                    FOR PUBLICATION

   UNITED STATES COURT OF APPEALS
        FOR THE NINTH CIRCUIT


 WILLIAM G. STEPHENS; NORMA                       No. 18-35908
 STEPHENS, husband and wife,
               Plaintiffs-Appellants,               D.C. No.
                                                 1:17-cv-00385-
                     v.                               BLW

 UNION PACIFIC RAILROAD
 COMPANY, a Delaware corporation,                   OPINION
                Defendant-Appellee.

        Appeal from the United States District Court
                  for the District of Idaho
      B. Lynn Winmill, Chief District Judge, Presiding

              Argued and Submitted July 9, 2019
                    Seattle, Washington

                     Filed August 28, 2019

Before: Paul J. Watford and Eric D. Miller, Circuit Judges,
      and Barbara Jacobs Rothstein, * District Judge.

                    Opinion by Judge Miller



    *
      The Honorable Barbara Jacobs Rothstein, United States District
Judge for the Western District of Washington, sitting by designation.
2                STEPHENS V. UNION PAC. R.R.

                          SUMMARY **


                    Idaho Law / Negligence

    The panel affirmed the district court’s summary
judgment in favor of Union Pacific Railroad in a plaintiff’s
action alleging that secondary exposure to asbestos exposure
caused his mesothelioma, and asserting negligence and
related claims under Idaho law.

    Plaintiff alleged that his father worked at a Union Pacific
roadhouse where he was exposed to asbestos, and that his
father carried the asbestos home and exposed plaintiff to
asbestos.

    Under Idaho law, the panel held that plaintiff failed to
create a genuine issue of fact on whether any asbestos
exposure that may have occurred was a substantial factor in
causing his mesothelioma. The panel held that in the context
of asbestos claims, the substantial-factor test requires
“demonstrating that the injured person had substantial
exposure to the relevant asbestos for a substantial period of
time.” McIndoe v. Huntington Ingalls, Inc., 817 F.3d 1170,
1176 (9th Cir. 2016).

    In an effort to establish causation, plaintiff relied on the
testimony of two experts. The panel agreed with the district
court that those opinions were insufficient. The panel held
that the experts had no basis to conclude that plaintiff was
exposed to asbestos with any regularity. The panel rejected

    **
       This summary constitutes no part of the opinion of the court. It
has been prepared by court staff for the convenience of the reader.
               STEPHENS V. UNION PAC. R.R.                   3

plaintiff’s assertion that Union Pacific had waived any
objection to the admissibility of one of the expert’s
testimony. The panel concluded that because plaintiff failed
to establish that he was regularly exposed to asbestos
attributable to Union Pacific, plaintiff could not create a
genuine issue of material fact whether his secondary
exposure was a substantial factor in causing his disease, and
he could not prevail on his negligence claim.


                         COUNSEL

Matthew P. Bergman (argued) and Ruby K. Aliment,
Bergman Draper Oslund, Seattle, Washington, for Plaintiffs-
Appellants.

Steven M. Crane (argued), Viiu Spangler Khare, Barbara S.
Hodous, and Ryan T. Moore, Berkes Crane Robinson & Seal
LLP, Los Angeles, California, for Defendant-Appellee.


                         OPINION

MILLER, Circuit Judge:

    William Stephens spent nearly 20 years working with
asbestos-containing products while employed at lumber
mills in Oregon. After being diagnosed with mesothelioma,
he brought an action in Oregon state court against his former
employers and other defendants that manufactured or used
asbestos-containing products, including the Union Pacific
Railroad Company. Stephens’s claims against Union Pacific
were dismissed for lack of personal jurisdiction, and he
ultimately settled the litigation in exchange for a substantial
payment from the other defendants.
4              STEPHENS V. UNION PAC. R.R.

    Stephens then brought the present action against Union
Pacific in the District of Idaho. In the late 1940s and early
1950s, when Stephens was a child, his father worked at a
Union Pacific roundhouse in Weiser, Idaho. Stephens
alleges that his father was exposed to asbestos at work and
then carried asbestos home on his clothes, exposing the rest
of his family. According to Stephens, that secondary
asbestos exposure caused his mesothelioma. Invoking the
district court’s diversity jurisdiction under 28 U.S.C. § 1332,
Stephens asserted negligence and related claims under Idaho
law.

