United States Court of Appeals, District of Columbia Circuit
September 26, 2017
Petitions for Review of a Final Rule of the Occupational
Safety & Health Administration
William L. Wehrum and Bradford T. Hammock argued the cause
for the Industry Petitioners. Susan F. Wiltsie, David Craig
Landin, Tressi L. Cordaro, Michael B. Schon and Linda E.
Kelly were with them on brief. Elizabeth C. Chandler Clements
entered an appearance.
Michael Connolly argued the cause for the
Petitioners-Intervenors Chamber of Commerce of the United
States, et al. William S. Consovoy, Steven P. Lehotsky and
Sheldon B. Gilbert were with him on brief.
Jeremiah A. Collins and Victoria L. Bor argued the cause for
the Union Petitioners. Randy S. Rabinowitz, Lynn K.
Rhinehart, Richard J. Brean and Ava Barbour were with them on
brief. Stephen A. Yokich entered an appearance.
Kristen M. Lindberg and Lauren S. Goodman, Senior Attorneys,
and Louise McGauley Betts, Attorney, United States Department
of Labor, argued the cause for the Respondents. On brief were
Nicholas C. Geale, Acting Solicitor of Labor, Heather R.
Phillips, Counsel for Appellate Litigation, Nathaniel I.
Spiller, Counsel for Health Standards, and Anne R. Godoy and
Allison G. Kramer, Senior Attorneys.
Victoria L. Bor argued the cause for the
Respondents-Intervenors. Jeremiah A. Collins, Randy S.
Rabinowitz, Lynn K. Rhinehart, Richard J. Brean and Ava
Barbour were with her on brief.
William L. Wehrum, Susan F. Wiltsie, David Craig Landin,
Bradford T. Hammock, Tressi L. Cordaro and Linda E. Kelly
were on brief for the Industry Respondent-Intervenors.
W. Jordan was on brief for the amici curiae The American
Thoracic Society, et al. in support of the respondent. Adam
Babich entered an appearance.
Before: Garland, Chief Judge, and Henderson and Tatel,
crystalline forms of silica,  a compound made of silicon and
oxygen, are commonly found in workplaces with rock, sand,
gravel, concrete, and brick. Exposure to silica is one of the
oldest known occupational hazards. And the health effects of
exposure to silica-most commonly silicosis, a progressive and
irreversible lung disease caused by the inflammatory effects
of silica-are not a thing of the past. "Currently,
silicosis is the most prevalent chronic occupational disease
in the world." Robbins & Cotran, Pathologic Basis of
Disease 690 (9th ed. 2015).
United States, more than two million workers are currently
exposed to some level of silica. In 2016, the Occupational
Safety and Health Administration (OSHA), an agency within the
United States Department of Labor, published a final rule
regulating workplace exposure to silica. Occupational
Exposure to Respirable Crystalline Silica, 81 Fed. Reg. 16,
285 (Mar. 25, 2016) (codified at 29 C.F.R. Pts. 1910, 1915,
and 1926) (Silica Rule or Rule). Petitions to review the Rule
came from both sides; a collection of industry petitioners
(Industry) believes OSHA impermissibly made the Rule too
stringent and several union petitioners (Unions) believe OSHA
improperly failed to make the Rule stringent enough.
petitioned for review of five issues: (1) whether substantial
evidence supports OSHA's finding that limiting
workers' silica exposure to the level set by the Rule
reduces a significant risk of material health impairment; (2)
whether substantial evidence supports OSHA's finding that
the Rule is technologically feasible for the foundry,
hydraulic fracturing, and construction industries; (3)
whether substantial evidence supports OSHA's finding that
the Rule is economically feasible for the foundry, hydraulic
fracturing, and construction industries; (4) whether OSHA
violated the Administrative Procedure Act (APA) in
promulgating the Rule; and (5) whether substantial evidence
supports two ancillary provisions of the Rule-one that allows
workers who undergo medical examinations to keep the results
confidential from their employers and one that prohibits
employers from using dry cleaning methods unless doing so is
infeasible. We reject all of Industry's challenges.
Unions petitioned for review of two parts of the Rule: (1)
the requirement that medical surveillance for construction
workers be provided only if the employee has to wear a
respirator for 30 days for one employer in a one-year period;
and (2) the absence of medical removal protections. We reject
the Unions' challenge to the construction standard's
30-day trigger for medical surveillance. We conclude that
OSHA failed to adequately explain its decision to omit
medical removal protections from the Rule and remand for
further consideration of the issue.
