                  FOR PUBLICATION
  UNITED STATES COURT OF APPEALS
       FOR THE NINTH CIRCUIT

In re: HANFORD NUCLEAR                  
RESERVATION LITIGATION,


BARBARA JEAN PHILLIPS,
                           Plaintiff,
                 and
WANDA BUCKNER; SHIRLEY                        No. 05-35648
CARLISLE,
               Plaintiffs-Appellants,          D.C. No.
                                            CV-91-03015-WFN
                  v.
E.I. DUPONT DE NEMOURS & CO., a
Delaware corporation; GENERAL
ELECTRIC CO., a New York
corporation; UNC NUCLEAR
INDUSTRIES, INC., a Delaware
corporation,
              Defendants-Appellees.
                                        




                             3583
3584    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION



In re: HANFORD NUCLEAR                  
RESERVATION LITIGATION,


BARBARA JEAN PHILLIPS,
                           Plaintiff,
                 and
GLORIA HOPE; CLARA REISS;
GLENDA WINSLOW; KATHRYN J.                    No.05-35651
GOLDBLOOM, aka Kathryn Janelle
Goldbloom,                                     D.C. No.
               Plaintiffs-Appellants,       CV-91-03015-WFN
                  v.
E.I. DUPONT DE NEMOURS & CO., a
Delaware corporation; GENERAL
ELECTRIC CO., a New York
corporation; UNC NUCLEAR
INDUSTRIES, INC., a Delaware
corporation,
              Defendants-Appellees.
                                        
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION   3585



In re: HANFORD NUCLEAR                  
RESERVATION LITIGATION,


BARBARA JEAN PHILLIPS,
                           Plaintiff,
                and
GLORIA HOPE; CLARA REISS;
GLENDA WINSLOW; WANDA
BUCKNER; KATHRYN J. VANCAMPEN,
aka Kathryn Janelle Goldbloom;                No. 05-35678
SHIRLEY CARLISLE,
               Plaintiffs-Appellees,           D.C. No.
                                            CV-91-03015-WFN
                 v.
E.I. DUPONT DE NEMOURS & CO., a
Delaware corporation; GENERAL
ELECTRIC CO., a New York
corporation,
             Defendants-Appellants,
                and
UNC NUCLEAR INDUSTRIES, INC., a
Delaware corporation,
                         Defendant.
                                        
3586   IN RE: HANFORD NUCLEAR RESERVATION LITIGATION



BARBARA JEAN PHILLIPS,                 
                          Plaintiff,
               and
STEVEN STANTON; GLORIA WISE,                 No. 05-35866
              Plaintiffs-Appellees,           D.C. No.
                v.                         CV-91-03015-WFN
E.I. DUPONT DE NEMOURS & CO.;
GENERAL ELECTRIC CO.,
           Defendants-Appellants.
                                       
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION   3587



PAMELA DURFEY; PAULENE ECHO             
HAWK; DOROTHY GEORGE, on their
own behalf and on behalf of a
class of similarly situated persons,
               Plaintiffs-Appellants,
                  v.
E. I. DUPONT DE NEMOURS & CO.,
a Delaware Corporation; GENERAL
ELECTRIC CO., a New York
Corporation, UNC NUCLEAR                      No. 05-35892
INDUSTRIES, INC., a Delaware                   D.C. No.
Corporation, ATLANTIC RICHFIELD             CV-93-03087-WFN
COMPANY, ATLANTIC RICHFIELD-
HANFORD CO., a Washington
Corporation; ROCKWELL
INTERNATIONAL CORP., a Delaware
Corporation; WESTINGHOUSE
HANFORD CORP., a Delaware
Corporation; WESTINGHOUSE
ELECTRIC CORP., a Pennsylvania
Corporation,
              Defendants-Appellees.
                                        
3588    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION



In re: HANFORD NUCLEAR                  
RESERVATION LITIGATION,


PAMELA DURFEY; PAULINE ECHO
HAWK; DOROTHY GEORGE, on their
own behalf and on behalf of a
class of similarly situated persons,
               Plaintiffs-Appellants,
                  v.
E.I. DUPONT DE NEMOURS & CO., a
Delaware corporation; GENERAL                 No. 05-35895
ELECTRIC CO., a New York
corporation; UNC NUCLEAR
                                               D.C. No.
                                            CV-91-03015-WFN
INDUSTRIES, INC., a Delware
corporation; ATLANTIC RICHFIELD
COMPANY; ATLANTIC RICHFIELD-
HANFORD CO., a Washington
corporation; ROCKWELL
INTERNATIONAL CORP., a Deleware
corporation; WESTINGHOUSE
HANFORD CORPORATION, a Delaware
corporation; WESTINGHOUSE
ELECTRIC CORP., a Pennsylvania
corporation,
              Defendants-Appellees.
                                        
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION   3589



In re: HANFORD NUCLEAR                 
RESERVATION LITIGATION,
                                             No. 06-35165
                                               D.C. No.
SHANNON C. RHODES,
                                           CV-91-03015-WFN
                Plaintiff-Appellant,
                v.                            ORDER
                                             AMENDING
E.I. DUPONT DE NEMOURS & CO., a             OPINION AND
Delaware corporation; GENERAL                 AMENDED
ELECTRIC CO., a New York                       OPINION
corporation,
             Defendants-Appellees.
                                       
       Appeal from the United States District Court
          for the Eastern District of Washington
      Wm. Fremming Nielsen, Senior Judge, Presiding

                  Argued and Submitted
          February 7, 2007—Pasadena, California

                  Filed August 14, 2007
                  Amended April 4, 2008

    Before: Mary M. Schroeder, Alfred T. Goodwin and
          Michael Daly Hawkins, Circuit Judges.

                Opinion by Judge Schroeder
3594    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
                          COUNSEL

Peter Nordberg, Berger & Montague, P.C., Philadelphia,
Pennsylvania; Roy S. Haber, Roy S. Haber, P.C., Eugene,
Oregon; Daniel Johnson and David Breskin, Short Cressman
& Burgess, PLLC, Seattle, Washington, for the plaintiffs-
appellants-appellees.

Christopher Landau, Kirkland & Ellis, LLP, Washington,
D.C., for the defendants-appellees-appellants.


                           ORDER

  The opinion in this matter filed on August 14, 2007, and
published at In re Hanford Nuclear Reservation Litigation,
497 F.3d 1005 (9th Cir. 2007), is amended as follows.

   On slip op. 9798, delete the last two sentences of the first
full paragraph and replace with the following:

    We hold that any Hanford Plaintiffs who filed inde-
    pendent suits pending class certification are entitled
    to class action tolling.

  On slip op. 9807, delete the last sentence of the first partial
paragraph and substitute the following:

    Congress did not have the benefit of any well-
    established common law principles relating to the
    government contractor defense when Congress
    drafted the relevant provisions of the PAA. The
    Supreme Court defined the defense only a few weeks
    before the PAA was signed into law.

  Delete the last sentence of the first partial paragraph on slip
op. 9808 and substitute the following:
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3595
    We therefore conclude that the government contrac-
    tor defense was not judicially well-established at the
    time Congress enacted the PAA.

   Delete the first full paragraph on slip op. 9808 and substi-
tute the following:

       Because Congress did not enact the PAA against
    a backdrop of well-established common law princi-
    ples that included the government contractor
    defense, we cannot grant immunity from liability.

       Defendants argue that even if the doctrine was not
    judicially well-established, Congress passed the
    1988 amendments to the Act with the defense in
    mind and intended that it apply. The defendants
    point to a provision in the PAA relating to under-
    ground detonation. The provision is 42 U.S.C.
    § 2210(d)(7), which states as follows:

         A contractor with whom an agreement of
         indemnification has been executed under
         paragraph (1)(A) and who is engaged in
         activities connected with the underground
         detonation of a nuclear explosive device
         shall be liable, to the extent so indemnified
         under this subsection, for injuries or dam-
         age sustained as a result of such detonation
         in the same manner and to the same extent
         as would a private person acting as princi-
         pal, and no immunity or defense founded in
         the Federal, State, or municipal character of
         the contractor or of the work to be per-
         formed under the contract shall be effective
         to bar such liability.

    Defendants contend the provision demonstrates that
    Congress intended to eliminate the modern govern-
3596   IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
    ment contractor defense for underground detonation
    and further contend that the provision demonstrates
    Congress intended the government contractors
    defense as we know it today to apply to all other
    claims arising out of nuclear incidents.

       The language of § 2210(d)(7) is not clear. It refers
    to “immunity or defense” founded on the “character
    of the contractor or of the work.” It thus appears to
    be referring to traditional sovereign immunity from
    any liability rather than the more sophisticated prin-
    ciples of accountability that underlie modern excep-
    tions of governmental tort liability. See Federal Tort
    Claims Act, 28 U.S.C. § 2680(a); Boyle, 487 U.S. at
    512.

       Even assuming, however, that Congress intended
    to ensure that the modern defense did not apply to
    underground detonation claims, it does not follow
    that Congress also intended, without saying so, that
    the defense would apply in all other situations. Such
    a result would conflict with the Congressional statu-
    tory aim to provide compensation for nuclear inju-
    ries.

       The defense is therefore inconsistent with Con-
    gressional purpose and the PAA preempts the
    defense for that reason as well. Congress drafted a
    precise, comprehensive litigation scheme for injuries
    sustained in a nuclear incident. The federal courts
    have recognized this Congressional intent. See
    O’Conner v. Commonwealth Edison Co., 13 F.3d
    1090, 1099 (7th Cir. 1994); In re TMI Litig. Cases,
    940 F.2d 832, 854-55 (3d Cir. 1991) (“In re TMI
    Litig.”). That scheme governs the conduct of this liti-
    gation. As the district court correctly concluded,
    “Congress has clearly spoken to the claims at issue
    in this case and its pronouncement, not a more gen-
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION          3597
    eral court-created common law defense[,] should
    govern the resolution of the claims.” In re Hanford
    Nuclear Reservation Litig., No. 91-3015, slip op. at
    11 (E.D. Wash. Mar. 30, 2004).

  Delete the last sentence of the first partial paragraph on slip
op. 9820, the first partial paragraph on that page, the para-
graph spanning slip op. 9820-21, and the first full paragraph
on slip op. 9821. Substitute the following:

       At the time this case was submitted for decision,
    the leading authority in this area was a district court
    opinion from the Southern District of New York. In
    re Worldcom, Inc. Sec. Litig., 294 F. Supp. 2d 431,
    451 (S.D.N.Y. 2003). The Sixth Circuit agreed with
    the reasoning of Worldcom and held that American
    Pipe tolling is not available to a plaintiff who files
    a separate action pending class certification. Wyser-
    Pratte Mgmt. Co. v. Telxon Corp., 413 F.3d 553, 569
    (6th Cir. 2005). So did the other district courts that
    dealt with the issue. See, e.g., In re Heritage Bond
    Litig., 289 F. Supp. 2d 1132, 1150 (C.D. Cal. 2003);
    see also Wyser-Pratte Mgmt. Co., 413 F.3d at 569
    (citing cases).