    The district court granted summary judgment in favor of
Union Pacific, concluding that Stephens failed to introduce
sufficient evidence to raise a genuine issue of fact on
(1) whether Stephens was exposed to asbestos attributable to
Union Pacific and (2) whether that exposure was a
substantial factor in causing his disease. We have
jurisdiction over Stephens’s appeal under 28 U.S.C. § 1291,
and we review the district court’s grant of summary
judgment de novo. See Colwell v. Bannister, 763 F.3d 1060,
1065 (9th Cir. 2014). We affirm.

    To survive a motion for summary judgment, a
nonmoving party must present “evidence from which a
reasonable jury could return a verdict in its favor.” Triton
Energy Corp. v. Square D Co., 68 F.3d 1216, 1221 (9th Cir.
1995); see Fed. R. Civ. P. 56(a). We must view the evidence
in the light most favorable to the nonmoving party.
Matsushita Elec. Indus. Co. v. Zenith Radio Corp., 475 U.S.
574, 587 (1986).

    Under those standards, we think it is a close question
whether Stephens presented sufficient evidence of exposure.
To establish liability for negligence under Idaho law, a
plaintiff must demonstrate “a causal connection between the
              STEPHENS V. UNION PAC. R.R.                5

defendant’s conduct and the resulting injury.” Johnson v.
Wal-Mart Stores, Inc., 423 P.3d 1005, 1008 (Idaho 2018).
Asbestos cannot cause injury in someone who is not exposed
to it, so in the context of asbestos-related negligence,
exposure is a necessary element of the claim. See, e.g.,
Menne v. Celotex Corp., 861 F.2d 1453, 1461–62 (10th Cir.
1988); Blackston v. Shook and Fletcher Insulation Co.,
764 F.2d 1480, 1482–83 (11th Cir. 1985).

    As the district court recognized, “Stephens himself
provides the only potential evidence of exposure.” Stephens
testified that when he was between six and eight years old,
he visited his father at work up to four times, where he
witnessed Union Pacific employees removing and replacing
insulation on steam engines. Stephens testified that the
workplace was dusty and that his father’s work clothes were
dusty when he came home. During a deposition, Stephens
was shown photographs of steam engines with exposed
insulation and he said that they looked familiar—but the
photographs were not taken at the Union Pacific roundhouse
in Weiser.

    For its part, Union Pacific admitted that during the
relevant time period, it likely used asbestos-containing
products. It also admitted that asbestos dust likely would
have been released if there was a “major overhaul” of a
steam engine, such as when its lagging was removed or
disturbed. And it admitted that if Stephens’s testimony is
correct—that is, if he saw steam engines at the roundhouse—
then those locomotives would have been insulated with
asbestos-containing products. But Union Pacific’s corporate
representative also testified that, apart from Stephens’s
account, there was no record of steam engines ever being
repaired or even located at the Union Pacific roundhouse in
Weiser.
6              STEPHENS V. UNION PAC. R.R.

    The parties dispute whether that evidence is sufficient,
but we need not resolve the question because we agree with
the district court that Stephens failed to create a genuine
issue of fact on whether any exposure that may have
occurred was a substantial factor in causing his disease.
When an injury has more than one possible cause, Idaho
requires a plaintiff to show that the defendant’s conduct was
a “substantial factor” in causing the injury. Garcia v.
Windley, 164 P.3d 819, 823 (Idaho 2007). The substantial-
factor test is more permissive than the “but-for” test applied
in cases involving only a single possible cause. The but-for
test asks whether an injury would have occurred “in a
hypothetical world absent the defendant’s alleged
negligence.” Newberry v. Martens, 127 P.3d 187, 190 (Idaho
2005). The substantial-factor test, by contrast, can be
satisfied even if the defendant’s negligence is one of several
factors that contributed to the injury, and even if the injury
would have happened without it. In other words, the
defendant’s negligence “need not be the sole factor, or even
the primary factor, in causing the plaintiff’s injuries, but
merely a substantial factor.” Fouche v. Chrysler Motors
Corp., 692 P.2d 345, 348 (Idaho 1984).