Occupational Safety and Health Act (OSH Act) authorizes the
Secretary of Labor (Secretary) to "promulgate, modify,
or revoke any occupational safety or health standard, "
29 U.S.C. § 655(b), by requiring conditions or the
adoption of practices, means, or methods "reasonably
necessary or appropriate to provide safe or healthful
employment and places of employment, " id.
§ 652(8). If the standard applies to toxic materials or
harmful physical agents, the Secretary "shall set the
standard which most adequately assures, to the extent
feasible, on the basis of the best available evidence, that
no employee will suffer material impairment of health or
functional capacity even if such employee has regular
exposure to the hazard" regulated by the standard
"for the period of his working life." Id.
§ 655(b)(5). The Secretary has delegated his authority
to OSHA. See 72 Fed. Reg. 31, 160 (June 5, 2007).
1971, OSHA adopted a standard regulating exposure to a
variety of substances, including silica. Occupational Safety
and Health Standards; National Consensus Standards and
Established Federal Standards, 36 Fed. Reg. 10, 466 (May 29,
1971). The 1971 rule established a permissible exposure limit
(PEL)-a time-weighted average of a worker's exposure
during a workday-of 100 micrograms per cubic meter
(µg/m3) in general industry and 250
µg/m3 in the construction industry.
See 81 Fed. Reg. at 16, 294. In the 1990s, OSHA
studied the efficacy of the 1971 rule regarding
silica-related health effects in the workplace and concluded
a new rule was needed. See id. at 16, 295.
2016, OSHA promulgated its final Silica Rule. 81 Fed. Reg.
16, 285. The Rule lowers the PEL to 50 µg/m3
for all covered industries, including as particularly
relevant here, the foundry, hydraulic fracturing, brick, and
construction industries. See 29 C.F.R. §§
1910.1053(c), 1926.1153(d)(1). Employers must assess silica
exposure levels in the workplace (or, for certain
construction industry tasks, adopt specific
"safe-harbor" practices) and, if necessary, must
implement engineering and work practice controls to keep
exposures below the PEL. Id. §§
1910.1053(f)(1), 1926.1153(c)(1), 1926.1153(d)(3)(i). If
engineering and work practice controls cannot reduce
exposures to the PEL, the employer must use controls to the
extent feasible and provide supplementary respirator
Silica Rule also establishes various ancillary provisions
including, again, as relevant here, housekeeping requirements
and medical surveillance requirements. Under the challenged
housekeeping provision, employers are prohibited from using
dry sweeping methods to clean worksites if doing so could
contribute to employee exposure to silica unless wet cleaning
methods are infeasible. Id. §§
1910.1053(h)(1), 1926.1153(f)(1). Under the challenged
medical surveillance provisions, employers must provide
medical screening to silica-exposed workers if certain
conditions are met. Most of the information from the medical
examinations, including medical professionals'
recommendations limiting the employee's exposure to
silica, are confidential and cannot be released to the
employer unless the employee authorizes disclosure.
Id. §§ 1910.1053(i)(6), 1926.1153(h)(6).
Finally, the Rule provides no medical removal protections to
workers whose doctors recommend either permanent or temporary
removal from silica exposure on the job.
compliance dates were established for each industry: June 23,
2017 for the construction industry, id. §
1926.1153(k); June 23, 2018 for the foundry industry,
id. § 1910.1053(l); and June 23, 2021
for the hydraulic fracturing industry, id.
first decide Industry's challenges. In order, we address
OSHA's significant risk findings, its technological
feasibility findings, its economic feasibility findings, the
procedural regularity of the Rule, and the challenged
ancillary provisions. The substantive issues are governed by
the "substantial evidence" standard, 29 U.S.C.
§ 655(f), under which we require OSHA to "identify
relevant factual evidence, to explain the logic and the
policies underlying any legislative choice, to state candidly
any assumptions on which it relies, and to present its
reasons for rejecting significant contrary evidence and
argument, " United Steelworkers of America v.
Marshall (Lead I), 647 F.2d 1189, 1207 (D.C. Cir. 1980).