       The Second Circuit, however, has vacated the dis-
    trict court decision in WorldCom. In re Worldcom
    Sec. Litig., 496 F.3d 245, 256 (2nd Cir. 2007). The
    Second Circuit has now held that American Pipe
    tolling does apply to plaintiffs who file their actions
    before a decision on class certification. The Second
    Circuit explained that applying American Pipe toll-
    ing to plaintiffs who filed individual suits before cer-
    tification is consistent with the purpose underlying
    statutes of limitations. Id. at 255. Statutes of limita-
    tions are intended to provide notice to defendants of
    a claim before the underlying evidence becomes
    stale. As the Supreme Court held in American Pipe,
3598    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
    however, the filing of a timely class action provides
    defendants with notice of the claim, so a follow-on
    individual suit cannot surprise defendants. Id.

       The Second Circuit also reasoned that although
    the American Pipe doctrine protects plaintiffs from
    being forced to file suit before the certification deci-
    sion, that doesn’t mean that plaintiffs who file before
    certification are not entitled to tolling. Id. at 256.
    They have a right to file at the time of their choosing
    and denying tolling would diminish that right. We
    find the Second Circuit’s reasoning persuasive and
    adopt it.

       We therefore conclude that members of the
    plaintiff-class who have filed individual suits are
    entitled to the benefits of American Pipe tolling.
    This includes Bellwether Plaintiff, Wise.

  On slip op. 9822, delete the first sentence of the first full
paragraph and replace it with the following:

    Although in Berg we referred to bodily injury as a
    jurisdictional prerequisite, id. at 1131-33, we used
    the term “jurisdictional” in the loose sense, perhaps
    too loose, to mean that medical monitoring claims
    were not compensable under the PAA absent physi-
    cal injury.

  On slip op. 9822, delete the last sentence of the first full
paragraph, and substitute the following:

    The district court in this case clearly had subject
    matter jurisdiction under the PAA to decide the
    issue; the district court simply did not have the
    power to grant the relief requested because the plain-
    tiffs have not suffered any physical injury.
         IN RE: HANFORD NUCLEAR RESERVATION LITIGATION     3599
   On slip op. 9836, second full paragraph, delete the phrase
“, and on statute of limitations grounds, the judgment in favor
of Plaintiff Wise”. In the fifth sentence of that paragraph,
replace the phrase “as well as the judgment in favor of Plain-
tiff Stanton” with “as well as the judgments in favor of Plain-
tiffs Stanton and Wise.”

  Defendant-Appellants’ Petition for Rehearing En Banc,
Defendant-Appellees’ Petition for Panel Rehearing, and
Plaintiffs’ Petition for Panel Rehearing and Rehearing En
Banc are denied.

  No subsequent petition for rehearing or rehearing en banc
may be filed.


                          OPINION

SCHROEDER, Circuit Judge:

I.   Introduction.

   The origins of this case trace back more than sixty years to
the height of World War II when the federal government
solicited Appellants E.I. DuPont de Nemours & Co., General
Electric, Inc., UNC Nuclear Industries, Inc., Atlantic Rich-
field Co., and Rockwell International Corp., (collectively
“Defendants”) to operate the Hanford Nuclear Weapons Res-
ervation (“Hanford”) in southeastern Washington. The Han-
ford Reservation was a plutonium-production facility that
helped make the atomic bomb that dropped on Nagasaki,
Japan in World War II.

   A regrettable Hanford byproduct was the radioiodine emit-
ted into the surrounding area. The plaintiffs in this litigation
are over two thousand residents who now claim that these
emissions, known as I-131, caused various cancers and other
3600    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
life-threatening diseases. The first group of plaintiffs filed a
complaint in 1990 under the federal statute governing nuclear
accidents, the Price-Anderson Act (“PAA”), claiming they
were entitled to damages for injuries arising from a nuclear
incident pursuant to 42 U.S.C. § 2210. The history is dis-
cussed in our earlier opinions in In re Hanford Nuclear Reser-
vation Litigation, 292 F.3d 1124 (9th Cir. 2002) (“In re
Hanford”); and Berg v. E.I. DuPont de Nemours & Co., 293
F.3d 1127 (9th Cir. 2002) (“Berg”). After almost two decades
of litigation, which already has included two appeals to this
court, the parties in 2005 agreed to a bellwether trial. The trial
was designed to produce a verdict that would highlight the
strengths and weaknesses of the parties’ respective cases and
thus focused on six plaintiffs (“Plaintiffs”) who were repre-
sentative of the larger group. The purpose of the trial was to
promote settlement and bring long-overdue resolution to this
litigation.

   Before us on appeal is a litany of issues stemming from the
bellwether trial. A threshhold issue is whether Defendants
may seek complete immunity under the common law govern-
ment contractor defense, because they were operating Han-
ford at the request of the federal government. We hold that the
defense is inapplicable as a matter of law, because Congress
enacted the PAA before the courts recognized the government
contractor defense, and the PAA provides a comprehensive
liability scheme that precludes Defendants’ reliance on such
a defense.

   In the alternative, Defendants argue that even if they are
not immune, they are not strictly liable for any I-131 emis-
sions, because the amounts of the emissions were within
federally-authorized levels; the plutonium-production process
was not an abnormally dangerous activity that would create
strict liability; and even if it were, Defendants qualify for the
“public duty” exception to strict liability. The district court
held that none of Defendants’ contentions were sufficient to
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3601
relieve them of strict liability for the injuries they caused. We
agree.

   With respect to the trial itself, the district court with admi-
rable diligence ruled on many issues of first impression. We
hold that under Washington law, the district court properly
instructed the jury that to impose liability, it had to find Han-
ford was the “but for” cause of Plaintiffs’ diseases and not
just a contributing cause under the more lenient “substantial
factor” test. The court also made a host of evidentiary rulings
that are before us on appeal. We hold that three of these rul-
ings constitute reversible error with respect to three of the
Bellwether Plaintiffs.

   There are statute of limitations issues as well. We hold that
any Hanford Plaintiffs who filed independent suits pending
class certification are entitled to class action tolling.

  Lastly, we hold that the district court properly dismissed
any medical monitoring claims as not cognizable under the
PAA. This is consistent with our decision in Berg, 293 F.3d
1127.

II.   Background.

   The United States government constructed Hanford during
World War II to manufacture plutonium for military purposes.
The facility was a component of the Army Corps of Engi-
neer’s secret Manhattan Project, with the primary objective of
developing an atomic bomb. In 1942, the Army Corps began
hiring civilian contractors to help build and operate the Han-
ford facility. It first recruited the University of Chicago Met-
allurgical Laboratory (“Met Lab”) to design the process and
equipment to produce plutonium. It then solicited E.I. DuPont
de Nemours & Co. (“DuPont”) to actually run the facility. It
is apparent the government itself did not have the expertise or
resources to operate Hanford.
3602    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
   DuPont initially refused. The government, however, per-
sisted and implored DuPont to run the plutonium-production
facility, because, as the government provided in DuPont’s
contract, the project was of the “utmost importance” and was
“necessary in facilitating the prosecution of the war.” DuPont
eventually acquiesced, stating it would run the facility out of
patriotic considerations. It accepted only one dollar as pay-
ment for its services. Several years later, the Hanford facility
successfully produced the plutonium that was used in 1945 to
drop the atomic bomb on Nagasaki and effectively end World
War II. (The bomb dropped on Hiroshima was uranium-
based, not plutonium-based).

   As part of the plutonium-production process, the Hanford
facility emitted I-131, a fission byproduct known as radioio-
dine. I-131 was known at the time to have potential adverse
health effects on humans. Accordingly, the Met Lab scientists
set tolerance doses for human exposure. For example, the Met
Lab determined that the human thyroid should not absorb
more than one rad per day for those individuals subject to
continuous exposure in the area. A rad is a measurement of
the amount of radioiodine absorbed into an organ or tissue.
On the basis of these safe exposure limit estimates, the Met
Lab approved a detailed operating procedure that would
ensure that the plutonium was produced within those emission
limits. The key to decreasing I-131 emissions was to allow for
longer cooling times of the uranium slugs used to produce the
plutonium. This strategy, however, often conflicted with the
federal government’s orders to increase plutonium produc-
tion.

   On September 1, 1946, DuPont transferred its duties to
General Electric (“GE”), which also agreed to earn no profit
from its work. GE ran the Hanford facility through the Cold
War. During the period of its operation, GE asked the federal
government to increase cooling times to allow for lower emis-
sions of I-131. By this time, Congress had established the
Atomic Energy Commission (“AEC”), see 42 U.S.C.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION      3603
§§ 2011-2013 (1946), and GE was bound by its determina-
tions. The AEC denied the request for longer cooling times,
and GE continued to produce plutonium consistent with gov-
ernment demands. By the 1950s, however, significant
improvements were made to the production process, and I-
131 emission levels dropped.

   In 1987, the United States Department of Energy (“DOE”)
created the Hanford Environmental Dose Reconstruction Proj-
ect (“HEDR”), overseen by the Center for Disease Control
and Prevention. The underlying purpose of the HEDR was to
estimate and reconstruct all radionuclide emissions from Han-
ford from 1944 to 1972 in order to ascertain whether neigh-
boring individuals and animals had been exposed to harmful
doses of radiation. Of particular concern to the HEDR were
the estimated doses of I-131 received by the thyroid glands of
humans, principally through consumption of milk from cows
that ingested contaminated vegetation on neighboring farms
and pastures. The HEDR concluded that I-131 emissions
peaked during the period from 1944 to 1946, when an esti-
mated 88% of Hanford’s total iodine emissions occurred.
HEDR explained that in later years, emissions declined
because of technological advances. In 1990, the Technical
Steering Panel of HEDR released a report entitled Initial Han-
ford Radiation Dose Estimates that publicly disclosed for the
first time that large quantities of radioactive and non-
radioactive substances had been released from Hanford,
beginning in the 1940s.

   This disclosure sparked a blaze of litigation. Thousands of
plaintiffs filed suit pursuant to the Price-Anderson Act, 42
U.S.C. § 2210(n)(2), which had been amended in 1988 to pro-
vide exclusive federal jurisdiction over all claims arising from
a nuclear incident, otherwise known as public liability actions.
The PAA allowed the plaintiffs to sue private parties, such as
DuPont, and to consolidate the claims in federal district court.
Id. While Congress wanted to ensure that victims of nuclear
incidents recovered compensation, it also included govern-
3604    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
ment indemnification provisions in the PAA to give private
parties an incentive to participate in the nuclear industry. See,
e.g., S. REP. NO. 100-70, at 14 (1988), reprinted in 1988
U.S.C.C.A.N. 1424, 1425-26.

   The PAA provides that although federal courts have exclu-
sive and original jurisdiction over claims stemming from
nuclear incidents, the substantive rules of decision are pro-
vided by the law of the state in which the nuclear incident
occurs. See 42 U.S.C. § 2014(hh). Plaintiffs therefore brought
tort claims under Washington law, asserting that because
Defendants were engaged in an abnormally dangerous activ-
ity, they were strictly liable for any Hanford-caused radiation
illness.