    While the substantial-factor test is a “liberal standard,”
Doe v. Sisters of Holy Cross, 895 P.2d 1229, 1233 (Idaho
1995), it is not without limit. See, e.g., Munson v. Idaho
Dep’t of Highways, 531 P.2d 1174, 1176–77 (Idaho 1975)
(affirming summary judgment because plaintiff failed to
create a genuine issue of fact on causation under substantial-
factor test). In the context of asbestos claims, we have held
that the substantial-factor test requires “demonstrating that
the injured person had substantial exposure to the relevant
asbestos for a substantial period of time.” McIndoe v.
Huntington Ingalls Inc., 817 F.3d 1170, 1176 (9th Cir.
2016). “Evidence of only minimal exposure to asbestos is
               STEPHENS V. UNION PAC. R.R.                    7

insufficient”; instead, the plaintiff must demonstrate “a high
enough level of exposure that an inference that the asbestos
was a substantial factor in the injury is more than
conjectural.” Id. (internal quotation marks omitted).

    Our decision in McIndoe involved federal maritime law,
but we believe that Idaho courts would apply the same
principles of tort causation that we applied in that case. Our
approach in McIndoe did not reflect a uniquely maritime rule
but instead was based on settled common-law principles
described in the Restatement. Id. at 1176 n.6, 1177.
Specifically, we relied on the Restatement (Third) of Torts
in concluding that liability cannot be based on “fleeting or
insignificant encounters” with asbestos. Id. at 1177. Our
reasoning in McIndoe paralleled that of Idaho decisions that
have relied on similar limiting principles articulated in the
Restatement (Second) of Torts. See, e.g., Beers v. Corp. of
Pres. of Church of Jesus Christ of Latter-Day Saints,
316 P.3d 92, 98 (Idaho 2013) (relying on the Second
Restatement to limit duty to protect third persons); Mico
Mobile Sales & Leasing, Inc. v. Skyline Corp., 546 P.2d 54,
57–58 (Idaho 1975) (relying on the Second Restatement to
limit liability resulting from a superseding cause). At least
on this point, the Third Restatement and the Second
Restatement are in accord. See Restatement (Third) of Torts:
Liab. for Physical & Emotional Harm § 36 cmt. a (2010)
(“[T]his Section preserves the limitation on liability that the
substantial-factor requirement in the prior Restatements
might have played in this situation.”). Given Idaho’s prior
reliance on the Restatement, and in light of the decisions of
courts in “numerous jurisdictions that employ the
substantial-factor standard to limit [the] scope of liability in
asbestos cases,” we believe that Idaho courts would apply
the substantial-factor test the same way we have. McIndoe,
817 F.3d at 1176 n.6. We therefore look to whether Stephens
8              STEPHENS V. UNION PAC. R.R.

has shown that exposure from Union Pacific sources was
“sufficiently sustained (or frequent) and intense to constitute
a proximate cause” of his mesothelioma. Id. (quoting Menne,
861 F.2d at 1461).

    In an effort to establish causation, Stephens relied on the
testimony of two experts: Dr. William Longo, a materials
scientist, and Dr. Andrew Churg, a pathologist. Dr. Longo
stated that “Stephens would have been exposed to significant
levels of airborne asbestos fibers” from his father’s work
clothes, and Dr. Churg, in turn, opined that “Stephens’
exposure to amosite asbestos from the Union Pacific
roundhouse . . . . was a substantial contributing cause of his
mesothelioma.” We agree with the district court that those
opinions are insufficient.

    Expert testimony cannot create a genuine issue of
material fact if it rests on assumptions that are not supported
by evidence. Federal Rule of Evidence 702(b) permits the
introduction of expert testimony only if “the testimony is
based on sufficient facts or data.” The expert’s opinion must
rest on “facts or data in the case that the expert has been
made aware of or personally observed,” not merely
assumptions and speculation. Fed. R. Evid. 703; see
Guidroz-Brault v. Mo. Pac. R.R. Co., 254 F.3d 825, 830–31
(9th Cir. 2001); DePaepe v. Gen. Motors Corp., 141 F.3d
715, 720 (7th Cir. 1998). A party’s own speculation is
insufficient to create a genuine issue of material fact, see
Nelson v. Pima Cmty. Coll., 83 F.3d 1075, 1081–82 (9th Cir.
1996), and a party cannot make it sufficient simply by
finding an expert who is willing to assume its correctness.