The APA governs the procedural challenge to ensure the Rule
is not promulgated "without observance of procedure
required by law." 5 U.S.C. § 706(2)(D).
turn to the Unions' challenges and address the 30-day
medical surveillance trigger in the construction standard and
the lack of medical removal protections in the general
industry standard. Where the Unions have failed to identify
evidence that their proposals would be feasible and generate
more than a de minimis benefit to worker health, we
reject them. See Building & Construction Trades
Department, AFL-CIO v. Brock (Asbestos), 838 F.2d 1258,
1271 (D.C. Cir. 1988). Where the Unions have met this initial
burden, we ask whether OSHA has supported its decision with
substantial evidence and otherwise engaged in reasoned
OSHA promulgates any permanent health or safety standard, it
must make a "threshold finding" that "it is at
least more likely than not that long-term exposure" to
the regulated substance at current exposure levels
"presents a significant risk of material
impairment" that "can be eliminated or lessened by
a change in practices." Industrial Union Department,
AFL-CIO v. American Petroleum Institute (Benzene), 448
U.S. 607, 642, 653 (1980) (plurality). The Supreme Court
has provided the guidepost that OSHA follows: a
one-in-a-thousand risk that exposure to the regulated
substance will be fatal can reasonably be considered
significant but a one-in-a-billion risk is likely not
significant. Id. at 655-56.
must support its significant risk finding with substantial
evidence. Id. at 653. Although it must rely on a
"body of reputable scientific thought" when
assessing risk, id. at 656, OSHA does not have to
"calculate the exact probability of harm" or
support its finding "with anything approaching
scientific certainty, " id. at 655-56. OSHA is
entitled to "some leeway" when its "findings
must be made on the frontiers of scientific knowledge."
Id. at 656. We "do not reweigh the evidence and
come to our own conclusion[s]; rather, we assess the
reasonableness of OSHA's conclusion." Public
Citizen Health Research Group v. Tyson (Ethylene Oxide),
796 F.2d 1479, 1495 (D.C. Cir. 1986).
promulgating the Silica Rule, OSHA conducted a Quantitative
Risk Assessment in which it reviewed toxicological,
epidemiological, and experimental studies about the adverse
health effects of silica exposure. 81 Fed. Reg. at 16, 380.
OSHA quantified the excess risk of silica-related health effects
assuming exposure over a working life (45 years) to various
levels of silica, including the original general industry PEL
of 100 µg/m3, the original construction PEL
of 250 µg/m3, and the new PEL of 50
µg/m3. Id. at 16, 300. OSHA
concluded that silica exposure significantly "increases
the risk of" four adverse health effects: silicosis and
other non-malignant respiratory disease (NMRD) mortality,
lung cancer mortality, silicosis morbidity, and renal disease
mortality. Id. at 16, 300, 16, 386-87. OSHA also
concluded that the risks at 50 µg/m3-the new
PEL-are lower than the risks at the original PELs of 100
µg/m3 and 250 µg/m3.
Id. at 16, 300. In total, OSHA estimated that the
Silica Rule will prevent 642 deaths and 918 cases of
silica-related disease each year. Id. at 16,
challenges OSHA's significant risk findings in three
ways. First, Industry attacks two parts of OSHA's
risk-assessment methodology. Second, it challenges OSHA's
findings on each of the four individual health risks.
Finally, Industry challenges OSHA's decision to include
the brick industry within the scope of the Rule. We reject
challenges two components of OSHA's risk-assessment
methodology: its no-threshold assumption and its failure to
account for a dose-rate effect. We uphold OSHA's
decisions on both.
Industry challenges OSHA's use of no-threshold
exposure-response models in its risk assessments for
silicosis and lung cancer. 81 Fed. Reg. at 16, 351. The
no-threshold concept means there is no exposure level below
which workers would not be expected to develop adverse health
effects. Id. OSHA did not definitively find that no
threshold exists. Instead, it found that if a threshold
exists it does so below the PEL, which justified its use of a
no-threshold model. OSHA supported its selection of the PEL
with studies showing that risks of lung cancer exist at 36
µg/m3 and 10 µg/m3, levels
lower than the PEL. Id. at 16, 351, 16, 356. To
OSHA, the studies showing risks below the PEL support its
conclusion that any threshold, if it exists, does so below
the PEL. See id. at 16, 351 ("As 36
µg/m3 is well below the previous industry
PEL of 100 µg/m3 and below the final PEL of
50 µg/m3, the . . . study showed no evidence
of an exposure-response threshold high enough to impact
OSHA's choice of PEL."). Industry, in contrast,
points to studies it claims not only show a threshold exists
but also show a threshold exists above the PEL. OSHA rejected
Industry's argument because the contrary studies used
non-reactive and poorly soluble particles-which silica is
not-and therefore the "findings regarding" the
particles "[cannot] be extrapolated to crystalline
silica." Id. at 16, 349. OSHA acknowledged
"there is considerable uncertainty" about whether a
threshold exists but found that "the weight of evidence
supports the view that, if there is a threshold, " it is
"likely lower than the" PEL. Id. at 16,
no-threshold assumption is supported by substantial evidence.