   On August 6, 1990, a group of plaintiffs filed a joint con-
solidated complaint in the Eastern District of Washington,
alleging a class action against Defendants. In 1991, the dis-
trict court consolidated any and all Hanford-related actions
pending in various courts, directed preparation of one consoli-
dated complaint, and designated specific lead counsel for all
parties. In an order dated September 22, 1994, the district
court addressed the issue of class certification and decided to
reserve decision under Federal Rule of Civil Procedure
23(b)(3) pending further discovery on causation issues.

   Accordingly, pending class certification, the litigation pro-
ceeded as a consolidated action. Throughout this period, the
district court entertained a handful of dispositive motions,
which led to two appeals to this court. In 2002, we heard In
re Hanford, 292 F.3d 1124, challenging the district court’s
dismissal of plaintiffs who could not establish they received
a “doubling dose” of radiation. We held that Plaintiffs’ claims
should proceed even if they could not show that Hanford radi-
ation doubled their risk of illness, and we remanded for trial.
Id. at 1139.

  We also heard the appeal in the related case, Berg, 293 F.3d
1127. The Berg plaintiffs had not yet suffered from any ill-
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION     3605
ness, but sued Defendants for medical monitoring. We held
that medical monitoring claims were not compensable under
the PAA and upheld the district court’s dismissal of those
actions with prejudice. Id. at 1133.

   After our decisions in In re Hanford and Berg, Judge Wil-
liam Fremming Nielsen steered the case toward resolution.
The parties agreed to proceed with a bellwether trial, hoping
it would reveal the strengths and weaknesses of their respec-
tive cases and thus pave the way for a settlement. The parties
eventually agreed on twelve bellwether plaintiffs. Six of these
plaintiffs had their claims dismissed on dispositive, pre-trial
motions. The remaining six plaintiffs went to trial in April
2005.

   The Bellwether Plaintiffs represent plaintiffs who suffer
from various thyroid diseases they claim were caused by radi-
ation emanating from Hanford. Plaintiffs Gloria Wise and
Steven Stanton have thyroid cancer. Plaintiffs Wanda Buck-
ner, Shirley Carlisle, and Kathryn Goldbloom suffer from
hypothyroidism, a condition that slows the body’s metabo-
lism. Hypothyroidism is most frequently caused by Hashimo-
to’s disease, an illness that Plaintiffs claim was caused by
Hanford radiation. Plaintiff Shannon Rhodes suffers from
lung cancer, which her doctors concluded was a form of
Hurthle cell thyroid cancer that had metastasized from a thy-
roid lobe previously removed.

   Prior to trial, the Bellwether Plaintiffs made several
motions to strike Defendants’ affirmative defenses. Defen-
dants first claimed that the government contractor defense
insulated them from all liability. The district court, in an
unpublished 2003 order, struck the defense under Federal
Rule of Civil Procedure 12(b), holding that the PAA displaced
any such defense as a matter of law. In a published order, the
court also ruled that plutonium production at Hanford was an
abnormally dangerous activity warranting strict liability under
Washington law. In re Hanford Nuclear Res. Litig., 350 F.
3606    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
Supp. 2d 871, 888 (E.D. Wash. 2004). It then limited the
issues at trial to causation and damages. Id.

   The primary dispute at trial was whether the amount of
radiation to which each plaintiff was exposed was sufficient
to be the cause-in-fact of his or her thyroid disease. There was
extensive testimony that I-131 radiation causes Hashimoto’s
disease, a cause of hypothyroidism, and that I-131 can also be
a contributing factor to thyroid cancer. The testimony
revealed, however, that to date epidemiological studies can
establish only that radiation of at least 100 rads is a contribut-
ing factor to thyroid illness. Some epidemiological studies
hypothesize that 40 rads might cause Hashimoto’s disease,
but there are no data beyond that threshold.

   Because many Plaintiffs were not exposed to radiation
above 40 rads, and no Plaintiff was exposed to radiation
above 100 rads, Plaintiffs had to present expert testimony that
scientific extrapolation permitted a finding of causation below
40 rads. Their primary experts were Dr. Terry Davies, an
endocrinologist, Dr. Sara Peters, a pathologist, Dr. F. Owen
Hoffman, a causation expert, and Dr. Colin Hill, a radiation
cell biologist. Plaintiffs also proffered the expert testimony of
epidemiologist Dr. A. James Ruttenber, but key parts of his
testimony relating to causation were excluded.

   After fourteen days of trial and four days of deliberations,
the jury found in favor of two plaintiffs, Steve Stanton and
Gloria Wise; the jury hung with respect to one plaintiff, Shan-
non Rhodes; and it found in favor of Defendants with respect
to the remaining three plaintiffs, Wanda Buckner, Shirley
Carlisle, and Kathryn Goldbloom. As damages for prevailing
plaintiffs, the jury awarded Stanton $227,508 and Wise
$317,251. Because the jury could not reach a verdict with
respect to Plaintiff Rhodes, the district court declared a mis-
trial. Rhodes re-tried her claims in front of a second jury in
November 2005, and the jury entered a defense verdict on all
counts.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION      3607
   Rhodes, along with the three non-prevailing plaintiffs in the
first trial, appeal a variety of evidentiary rulings, as well as
the district court’s jury instruction that under Washington law
Plaintiffs had to prove “but-for” causation, rather than “sub-
stantial factor” causation. Defendants appeal the judgments
entered in favor of the two prevailing plaintiffs, claiming the
district court erred as a matter of law in striking the govern-
ment contractor defense. In the alternative, Defendants argue
that Plaintiffs may not proceed under a strict liability theory,
because the I-131 emissions were within federally-authorized
levels. They also contend the plutonium-production process
was not an abnormally dangerous activity under Washington
law and, even if it were, that Defendants qualify for the nar-
row “public duty” exception to strict liability.

   Defendants also contend that prevailing Plaintiff Wise’s
suit was untimely under Washington’s statute of limitations.
See Am. Pipe & Constr. Co. v. Utah, 414 U.S. 538 (1974).
Wise had filed an independent action in 1997, likely beyond
the statutory period, but Defendants did not invoke American
Pipe and the district court allowed Wise’s claim to proceed as
part of the pending class action.

   Apart from the issues relating to the Bellwether Plaintiffs,
Plaintiffs Pamela Durfey, Paulene Echo Hawk, and Dorothy
George, who do not yet have symptoms of any thyroid dis-
ease, sued Defendants for the costs of medical monitoring.
The district court, following this court’s decision in Berg, 293
F.3d at 1132-33, held that the PAA precluded any medical
monitoring claims that were unaccompanied by physical
injury. Rather than remanding those claims to state court,
however, the district court held that the PAA bestowed exclu-
sive jurisdiction in the federal courts for claims arising from
a nuclear incident, and that therefore the PAA’s provisions
preempted any state-derived medical monitoring claim.
Accordingly, it directed entry of final judgment for DuPont
under Federal Rule of Civil Procedure 54(b). The plaintiffs
appeal this dismissal.
3608     IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
III.   The Government Contractor Defense.

   The overarching issue before us is Defendants’ contention
that the government contractor defense is available to them as
a matter of law and that it provides complete immunity from
liability if its substantive requirements are satisfied. The dis-
trict court held that the affirmative defense was inapplicable
as a matter of law because the provisions of the PAA cannot
be reconciled with the defense and implicitly displace it. We
review de novo the district court’s conclusion that the affirma-
tive defense is unavailable, United States v. Griffin, 440 F.3d
1138, 1143 (9th Cir. 2006), and we reach the same conclu-
sion.

    [1] The government contractor defense is by now an estab-
lished component of federal common law, but it was first rec-
ognized by the Supreme Court less than twenty years ago in
Boyle v. United Techs. Corp., 487 U.S. 500 (1988). The
defense is intended to implement and protect the discretionary
function exception of the Federal Tort Claims Act (“FTCA”),
28 U.S.C. § 2680(a), which was enacted after World War II.
The defense allows a contractor-defendant to receive the ben-
efits of sovereign immunity when a contractor complies with
the specifications of a federal government contract. Boyle,
487 U.S. at 511-12. As the Court said in Boyle, “[i]t makes
little sense to insulate the Government against financial liabil-
ity for the judgment that . . . equipment is necessary when the
Government produces the equipment itself, but not when it
contracts for the production.” Id. at 512.

   As a threshold matter, we agree with Defendants that the
government contractor defense applies not only to claims
challenging the physical design of a military product, but also
to the process by which such equipment is produced. Accord-
ingly, a contractor who agrees to operate a production facility
pursuant to government specifications may qualify for the
defense.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION       3609
   The issue here, however, is whether the PAA preempts reli-
ance on the common law doctrine, either because the defense
contradicts the federal statute or because the statute predates
the defense. Congress is presumed to “legislate against a
background of common-law adjudicatory principles.” Astoria
Fed. Sav. & Loan Ass’n v. Solimino, 501 U.S. 104, 108
(1991). A federal statute enacted after a common law doctrine
has been established will not therefore abrogate the federal
common law rule unless the statute speaks directly to the
question addressed by common law. United States v. Texas,
507 U.S. 529, 534 (1993). This is because “where a common-
law principle is well established . . . the courts may take it as
given that Congress has legislated with an expectation that the
principle will apply except when a statutory purpose to the
contrary is evident.” Astoria, 501 U.S. at 108 (internal quota-
tions and citations omitted) (holding that the court should
assume Congress drafted the Age Discrimination in Employ-
ment Act with the common law administrative estoppel doc-
trine in mind); see also Pasquantino v. United States, 544
U.S. 349, 359 (2005).

   [2] Whether the PAA preempts the government contractor
defense is therefore a two-step inquiry. We must first deter-
mine whether the government contractor defense was well-
established at the time Congress enacted the operative version
of the PAA. If so, we must determine whether a statutory pur-
pose contrary to the government contractor defense is evident.

   The defense fails the first inquiry. Defendants are not enti-
tled to the government contractor defense, because the statute
predates clear judicial recognition of any such defense. In
addition, the statute’s comprehensive liability scheme is
patently inconsistent with the defense and precludes its opera-
tion in this case.

  [3] The Supreme Court’s decision in Boyle was filed on
June 27, 1988. Less than two months later, the PAA was
amended, on August 20, 1988, to include the pertinent lan-
3610    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
guage establishing exclusive federal jurisdiction for all public
liability claims arising from nuclear incidents. While the gov-
ernment contractor defense was technically a recognized com-
mon law principle at the time Congress enacted the PAA, it
was hardly a well-established doctrine. See Astoria, 501 U.S.
at 108. (noting that courts should presume Congress legislated
with an expectation that a common law doctrine would apply
only if the common-law principle was well-established). Con-
gress did not have the benefit of any well-established com-
mon law principles relating to the government contractor
defense when Congress drafted the relevant provisions of the
PAA. The Supreme Court defined the defense only a few
weeks before the PAA was signed into law.

