convinces me that the
circumstances would sufficiently offset the dangers of using”
NPESE in high risk areas. Rosebud Order II at 7–8 (emphasis
added). Moreover, MSHA has, by regulation, applied its
permissibility requirements to equipment other than that
which “cuts into coal.” Canyon Fuel Order at 41. See, e.g.,
30 C.F.R. § 75.500(d) (“All . . . electric face equipment which
                               24
is taken into or used inby the last crosscut of any coal mine”
must be permissible) (emphasis added). The petitioners’
contention that the NPESE—although non-permissible—is
relatively safe suggests only that this condition is less
necessary than others, not that it is arbitrary. In addition, the
Assistant Secretary noted that the petitioners used the Twenty
Mile consent order, Case No. 2007-MSA-00002 (Dep’t of
Labor Dec. 5, 2007), as a template for their petition and
Twenty Mile included the same condition.

2. Methane detection and shutdown requirement guards
against explosions

     The petitioners next contend that, because the ALJ
imposed a condition that operators cease using NPESE if the
methane level reaches a 1% concentration and, because a 5%
concentration is the minimum concentration necessary for
ignition, the requirement that production cease during NPESE
use is arbitrary. The Assistant Secretary amply rebutted this
argument.      He noted that although the 1% methane
concentration condition “provide[s] some protection from the
increased risk of a methane ignition posed by using non-
permissible equipment . . . [it is] not enough.” Rosebud Order
I at 35–36. As he explained, the record indicated that
methane detectors are not always properly calibrated and also
may fail. Moreover, he cited testimony that a “lag time”
exists between an increase in methane concentration and its
detection. Rosebud Order II at 8. Thus, if there were a
“sudden inundation of methane,” the methane detector might
not register it before an explosive quantity accumulated near
the NPESE. Id.

3. NPESE will not encounter methane or float coal dust

    The petitioners next contend that, as a matter of practice,
surveying generally does not occur in areas where methane
                                  25
and coal dust are present and that, even when it does, the
ventilation systems will prevent an explosion. First, we note
that much of this argument is equivocal.24 To second-guess
the Assistant Secretary on this ground would require us to
weigh the evidence de novo and usurp MSHA’s statutorily
conferred authority to determine whether a specific mine
hazard—once its existence is conceded—is substantial
enough to impose restrictions. See, e.g., Partington v. Houck,
723 F.3d 280, 291 (D.C. Cir. 2013) (“we do not substitute our
judgment for that of the agency or evaluate de novo” its
factual findings).

    In any event, the Assistant Secretary adequately
addressed the objection with a reasoned explanation. First, he
observed that the record was ambiguous about whether
surveying sometimes occurred in the areas the petitioners
claimed to avoid.25 Moreover, he observed that nothing in the


     24
       See, e.g., Pet’rs’ Br. at 44 (there is “little or no exposure to
either” dust or methane) (emphasis added); id. at 45 (in “most
instances, the surveying equipment will be positioned upwind of
the continuous miner and thus not exposed in any way to methane
or dust”) (emphasis added); id. (“surveyors are generally upwind of
the entry where production is occurring”) (emphasis added); id. at
46 (“it is clear that the instrument will not often be in close
proximity downwind of the continuous miner”) (emphasis added).
     25
        See Rosebud Order II at 4 n.2 (“Although initially stating
that he did not survey in the entry where the continuous miner is
mining, . . . Groff then testified that ‘usually we coordinate
ourselves in different entries.’ ” (emphasis in original)); id. at n.3
(“The evidence does not support Rosebud’s assertion that surveying
is not conducted close to the face. . . . Groff testified that he has
taken shots as close as 50 feet from the face.”); id. at 7 n.4 (“The
evidence does not support Rosebud’s assertion that surveying
equipment is always used in the middle of the entry. . . . [Groff]
                                  26
ALJ orders “require[d] that the equipment be used” only in
the areas identified by the petitioners—i.e., in different mine
entries, a sufficient distance from the face or in the middle of
mine entries. Canyon Fuel Order at 41–42. Regarding
whether surveying often or always occurred upwind of
production, the Assistant Secretary noted the same ambiguity,
i.e., that the petitioners occasionally surveyed downwind. See
id. at 42–43 n.18 (Canyon Fuel expert “testified that when one
surveys in the longwall tailgate return production is ‘most
always’ upstream.”) Moreover, the conditions of use did not
require that surveying equipment be used only outside the
designated areas—that the Assistant Secretary was unmoved
by the assertion that this would almost always be the case was
not arbitrary.26

