                                       PRECEDENTIAL

      UNITED STATES COURT OF APPEALS
           FOR THE THIRD CIRCUIT
                ___________

                    No. 10-2108
                    ___________

      GLENN GATES; DONNA GATES, H/W,
         ON BEHALF OF THEMSELVES
    AND ALL OTHERS SIMILARLY SITUATED

                          v.

       ROHM AND HAAS COMPANY;
      MORTON INTERNATIONAL, INC, ;
     ROHM AND HAAS CHEMICALS LLC;
  HUNTSMAN; HUNTSMAN POLYURETHANES;
    MODINE MANUFACTURING COMPANY

              Glenn Gates, Donna Gates,
                                      Appellants
             _______________________

    On Appeal from the United States District Court
       for the Eastern District of Pennsylvania
         D.C. Civil Action No. 06-cv-01743
            (Honorable Gene E.K. Pratter)
                  ______________

              Argued November 3, 2010
Before: SCIRICA, RENDELL and ROTH, Circuit Judges.
                 (Filed: August 25, 2011)

LOUIS C. RICCIARDI, ESQUIRE (ARGUED)
Trujillo Rodriguez & Richards
1717 Arch Street, Suite 3838
Philadelphia, Pennsylvania 19103

AARON J. FREIWALD, ESQUIRE
Layser & Freiwald
1500 Walnut Street, 18th Floor
Philadelphia, Pennsylvania 19102
      Attorneys for Appellants

CARL A. SOLANO, ESQUIRE (ARGUED)
NILAM A. SANGHVI, ESQUIRE
SAMUEL W. SILVER, ESQUIRE
RALPH G. WELLINGTON, ESQUIRE
Schnader Harrison Segal & Lewis
1600 Market Street, Suite 3600
Philadelphia, Pennsylvania 19103
      Attorneys for Appellees,
      Rohm and Haas Company,
      Morton International, Inc.,
      Rohm and Haas Chemicals LLC
                    _________________

               OPINION OF THE COURT
                  _________________

SCIRICA, Circuit Judge.

      This is an interlocutory appeal under Fed. R. Civ. P.
23(f) from the denial of class certification for medical




                            2
monitoring and property damage. Plaintiffs aver chemical
companies dumped an alleged carcinogen at an industrial
complex near their residences. The District Court found
individual issues predominated on exposure, causation, and
the need for medical monitoring and also found individual
issues predominated as to a liability-only issue class for the
property damage claims.

                               I.

        Named plaintiffs Glenn and Donna Gates are residents
of McCullom Lake Village, Illinois, a primarily residential
area of approximately 2000 people and 400 homes.
Defendants are chemical companies that owned and operated
a facility in Ringwood, Illinois, one mile north of McCullom
Lake Village. According to plaintiffs, defendants dumped
wastewater containing vinylidene chloride into a nearby
lagoon that seeped into an underground aquifer where it
degraded into vinyl chloride, a carcinogen. Plaintiffs contend
vinyl chloride evaporated into the air from the shallow aquifer
and was swept by the wind over McCullom Lake Village.

        Plaintiffs seek certification of two classes: (1) a class
seeking medical monitoring for village residents exposed to
the airborne vinyl chloride between 1968 and 2002, and (2) a
liability-only issue class seeking compensation for property
damage from the exposure. At issue is whether the District
Court erred in finding individual issues barred certification of
the proposed trial classes under Fed. R. Civ. P. 23(b)(2) or
23(b)(3). We will affirm.

                               A.

       From 1951 to 2005, defendant Morton International




                               3
owned and operated the Ringwood facility. In June 1999,
defendant Rohm & Haas Co. acquired Morton and from 2005,
defendant Rohm & Haas Chemicals, LLC, a wholly-owned
subsidiary of Rohm & Haas Co., has operated the Ringwood
facility.1

       Morton made use of vinylidene chloride at the
Ringwood facility and from 1960 to 1978, disposed
wastewater containing vinylidene chloride into an on-site
lagoon. In 1973, tests of the shallow aquifer under the
Ringwood facility showed elevated levels of ammonia and
chloride. This shallow aquifer does not extend under
McCullom Lake Village. In 1978, Morton ceased using the
on-site lagoon and covered it.

        In 1984, Morton conducted an environmental
assessment of the Ringwood facility and installed nineteen
monitoring wells at the facility. Samples from these wells
contained vinylidene chloride and vinyl chloride.
Subsequently, more than ninety monitoring wells were
installed in the area around the Ringwood facility. 2 To date,
neither vinylidene chloride nor vinyl chloride has been
detected in tests of residential wells in McCullom Lake
Village used to obtain drinking water. Plaintiffs contend
1
  Additional     defendants    Huntsman    and    Huntsman
Polyurethanes were dismissed by stipulation without
prejudice and defendant Modine reached a class settlement
that the District Court approved.
2
  In 1991, Morton voluntarily enrolled the Ringwood facility
in the Illinois Environmental Protection Agency‟s
remediation program, an ongoing process. The remediation
plan for the shallow aquifer involves using wells and a
wastewater treatment plant to decontaminate the water.




                              4
these chemicals may be present at undetectable levels.

                              B.

       In 2006, named plaintiffs filed a complaint alleging
there were multiple pathways of contamination from multiple
chemicals including vinyl chloride.3 The putative classes
include only those with economic injury or exposure.
Persons alleging physical injury (including brain cancer) are
excluded from the classes.

        Despite asserting multiple potential pathways of
contamination, plaintiffs limited their arguments at class
certification to a single chemical, vinyl chloride, and a single
pathway, via a shallow aquifer into the air. A deeper aquifer
runs underneath the Ringwood facility, but the parties dispute
whether it has become contaminated and whether the aquifer
flows to the village. Plaintiffs originally alleged this deeper
aquifer (“deeper plume”) carried vinyl chloride to the ground
water under the village. They also alleged “air stripping”
equipment used to remove contamination from the facility‟s
groundwater caused contaminants to be released into the air.

        Despite asserting several claims for relief including
medical monitoring, property damage claims, relief under the
Comprehensive Environmental Response, Compensation, and
Liability Act (CERCLA), 42 U.S.C. § 9601 et seq., the
Illinois Environmental Protection Act, 415 Ill. Comp. Stat. §
5/1 et seq., and state-law fraudulent misrepresentation and
willful and wanton misconduct claims, plaintiffs chose to
proceed on a class basis only on the medical monitoring and

3
 The other contaminants included trichloroethylene (TCE),
and 1, 1-Dichloroethylene (DCE) both industrial solvents.




                               5
property damage claims and, as noted, solely with regard to
vinyl chloride exposure. The proposed medical monitoring
class includes:

       All individuals who lived for one year or more
       in total (whether consecutively or not) within
       McCullom Lake Village during the time period
       from January 1, 1968 to December 31, 2002.
       Excluded from the class are individuals for
       whom brain cancer has been detected and
       individuals bringing claims in any court of
       competent jurisdiction arising out of exposure
       to chlorinated solvents.