Although Industry claims OSHA's position is inconsistent
with common sense and "mounting judicial
skepticism" of no-threshold models, citing to several
district court and state court cases disapproving a
no-threshold approach, Industry Br. at 28-29, OSHA's
position is in line with our precedent. In Ethylene
Oxide, we upheld a no- threshold model based on
OSHA's having found evidence of adverse health effects at
levels of exposure to ethylene oxide below the established
PEL, then extrapolating that evidence to assume no threshold
of ethylene oxide exposure existed below which risks did not
exist and rejecting two contrary comments that purportedly
showed a threshold did exist. 796 F.2d at 1500. As in
Ethylene Oxide, Industry presents, and urges us to
adopt, "one side of the debate." Id. But
OSHA has explained why it rejected Industry's side of the
debate, presented the other side of the debate, and supported
it with evidence from which a reasonable conclusion could be
made, as OSHA did here, that no threshold of safe exposure to
silica exists. We cannot "choose a particular side as
the 'right' one" in a scientific dispute.
Id. Accordingly, OSHA's no-threshold assumption
satisfies our substantial evidence test.
Industry challenges OSHA's decision not to include a
dose-rate effect in the model, which means OSHA assessed
health risks based on the cumulative amount of silica
exposure without accounting for the intensity of exposures.
81 Fed. Reg. at 16, 375. OSHA took its position "because
each of the key . . . studies" OSHA relied on used
cumulative exposure as the only metric. Id. at 16,
374-75. Multiple commenters supported the notion that
"cumulative exposure is a reasonable and practical
choice" and that cumulative exposure "is often the
best predictor of chronic disease." Id. at 16,
375. Competing commenters argued that OSHA's risk
assessment should account for the intensity of exposures.
Id. Industry relied on studies showing that not
accounting for a dose-rate effect "could overestimate
risk at lower concentrations." Id. The studies
supporting Industry's position, however, largely observed
an intensity-based effect at 500 µg/m3 and
2, 000 µg/m3, exposure levels so "far
above the previous PEL, " id. at 16, 395, that
OSHA determined the studies were of little use to the
"exposure range of interest"- 25 to 500
µg/m3, id. at 16, 376.
Ethylene Oxide, we upheld OSHA's decision not to
include a dose-rate effect in its model when faced with
"competing technical opinions" about whether the
amount or the intensity of ethylene oxide exposure mattered
more. Ethylene Oxide, 796 F.2d at 1504. OSHA did the
same in its Silica Rule: it took competing evidence, favored
one side, and explained the reasons for its decision. We
"cannot expect OSHA to [locate and use] absolutely
conclusive studies on these difficult medical issues"
and we must uphold OSHA's choice, even in the face of
"controverted" evidence, if it falls within a
"zone of reasonableness." Lead I, 647 F.2d
at 1253 (quoting Hercules, Inc. v. EPA, 598 F.2d 91,
107 (D.C. Cir. 1978)). We believe OSHA's conclusions on
handling the purported dose-rate effect are reasonable.
"[C]ourts cannot interfere with reasonable
interpretations of equivocal evidence, " Ethylene
Oxide, 796 F.2d at 1505, and therefore we do not
Adverse Health Effects
noted earlier, OSHA concluded that long-term silica exposure
above the PEL presents a significant risk of four discrete
adverse health effects: (1) silicosis and NMRD mortality; (2)
lung cancer mortality; (3) silicosis morbidity; and (4) renal
disease mortality. 81 Fed. Reg. at 16, 300, 16, 386- 87.
Industry challenges OSHA's findings as to all four.
Industry acknowledged at oral argument that, to prevail, it
would have to show none of the discrete findings is supported
by substantial evidence. We address each in turn. We conclude
OSHA's significant risk findings as to the first three
adverse health effects are supported by substantial evidence,
which supports OSHA's overall finding of a significant
not reach OSHA's finding with respect to renal disease
Silicosis or Non-Malignant Respiratory Disease Mortality
is a progressive, irreversible lung disease caused by the
inflammatory effects of silica in the lungs. OSHA found that
silica exposure at the original PEL of 100
µg/m3 created an excess risk of silicosis
mortality for 11 in 1, 000 workers that would be reduced to 7
in 1, 000 workers at the Rule's PEL of 50
µg/m3. 81 Fed. Reg. at 16, 303, 16, 312.