   The origins of the defense in the cases antecedent to Boyle
do not materially affect this analysis. In 1940, the Supreme
Court arguably planted the seeds of the government contractor
defense in Yearsley v. W.A. Ross Constr. Co., 309 U.S. 18,
20-21 (1940). It held that an agent of the government could
not be held liable under the Takings Clause for the defective
construction of a dam that damaged land, as long as the agent
followed government specifications for the dam’s construc-
tion. The Court limited the applicability of the defense to
principal-agent relationships where the agent had no discre-
tion in the design process and completely followed govern-
ment specifications. Nothing in Yearsley extended immunity
to military contractors exercising a discretionary governmen-
tal function. See Boyle, 487 U.S. at 524-25 (J. Brennan, dis-
senting) (Yearsley is “a slender reed on which to base so
drastic a departure from precedent” . . . . “[It] has never been
read to immunize the discretionary acts of those who perform
service contracts for the Government”).

   While some circuit courts began extending the Yearsley
doctrine to military contractors as early as the 1960s, see
McKay v. Rockwell Int’l Corp., 704 F.2d 444, 448-49 (9th
Cir. 1983) (citing cases), other circuits held that Yearsley was
clearly limited to principal-agent relationships and did not
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION       3611
apply to military contractors, see, e.g., Bynum v. FMC Corp.,
770 F.2d 556, 564 (5th Cir. 1985). In light of the conflicting
authority on the matter, it is clear that neither the scope nor
the contours of the defense were well-defined until the
Supreme Court’s 1988 decision in Boyle. We therefore con-
clude that the government contractor defense was not judi-
cially well-established at the time Congress enacted the PAA.

   Because Congress did not enact the PAA against a back-
drop of well-established common law principles that included
the government contractor defense, we cannot grant immunity
from liability.

   Defendants argue that even if the doctrine was not judi-
cially well-established, Congress passed the 1988 amend-
ments to the Act with the defense in mind and intended that
it apply. The defendants point to a provision in the PAA relat-
ing to underground detonation. The provision is 42 U.S.C.
§ 2210(d)(7), which states as follows:

    A contractor with whom an agreement of indemnifi-
    cation has been executed under paragraph (1)(A) and
    who is engaged in activities connected with the
    underground detonation of a nuclear explosive
    device shall be liable, to the extent so indemnified
    under this subsection, for injuries or damage sus-
    tained as a result of such detonation in the same
    manner and to the same extent as would a private
    person acting as principal, and no immunity or
    defense founded in the Federal, State, or municipal
    character of the contractor or of the work to be per-
    formed under the contract shall be effective to bar
    such liability.

Defendants contend the provision demonstrates that Congress
intended to eliminate the modern government contractor
defense for underground detonation and further contend that
the provision demonstrates Congress intended the government
3612    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
contractors defense as we know it today to apply to all other
claims arising out of nuclear incidents.

   The language of § 2210(d)(7) is not clear. It refers to “im-
munity or defense” founded on the “character of the contrac-
tor or of the work.” It thus appears to be referring to
traditional sovereign immunity from any liability rather than
the more sophisticated principles of accountability that under-
lie modern exceptions of governmental tort liability. See Fed-
eral Tort Claims Act, 28 U.S.C. § 2680(a); Boyle, 487 U.S. at
512.

   Even assuming, however, that Congress intended to ensure
that the modern defense did not apply to underground detona-
tion claims, it does not follow that Congress also intended,
without saying so, that the defense would apply in all other
situations. Such a result would conflict with the Congressio-
nal statutory aim to provide compensation for nuclear injuries.

   The defense is therefore inconsistent with Congressional
purpose and the PAA preempts the defense for that reason as
well. Congress drafted a precise, comprehensive litigation
scheme for injuries sustained in a nuclear incident. The fed-
eral courts have recognized this Congressional intent. See
O’Conner v. Commonwealth Edison Co., 13 F.3d 1090, 1099
(7th Cir. 1994); In re TMI Litig. Cases, 940 F.2d 832, 854-55
(3d Cir. 1991) (“In re TMI Litig.”). That scheme governs the
conduct of this litigation. As the district court correctly con-
cluded, “Congress has clearly spoken to the claims at issue in
this case and its pronouncement, not a more general court-
created common law defense[,] should govern the resolution
of the claims.” In re Hanford Nuclear Reservation Litig., No.
91-3015, slip op. at 11 (E.D. Wash. Mar. 30, 2004).

   [4] Congress enacted the PAA with twin goals in mind: to
provide an incentive to contractors to participate in the
nuclear industry by limiting their liability, and to compensate
victims of nuclear accidents. See, e.g., Pub. L. No. 100-408,
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3613
102 Stat. 1066 (1988); S. REP. NO. 100-218, at 4-13 (1987),
reprinted in 1988 U.S.C.C.A.N. 1476, 1479-88. The Act
placed Plaintiffs’ state law claims in federal court and pro-
vided indemnification of Defendants from the federal govern-
ment for any liability to victims of nuclear incidents. See 42
U.S.C. § 2210; S. REP. NO. 100-218, at 13, reprinted in 1988
U.S.C.C.A.N. at 1484, 1488. To allow those entitled to
indemnity as government contractors to disclaim any liability
because they are government contractors would be inconsis-
tent with the goal of the PAA to provide compensation to vic-
tims of nuclear incidents. We will not assume that in enacting
the PAA’s comprehensive scheme, Congress intended, yet
failed to state in the Act, that victims of nuclear incidents can-
not recover tort damages from nuclear operators when the
operators were pursuing government goals. Accordingly, we
hold that the government contractor defense is inapplicable as
a matter of federal law and affirm the district court’s ruling
on this key issue.

IV.   Strict Liability.

   Defendants next argue that the district court erred as a mat-
ter of Washington state law in holding Defendants strictly lia-
ble for any I-131 emissions from the Hanford facility.
Defendants challenge that ruling on three grounds: (1) that
strict liability pursuant to Washington state law may not be
imposed under the PAA if Defendants released I-131 within
federally-authorized emission levels; (2) even if state liability
law applies, the Hanford activity did not meet the “abnor-
mally dangerous activity” test that warrants strict liability; and
(3) even if Washington courts would apply a strict liability
regime, Defendants would be exempted under the “public
duty” exception that applies generally to heavily regulated
entities doing potentially hazardous work. For the reasons
below, we affirm the district court’s imposition of strict liabil-
ity.
3614    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
  A.   Federally-Authorized Emissions.

   [5] It is not disputed that the federal government is in
charge of nuclear safety. “[T]he safety of nuclear technology
[is] the exclusive business of the Federal Government,” which
has “occupied the entire field of nuclear safety concerns.”
Koller v. Pinnacle West Capital Corp., 2007 U.S. Dist.
LEXIS 9186 (D. Ariz. Feb. 7, 2007) (second alteration in
original) (quoting Pac. Gas Elec. Co. v. State Energy Res.
Conservation & Dev. Comm’n, 461 U.S. 190, 208, 212
(1983)). Every federal circuit that has considered the appro-
priate standard of care under the PAA has concluded that
nuclear operators are not liable unless they breach federally-
imposed dose limits. See, e.g., O’Conner, 13 F.3d at 1105; In
re TMI Litig., 940 F.2d at 859; Roberts v. Fla. Power & Light
Co., 146 F.3d 1305, 1308 (11th Cir. 1998); Nieman v. NLO,
Inc., 108 F.3d 1546, 1553 (6th Cir. 1997).

   [6] Defendants are thus correct insofar as they point out
that the clear weight of authority supports the principle that
federal law preempts states from imposing a more stringent
standard of care than federal safety standards. Strict liability
may not be imposed for I-131 releases within federally-
authorized limits, because any federal authorization would
preempt state-derived standards of care. To allow a jury to
decide on the basis of a state’s reasonableness standard of
care would “put juries in charge of deciding the permissible
levels of radiation exposure and, more generally, the ade-
quacy of safety procedures at nuclear plants—issues that have
explicitly been reserved to the federal government.” In re TMI
Gen. Publ. Utils. Corp., 67 F.3d 1103, 1115 (3d Cir. 1995)
(citing Pacific Gas, 461 U.S. at 212). This result would under-
mine the purpose of a comprehensive and exclusive federal
scheme for nuclear incident liability.

   Defendants then go further, however, and argue that the
district court in this case permitted the jury to substitute its
view of a reasonable emission standard for a government
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION       3615
standard. The problem with Defendants’ argument is that no
federal standards governing emission levels existed at the
time of the I-131 emissions. Defendants try to remedy this
problem by pointing to “tolerance doses” recommended and
implemented by military and government scientists working
on the Hanford project and ask us to equate such recommen-
dations with federally-authorized emission levels. They are
not the same.

   These tolerance doses, although established under the aegis
of the United States Army, did not carry the force of law and
thus cannot provide the basis for a safe harbor from liability.
They amounted to no more than site-specific safety rules. The
United States Army instructed the Manhattan Engineering
District to set forth standard, internal operating procedures for
the plutonium-production process at Hanford. The tolerance
doses were part of these procedures. The Met Lab scientists
calculated what they thought were the outer limits of safe
exposure at the plant. These internal guidelines were, how-
ever, exactly and only what they claimed to be: internal. They
were not comprehensive, federal standards governing emis-
sion levels on which Defendants could rely to relieve them
from liability for harm they caused.

   Defendants are correct that it would not have been possible
for an agency to establish emission levels in the early 1940s,
because the Atomic Energy Act was not enacted until 1954
and the Nuclear Regulatory Commission was created in 1974.
In fact, the emissions occurred even prior to the enactment of
the Administrative Procedure Act in 1946. This history, how-
ever, undermines Defendants’ position, because it highlights
the absence of any federal machinery to promulgate legal
standards on which Defendants could have reasonably relied
to insulate them from liability to those living and breathing
twenty-four hours a day in the area surrounding Hanford. The
need for such standards was not recognized until many years
later.
3616      IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
  B.     Abnormally Dangerous Activity.

  Defendants next argue that even if state law standards
apply in this case, the district court erred by holding that
Washington tort law would impose strict liability. Specifi-
cally, Defendants contend that operating the Hanford facility
does not constitute an “abnormally dangerous activity” under
Washington law. We review de novo the question of whether
an activity is abnormally dangerous, Langan v. Valicopters,
Inc., 567 P.2d 218, 221 (Wash. 1977), and we affirm.

  Washington has adopted the Restatement (Second) of
Torts, sections 519 and 520, which outline the strict liability
regime for abnormally dangerous activities. Klein v. Pyrodyne
Corp., 810 P.2d 917, 920 (Wash. 1991); New Meadows Hold-
ing Co. v. Wash. Water Power Co., 687 P.2d 212, 215 (Wash.
1984). Section 519 provides:

       (1) One who carries on an abnormally dangerous
       activity is subject to liability for harm to the person,
       land, or chattels of another resulting from the activ-
       ity, although he has exercised the utmost care to pre-
       vent such harm.

       (2) Such strict liability is limited to the kind of
       harm, the risk of which makes the activity abnor-
       mally dangerous.

Section 520 lists the factors to be used when determining
what constitutes an abnormally dangerous activity:

       (a) Whether the activity involves a high degree of
       risk of some harm to the person, land or chattels of
       another;

       (b) Whether the gravity of the harm which may
       result from it is likely to be great;
          IN RE: HANFORD NUCLEAR RESERVATION LITIGATION     3617
    (c) Whether the risk cannot be eliminated by the
    exercise of reasonable care;

    (d) Whether the activity is not a matter of common
    usage;

    (e) Whether the activity is inappropriate to the
    place where it is carried on; and

    (f)    The value of the activity to the community.