The proposed property damage class includes:

       All persons who presently own real property
       within McCullom Lake Village, or who owned
       real property within McCullom Lake Village as
       of April 25, 2006 (the date of the filing of the
       complaint) through the present. Excluded from
       the Class are individuals who have already
       brought claims in any court of competent
       jurisdiction arising out of exposure to
       chlorinated solvents.

Plaintiffs sought certification of only these classes.

       At the class certification hearing both parties submitted
expert evidence.4 Plaintiffs relied on a report from Paolo

4
 By stipulation the parties agreed to delay consideration of a
pending omnibus Daubert motion regarding their proposed
experts. In the interim, we decided In re Hydrogen Peroxide




                                6
Zannetti and a report and testimony from Gary Ginsberg.
Zannetti, an expert in modeling dispersion of air pollution,
submitted a report estimating the dispersion of vinyl chloride
over the village based on data from the monitoring wells.
Ginsberg, a toxicologist at the Connecticut Department of
Public Health, presented a risk assessment of exposure to
vinyl chloride.

       To measure the exposure from pollutants such as vinyl
chloride, the experts modeled the exposure of residents
compared to their background levels of exposure absent the
alleged pollution attributable to the defendants.5 Plaintiffs
contend the natural background level is 0.042 micrograms per
cubic meter (“µ/m3”), a measure contained in the federal
Environmental Protection Agency‟s 1999 National-Scale Air
Toxics Assessment.

       Zannetti‟s report modeled the emissions over the

Antitrust Litig., 552 F.3d 305 (3d Cir. 2008), which required a
“rigorous analysis” of the proposed classes in light of the
requirements for class certification. Id. at 309. The District
Court ordered supplementary briefing and informed the
parties that they may need to address the reliability of expert
evidence to the extent it related to class certification issues.
The District Court‟s analysis turned largely on whether the
experts‟ opinions qualified as common proof and not whether
their methods were reliable.
5
  Exposure is compared to background levels unless the
defendant‟s contamination is so severe that it alters the
baseline background level. See In re Paoli R.R. Yard PCB
Litig., 113 F.3d 444, 461 (3d Cir. 1997). The District Court
found the expert testimony did not meet that standard and
plaintiffs do not challenge that finding on appeal.




                               7
village using data from monitoring wells to develop models
for the concentration of vinyl chloride in the air during four
time periods, 1940-67, 1968-89, 1990-96, and 1997-2006.
Included in his report are maps of the village with isopleth
lines6 showing the concentration of vinyl chloride exposure
for persons within the isopleth during each time period. The
isopleths are based on his “high scenario,” which was an
estimate based on the highest single recorded concentration at
each monitoring site. He also developed a scenario he termed
the “low scenario,” which extrapolated exposure from the
average of all recorded concentrations at each site. Zannetti
used the highest recorded data because, in his opinion, the
contamination had ended by the time the monitoring began
and the historical levels were expected to be significantly
higher than those measured. The exposure at the part of the
village closest to the shallow plume ranged from 0.0266 µ/m3
to 0.210 µ/m3 in the “high scenario” and 0.00554 µ/m3 to
0.0159 µ/m3 in the “low scenario.”7


6
  Isopleths are lines on a map joining points of equal value to
show distributions of a specific variable, such as the use of
contour lines on a topographical map to show elevation. The
isopleth lines here demark areas in and around the village
where the estimated concentration of vinyl chloride within the
line equals or exceeds the stated value.
7
  Three isopleth maps show the concentration during the class
period. The isopleth modeling the high emission scenario
from 1968 to 1989 shows a small fraction of the Village in an
isopleth of 0.20 µ/m3 with the remainder of the Village in an
isopleth of 0.08 µ/m3. The isopleth of the period from 1990
to 1996 shows a significant portion of the Village in an
isopleth of 0.08 µ/m3 and the rest in an isopleth of 0.022




                              8
       Ginsberg testified that the average amount of exposure
for residents of the village over a twenty-five year period
from the shallow plume would be 0.127 µ/m3 (in addition to
any background exposure). Ginsberg arrived at this figure by
averaging the concentrations in Zannetti‟s isopleths based on
the “high scenario.”8 The “high scenario” extrapolated
exposure levels based on maximum detected concentration at
monitoring wells from 1985 to 1990. He used the “high
scenario” because “the contamination was likely higher in the
past.” In his view, the scenario still probably underestimated
the exposure. If the “low scenario” were used the average
exposure for a twenty-five year period would be 0.011 µ/m3.

       Ginsberg disclaimed that his report9 was conclusive as


µ/m3. The last isopleth map showing the period from 1997 to
2006 shows a minority of the village in an isopleth of 0.026
µ/m3 and the remainder in an isopleth of 0.008 µ/m3.
8
  The District Court noted Ginsberg described his calculation
of the 0.127 figure “in two different, contradictory ways” at
the hearing and during his deposition. At his deposition
Ginsberg testified he used Zannetti‟s “high scenario” which is
calculated only for the point of the Village closest to the
plume. But at the hearing, Ginsberg testified he averaged the
two ends of the isopleth distribution—the point closest to the
contamination and the point at the furthest end of the village.
The District Court found that either explanation would not
change the fact the number represents an average of class
members‟ exposure.
9
  The bulk of Ginsberg‟s report provides a detailed analysis of
the carcinogenic nature of vinyl chloride. The defendants
dispute whether vinyl chloride poses a cancer risk to humans.
We need not address the issue as it presents a merits




                              9
to individual cases. At one point during his hearing
testimony, Ginsberg stated the hypothetical risk calculations
are “not meant to predict risk for a single individual under
any specific scenario” because of “individual or personal
variability—susceptibility.”

       The District Court denied class certification for both
classes. It found the medical monitoring class lacked the
cohesiveness needed to maintain a class under Rule 23(b)(2)
and that common issues of law and fact did not predominate
as required under Rule 23(b)(3). Both failed for the same
reason—the “common” evidence proposed for trial did not
adequately typify the specific individuals that composed the
two classes. The court also found the remaining individual
issues would require trial, undoing any efficiencies of class
proceedings and possibly leading a second jury to reconsider
evidence presented to the jury in the class proceeding.

        The court found plaintiffs failed to present common
proof of three issues critical to recovering on the medical
monitoring claim—(1) that plaintiffs suffered from exposure
greater than normal background levels, (2) the proximate
result of which is significantly increased risk of developing a
serious disease, and (3) whether the proposed medical
monitoring regime is reasonably medically necessary.