    Regarding ventilation, the Assistant Secretary noted that
MSHA regulations already require ventilation so that it does
not “offset the decrease in safety from using” NPESE.
Rosebud Order II at 6. In addition, “ventilation systems do
not always work effectively and operators do not always
comply with ventilation requirements.” Id. Ventilation is but
one of many “redundant safety measures . . . the Mine Act
and its standards require” to guard “against ignitions and
explosions.” Id. at 5–6; see also Canyon Fuel Order at 40


acknowledged that he does not always set up in the middle of the
entry.”).
     26
        It is unclear from the record whether the risk of NPESE use
is mitigated entirely if its use is limited to, inter alia, areas upwind
of production or in entries where production is not occurring. The
Assistant Secretary did not reach this issue and thus we need not
reach it. The petitioners do not argue that it was arbitrary to impose
the cessation of production condition in lieu of a condition
requiring, for example, that surveyors always remain upwind of
production.
                              27
(“One of the most frequently cited violations is the failure to
comply with ventilation requirements.”). In addition, even if
the ventilation system functioned properly, the Assistant
Secretary concluded that it captured only “significant
amount[s] of dust and methane”—not all of it. Id. Record
evidence supports his conclusion. See id.at 40 n.14 (citing
ALJ hearing transcript).

4. NPESE has low ignition potential

    The petitioners also argue that NPESE is unlikely to
cause an explosion. See, e.g., Pet’rs’ Br. at 52 (although
NPESE is not “permissible,” it nonetheless “has a very low
potential for ignition of methane or coal dust”); id. at 53
(NPESE “does not generate heat”). Substantial evidence
supports the Assistant Secretary’s rejection of this argument.

     The Assistant Secretary considered—and rejected—
expert testimony on the relative ignition potential of the
equipment. For example, he noted that as part of the test for
determining whether equipment is permissible, “MSHA
layers dust onto components to see if dust will smolder.”
Canyon Fuel Order at 35. Smoldering corresponds to
overheating, which can result in ignition. Granted, record
evidence suggested that if there is significant overheating,
“components inside the devices would ‘likely’ fail, the
equipment would not function, and there would be no safety
hazard.” Id. But the Assistant Secretary observed that the
evidence was equivocal and not supported with test results.
There was also testimony indicating that “if there were
internal sparking or overheating it would not be detected.” Id.
at 36. The Assistant Secretary further observed that the safety
warning contained in the manual indicated that certain
equipment “[m]ay ignite explosively.” Id.
                               28
     The petitioners supplement their argument about the
equipment’s relative safety with the observation that it cannot
create sparks. See Pet’rs’ Br. at 63 (“[U]nlike a continuous
miner or roofbolter, [NPESE] creates no sparks.”). But see id.
at 53 (“[T]he changing of batteries has a potential for creating
sparks.”). They argue, therefore, that “dust or methane would
necessarily have to enter the instrument” in order for an
explosion to occur. Id. at 63. But the Assistant Secretary
disagreed and record evidence supports his skepticism. For
example, Ryder “acknowledged that non-permissible
electronic surveying equipment can spark if there is
something wrong with the device such as a loose connection.”
Canyon Fuel Order at 28 n.8. And a MSHA witness “testified
that batteries in the equipment can short out and cause an
arc.” Id.

5. Methane and dust will not enter NPESE electrical
compartments

     Based on their dubious contention that sparking cannot
occur, the petitioners argue that ignition can result only if dust
or methane gets into the NPESE. See Pet’rs’ Br. at 63 (“dust
or methane would necessarily have to enter the instrument”
for ignition to occur). And the petitioners contend that the
devices were adequately sealed and that the ALJ-imposed
condition requiring updating of equipment sufficiently
guarded against degradation of seals. The Assistant Secretary
concluded that the record rebutted this claim.

    The premise that the devices were well-sealed was based
on Ryder’s faulty water immersion and dust swab tests. As
the Assistant Secretary explained, the test results were
performed on equipment different from that the petitioners
sought to use. Ryder claimed that the equipment he inspected
was substantially similar to the petitioners’ but he “did not
                               29
take apart” the latter; and Huntley testified that, absent such
an examination, it would be difficult to conclude that it was
similar. Rosebud Order I at 29. Moreover, even assuming
Ryder tested sufficiently similar devices, Huntley testified
that it was “suspect” to use water as a surrogate for gas and, in
any event, moisture was found in all of the equipment Ryder
tested. Rosebud Order I at 30. Although Ryder testified that
the water entered only because the seals were degraded, the
Assistant Secretary observed that there was no record
evidence documenting how long it took a seal to degrade.
And, again, the petitioners had the burden of proof. 30 C.F.R.
§ 44.30(b).