RESTATEMENT (SECOND) OF TORTS §§ 519-20 (1977). A court
does not have to weigh each of the elements listed in § 520
equally. Langan, 567 P.2d at 221. One factor, alone, however,
is generally not sufficient to find an activity abnormally dan-
gerous. Id.

    [7] Defendants argue that at the time of the emissions in the
1940s, they did not know the risks that were attributable to
radioiodine exposure, and therefore § 520’s factors (a)-(c)
cannot be weighed against them. Any possible injury from
radiation, however, need not have been actually known by
Defendants at the time of exposure in order to impose strict
liability. Under Washington law, if the actual harm fell within
a general field of danger which should have been anticipated,
strict liability may be appropriate. Whether an injury should
have been anticipated does not depend on whether the particu-
lar harm was actually expected to occur. Koker v. Armstrong
Cork, Inc., 804 P.2d 659, 667-68 (Wash. Ct. App. 1991). It is
sufficient that “the risk created [be] so unusual, either because
of its magnitude or because of the circumstances surrounding
it. . . .” Langan, 567 P.2d at 221.

   [8] There is no question that Defendants should have antici-
pated some of the many risks associated with operating a
nuclear facility, creating plutonium, and releasing I-131 into
the atmosphere. It is exactly because of these risks, and the
potential exposure to liability arising from them, that the gov-
3618    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
ernment contracted with Defendants to limit liability in case
of an accident. For these same reasons, the Met Lab scientists
recommended dosage limits.

   [9] We agree with the district court that Defendants’ con-
duct at Hanford was an abnormally dangerous activity under
the § 520 factors. There was a high degree of risk to people
and property associated with the Hanford facility and the
gravity of any harm was likely to be great. See RESTATEMENT
(SECOND) OF TORTS § 520. Regardless of Defendants’ efforts
to exercise reasonable care, some I-131 would be released,
and developing plutonium is hardly an activity of common
usage. While the value to the community at large, i.e., the
nation, of developing an atomic bomb was perceived as high
and there is pragmatically no very appropriate place to carry
on such an activity, the § 520 factors on balance support hold-
ing that Defendants’ activities were abnormally dangerous.

  C.   Public Duty Exception to Strict Liability.

   Defendants’ final defense is that even if their conduct con-
stituted an abnormally dangerous activity, they are exempted
from strict liability under Washington law pursuant to the
“public duty” exception. See RESTATEMENT (SECOND) OF TORTS
§ 521. While this issue presents a close question, we conclude
that Defendants do not qualify for the exception.

  Section 521 of the Restatement provides:

    The rules as to strict liability for abnormally danger-
    ous activities do not apply if the activity is carried on
    in pursuance of a public duty imposed upon the actor
    as a public officer or employee or as a common car-
    rier.

Id. As a threshold matter, Washington courts have not yet
adopted § 521. We must therefore decide what the Washing-
ton Supreme Court would likely do if confronted with the
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3619
issue. See NLRB v. Calkins, 187 F.3d 1080, 1089 (9th Cir.
1999). We hold that the court would likely adopt the public
duty exception.

   Although they have never explicitly adopted § 521, Wash-
ington courts have adopted § 519, which governs abnormally
dangerous activities generally. The comments to § 519 indi-
cate that the public duty exception is part and parcel of strict
liability. Comment “a” to § 519 states that “[t]he general rule
stated in this Section is subject to exceptions and qualifica-
tions, too numerous to be included within a single section. It
should therefore be read together with §§ 520 to 524A, by
which it is limited.” RESTATEMENT (SECOND) OF TORTS, § 519
cmt. a. Comment “d” further limits the scope of strict liability
and states that persons are accountable only for abnormally
dangerous activities they undertake “for [their] own pur-
poses.” Id. § 519 cmt. d. A key corollary to this point is that
strict liability does not apply to activities carried on in pursu-
ance of a public duty the actor was legally obligated to per-
form. See id. § 521.

   Although Washington could adopt § 519 without adopting
the numerous exceptions found in §§ 521-524A, it is unlikely
that it would do so. Washington adopted wholesale the abnor-
mally dangerous activity doctrine and its exceptions when
they existed in the First Restatement. See, e.g., Epperly v.
Seattle, 399 P.2d 591, 595 (Wash. 1965); Foster v. Preston
Mill Co., 268 P.2d 645, 647 (Wash. 1954). Furthermore, of
the states that have adopted §§ 519-20, the vast majority has
also adopted the subsequent exceptions.

   [10] Although widely adopted, the courts that have applied
the public duty exception have generally done so only to the
extent a defendant was legally required to perform the
ultrahazardous activity. See RESTATEMENT (SECOND) OF TORTS,
§ 521, cmt. a. The Washington Supreme Court’s decision in
Siegler v. Kuhlman, 502 P.2d 1181 (Wash. 1972), supports
such an application of the public duty doctrine here. The
3620    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
defendants in Siegler were a trucking company for Texaco
and its driver, and the company was not legally obligated as
a common carrier to carry materials that eventually caused an
explosive, fatal accident on a highway. The Washington court
held that the activity was abnormally dangerous and that the
defendants could be held strictly liable for the accident. It is
therefore most likely that the Washington Supreme Court
would apply strict liability when the defendant was perform-
ing a dangerous activity for “his own purpose,” and would
apply the public duty exception only in the appropriate case
when the defendant was engaged in a legally-obligated activ-
ity, such as a regulated common carrier bound to carry haz-
ardous substances.

   Defendants argue that in light of the exceptional and patri-
otic circumstances under which they operated Hanford, we
should treat them as analogous to public employees who
would qualify for the exception. Although Defendants are
correct that we generally do not parse the language of a
restatement as meticulously as that of a statute, and we will
apply it “when the purposes it seeks to serve dictate its appli-
cation,” McKay, 704 F.2d at 447, Defendants do not satisfy
the exception’s purpose in this case. Defendants are not public
officers or employees or common carriers, see RESTATEMENT
(SECOND) OF TORTS § 521, and they were not legally obligated
to operate Hanford.

   The prototypical example of a defendant entitled to the
public duty exception is a utility company that is legally
required to transport an ultrahazardous good, such as electric-
ity, and causes injury to someone during transport. Courts
have recognized a public duty exception in such cases,
because common carriers must accept, carry, and deliver all
goods offered to them for transport within the scope of the
operating authority set forth in their permits. See, e.g., 16
U.S.C. § 824 et. seq. (granting the Federal Energy Regulatory
Commission authority to establish guidelines for common
carriers of electricity in interstate commerce); United States v.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION       3621
W. Processing Co., 756 F. Supp. 1416, 1421 (W.D. Wash.
1991). They cannot discriminate against customers or refuse
to accept commodities that may be dangerous for transport.
Id.

    [11] The case law therefore illustrates that the duty
involved is the legal obligation to perform the abnormally
dangerous activity in accordance with government orders.
See, e.g., EAC Timberlane v. Pisces, Ltd., 745 F.2d 715, 721
n.12 (1st Cir. 1984) (noting that the public duty must be one
imposed on the actor) (citing Actiesselskabet Ingrid v. Central
R.R. Co. of New Jersey, 216 F. 72 (2d Cir. 1914); Town of
East Troy v. Soo Line R.R. Co., 409 F. Supp. 326, 329 (E.D.
Wis. 1976) (no strict liability for spillage of carbolic acid by
derailment of common carrier train); Christ Church Parish v.
Cadet Chem. Corp., 199 A.2d 707, 708-09 (Conn. Super. Ct.
1964) (transportation of twenty tons of various chemical sub-
stances); Pecan Shoppe of Springfield v. Tri-State Motor
Transit Co., 573 S.W.2d 431, 438-39 (Mo. Ct. App. 1978)
(transporter of explosives); Pope v. Edward M. Rude Carrier
Corp., 75 S.E.2d 584, 595-96 (W. Va. 1953) (transporter of
explosives). Qualifying entities must be operating pursuant to
the mandate and control of the government; they must have
little discretion over the manner in which they conduct their
activities. See Actiesselskabet Ingrid, 216 F. at 78 (“It cer-
tainly would be an extraordinary doctrine for courts . . . to say
that a common carrier is under legal obligation to transport
dynamite and is an insurer against any damage which may
result in the course of transportation, even though it has been
guilty of no negligence which occasioned the explosion which
caused the injury.”); Pope, 75 S.E. 2d at 591-92 (holding no
strict liability for common carrier transport of explosives); but
see Lamb v. Martin Marietta Energy Sys., Inc., 835 F. Supp.
959 (W.D. Ky. 1993) (applying the public duty exception to
a nuclear facility because under Kentucky law the public duty
exception includes entities engaged in activities of public
necessity even when there is no legal duty to perform them).
3622         IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
  [12] There was no government mandate here. The events
giving rise to this litigation occurred before the government
developed rules or the ability to control nuclear facilities. The
government was relying on the expertise of defendants and
not vice versa.

   [13] We should not confuse the legal concept of a public
duty with popular notions of patriotic duty taken at personal
sacrifice. Defendants may well have been acting at the gov-
ernment’s urging during wartime. The public duty exception,
however, was developed under state law in recognition of the
need to protect private actors who are legally required to
engage in ultrahazardous activities. No matter how strongly
Defendants may have felt a patriotic duty, they had no legal
duty to operate Hanford, and they are, therefore, not entitled
to the public duty exception. The district court correctly found
defendants subject to strict liability.

V.        Statute of Limitations.

     A.     Waiver.

   Defendants argue for the first time on appeal that Bell-
wether Plaintiff Gloria Wise’s lawsuit was untimely, because
pursuant to Am. Pipe & Constr. Co. v. Utah, 414 U.S. 538
(1974), Wise lost the benefit of any statute of limitations toll-
ing when she filed a separate, individual suit prior to the
denial of class certification. Defendants arguably have waived
this claim, because they did not raise the issue below suffi-
ciently for the district court to rule on the matter. McMillan
v. United States, 112 F.3d 1040, 1047 (9th Cir. 1997); In re
E.R. Fegert, Inc., 887 F.2d 955, 957 (9th Cir. 1989). We have
discretion, however, to overlook any waiver. See United
States v. Bynum, 327 F.3d 986, 990 (9th Cir. 2003). We exer-
cise such discretion in this case, because the issue of whether
Wise’s lawsuit was tolled pending class certification is a
purely legal question, see id.; both parties concede that the
issue is “important,” and addressing the issue is consistent
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION       3623
with the purpose of a bellwether trial in clarifying and stream-
lining the relevant issues. The issue should therefore be set-
tled now.