      The court found the proposed expert evidence
demonstrating the first element—exposure greater than
normal background levels—did not reflect the exposure of
any specified individuals within the class. The court rejected
Ginsberg‟s risk analysis and use of the 0.127 µ/m3 figure

determination that does not alter the analysis of the propriety
of class certification.




                              10
because it represented an average exposure, not the exposure
of any actual class member. The court also rejected as
insufficient Zannetti‟s isopleths because the maps assumed a
constant value for exposure during lengthy time periods. It
found the isopleths were “overly simplistic” and averaged the
class members‟ exposures, rendering them unsuitable as
common proof.

       The court found no common proof of minimum
exposure level above which class members were at an
increased risk of serious disease. The court rejected the
proposed value of 0.07 µ/m3—the EPA‟s regulatory standard
for exposure to a mixed population of children and adults—
because 0.07 µ/m3 is a precautionary value below which a
mixed population is likely to be safe. It does not establish the
converse, the required element—the point at which class
members would likely be at risk.

       The court doubted that putative “common” proof could
demonstrate whether the proposed monitoring regime is
reasonably medically necessary. Plaintiffs wanted class
members to receive serial MRIs to scan for cancerous tumors
or CAT scans, if MRIs would pose health risks. The court
did not believe a regime could be developed using common
proof because of class members‟ differing ages, medical
histories, genetic predispositions, and tolerance of serial
MRIs.

       The court also denied certification of the property
damage class, finding similar defects with the “common”
proof. The court noted “[p]laintiffs rely on the same expert
testimony that they offered to support their medical
monitoring claim.” The court refused to certify a liability-
only class because the common evidence could not establish




                              11
contamination at each property that was attributable to the
defendants.

                               II.

       The District Court‟s reasoned analysis of the denial of
class certification makes clear it did not abuse its discretion.
“Factual determinations necessary to make Rule 23 findings
must be made by a preponderance of the evidence.”
Hydrogen Peroxide, 552 F.3d at 320. We review the District
Court‟s findings for abuse of discretion. Id. at 312. A district
court abuses its discretion if its “decision rests upon a clearly
erroneous finding of fact, an errant conclusion of law or an
improper application of law to fact.” Id. (internal quotation
marks omitted). Plaintiffs sought certification of the medical
monitoring class under either Rule 23(b)(2) or 23(b)(3). We
will first address denial of class certification under Rule
23(b)(2).

                               A.

        Rule 23(b)(2) applies when “the party opposing the
class has acted or refused to act on grounds that apply
generally to the class, so that final injunctive relief or
corresponding declaratory relief is appropriate respecting the
class as a whole.” Fed. R. Civ. P. 23(b)(2). The Supreme
Court recently clarified “Rule 23(b)(2) applies only when a
single injunction or declaratory judgment would provide
relief to each member of the class.” Wal-Mart Stores, Inc., v.
Dukes, --- U.S. ----, 131 S. Ct. 2541, 2557 (2011). Rule
23(b)(2), in contrast to (b)(3), “does not authorize class
certification when each class member would be entitled to an
individualized award of monetary damages.” Id. But the
Court did not conclusively decide “whether there are any




                               12
forms of „incidental‟ monetary relief that are consistent with
the interpretation of Rule 23(b)(2) we have announced and
that comply with the Due Process Clause.” Id. at 2561.

                               1.

       Medical monitoring cannot be easily categorized as
injunctive or monetary relief. A medical monitoring cause of
action allows those exposed to toxic substances to recover the
costs of periodic medical appointments and the costs of tests
to detect the early signs of diseases associated with exposure.
The few states that recognize medical monitoring as a remedy
recognize it as a cause of action, like Pennsylvania, Redland
Soccer Club, Inc. v. Dep’t of the Army, 696 A.2d 137, 142
(Pa. 1997), or treat it as a type of relief granted in connection
with a traditional tort cause of action, see, e.g., Bourgeois v.
A.P. Green Indus., Inc., 716 So.2d 355, 359 (La. 1998).10
10
  See Principles of the Law of Aggregate Litigation § 2.04
reporter‟s notes cmt. b, at 124 (2010) (“As a matter of
substantive law, courts are split on the viability of, and proper
approach to medical monitoring actions.”); 7AA Charles Alan
Wright, Arthur R. Miller & Mary Kay Kane, Federal Practice
& Procedure § 1775, at 55-56 (3d ed. 2005) (“One type of
order about which there is some disagreement in the courts is
a request for medical monitoring. Some courts have deemed
that request the equivalent of one for an injunction; others
have treated it as a form of damage relief.” (footnote
omitted)); 1 Joseph M. McLaughlin, McLaughlin on Class
Actions: Law and Practice § 5:18, at 5-70 (3d ed. 2006)
(“Medical monitoring is a controversial, cutting-edge concept
that has not undergone widespread scrutiny in the state courts,
let alone gained widespread acceptance.”). Only a handful of
states have allowed plaintiffs to recover the costs of medical




                               13
The remedy of medical monitoring has divided courts on
whether plaintiffs should proceed under Rule 23(b)(2) or Rule
23(b)(3).11 The Pennsylvania Supreme Court has endorsed
awarding medical monitoring damages as a trust fund which
“compensates the plaintiff for only the monitoring costs
actually incurred.” Redland Soccer Club, 696 A.2d at 142
n.6. It has not yet decided whether medical monitoring
awards can be in the form of a lump-sum verdict. Id. We
have previously reviewed the certification of a Pennsylvania-
law medical monitoring class under Rule 23(b)(2) without
comment on whether medical monitoring claims are
predominately claims for injunctive or monetary relief. See
Barnes v. Am. Tobacco Co., 161 F.3d 127, 143 (3d Cir.
1998).

      The District Court here denied certification under both


monitoring without other physical injury. See Burns v.
Jaquays Mining Corp., 752 P.2d 28, 33-34 (Ariz. Ct. App.
1987); Potter v. Firestone Tire and Rubber Co., 863 P.2d
795, 822-23 (Cal. 1993); Ayers v. Twp. of Jackson, 525 A.2d
287, 314 (N.J. 1987); Redland Soccer Club, Inc. v. Dep’t of
the Army, 696 A.2d 137, 142 (Pa. 1997); Hansen v. Mountain
Fuel Supply Co., 858 P.2d 970, 979-80 (Utah 1993); Bower v.
Westinghouse Elec. Corp., 522 S.E.2d 424, 429-30 (W.Va.
1999); see also Adkins v. Thomas Solvent Co., 487 N.W.2d
715, 720 (Mich. 1992) (recognizing threats or impending
threats to health as actionable under a private nuisance cause
of action).
11
   Compare, e.g., Boughton v. Cotter Corp., 65 F.3d 823, 827
(10th Cir. 1995) with Zinser v. Accufix Research Inst. Inc.,
253 F.3d 1180, 1194-96, amended, 273 F.3d 1266 (9th Cir.
2001).