Other NMRD caused by silica exposure include emphysema,
chronic obstructive pulmonary disease, and chronic
bronchitis. Id. at 16, 304. OSHA found that silica
exposure at the 100 µg/m3 PEL created an
excess risk of NMRD mortality (including silicosis mortality)
for 85 in 1, 000 workers that would be reduced to 44 in 1,
000 workers at the Rule's PEL of 50
µg/m3. Id. at 16, 303. Both
Industry and the Chambers Intervenors  challenge
OSHA's findings on silicosis and NMRD mortality.
support its findings on silicosis and NMRD mortality, OSHA
relied on two studies: the Mannetje study, which showed a
statistically significant association between silicosis
mortality and cumulative exposure to silica, and the Park
study, which quantified the relationship between silica
exposure and NMRD mortality. Id. at 16, 317.
Industry's objections to OSHA's conclusions primarily
attack the reliability of the Park study. Industry claims the
Park study (1) focused on workers with cumulative exposure
levels far above what workers typically faced under the
original PEL and (2) produced results that were likely skewed
by smoking because the study had smoking data for only
one-half of the studied workers.
rulemaking, OSHA addressed both criticisms. On the first
point, OSHA acknowledged "some uncertainty in using
models heavily influenced by exposures above the previous
PEL" but noted that the average cumulative exposure of
the studied workers was "lower than what the final rule
would permit over 45 years of exposure." Id. at
16, 318. Accordingly, OSHA "[dis]agree[d] that the Park
study should be discounted" and instead concluded that
the study was both relevant and appropriate to rely on.
Id. On the second point, OSHA acknowledged that
"comprehensive smoking data would be ideal" but
assessed the Park study's mechanics in detail and
concluded that the risk estimates were "not likely to be
exaggerated due to [studied workers'] smoking
our substantial evidence standard, OSHA has a duty to
"present its reasons for rejecting significant contrary
evidence and argument." Lead I, 647 F.2d at
1207. OSHA acknowledged and adequately responded to
Industry's criticisms of the Park study. Even if the Park
study was "flawed in some way, " OSHA is not
precluded from relying on imperfect evidence so long as it
"recognize[s] and account[s] for the methodological
weaknesses" of the evidence. Ethylene Oxide,
796 F.2d at 1487; see id. at 1495 ("While some
of OSHA's evidence suffers from shortcomings, such
incomplete proof is inevitable when the Agency regulates on
the frontiers of scientific knowledge."). OSHA did
recognize and account for the weaknesses of the two studies
it relied on here.
Chambers, meanwhile, present a record of death certificates
and their listed cause of death that shows
silicosis-attributed deaths dropped from 1, 065 in 1968
(three years before the 1971 PEL was implemented) to 123 in
2007. The decline, according to the Chambers, shows that the
current risks are due not to exposure levels at the 1971 PEL
but instead are due to pre-1971 exposures or exposures
occurring in violation of the 1971 PEL. Thus, the Chambers
argue, the 1971 rule is working and there is no need for a
here again, OSHA adequately explained why it rejected this
evidence. First, OSHA concluded that the death certificate
data underreported risks after one commenter found that
silicosis was listed as the cause of death for only 14
percent of people with confirmed silicosis. 81 Fed. Reg. at
16, 328. Second, the death certificate data "d[id] not
include information about exposure" levels for those
who died as a result of silicosis, which made the data
"inadequate and inappropriate for" setting a
standard regulating silica at particular exposure levels.
Id. at 16, 326. Indeed, the agency that compiled and
analyzed the death certificate data testified that relying on
the death certificates to show no significant risk exists
would be a "misuse" of the data. Id.
OSHA responded directly to the Chambers' arguments that
the death certificate data showed the risks of silica
exposure are no longer significant. OSHA acknowledged that
silicosis-related deaths have dropped since 1968 but pointed
to evidence showing that the decline leveled off at
approximately 90 to 180 deaths per year since 2000.