   We recognize that the results of the Hanford bellwether
trial are not binding on the remaining plaintiffs. See, e.g., In
re Hanford Nuclear Res. Litig., No. 91-03015, Dkt. #1294 at
164-65 (E.D. Wash. June 3, 2003). Nevertheless, according to
counsel for the parties, the purpose of the bellwether trial was
to “establish the relative strengths and weaknesses of the par-
ties, spread out mainly for settlement purposes. . . .” Id. at
155, 165. The bellwether trial was meant to be a “learning
process.” Id. at 161. The parties also state in their briefs that
there are “numerous” other plaintiffs who face a similar stat-
ute of limitations hurdle. It would defeat the purpose of a bell-
wether trial and only deter settlement longer to refrain from
deciding this purely legal issue at the earliest possible stage.
Our resolution will save any potentially time-barred plaintiffs
from expending additional resources and energy on futile
legal proceedings.

  B.   Application of American Pipe.

   In April 1994, the Hanford plaintiffs moved for class certi-
fication. The district court reserved ruling on certification
under the opt-out provision of Federal Rule of Civil Proce-
dure 23(b)(3). Plaintiffs eventually withdrew their request for
certification, but not until May 2003, after the first series of
appeals was decided by this circuit. Accordingly, the district
court held that the statute of limitations was tolled for all
putative class members from April 1994 until May 2003.

   Plaintiff Wise was diagnosed with thyroid cancer in April
1993, three years after the first class action complaints were
filed and one year before Plaintiffs moved for class certifica-
tion. Wise filed an individual suit in district court in July
1997. If the date of diagnosis is the triggering date for the
statute of limitations, her individual suit was apparently
3624    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
untimely, because Washington’s statute of limitations for per-
sonal injury claims is three years. Wise’s suit would have
been timely only if she was entitled to the tolling that began
for the class action plaintiffs on April 15, 1994. Defendants
now contend that Wise forfeited these tolling benefits when
she filed her individual suit prior to the district court granting
or denying class certification. Wise contends she is entitled to
tolling as a member of the class pursuant to American Pipe.

   [14] In American Pipe, 414 U.S. at 554, the Supreme Court
held that the commencement of a class action suspends the
applicable statute of limitations as to all asserted members of
the class who would have been parties had the suit been per-
mitted to continue as a class action. Id. The tolling period
ends, and the statute runs anew, once class certification is
granted or denied. Tosti v. City of Los Angeles, 754 F.2d
1485, 1488 (9th Cir. 1985). The issue in this case, which is
one of first impression in our circuit, is whether American
Pipe also permits tolling for a plaintiff who files a separate
action pending class certification.

   The Sixth Circuit, which is the only circuit to have
addressed the issue directly, has also said no. It held that the
purposes of class action tolling under American Pipe “are not
furthered when plaintiffs file independent actions before deci-
sion on the issue of class certification.” Wyser-Pratte Mgmt.
Co., Inc. v. Telxon, 413 F.3d 553, 569 (6th Cir. 2005). Such
purposes are only furthered “when plaintiffs delay until the
certification issue has been decided.” Id. The Second Circuit
tangentially reached a similar conclusion two decades earlier,
when it stated that “[t]he policies behind Rule 23 and Ameri-
can Pipe would not be served, and in fact would be disserved,
by guaranteeing a separate suit at the same time that a class
action is ongoing.” Glater v. Eli Lilly & Co., 712 F.2d 735,
739 (1st Cir. 1983). Countless federal district courts have
come to the similar conclusion that “[a]pplying the tolling
doctrine to separate actions filed prior to class certification
would create the very inefficiency that American Pipe sought
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3625
to prevent.” In re Worldcom, Inc. Sec. Litig., 294 F. Supp. 2d
431, 451 (S.D.N.Y. 2003) (citing cases); see also In re Heri-
tage Bond Litig., 289 F. Supp. 2d 1132, 1150 (C.D. Cal.
2003).

   At the time this case was submitted for decision, the lead-
ing authority in this area was a district court opinion from the
Southern District of New York. In re Worldcom, Inc. Sec.
Litig., 294 F. Supp. 2d 431, 451 (S.D.N.Y. 2003). The Sixth
Circuit agreed with the reasoning of Worldcom and held that
American Pipe tolling is not available to a plaintiff who files
a separate action pending class certification. Wyser-Pratte
Mgmt. Co. v. Telxon Corp., 413 F.3d 553, 569 (6th Cir. 2005).
So did the other district courts that dealt with the issue. See,
e.g., In re Heritage Bond Litig., 289 F. Supp. 2d 1132, 1150
(C.D. Cal. 2003); see also Wyser-Pratte Mgmt. Co., 413 F.3d
at 569 (citing cases).

   [15] The Second Circuit, however, has vacated the district
court decision in WorldCom. In re Worldcom Sec. Litig., 496
F.3d 245, 256 (2nd Cir. 2007). The Second Circuit has now
held that American Pipe tolling does apply to plaintiffs who
file their actions before a decision on class certification. The
Second Circuit explained that applying American Pipe tolling
to plaintiffs who filed individual suits before certification is
consistent with the purpose underlying statutes of limitations.
Id. at 255. Statutes of limitations are intended to provide
notice to defendants of a claim before the underlying evidence
becomes stale. As the Supreme Court held in American Pipe,
however, the filing of a timely class action provides defen-
dants with notice of the claim, so a follow-on individual suit
cannot surprise defendants. Id.

   The Second Circuit also reasoned that although the Ameri-
can Pipe doctrine protects plaintiffs from being forced to file
suit before the certification decision, that doesn’t mean that
plaintiffs who file before certification are not entitled to toll-
ing. Id. at 256. They have a right to file at the time of their
3626     IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
choosing and denying tolling would diminish that right. We
find the Second Circuit’s reasoning persuasive and adopt it.

   [16] We therefore conclude that members of the plaintiff-
class who have filed individual suits are entitled to the bene-
fits of American Pipe tolling. This includes Bellwether Plain-
tiff, Wise.

VI.    Medical Monitoring Claims.

  Plaintiffs Pamela Durfey, Paulene Echo Hawk, and Doro-
thy George’s only claim on appeal is for medical monitoring.
They do not yet have any diseases attributable to Hanford
radiation. Because in all relevant respects Plaintiffs are analo-
gous to the plaintiffs who requested medical monitoring in
2002 in Berg, 293 F.3d 1127, Plaintiffs’ claims were origi-
nally stayed pending this court’s decision in that case.

   We then decided Berg, in which we held that claims for
medical monitoring are not compensable under the PAA,
because they do not constitute claims of “bodily injury, sick-
ness, disease, or death . . .” Berg, 293 F.3d at 1132-33 (citing
42 U.S.C. § 2014(q)). After our decision, Plaintiffs in this
case asked the district court to remand their medical monitor-
ing claims to state court. They claimed that Berg abrogated
subject matter jurisdiction in federal court for all medical
monitoring claims. Defendants opposed remand, arguing that
Berg did not remove the district court’s subject matter juris-
diction, but held only that a medical monitoring claim was not
cognizable under the PAA.

   [17] Although in Berg we referred to bodily injury as a
jurisdictional prerequisite, id. at 1131-33, we used the term
“jurisdictional” in the loose sense, perhaps too loose, to mean
that medical monitoring claims were not compensable under
the PAA absent physical injury. We have been guilty of such
expansive use of the term before. See Khalsa v. Weinberger,
779 F.2d 1393, 1396 n.2 (9th Cir. 1985) (“. . . jurisdiction has
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION          3627
many possible meanings, ranging from subject matter juris-
diction to the power to grant the relief requested . . . ”). The
district court in this case clearly had subject matter jurisdic-
tion under the PAA to decide the issue; the district court sim-
ply did not have the power to grant the relief requested
because the plaintiffs have not suffered any physical injury.

    [18] The PAA is the exclusive means of compensating vic-
tims for any and all claims arising out of nuclear incidents.
Berg, 293 F.3d at 1132; In re TMI Litig., 940 F.2d at 854; see
also 42 U.S.C. § 2014(hh), (w) (federal courts have jurisdic-
tion over public liability actions, defined as “any suit asserting
. . . any legal liability arising out of or resulting from a nuclear
accident”) (emphasis added). This result is consistent with
Congress’s explicit intent in enacting the 1988 Amendments
and avoiding piecemeal litigation arising from nuclear inci-
dents. We therefore affirm the district court’s exercise of
jurisdiction over Plaintiffs’ medical monitoring claims and its
conclusion pursuant to our decision in Berg that they were not
compensable under the Act. The district court properly denied
Plaintiffs’ request for a remand to state court.

VII.   First Bellwether Trial.

   The remaining issues on appeal stem from a variety of legal
and evidentiary rulings in the two trials. Three of the six Bell-
wether Plaintiffs, Goldbloom, Buckner, and Carlisle, lost at
the first trial. Three of their evidentiary challenges constitute
reversible error; the remaining arguments are meritless.

  A.   Causation.

  We first decide whether under Washington law the district
court properly instructed the jury in the bellwether trial on
“but-for,” and not “substantial factor,” causation. Plaintiffs
contend that the more lenient substantial factor test should
apply because other factors could have contributed to their ill-
nesses, such as smoking and genetics. We review the district
3628    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
court’s application of Washington law de novo, Ostad v. Or.
Health Scis. Univ., 327 F.3d 876, 883 (9th Cir. 2003), and we
affirm the district court’s instruction on but-for causation.

   Under the PAA, Washington state law controls the standard
of causation to be used in this case. See 42 U.S.C. § 2014(hh).
(“A public liability action shall be deemed to be an action
arising under section 170 [42 U.S.C. § 2210], and the substan-
tive rules for decision in such action shall be derived from the
law of the State in which the nuclear incident involved occurs,
unless such law is inconsistent with the provisions of such
section.”). Washington courts will depart from the standard
but-for causation instruction in favor of the substantial factor
test only in three rare circumstances: (1) the plaintiff was
excusably ignorant of the identity of the tortfeasor who
caused his injury; (2) the plaintiff probably would have been
injured anyway, but lost a significant chance of avoiding the
injury; or (3) the plaintiff has been injured by multiple inde-
pendent causes, each of which would have been sufficient to
cause the injury. Gausvik v. Abbey, 107 P.3d 98, 108 (Wash.
Ct. App. 2005); see also Daugert v. Pappas, 704 P.2d 600,
605-06 (Wash. 1985).

   The parties agree that the first and second exceptions are
not at issue here. Plaintiffs know the identity of the tortfeasors
and had no chance to avoid injury. See, e.g., Lockwood v.
AC&S, Inc., 744 P.2d 605, 613 (Wash. 1987) (applying the
substantial factor test when there is no doubt that asbestos was
the cause of a plaintiff’s asbestosis, but the plaintiff cannot
identify which manufacturer is responsible); Mavroudis v.
Pittsburgh-Corning Corp., 935 P.2d 684, 687 (Wash. Ct.
App. 1997).