                             14
subsections for reasons unrelated to the injunctive or
monetary nature of the relief sought. In light of the Supreme
Court‟s recent decision in Wal-Mart Stores, Inc., v. Dukes, ---
U.S. ----, 131 S. Ct. 2541 (2011), we question whether the
kind of medical monitoring sought here can be certified under
Rule 23(b)(2) but we do not reach the issue. As noted, the
Court held “Rule 23(b)(2) applies only when a single
injunction or declaratory judgment would provide relief to
each member of the class” but left open the question
“whether there are any forms of „incidental‟ monetary relief
that are consistent with the interpretation of Rule 23(b)(2) we
have announced and that comply with the Due Process
Clause.” Id. at 2557, 2561 (quoting Allison v. Citgo
Petroleum Corp., 151 F.3d 402, 415 (5th Cir. 1998)). If the
plaintiffs prevail, class members‟ regimes of medical
screenings and the corresponding cost will vary individual by
individual. But we need not determine whether the monetary
aspects of plaintiffs‟ medical monitoring claims are incidental
to the grant of injunctive or declaratory relief. “[A] single
injunction or declaratory judgment” cannot “provide relief to
each member of the class” proposed here, id. at 2557, due to
individual issues unrelated to the monetary nature of the
claim. For its part, the District Court found certification
improper under either category for reasons apart from the
monetary nature of plaintiffs‟ claims.

                              2.

       Although Rule 23(b)(2) classes need not meet the
additional predominance and superiority requirements of Rule
23(b)(3), “it is well established that the class claims must be
cohesive.” Barnes, 161 F.3d at 143. Rule 23(b)(2) requires
that “the party opposing the class has acted or refused to act
on grounds that apply generally to the class.” Fed. R. Civ. P.




                              15
23(b)(2). “The key to the (b)(2) class is „the indivisible
nature of the injunctive or declaratory remedy warranted—the
notion that the conduct is such that it can be enjoined or
declared unlawful only as to all of the class members or as to
none of them.‟” Wal-Mart Stores, Inc, 131 S. Ct. at 2557
(quoting Richard A. Nagareda, Class Certification in the Age
of Aggregate Proof, 84 N.Y.U. L. Rev. 97, 132 (2009)).
“Indeed, a (b)(2) class may require more cohesiveness than a
(b)(3) class.” Barnes, 161 F.3d at 142.12

       As all class members will be bound by a single
judgment, members of a proposed Rule 23(b)(2) injunctive or
declaratory class must have strong commonality of interests.
The Supreme Court in Wal-Mart recently highlighted the
importance of cohesiveness in light of the limited protections
for absent class members under subsections (b)(1) and (b)(2):

       Classes certified under (b)(1) and (b)(2) share
       the most traditional justifications for class
       treatment—that individual adjudications would
       be impossible or unworkable, as in a (b)(1)
12
  Commentators have noted that certification requirements
under Rule 23(b)(2) are more stringent than under (b)(3). See
7AA Charles Alan Wright, Arthur R. Miller & Mary Kay
Kane, Federal Practice and Procedure, § 1784.1, at 343 (3d
ed. 2005) (“[T]he common-question and superiority standards
of Rule 23(b)(3) are in some ways much less demanding than
that of either Rule 23(b)(1) or Rule 23(b)(2) . . . .”); see also 1
Joseph M. McLaughlin, McLaughlin on Class Actions: Law
and Practice § 5:15, at 5-57 (3d ed. 2006) (“[I]t is well
established that a rule 23(b)(2) class should actually have
more cohesiveness than a Rule 23(b)(3) class.” (internal
quotations omitted)).




                                16
       class, or that the relief sought must perforce
       affect the entire class at once, as in a (b)(2)
       class. For that reason these are also mandatory
       classes: The Rule provides no opportunity for
       (b)(1) or (b)(2) class members to opt out, and
       does not even oblige the District Court to afford
       them notice of the action.

Wal-Mart Stores, Inc., 131 S. Ct. at 2558 (footnote omitted).
The “disparate factual circumstances of class members” may
prevent a class from being cohesive and, therefore, make the
class unable to be certified under Rule 23(b)(2). Carter v.
Butz, 479 F.2d 1084, 1089 (3d Cir. 1973).

       Because causation and medical necessity often require
individual proof, medical monitoring classes may founder for
lack of cohesion. See In re St. Jude Med. Inc., 425 F.3d 1116,
1122 (8th Cir. 2005); Ball v. Union Carbide Corp., 385 F.3d
713, 727-28 (6th Cir. 2004); Zinser v. Accufix Research Inst.,
Inc., 253 F.3d 1180, 1195-96, amended, 273 F.3d 1266 (9th
Cir. 2001); Barnes, 161 F.3d at 143-46; Boughton v. Cotter
Corp., 65 F.3d 823, 827 (10th Cir. 1995). Rule 23(b)(2)
“does not authorize class certification when each class
member would be entitled to an individualized award of
monetary damages.”13 Wal-Mart, 131 S. Ct. at 2557.

       The District Court found individual issues were
significant to certain elements of the medical monitoring
claims here. To prevail on a medical monitoring claim under


13
  As noted, the Court left open whether monetary awards
incidental to the grant of declaratory or injunctive relief were
permissible. Id. at 2561.




                              17
Pennsylvania law,14 plaintiffs must prove:

       (1) exposure greater than normal background
       levels;
       (2) to a proven hazardous substance;
       (3) caused by the defendant‟s negligence;
       (4) as a proximate result of the exposure,
       plaintiff has a significantly increased risk of
       contracting a serious latent disease;
       (5) a monitoring procedure exists that makes the
       early detection of the disease possible;
       (6) the prescribed monitoring regime is different
       from that normally recommended in the absence
       of the exposure; and
       (7) the prescribed monitoring regime is
       reasonably       necessary       according     to
       contemporary scientific principles.

Redland Soccer Club, 696 A.2d at 145-46. “Expert testimony
is required to prove these elements.” Sheridan v. NGK
Metals Corp., 609 F.3d 239, 251 (3d Cir. 2010) (citing
Redland Soccer Club, 696 A.2d at 145-46). The District
Court identified individual issues that would eclipse common
issues in at least three of the required elements, noting several
potential variations in proving exposure above background, a
significantly increased risk of a serious latent disease, and the
reasonable necessity of the monitoring regime. Plaintiffs

14
  Neither party challenges the trial court‟s conclusion that
Pennsylvania law applies or that, if Illinois law applied, the
Illinois Supreme Court would adopt a cause of action for
medical monitoring with the same essential elements as
Pennsylvania law.