Id. at 16, 324. This evidence "suggest[s] that
the number of silicosis deaths . . . may be stabilizing,
" id., which also suggests that the significant
risk of silicosis mortality would not disappear if OSHA
simply let the 1971 PEL run its course, as Industry argued,
id. at 16, 325. OSHA also pointed to evidence
showing that the decline in silicosis-related deaths tracks
the decline in high-exposure jobs as much as it tracks
improved working conditions, further suggesting that OSHA
"still h[as] work to do" to make silica exposure
safe. Id. at 16, 325-26. Thus, although OSHA agreed
that the death certificate data was "useful for
providing context and an illustration of a significant
general trend in the reduction of deaths associated with
silicosis over the past four to five decades, " the
"limited and incomplete" data made reliance on the
death certificates "inappropriate." Id. at
16, 330. OSHA "described in some detail [its] reasons
for choosing between competing alternatives."
Asbestos, 838 F.2d at 1266. Accordingly, OSHA has
met its burden to identify the evidence it relied on and
explain why it rejected contrary evidence.
Lung Cancer Mortality
found that silica exposure at the 100 µg/m3
PEL created an excess risk of lung cancer mortality equal to
11 to 54 deaths per 1, 000 workers that would be reduced to
an excess risk of 5 to 23 deaths per 1, 000 workers at the 50
µg/m3 PEL. 81 Fed. Reg. at 16, 338. Industry
argues the conclusion hinges on OSHA's unsupported
assumption that silica exposure directly increases the risk
of lung cancer in the absence of silicosis. That is, if the
risk of lung cancer depends on preexisting silicosis, then
silica exposure alone does not create an independent risk of
points to evidence that asserts the association between
silicosis and lung cancer is "more compelling" than
the association between silica exposure and lung cancer.
Joint Appendix (J.A.) 3027. But the mere suggestion in some
evidence that silicosis is a necessary precursor of lung
cancer does not bind the agency. See Ethylene Oxide,
796 F.2d at 1504 (noting that suggestive statements "do
not amount to a scientific certainty binding on the
agency"). Meanwhile, OSHA also cites to numerous studies
that show silica exposure can lead directly to lung cancer.
81 Fed. Reg. at 16, 309 (recapping and summarizing findings).
As one commenter put it, the literature OSHA relied on shows
"silica has been established as a cause of lung
cancer." J.A. 7815. We lack the technical expertise to
second-guess OSHA's judgment when it "review[ed] all
sides of the issue and reasonably resolve[d] the
matter." Ethylene Oxide, 796 F.2d at 1500. We
do not second-guess OSHA's conclusions here.
specifically challenges OSHA's decision to give weight to
a 2004 Attfield and Costello study, which showed there is an
association between silica exposure and lung cancer, instead
of a 2011 Vacek study showing there is no such association.
81 Fed. Reg. at 16, 338. Industry provides a laundry list of
reasons why it believes the Vacek study is better: it is more
recent, covered more workers, covered more years, and used
more detailed information. But OSHA explained its reasons for
rejecting the Vacek study. Among them: the Vacek study found
an unexplained significant excess risk of lung cancer that
called into question all of its results and had a low risk
estimate for a particular type of worker (channel bar
operators) that OSHA concluded had major consequences for the
entire exposure analysis. Id. at 16, 335-37.
Moreover, OSHA provided affirmative reasons for choosing the
Attfield and Costello study. Most importantly, OSHA reasoned,
that study accounted for a healthy worker survivor effect-the
tendency of healthy workers to remain in the workforce longer
than ill workers and therefore face more exposure than ill
workers, which "may" make the "risk of disease
at higher exposures" improperly "appear to be
constant or decrease"- but the Vacek study did not
assess the healthy worker survivor effect. Id. at
16, 336. "We have then, at worst, the ordinary situation
of controverted evidence, in which we must defer to the
reasonable and conscientious interpretations of the
agency." Lead I, 647 F.2d at 1258.
support its finding of a significant risk of silicosis
morbidity, OSHA relied on five studies that showed an excess
risk between 60 and 773 cases of silicosis morbidity per 1,
000 workers at a level of 100 µg/m3 that
would be reduced to an excess risk between 20 and 170 cases
of silicosis morbidity per 1, 000 workers at a level of 50
µg/m3. 81 Fed. Reg. at 16, 317. The variance
among studies, according to Industry, "suggests that
none of [the studies] is a reliable guide to a correct
quantification" of exposures and therefore none of the
studies can support a finding of a significant risk of
silicosis morbidity. J.A. 3368. OSHA concluded the results of