   [19] Plaintiffs therefore appear to rely on the third type of
substantial factor causation, which applies when there have
been “multiple, independent causes,” each of which alone is
sufficient to cause the injury. Gausvik, 107 P.3d at 108. There
are two requirements they must satisfy (1) there must have
             IN RE: HANFORD NUCLEAR RESERVATION LITIGATION   3629
been multiple causes of the injury; and (2) any one cause
alone was sufficient to cause the injury. Id. Plaintiffs can not
satisfy the second requirement. Plaintiffs instead ask us to
expand the substantial factor doctrine and apply the test when
there are potentially multiple causes of each plaintiff’s injury,
such as radiation, smoking, genetics, or pregnancy, even
though Plaintiffs cannot show that Hanford radiation alone
would have been sufficient to cause the injury. Their reading
of Washington law would allow the substantial factor test to
supplant but-for causation in virtually all toxic tort cases.
Such a result is inconsistent with existing Washington law,
which applies the substantial factor test in very limited cir-
cumstances. See also RESTATEMENT (THIRD) OF TORTS § 26
cmt. j (Proposed Final Draft 2005) (eliminating the substantial
factor test). We therefore hold that the district court properly
instructed the jury on but-for causation.

  B.        Evidentiary Rulings Constituting Reversible Error.

       i.    Cross-Examination of Dr. Davies.

   Plaintiffs raise two issues with respect to Defendants’
cross-examination of Plaintiffs’ endocrinologist expert Dr.
Terry Davies, in the first bellwether trial. The first issue con-
cerns the district court’s ruling that Dr. Davies could not tes-
tify that he authored articles on I-131’s effect on thyroid cells.
The second issue is Defendants’ cross-examination of Dr.
Davies with deposition testimony of a non-testifying expert.
These errors surrounding Dr. Davies’ testimony, taken
together, were prejudicial to Plaintiffs’ case. We therefore
must remand for a new trial.

   With respect to Dr. Davies’ pre-litigation scholarship, the
district court barred Plaintiffs from asking him whether he has
“published any peer reviewed articles or papers regarding the
capacity of I-131 to kill or damage thyroid cells.” The record
reveals that the district court believed Dr. Davies could not
testify about any pre-litigation articles on this subject, because
3630    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
Dr. Davies had not written any peer-reviewed scientific article
concluding that doses at less than 100 rads can cause autoim-
mune disease.

   This ruling was an abuse of discretion, because it deprived
the jury of testimony from Dr. Davies about the extent of his
pre-litigation expertise regarding causes of thyroid illness.
That Dr. Davies had not written any articles specifically
directed to causation below 100 rads does not mean Dr.
Davies’ work on causation generally was inadmissible. Plain-
tiffs’ key witness on causation extensively researched and
authored scholarship on the capacity of I-131 emissions to kill
thyroid cells, and the jury was entitled to know the reach of
his expertise.

   Standing alone, this error might not be prejudicial; there is,
however, a more serious problem with the presentation of Dr.
Davies’ testimony. Defendants were allowed to impeach one
of Plaintiffs’ key expert witnesses with inadmissible evidence,
hearsay statements that Defendants themselves successfully
excluded from Plaintiffs’ case-in-chief.

   Prior to trial, Plaintiffs proffered the deposition testimony
of epidemiologist Dr. A. James Ruttenber. According to
Plaintiffs, Dr. Ruttenber would have testified that although
current epidemiological studies prove only that radiation
above 40 rads can cause thyroid disease, those studies do not
preclude causation at lower dosages. On Defendants’ motion,
the district court excluded this testimony, finding it was too
speculative. Having learned that the most probative part of
Dr. Ruttenber’s testimony was no longer admissible, Plaintiffs
chose not to call him to the stand.

   Plaintiffs, instead, called Dr. Davies to testify that low dos-
ages of radiation could cause thyroid disease. Dr. Davies had
relied on some of Dr. Ruttenber’s dosage estimates in prepar-
ing his pretrial expert report, but he never read nor relied on
Dr. Ruttenber’s deposition in rendering his expert opinion.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION         3631
Even though Dr. Ruttenber’s causation testimony has been
ruled inadmissible prior to trial, defense counsel on cross-
examination used Dr. Ruttenber’s deposition to impeach Dr.
Davies’ testimony. Among many other questions, defense
counsel asked the following regarding Dr. Ruttenber’s deposi-
tion:

    Q: And doctor, are you aware that Doctor Rutten-
    ber has said that the epidemiological literature can
    only show an increased risk of autoimmune thyroidi-
    tis down to 40 rads? . . . .

    Q: Did Doctor Ruttenber ever identify to you any
    epidemiological studies that reported that doses at 10
    rad increased the risk of autoimmune thyroid dis-
    ease? . . .

    Q: Yeah, and page 75 at the bottom, lines 23 to 25,
    “Doctor Ruttenber, is it your testimony that the epi-
    demiology gets you down to increased risk of
    autoimmune thyroiditis at point 4 gray?” . . . [ ] now,
    were you aware that Doctor Ruttenber had taken that
    position?”

Plaintiffs made continuing objections to this cross-
examination, but the district court permitted the questions.

   Dr. Ruttenber’s statements should not have been used to
impeach Dr. Davies because they were inadmissible hearsay
on which Dr. Davies did not rely. We agree with the Fifth Cir-
cuit that reports of other experts cannot be admitted even as
impeachment evidence unless the testifying expert based his
opinion on the hearsay in the examined report or testified
directly from the report. Bryan v. John Bean, 566 F.2d 541,
546-47 (5th Cir. 1978) (citing Box v. Swindle, 306 F.2d 882
(5th Cir. 1962)); see also United States v. Layton, 549 F.
Supp. 903, 920-21 (N.D. Cal. 1982). Because Dr. Davies did
not rely on Dr. Ruttenber’s deposition, and because the trial
3632      IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
court had excluded the deposition testimony as inadmissible
hearsay, Defendants should not have been allowed to use the
testimony to impeach Dr. Davies’ credibility.

   Defendants urge us to find that the error is harmless.
Defendants, however, overlook their own emphasis on the
significance of Dr. Davies’ testimony and his credibility to the
jury. Defendants read a large portion of Dr. Davies’ cross-
examination back to the jury in closing arguments, highlight-
ing how Dr. Ruttenber’s deposition testimony rendered Dr.
Davies’ testimony not credible. They then stated:

    [I]f you have Ruttenber saying 40 rads as of today,
    based on his review of radiation epidemiology, how
    does Davies get away with saying at 10 rads their
    dose; we say it’s closer to 6. How does Davies get
    away with offering an opinion on causation? It’s just
    not credible.

   [20] Dr. Davies was the key witness on causation and
Defendants’ strategy was to cast doubt on his opinion. They
did just that by improperly using Dr. Ruttenber’s deposition.
The prejudice to Plaintiffs was exacerbated by the court’s rul-
ing that Plaintiffs were unable to rehabilitate Dr. Davies’
credibility with evidence of his pre-litigation, peer-reviewed
articles on causation. Dr. Davies’ endocrinologist testimony
on causation was particularly probative, because Plaintiffs
already had lost the key expert’s epidemiological testimony,
Dr. Ruttenber’s opinion, regarding causation. We thus have
no choice but to reverse the verdicts against Plaintiffs Gold-
bloom, Carlisle, and Buckner and remand for a new trial.

    ii.   Hurthle Cell Evidence.

   There is an additional ground for reversal with respect to
Plaintiff Goldbloom. The district court erred when it
instructed the jury to disregard Dr. Peters’ expert testimony
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION      3633
that Goldbloom’s thyroid contained Hurthle cells, which are
indicative of some kind of injury to the thyroid.

   Although Dr. Peters’ pretrial expert report contained no
evidence regarding the presence of Hurthle cells in Gold-
bloom’s thyroid, defense counsel did not object to the testi-
mony at trial. Partway through deliberations, the jury astutely
asked whether it could consider Dr. Peters’ testimony even
though evidence regarding Hurthle cells was not in the expert
report. The district court held a conference with the parties to
discuss the jury’s question. Plaintiffs argued that the jury
should be able to consider the Hurthle cell evidence, because
Defendants did not object to its admission at trial. Defendants
argued that the court should instruct the jury to disregard the
evidence, because it was not contained in Dr. Peters’ pretrial
report. The district court agreed with Defendants and
instructed the jury to disregard the Hurthle cell testimony.

   This ruling was erroneous. It is a rare circumstance when
the court may exclude evidence after the close of the parties’
cases. Jerden v. Amstutz, 430 F.3d 1231, 1236-37 (9th Cir.
2005), amended on other grounds, 2006 U.S. App. LEXIS
673 (9th Cir. Jan. 12, 2006). The reasons for such caution are
clear. If the parties have already rested, they no longer have
a chance to provide a curative response to the excluded evi-
dence. Id. at 1237. They also do not have a chance to present
the testimony in another fashion, such as calling an additional
witness. Id. (citing Bartleson v. United States, 96 F.3d 1270,
1278 (9th Cir. 1996)). And most pertinent to this case, it is
impossible to erase from the jury’s minds any arguments that
were made during closing summation about the belatedly-
excluded evidence.

  In this case, not only did Defendants fail to object to the
Hurthle cell evidence, they used the evidence themselves,
pointing to the testimony during closing summation as evi-
dence that Dr. Peters was not a reliable witness. Defendants
should not have been allowed to reap the benefit of a tardy
3634      IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
exclusion of Plaintiff’s evidence after they used the same evi-
dence in their closing argument as a basis for impeachment.

   [21] The error in excluding the evidence was prejudicial.
Plaintiff Goldbloom did not have an opportunity to remedy
the error, because the evidence was not excluded until after
the parties had rested. Her only remedy was to seek a mistrial.
The error in excluding the Hurthle cell evidence, when cou-
pled with the errors above, thus warrants a new trial.

  C.      Remaining Evidentiary Challenges.

  Plaintiffs’ remaining claims are all challenges to various
evidentiary rulings. To help facilitate the bellwether process,
we briefly address each, although none has merit.

    i.     Dr. Ruttenber’s Testimony.

   [22] The district court did not err in prohibiting Plaintiffs’
epidemiologist expert, Dr. Ruttenber, from testifying that sci-
entific extrapolation supports a finding that radiation below
40 rads could cause hypothyroidism or autoimmune thyroidi-
tis. Plaintiffs assert that the district court’s rulings impermiss-
ibly required epidemiological studies to be a prerequisite to
causation testimony. See In re Hanford, 292 F.3d 1124; Ken-
nedy v. Collagen Corp., 161 F.3d 1226, 1229-30 (9th Cir.
1998). The court’s ruling, however, did not go that far. The
court only precluded Dr. Ruttenber from, first, speculating
that such extrapolation would likely produce results showing
causation below 40 rads, and, second, stating that current epi-
demiological data do not contradict or prevent such a finding.
Because the data Dr. Ruttenber used to make these two con-
clusions were not reliable, see Daubert v. Merrell Dow
Pharms., 509 U.S. 579, 589 (1993), the district court did not
abuse its discretion in excluding Dr. Ruttenber’s opinion.

    ii.    Alternative Causes of Thyroid Disease.