                               18
contend the court misinterpreted and improperly evaluated the
evidence on the merits, rather than under a class certification
standard, an error compounded by the parties‟ stipulation that
consideration of Daubert issues would be put off until after
class certification.

                               3.

       The District Court did not err in considering whether
the proposed common proof would accurately reflect the
exposure of individual members of the class to vinyl
chloride.15 “Frequently the „rigorous analysis‟ will entail
some overlap with the merits of the plaintiff‟s underlying
claim.” Wal-Mart Stores, Inc, 131 S. Ct. at 2551. “[T]he
court may „consider the substantive elements of the plaintiffs‟
case in order to envision the form that a trial on those issues
would take.‟” Hydrogen Peroxide, 552 F.3d at 317 (quoting
Newton v. Merrill Lynch, Pierce, Fenner & Smith, Inc., 259
F.3d 154, 166 (3d Cir. 2001)).

       Plaintiffs proposed to show the exposure of class
members through the expert opinions of Zannetti and
Ginsberg. On appeal, plaintiffs contend the court failed to
concentrate on Zannetti‟s isopleths and failed to recognize
that the isopleths provide average exposure per person, not a
class-wide average across class members.

15
  Plaintiffs were aware that the issues of class certification
were linked to the merits of their claims. In their reply brief
to the District Court, plaintiffs stated “[a]lthough typically a
party moving for class certification need not present expert
opinions on the merits (as opposed to the experts‟ proposed
methodologies), in this case, merits and certification are
closely linked.”




                              19
       The District Court found that the isopleths could not
constitute common proof of exposure above background
levels. It noted several problems—that the isopleths only
showed average daily exposure, not minimum exposure, used
average exposure over very long periods of time when
exposure likely varied, and could not show that every class
member was exposed above background.16

       Instead of showing the exposure of the class member
with the least amount of exposure, plaintiffs‟ proof would
show only the amount that hypothetical residents of the
village would have been exposed to under a uniform set of
assumptions without accounting for differences in exposure
year-by-year or based upon an individual‟s characteristics. At
most, the isopleths show the exposure only of persons who
lived in the village for the entire period the isopleth represents
and who behaved according to all assumptions that Zannetti
made in creating the isopleth.

       Plaintiffs cannot substitute evidence of exposure of
actual class members with evidence of hypothetical,
composite persons in order to gain class certification. Cf.
Principles of the Law of Aggregate Litigation § 2.02 cmt. d,
at 89 (2010) (“Aggregate treatment is thus possible when a
trial would allow for the presentation of evidence sufficient to
demonstrate the validity or invalidity of all claims with
respect to a common issue under applicable substantive law,
16
  While plaintiffs argue the court committed error by
describing their evidence as “averages,” plaintiffs themselves
stated in their reply brief to the District Court that “[p]laintiffs
will prove that daily average levels of vinyl chloride during
the defined periods of time migrated from defendants‟
manufacturing sites to the Village.” (emphasis added).




                                20
without altering the substantive standard that would be
applied were each claim to be tried independently and without
compromising the ability of the defendant to dispute
allegations made by claimants or to raise pertinent substantive
defenses.”). The evidence here is not “common” because it is
not shared by all (possibly even most) individuals in the class.
Averages or community-wide estimations would not be
probative of any individual‟s claim because any one class
member may have an exposure level well above or below the
average.

        Attempts to meet the burden of proof using modeling
and assumptions that do not reflect the individual
characteristics of class members have been met with
skepticism. See In re Fibreboard Corp., 893 F.2d 706, 712
(5th Cir. 1990) (“It is evident that these statistical estimates
deal only with general causation, for population-based
probability estimates do not speak to a probability of
causation in any one case; the estimate of relative risk is a
property of the studied population, not of an individual‟s
case.” (internal quotation omitted) (emphasis in original)); In
re “Agent Orange” Prod. Liab. Litig. MDL No. 381, 818 F.2d
145, 165 (2d Cir. 1987) (noting that “generic causation and
individual circumstances concerning each plaintiff and his or
her exposure to Agent Orange . . . appear to be inextricably
intertwined” and expressing concern that if the class had been
certified for trial “the class action would have allowed generic
causation to be determined without regard to those
characteristics and the individual‟s exposure”); see also 2
Joseph M. McLaughlin, McLaughlin on Class Actions: Law
and Practice § 8:9, at 8-55 to -57 (3d ed. 2006) (“Permitting a
class to proceed with its suit without linking its proof to even
a single class member would contravene the overwhelming




                              21
authority recognizing the individualized nature of the
causation inquiry in mass tort cases.”).

        There are several reasons the amount of vinyl chloride
exposure for class members would differ from the exposure
estimated by Zannetti‟s isopleths. Levels of vinyl chloride
varied within the periods the isopleth measures. Zannetti
assumes one constant level of exposure for 1968 to 1989,
another for 1990 to 1996 and a third for 1997 to 2006. But
another part of Zannetti‟s report notes the temporal level of
exposure varied drastically—even hourly. He states “hourly
concentration impacts are frequently one order of magnitude
(i.e., 10 times) greater and even two orders of magnitude (i.e.,
100 times) greater than the annual average.” The implication
of Zannetti‟s statement is that for the average to be at the
calculated level there would be periods when the
concentration would be significantly lower than the period
average, in addition to the periods when the concentration is
significantly higher. This fluctuation makes the specific
period of time and amount of time class members were in the
village critical in deciding whether they were exposed to
higher than background levels. As the court noted, within
each of these periods, the exposure varied and only persons
residing within the village the entire period would have their
personal average exposure equal the average exposure within
the isopleth lines.

       Plaintiffs‟ experts contended that, because the
dumping of vinylidene chloride stopped in 1978, the
concentration of vinyl chloride fell during much of the class
period. But under the plaintiffs‟ proposed modeling and
isopleths, a class member who lived in the village from 1988-
89—a full decade after the dumping ended—would be
assumed to have been exposed to the same concentration of




                              22
vinyl chloride as a person living in the same neighborhood
from 1968-69 when dumping occurred.

        Moreover, the isopleths do not reflect that different
persons may have different levels of exposure based on
biological factors or individual activities over the class
period. Factors which affect a person‟s exposure to toxins
can include activity level, age, sex, and genetic make-up. See
Federal Judicial Center, Reference Manual on Scientific
Evidence 430 (2d ed. 2000). On cross-examination, Ginsberg
stated that “[s]ome people will have higher breathing rates per
body weight, et cetera,” which would create a disparity
between the concentrations of vinyl chloride (based on
estimated exposure as opposed to actual exposure).