     The petitioners argue that, even if dust or methane can
enter the electrical compartments, the openings “are
sufficiently small in most cases to prevent the escape of flame
outside the compartment.” Pet’rs’ Br. at 58. We once again
note the petitioners’ equivocal language and also observe that
the Assistant Secretary referenced testimony rebutting this
contention.       See Rosebud Order I at 31 (“I credit
Huntley’s . . . testimony that internal pressures from an
ignition could create larger openings.”).

6. Other petitions

     The petitioners next contend that the Assistant Secretary
improperly analogized to other petitions in imposing the
condition that coal production cease when surveying occurs in
high-risk areas. We need make only two brief observations.
First, we question the relevance of this claim. The petitioners
contend, for example, that MSHA “permits photography [in
high-risk areas] with less extensive requirements than the
[NPESE] petitions and permits cutting and welding under less
extensive conditions which do not involve cessation of
production.” Pet’rs’ Br. at 60 n.23. But we have no basis on
                              30
this record to conclude either that that equipment poses the
same (or greater) risk as the NPESE or that the conditions
imposed on the use of that equipment, even if not identical,
are not nonetheless more stringent. Even if we could reach
those conclusions, they do not establish, on their own, that the
condition MSHA placed on NPESE is arbitrary. Second, the
petitioners apparently encouraged the Assistant Secretary to
rely on other petitions such as Twenty Mile. See Canyon Fuel
Order at 40 (“Canyon Fuel expert witness Hartsog
acknowledged . . . reli[ance] on other granted-petitions [sic]
for modification of permissibility standards that allow the use
of diagnostic and testing equipment in high risk areas as well
as the modification in In re Twentymile Coal Co.”); Rosebud
Order I at 39 (“Rosebud mining engineer Cobaugh
acknowledged that the Twentymile consent agreement was a
template for Rosebud’s petitions for modification in this
case.”). And the Twenty Mile petition did involve NPESE.
The petitioners now contend that Twenty Mile was “never
subjected to the test of litigation and a decision by an
impartial ALJ.” Pet’rs’ Br. at 60. Although accurate, their
backtracking does little to establish that the conditions are
arbitrary or capricious. The Assistant Secretary’s conditions
are supported by the record before him and his reference to
Twenty Mile was little more than an aside. See Rosebud
Order I at 39 (“I also note that the same requirement is
contained in the Consent Agreement in [Twenty Mile].”).

7. Reliance on device warning

     The petitioners also argue that the Assistant Secretary
improperly relied on the manufacturer’s warning inasmuch as
neither MSHA nor the manufacturer could explain its basis.
The petitioners again overlook that they bear the burden of
proof in the modification petition process. See 30 C.F.R.
§ 44.30(b). And, in any event, it was not arbitrary for the
                                31
Assistant Secretary to rely on the warning applicable to the
very equipment the petitioners sought to use.              The
manufacturer, after all, “is in the best position to know about
the ignition risks of the equipment it manufactures.” Rosebud
Order I at 34; see also Canyon Fuel Order at 37 (“[T]he
manufacturers of the equipment are in the best position to
evaluate its ignition potential.”).

              B. FLOAT COAL DUST CONDITION

     The petitioners separately argue that the condition
prohibiting surveying in high-risk areas when float coal dust
is in suspension is arbitrary. It is uncontested that this
condition enhances mine safety. What is at issue is whether
the Assistant Secretary reasonably concluded that it is
necessary. We note, first, that the petitioners’ arguments
repeat earlier contentions. See Pet’rs’ Br. at 63 (“[T]here is
nothing about use of a surveying instrument that liberates dust
or methane.”); id. (“it creates no sparks”); id. at 65 (for
explosion to occur “dust must still find its way into the insides
of the electronic surveying instrument which is highly
unlikely”). Only two contentions require analysis: the
condition is unclear and impossible to implement and the
condition is self-regulating because surveying becomes
impossible at a dust concentration well below an explosive
point.