  [23] The district court also did not err in permitting Defen-
dants to cross-examine Plaintiffs’ experts on alternative
           IN RE: HANFORD NUCLEAR RESERVATION LITIGATION     3635
causes of thyroid disease. Plaintiffs’ chosen methodology for
proving causation was differential diagnosis, which is a pro-
cess by which an expert compiles a comprehensive list of
potential causes and then engages in a process of elimination
to reach the most likely cause. See Clausen v. M/W New Car-
issa, 339 F.3d 1049, 1057-58 (9th Cir. 2003). Plaintiffs’
experts enumerated possible causes of Plaintiffs’ diseases,
such as genetic predisposition, pregnancy, and stress, and then
eliminated them, leaving radioiodine as the only probable
cause. Having put these alternative causes at issue, Plaintiffs
could not expect Defendants not to question the experts’
rejection of them. Defendants were entitled to impeach the
experts’ methodology and their underlying conclusions.

    iii.    Indemnification.

   [24] Plaintiffs challenge the district court’s ruling that
Plaintiffs could not tell the jury that the federal government
would indemnify Defendants for any liability imposed. Evi-
dence of indemnification is generally inadmissible but may be
used to show prejudice or bias of a witness. See, e.g., FED. R.
EVID. 411. The only evidence to which a question of govern-
ment bias might have arisen, however, was the Hanford Envi-
ronment Dose Reconstruction Project (“HEDR”) and
Plaintiffs stipulated to the accuracy of this document prior to
trial. That stipulation is binding. U.S. Dep’t of Labor v. Kerr-
McGee Chem. Corp., 15 OSHC (BNA) 2070 (9th Cir. 1993);
Am. Title Ins. Co. v. Lacelaw Corp., 861 F.2d 224, 226 (9th
Cir. 1988). There was thus no permissible ground on which
Plaintiffs could introduce evidence of indemnification. The
district court did not err in its application of Rule 411.

    iv.     Dr. Hill’s Testimony.

  [25] Plaintiffs’ final contention on appeal is that the district
court abused its discretion when it prohibited Plaintiffs’
expert Dr. Hill from comparing radioiodine causation in ani-
mals to causation in humans. This ruling was not an abuse of
3636     IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
discretion, because the information was not contained in Dr.
Hill’s pretrial expert report and Defendants were not allowed
to question Dr. Hill regarding animal studies and thyroid dis-
ease at his deposition.

VIII.   Rhodes’ Second Trial.

   Plaintiff-Appellant Shannon Rhodes suffers from several
thyroid problems, the most serious of which is thyroid cancer.
Pursuant to the stipulation prior to trial, Rhodes was exposed
to Hanford radiation of 2.3 to 17 rads. At the bellwether trial,
she offered the testimony of her expert witness, Dr. Inder
Chopra, who testified that it was more probable than not that
Hanford radiation caused Rhodes’ cancer. The jury failed to
reach a decision on Rhodes’ claims, however, and the trial
court was forced to order a mistrial. In November 2005,
Rhodes tried her case for a second time, and the second jury
rendered a verdict for Defendants. Rhodes’ appeals several
legal and evidentiary rulings from her second trial.

  A.    Juror Misconduct.

   [26] Rhodes claims that the district court erred in denying
her post-trial motion for a new trial, because the jury improp-
erly considered extrinsic evidence during its deliberations.
Rhodes proffers the signed affidavit of a juror stating that
“during deliberations and immediately after the first of several
votes were taken on the issue of causation, one of the jurors
announced that this was Mrs. Rhodes second trial on the very
issues that we were to decide and that she had lost in the first
trial.”

   A party is entitled to a new trial when the jury obtains or
uses evidence that has not been introduced during trial only
if the improperly considered evidence was extrinsic. United
States v. Keating, 147 F.3d 895, 900 (9th Cir. 1998). The dis-
trict court looked at the trial record as a whole and decided
that any evidence regarding Rhodes’ previous trial came from
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION        3637
accidental, off-hand comments during the second trial, and
thus the evidence was not extrinsic.

   We have conducted an independent review of the record,
United States v. Madrid, 842 F.2d 1090, 1092 (9th Cir. 1988),
and we affirm. There were several instances during Rhodes’
trial when the first litigation was mistakenly mentioned by
either a witness or an attorney and from which the jurors
could have inferred that Rhodes had litigated these same
claims before and “lost.” We agree with the district court that
because the evidence was not extrinsic, there was no juror
misconduct.

  B.   Voir Dire.

   [27] The district court did not err in refusing to hold an evi-
dentiary hearing regarding possible juror misconduct during
voir dire. A motion for a new trial based on juror dishonesty
during voir dire requires a showing that (1) a juror failed to
answer honestly a material question; and (2) a correct answer
would have provided a valid basis for a challenge for cause.
Price v. Kramer, 200 F.3d 1237, 1254 (9th Cir. 2000). Other
than the affidavit, which stated that a juror admitted during
deliberations that he read about Rhodes’ prior trial, Rhodes
does not proffer any evidence that the juror lied during voir
dire. The juror who read about Rhodes’ trial easily could have
done so after the trial began. He also could have read about
it in the medical report that was admitted as evidence, in
which Rhodes’ own doctor at the time of the first bellwether
trial noted that Rhodes was stressed because her litigation was
not going well. Rhodes has not established any juror miscon-
duct entitling her to a new trial.

  C.   Reference to Impeachment Evidence as “Totally
       Collateral.”

  During trial, the district court referred to evidence pres-
ented by Rhodes to impeach a defense expert as “totally col-
3638    IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
lateral.” The district court later went on the record and
admitted his comment was a mistake. Rhodes contends that
the district court’s comment unfairly prejudiced the jury in
favor of the defense, entitling her to a new trial. There was no
undue prejudice.

   [28] We review a judge’s comments during trial for an
abuse of discretion and reverse only if they “projected to the
jury an appearance of advocacy or partiality.” United States
v. Parker, 241 F.3d 1114, 1119 (9th Cir. 2001). A trial court
has substantial leeway in overseeing the presentation of evi-
dence, because it is most familiar with the dynamics of a pro-
ceeding and the dangers of jury confusion. See, e.g., S.M. v.
J.K., 262 F.3d 914, 919 (9th Cir. 2001). The district court’s
reference to certain Plaintiff’s testimony as “totally collateral”
when sustaining Defendants’ objection, although an admitted
mistake, did not project an appearance of advocacy or partial-
ity that warrants reversal in this case.

  D.   “Best Estimate” Requirement.

   Plaintiff next contends that the district court erred in requir-
ing Dr. Hoffman to give a “best estimate” of the dose of radi-
ation received by Rhodes. At the bellwether trial, Dr.
Hoffman testified to a range of possible dosages for each indi-
vidual with a certain percentage of confidence (i.e. a confi-
dence interval). Defendants objected to this use of confidence
intervals in Rhodes’ second trial, claiming they would mis-
lead the jury and encourage them mistakenly to believe that
there was a high likelihood that Rhodes was exposed to radia-
tion near the upper-boundary of the dosage range, when in
fact, the expert was only capable of endorsing the entire range
with ninety-percent confidence. Plaintiff agreed to the district
court’s suggestion that Dr. Hoffman testify to a range of pos-
sible dosage exposure, but also give a “best estimate” of indi-
vidual dosage exposure, which was the average of the range.
Rhodes now argues that this ruling was an abuse of discretion.
        IN RE: HANFORD NUCLEAR RESERVATION LITIGATION          3639
   [29] The district court has discretion to exclude evidence
when its probative value is substantially outweighed by the
danger of misleading the jury or confusing the issues. See
FED. R. EVID. 403. This is especially true with respect to
expert witnesses. Daubert, 509 U.S. at 595; Daubert v. Mer-
rell Dow Pharms., Inc., 43 F.3d 1311, 1321 n.17 (9th Cir.
1995). In reviewing the record of the pre-trial hearing and the
application of the “best estimate” testimony at both trials, the
district court struck a fair balance between probative and mis-
leading testimony. It allowed Dr. Hoffman to explain his con-
fidence interval and to give a range of possible dosage
exposure. It also helped the jury focus on the significance of
such an interval, by having Dr. Hoffman highlight the average
of the possible range. This ruling was not an abuse of discre-
tion.

  E.   Cross-Examination of Dr. Hoffman.

  [30] The district court did not abuse its discretion in allow-
ing Defendants to cross-examine Dr. Hoffman with testimony
he gave at the first bellwether trial. At the first trial, Dr. Hoff-
man testified to each plaintiff’s “probability of causation”
(“PC”). Plaintiff Rhodes apparently decided that Dr. Hoff-
man’s testimony on this issue was more harmful than helpful,
and in a pre-trial motion for her second trial, she asked the
court to exclude all references by either party to Dr. Hoff-
man’s PC analysis. Rhodes, however, still wanted Dr. Hoff-
man to testify on other topics.

   The district court properly denied the motion. Dr. Hoff-
man’s testimony at the first bellwether trial was an admission
of a party opponent under Federal Rule of Evidence
801(d)(2)(C). Glendale Fed. Bank, FSB v. United States, 39
Fed. Cl. 422, 424-25 (1997). Plaintiff cannot now exclude
trial testimony that she, herself, proffered. Defendants were
properly permitted to cross-examine Dr. Hoffman on his pre-
vious PC analysis.
3640     IN RE: HANFORD NUCLEAR RESERVATION LITIGATION
  F.    Cross-Examination of Dr. Chopra.

   Rhodes next urges us to hold that the district court abused
its discretion in allowing Defendants to use Dr. Hoffman’s PC
analysis to cross-examine Dr. Chopra and impeach her credi-
bility. We agree with Rhodes that this ruling was an abuse of
discretion, but because the error had little or no effect on the
verdict, there was no reversible error.

   Opposing counsel may cross-examine an expert on the facts
or data on which his opinion was based. See FED. R. EVID.
703; United States v. Preciado-Gomez, 529 F.2d 935, 942
(9th Cir. 1976). Because Dr. Chopra did not rely on Dr. Hoff-
man’s PC data, it should not have been admitted under that
rule. See, e.g., Bryan, 566 F.2d at 547; Layton, 549 F. Supp.
at 920; Briggs v. Chi. G. W. R. Co., 57 N.W.2d 572, 583-84
(Minn. 1953).

  Rhodes, however, does not show how the PC evidence
could have materially affected the jury’s verdict. Defendants
had already undermined Dr. Chopra’s credibility by highlight-
ing the doctor’s lack of due diligence in uncovering Rhodes’
medical history. There is no reason to believe that Defen-
dants’ use of Dr. Hoffman’s testimony had any additional,
much less, any substantial effect on the verdict.

                        *      *     *

 [31] The jury’s defense verdict in Rhodes’ second trial
must be affirmed.

IX.    Conclusion.

   We are mindful of the time and resources both the district
court and the parties have expended in this protracted litiga-
tion. We also realize that resolution is needed. We affirm the
district court’s major rulings. These relate to the government
contractor defense, strict liability, and causation. We also
       IN RE: HANFORD NUCLEAR RESERVATION LITIGATION    3641
affirm the district court’s ruling on the medical monitoring
claims and the judgment against Plaintiff Rhodes, as well as
the judgments in favor of Plaintiffs Stanton and Wise. We
reverse, on evidentiary grounds, the judgments against Plain-
tiffs Buckner, Carlisle, and Goldbloom. We remand those
matters for further proceedings.

  Each party to bear its own costs on appeal.

  AFFIRMED IN PART; REVERSED AND REMANDED
IN PART.