        Each person‟s work, travel, and recreational habits
may have affected their level of exposure to vinyl chloride.
Ginsberg admitted that differences in the amount of time
spent outside the village would create different average
concentrations to which the class members were exposed. A
person who worked outside the village would have been
exposed less than a stay-at-home parent, or retiree. The
isopleths assume exposure to the same concentration for class
members who may have spent very different amounts of time
in the village.

       Plaintiffs argue unconvincingly that the isopleths
reflect average exposure of individuals rather than a
classwide average. They contend the isopleth represents a
concentration which is the “least exposure of anyone within
the area circumscribed by the isopleth line.” But one cannot
evaluate the accuracy of this claim unless plaintiffs presented
some way to measure the actual minimum levels of exposure
of individual class members. Plaintiffs‟ model assumes away




                              23
relevant variations between the hundreds of residents within
the same isopleth lines that would result in exposure to
different concentrations of vinyl chloride.17 Their model does
not provide individual average exposures of actual class
members.

                              4.

       The District Court did not abuse its discretion in
finding plaintiffs would be unable to prove a concentration of
vinyl chloride that would create a significant risk of
contracting a serious latent disease for all class members.
Nor was there common proof that could establish the danger
point for all class members.

        The court identified two problems with the proposed
evidence.      First, it rejected the plaintiffs‟ proposed
threshold—exposure above 0.07 µ/m3, developed as a
regulatory threshold by the EPA for mixed populations of
adults and children—as a proper standard for determining
liability under tort law. Second, the court correctly noted,
even if the 0.07 µ/m3 standard were a correct measurement of
the aggregate threshold, it would not be the threshold for each
class member who may be more or less susceptible to
diseases from exposure to vinyl chloride.18


17
   Zannetti‟s report does not list the assumptions made that
would affect the concentration of exposure, such as the
amount of time spent in the village. Ginsberg, in reaching his
average, assumed the residents were present 350 days in a
year for 24 hours-a-day, 7 days-a-week for twenty-five years.
18
   Plaintiffs‟ experts agreed risk levels varied by individual.
Melissa Neiman, a board certified neurosurgeon, noted




                              24
       Although the positions of regulatory policymakers are
relevant, their risk assessments are not necessarily conclusive
in determining what risk exposure presents to specified
individuals. See Federal Judicial Center, Reference Manual
on Scientific Evidence 413 (2d ed. 2000) (“While risk
assessment information about a chemical can be somewhat
useful in a toxic tort case, at least in terms of setting
reasonable boundaries as to the likelihood of causation, the
impetus for the development of risk assessment has been the
regulatory process, which has different goals.”); id. at 423
(“Particularly problematic are generalizations made in
personal injury litigation from regulatory positions. . . . [I]f
regulatory standards are discussed in toxic tort cases to
provide a reference point for assessing exposure levels, it
must be recognized that there is a great deal of variability in
the extent of evidence required to support different
regulations.”).



“[i]ndividuals in the class will likely have varying degrees of
risk regarding the development of brain tumors,” although in
her opinion all exposed persons would have a higher risk than
the average non-exposed person. Ginsberg‟s report states
exposure to vinyl chloride has “greater cancer effects in early
life for liver and other tumor sites.” Therefore, exposure at
the screening target of 0.07 µ/m3 used for “mixed
populations” may pose little risk for healthy adults, but may
pose a great risk for children. For example, Ginsberg testified
that EPA Region 3 considers 0.16 µ/m3 as the “de minimis
risk threshold” but, according to Ginsberg, the EPA uses “a
lower screening level [the 0.07 µ/m3 standard] for
community risk if it‟s a mixed population, meaning young
children and adults.”




                              25
        Thus, plaintiffs could not carry their burden of proof
for a class of specific persons simply by citing regulatory
standards for the population as a whole. Cf. Wright v.
Willamette Indus., Inc., 91 F.3d 1105, 1107 (8th Cir. 1996)
(“Whatever may be the considerations that ought to guide a
legislature in its determination of what the general good
requires, courts and juries, in deciding cases, traditionally
make more particularized inquiries into matters of cause and
effect.”).

       Plaintiffs have failed to propose a method of proving
the proper point where exposure to vinyl chloride presents a
significant risk of developing a serious latent disease for each
class member. Plaintiffs propose a single concentration
without accounting for the age of the class member being
exposed, the length of exposure, other individual factors such
as medical history, or showing the exposure was so toxic that
such individual factors are irrelevant. The court did not abuse
its discretion in concluding individual issues on this point
make trial as a class unfeasible, defeating cohesion.

                               5.

        Nor did the court abuse its discretion in determining
individual issues defeat cohesion with respect to whether the
proposed monitoring regime is reasonably medically
necessary. We have been skeptical that the necessity for
individuals‟ medical monitoring regimes can be proven on a
class basis. See Barnes, 161 F.3d at 146 (“Although the
general public‟s monitoring program can be proved on a
classwide basis, an individual‟s monitoring program by
definition cannot.”); see Principles of the Law of Aggregate
Litigation § 2.04 reporter‟s notes cmt. b, at 126 (2010)
(“[A]fter Barnes, courts often have withheld class




                              26
certification for medical monitoring due to the presence of
individualized issues . . . .”). Plaintiffs‟ proposed common
evidence and trial plan do not resolve any of the issues in
proving medical necessity on an aggregate basis.

        The District Court did not err in rejecting plaintiffs‟
conclusory allegation they could prove the need for serial
MRIs on a classwide basis. There were conflicting expert
reports. Ginsberg‟s report contended class members were at
increased risk due to exposure but did not discuss possible
monitoring and treatment regimes. Melissa Neiman, a
neurosurgeon, suggested that serial MRIs and neurological
examinations can be used to detect types of brain cancer
associated with exposure to vinyl chloride without
explanation of their effectiveness or potential risk. None of
plaintiffs‟ experts addressed how medical monitoring would
proceed. Defendants‟ expert Peter Valberg, a toxicologist,
maintained the negative health effects of screening may
outweigh any potential benefits. Another defense expert,
Henry Friedman, a neuro-oncologist, contended a regime of
serial MRIs would be contraindicated and potentially risky
because the contrast agent used for MRIs poses dangers to
those with kidney disease. The court did not err in crediting
defense experts‟ detailed discussions of why the medical
monitoring regime would present individual rather than
common issues. See Hydrogen Peroxide, 552 F.3d at 323
(“Weighing conflicting expert testimony at the certification
stage is not only permissible; it may be integral to the
rigorous analysis Rule 23 demands.”).