     The petitioners rely on the Administrator’s statements in
his denial of their original petitions that “it is not possible for
the petitioner to implement this action item [because] [f]loat
coal dust cannot be entirely eliminated during the cutting
process of mining. . . . Unless all mining were to cease, float
coal dust would be generated.” Administrator’s Proposed
Decision and Order at 6, Parkwood Res. Inc., Docket No. M-
2008-054-C (Dep’t of Labor Jan. 29, 2010). But, given that
                                 32
the Assistant Secretary has required coal production to cease
while surveying is conducted in the high-risk areas, the
petitioners’ point is weakened. And we have found no other
record support for this argument.27 Regarding whether the
condition is clear enough to be implemented, the Assistant
Secretary resolved its vagueness by noting that a “visual
determination” suffices to determine if dust is in suspension.
Rosebud Order II at 11 n.7.

     The petitioners also contend that this condition is
unnecessary because it is “self-regulating.” Pet’rs’ Br. at 63.
They claim that “far less than a sufficient amount of dust to be
explosive would preclud[e] surveying” by reducing visibility
below levels necessary for surveying. Id. But the Assistant
Secretary reasonably rejected this argument. As he explained,
“coal dust can be rapidly placed in suspension . . . [and] even
a vigilant surveyor may not have the time to de-energize his
instrument before it encounters an explosive concentration of
coal dust.” Rosebud Order I at 33.

          C. VIABLE MECHANICAL SURVEYING EQUIPMENT

     The final condition under challenge is that the petitioners
must switch to viable mechanical surveying equipment when
it becomes commercially available. We first note that it is
MSHA’s position that the use of NPESE, under the conditions
of use imposed by the Assistant Secretary’s two orders, is no
more dangerous than the use of mechanical surveying
     27
        The petitioners argue in the alternative that the prohibition
on surveying in high-risk areas while production is ongoing renders
this condition redundant. But the record reflects that coal dust can
also be placed in suspension from “methane explosions, bumps,
fans, roof falls, brushing up against insufficiently rock-dusted float
coal dust, and the exhaust from large pieces of equipment.”
Canyon Fuel Order at 35.
                              33
equipment. See Rosebud Order I at 44 (“I have found that the
[NPESE], including the modifications and additional
conditions in the [ALJ’s] decision and order, as modified and
supplemented by the conditions in this decision and order,
will at all times promote the same safety goals as the original
standards [allowing mechanical equipment] with no less than
the same degree of success.”). If that were not so, the
modification grant here would be improper. See S. Ohio Coal
Co., 928 F.2d at 1202 (modification must “promote the same
safety goals as the original standard with no less than the
same degree of success.”). And the petitioners contend that
NPESE (with the conditions of use) is not only as safe as, but
safer than, mechanical surveying equipment.

     The petitioners make two arguments to suggest that
mechanical surveying equipment, even when “viable,” is less
safe than NPESE. First, they argue that surveying with
NPESE is faster and thus surveyors are exposed to the
dangers of mines for less time than they would be with
mechanical equipment. But the Assistant Secretary observed
that this assertion was unsupported by data, see Rosebud
Order I at 45 n.25 (“The evidence concerning the increased
likelihood of injury from the asserted increase in exposure
time is general and not quantified and does not establish that
the increase in exposure time would result in anything more
than an insubstantial decrease in safety.”), and it did not
consider “the additional time needed to comply with the
conditions for use” of NPESE, id.

    The petitioners also assert that even “viable” mechanical
surveying equipment will have inferior accuracy. The record
supports this assertion, compare Rosebud Order I at 44 n.23
(suggesting “1 foot-in-10,000 feet accuracy levels” viable)
with Petition for Modification Stipulations ¶ 21, In re
Rosebud Mining Co., Docket Nos. 2010-MSA-1, 2011-MSA-
                             34
2, -11, -12 (reflecting NPESE achieved 1 foot in 81,507 feet
accuracy), but, even assuming the accuracy gap is more than
de minimis, we have no way to measure its impact on mine
safety. See Rosebud Order I at 44 n.23 (expert testimony
reflecting that “there are no safety issues when surveying
equipment achieves 1 foot-in-10,000 feet accuracy levels.”).
Thus, whatever accuracy gain is made by using NPESE, it is
not plain that it improves mine safety more than would viable
mechanical equipment.

     Finally, the Assistant Secretary identified a mine safety
risk from the use of NPESE that would not exist with viable
mechanical surveying equipment—the use of “MSHA’s
limited resources . . . spent ensuring compliance with the
terms and conditions” of use. Rosebud Order I at 45.
Because MSHA must assess what effect modifications will
have on “overall mine safety,” S. Ohio Coal Co., 928 F.2d at
1202, the preservation of finite resources for use in ensuring
compliance with other standards is a reasonable basis upon
which to include this condition.

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

                                                  So ordered.