    Here, a key premise of both experts’ analyses was that
Stephens was frequently exposed to asbestos. For example,
Dr. Longo explained in some detail how Stephens’s father’s
exposure to asbestos would result in contamination at home,
               STEPHENS V. UNION PAC. R.R.                  9

which in turn would result in exposure to Stephens.
Similarly, Dr. Churg opined that Stephens’s exposure to his
father’s work clothes would have been sufficient on its own
to cause his disease because that secondary exposure was
“much more frequent” than Stephens’s “occasional visits” to
his father’s workplace. But both Dr. Longo and Dr. Churg
admitted that they had no knowledge of the degree to which
Stephens’s father was exposed to asbestos at work. Without
knowing that, they had no basis to conclude that Stephens
was exposed to asbestos at home with any regularity.

    Even when reasonable inferences are drawn in
Stephens’s favor, the evidence in the record does not support
the experts’ assumption that Stephens’s father was regularly
exposed to asbestos. As we have explained, the evidence
was, at best, barely sufficient to show any exposure at all.
Stephens testified that on no more than four occasions he
saw insulation being removed and replaced on steam engines
at the Union Pacific roundhouse. While Stephens testified
that his father’s work clothes were dusty, that does not
establish the frequency with which he was exposed to
asbestos dust. The lack of detail in Stephens’ testimony
about his father’s job is certainly understandable given that
Stephens was between six and eight years old at the time—
more than 60 years ago—but whatever the reason, Stephens
did not recall his father’s job title or duties, and he had no
basis for saying whether his father would have been working
in close proximity to steam engines. Stephens points to
admissions by Union Pacific that asbestos-containing steam
engines underwent regular maintenance, but those
admissions establish only that such work occurred regularly
systemwide; they do not say anything about how often it
occurred at the Weiser roundhouse.
10             STEPHENS V. UNION PAC. R.R.

    Stephens asserts that Union Pacific waived any objection
to the admissibility of Dr. Churg’s testimony, but that is
incorrect. In its reply in support of its summary judgment
motion, Union Pacific argued that “Dr. Churg did not base
his opinion on sufficient facts or data,” and it urged the
district court to exclude his opinion. In so arguing, Union
Pacific correctly identified the fundamental problem in
Stephens’s case: the lack of evidence about exactly what
happened at the Weiser roundhouse in the 1940s and 1950s.
That is not a subject on which Stephens’s experts have any
expertise—or any other basis for knowledge—so their
testimony cannot fill the evidentiary gap.

    Stephens notes that Idaho courts have cautioned against
“second-guessing the facts upon which . . . experts choose to
rely.” Earl v. Cryovac, A Div. of W.R. Grace Co., 772 P.2d
725, 728 (Idaho Ct. App. 1989). In Earl, the court considered
the standards applicable to the admission of expert testimony
under Idaho law. But the “standards for admitting expert
evidence” in a diversity case are “matters that fall on the
procedural side of the Erie divide” and are governed by
federal law, not state law. Wallace v. McGlothan, 606 F.3d
410, 419 (7th Cir. 2010); see also Wackman v. Rubsamen,
602 F.3d 391, 400 n.2 (5th Cir. 2010). In any event, even if
Dr. Longo and Dr. Churg were permitted to testify as
experts, their assumption of facts that Stephens has failed to
establish cannot, on its own, create a dispute of material fact.
And the court in Earl recognized that an expert “must be
accorded substantial deference in the selection of data upon
which he chooses to base his opinion,” a proposition entirely
consistent with our holding that the expert’s opinion must
rest on actual data, not unfounded assumptions. Earl,
772 P.2d at 728 (emphasis added).
              STEPHENS V. UNION PAC. R.R.              11

   Stephens failed to establish that he was regularly
exposed to asbestos attributable to Union Pacific. Without
such evidence, Stephens cannot create a genuine issue of
material fact on whether his secondary exposure was a
substantial factor in causing his disease, and he cannot
prevail on his negligence claims.

   AFFIRMED.