      Plaintiffs‟ proposed common evidence and trial plan
would not be able to prove the medical necessity of plaintiffs‟
proposed monitoring regime without further individual
proceedings to consider class members‟ individual




                              27
characteristics and medical histories and to weigh the benefits
and safety of a monitoring program. Plaintiffs cannot show
the cohesiveness required for certification of a Rule 23(b)(2)
class. The court did not abuse its discretion in refusing to
certify a class that would be able to resolve few if any issues
that would materially advance resolution of the underlying
claims.

                              B.

                              1.

     Plaintiffs also sought certification under Rule 23(b)(3).
The requirements of predominance and superiority19 for

19
  Rule 23(b)(3) requires “that the questions of law or fact
common to class members predominate over any questions
affecting only individual members, and that a class action is
superior to other available methods for fairly and efficiently
adjudicating the controversy.” Fed. R. Civ. P. 23(b)(3). It
lists factors relevant to the predominance and superiority
requirements:
        (A) the class members‟ interests in individually
        controlling the prosecution or defense of
        separate actions;
        (B) the extent and nature of any litigation
        concerning the controversy already begun by or
        against class members;
        (C) the desirability or undesirability of
        concentrating the litigation of the claims in the
        particular forum; and
        (D) the likely difficulties in managing a class
        action.
Id.




                              28
maintaining a class action under Rule 23(b)(3) are less
stringent than the cohesiveness requirement of Rule
23(b)(2).20 See Barnes, 161 F.3d at 143; In re St. Jude Med.
Inc., 425 F.3d at 1121. But the two inquiries are similar.
“The Rule 23(b)(3) predominance inquiry tests whether
proposed classes are sufficiently cohesive to warrant
adjudication by representation.” Amchem, 521 U.S. at 623.

       Courts have generally denied certification of medical
monitoring classes when individual questions involving
causation and damages predominate over (and are more
complex than) common issues such as whether defendants
released the offending chemical into the environment. See In
re St. Jude Med., Inc., 522 F.3d 836, 840 (8th Cir. 2008)
(reversing the decision to certify a Rule 23(b)(3) class for
“the highly individualized remedy of medical monitoring”);
see generally 7AA Charles Alan Wright, Arthur R. Miller,
Mary Kay Kane & Richard L. Marcus, Federal Practice &
Procedure § 1782 (3d ed. 2005) (“[T]o the extent that
different injuries are alleged to have occurred to different
class members and over different periods of time, it is
difficult to show that common issues predominate and that a
class action would be superior.”).

       As discussed, the inquiries into whether class members
were exposed above background levels, whether class
members face a significantly increased risk of developing a
serious latent disease, and whether a medical monitoring
regime is reasonably medically necessary all require
considering individual proof of class members‟ specific
20
  The parties do not address the court‟s findings that a class
action would not be a superior method of adjudication. In
any event, we see no error here.




                             29
characteristics. The court did not abuse its discretion in
finding individual issues predominate over any issues
common to the class.

                               2.

        Plaintiffs contend an alternative class could have been
certified. They offer three possible modifications—only one
of which they presented to the District Court. Plaintiffs
suggested in a footnote in their trial reply brief that, if their
proposed common proof were insufficient, they could create
isopleths measuring exposure in each calendar year. The jury
would then use these yearly isopleths to determine if
residents‟ exposure levels in that year satisfied the elements
of Pennsylvania‟s medical monitoring cause of action.

        The court did not err in rejecting plaintiffs‟ alternative
class definition. “A party‟s assurance to the court that it
intends or plans to meet the requirements is insufficient.”
Hydrogen Peroxide, 552 F.3d at 318 (citing Newton, 259 F.3d
at 191). “Rule 23 does not set forth a mere pleading standard.
A party seeking class certification must affirmatively
demonstrate his compliance with the Rule—that is, he must
be prepared to prove that there are in fact sufficiently
numerous parties, common questions of law or fact, etc.”
Wal-Mart Stores, Inc., 131 S. Ct. at 2551. “[A] district court
errs as a matter of law when it fails to resolve a genuine legal
or factual dispute relevant to determining the requirements.”
Hydrogen Peroxide, 552 F.3d at 320. Plaintiffs did not
present yearly isopleths to the trial court, did not show such
isopleths to be feasible given the limited data available, and
did not explain how these yearly determinations would
correspond to actual class members.




                               30
        On appeal, plaintiffs suggest for the first time two
further refinements to their class definition. Plaintiffs
contend common issues would predominate if the class
definition were (1) amended to include only class residents
who lived in the village for the entire period represented by
the isopleths presented to the trial court, or (2) amended to
include only class members who lived in the village for an
entire calendar year and yearly isopleths were created. These
alternatives are not properly before us, having never been
presented to the trial court. Even if we were to consider
plaintiffs‟ arguments, their alternatives do not resolve the
defects in the isopleths nor provide enough detail to
determine how the claims of such a class would be tried.

                              C.

                               1.

       Plaintiffs also sought certification of a Rule 23(b)(3)
class of property owners who allegedly suffered loss in
property values due to defendants‟ contamination under
theories of public nuisance, private nuisance, strict liability,
CERCLA, conspiracy, negligence, negligence per se, and
trespass. The court noted “[p]laintiffs rely on the same expert
testimony that they offered to support their medical
monitoring claim.” Accordingly, it found common questions
did not predominate over individual questions because
“[a]lthough many aspects of [p]laintiffs‟ claims may be
common questions, the parties agree that resolution of those
questions leaves significant and complex questions
unanswered, including questions relating to causation of
contamination, extent of contamination, fact of damages, and
amount of damages.”




                              31
       The District Court properly explained its reasons for
finding that individuals issues predominated over common
issues. Plaintiffs cannot fault the court for failing to examine
each element of their purported causes of action when they
failed to present arguments or propose common proof for
each element. As the arguments for certification of the
property class relied on the same purported “common”
evidence as the medical monitoring class, the court did not err
by denying certification of the property damage class.

        The trial court properly considered and rejected the
arguments plaintiffs did make. Plaintiffs rely on other
instances of property contamination where the courts found
common issues predominated. But those cases presented
simpler theories of contamination or discrete incidents of
contamination. In Mejdrech v. Met-Coil Systems Corp., 319
F.3d 910 (7th Cir. 2003), the plaintiffs alleged the improper
handling of chemicals contaminated the soil and groundwater
beneath their properties. The court certified an issue class on
the defendant‟s negligence and the extent of contamination
but left damages to be resolved individually. But the Seventh
Circuit, in affirming the order certifying the class, noted the
question of the “geographical scope of the contamination”
was “not especially complex.” Id. at 912.

        Similarly, in Sterling v. Velsicol Chemical Corp., 855
F.2d 1188 (6th Cir. 1988), the plaintiffs alleged groundwater
contamination that could be discovered merely by testing
local wells. Id. at 1193. The Sixth Circuit affirmed the
certification order but noted a class action is only suited for
situations when “the cause of the disaster is a single course of
conduct which is identical for each of the plaintiffs.” Id. at
1197. The court warned:




                              32
       In complex, mass, toxic tort accidents, where no
       one set of operative facts establishes liability,
       no single proximate cause equally applies to
       each potential class member and each
       defendant, and individual issues outnumber
       common issues, the district court should
       properly question the appropriateness of a class
       action for resolving the controversy.

Id. Not all claims of property damage based on exposure are
alike. Single instances or simple theories of contamination
may be more apt for consolidated proceedings than extensive
periods of contamination with multiple sources and various
pathways. See In re Methyl Tertiary Butyl Ether (“MTBE”)
Prods. Liab. Litig., 241 F.R.D. 435, 447 (S.D.N.Y. 2007)
(certifying class for damage to property from water
contamination but noting “[c]ourts have repeatedly drawn
distinctions between proposed classes involving a single
incident or single source of harm and proposed classes
involving multiple sources of harm occurring over time”);
Reilly v. Gould, Inc., 965 F. Supp. 588, 602 (M.D. Pa. 1997)
(noting in refusing to certify a property damage class “it is the
presence of additional individualized factors affecting
individual plaintiffs which wreaks havoc on the notion that all
plaintiffs‟ injuries have been caused solely by the defendant‟s
actions”).

       Here, plaintiffs contend varied levels of vinylidene
chloride at various times seeped into a shallow aquifer,
degraded into vinyl chloride, diffused from the aquifer to the
ground above, and evaporated into the air to be carried over
the village. Given the potential difference in contamination
on the properties, common issues do not predominate. Cf.
Fisher v. Ciba Specialty Chems. Corp., 238 F.R.D. 273, 305




                               33
n.70 (S.D. Ala. 2006) (“[A] property-by-property inquiry will
unquestionably be necessary to determine whether that source
and that pathway have any bearing on the experience of a
particular property owner within the Proposed Class Area.”).
The District Court did not abuse its discretion in finding the
property damage class members‟ individual issues
predominated over the issues common to the class.

                               2.

       Alternatively, plaintiffs contend that, even if common
issues do not predominate, the court should have certified an
“issue only” class on liability. The court found an issue class
was not feasible and would not advance the resolution of
class members‟ claims. The court noted both the fact of
damages and the amount of damages “would remain
following the class-wide determination of any common
issues,” and further that causation and extent of
contamination would need to be determined at follow-up
proceedings. Due to the numerous individual issues that
would remain, the court declined to certify a liability-only
class.

       “[A] court‟s decision to exercise its discretion under
Rule 23(c)(4),[21] like any other certification determination
under Rule 23, must be supported by rigorous analysis.”
Hohider v. United Parcel Serv., Inc., 574 F.3d 169, 200-01
(3d Cir. 2009). Rule 23(c)(4) “both imposes a duty on the
court to insure that only those questions which are appropriate
for class adjudication be certified, and gives it ample power to

21
  Fed. R. Civ. P. 23(c)(4) states: “Particular Issues. When
appropriate, an action may be brought or maintained as a
class action with respect to particular issues.”




                              34
„treat common things in common and to distinguish the
distinguishable.‟” Chiang v. Veneman, 385 F.3d 256, 267 (3d
Cir. 2004) (quoting Jenkins v. United Gas Corp., 400 F.2d 28,
35 (5th Cir. 1968)).         “The interaction between the
requirements for class certification under Rule 23(a) and (b)
and the authorization of issue classes under Rule 23(c)(4) is a
difficult matter that has generated divergent interpretations
among the courts.” Hohider, 574 F.3d at 200 n.25.

        Courts have disagreed over the extent to which the
ability to certify issue classes alters the predominance
requirement. Some appellate courts have viewed Rule
23(c)(4) as a “housekeeping rule” allowing common issues to
be certified only when the cause of action, taken as a whole,
meets the predominance requirement. See Castano v. Am.
Tobacco Co., 84 F.3d 734, 745 n.21 (5th Cir. 1996). Others
have allowed certification of issue classes even if common
questions do not predominate for the cause of action as a
whole. See In re Nassau County Strip Search Cases, 461
F.3d 219, 226 (2d Cir. 2006); Gunnells v. Healthplan Servs.,
Inc., 348 F.3d 417, 439 (4th Cir. 2003); Valentino v. Carter-
Wallace, Inc., 97 F.3d 1227, 1234 (9th Cir. 1996). We noted
the split of authority in Hohider. 574 F.3d at 200 & n.25.

        The District Court here found “resolution of [common]
questions leaves significant and complex questions
unanswered.” We agree, as the common issues here are not
divisible from the individual issues. See Hohider, 574 F.3d at
200 n.25. Following Hohider, the District Court conducted a
rigorous analysis on the effect “partial certification would
have on the class action going forward.” Id. at 202. In
Hohider, we provided relevant considerations on when a
district court may wish to carve at the joints to form issue
classes and cited the ALI‟s Proposed Final Draft of the




                              35
Principles of the Law of Aggregate Litigation. The ALI‟s
final draft preserved and expanded its discussion of these
important considerations. See Principles of the Law of
Aggregate Litigation §§ 2.02-05, 2.07-2.08 (2010).

       Rather than joining either camp in the circuit
disagreement, we believe the considerations set forth in
Hohider and more recently in the Final Draft of the ALI‟s
Principles of Aggregate Litigation provide the most sound
guidance in resolving this complicated area of class action
procedure.

        In light of the adoption of the Final Draft of the
Principles of Aggregate Litigation, when deciding whether or
not to certify an issue class, the trial court should consider:
the type of claim(s) and issue(s) in question; the overall
complexity of the case; the efficiencies to be gained by
granting partial certification in light of realistic procedural
alternatives; the substantive law underlying the claim(s),
including any choice-of-law questions it may present and
whether the substantive law separates the issue(s) from other
issues concerning liability or remedy; the impact partial
certification will have on the constitutional and statutory
rights of both the class members and the defendant(s); the
potential preclusive effect or lack thereof that resolution of
the proposed issue class will have; the repercussions
certification of an issue(s) class will have on the effectiveness
and fairness of resolution of remaining issues; the impact
individual proceedings may have upon one another, including
whether remedies are indivisible such that granting or not
granting relief to any claimant as a practical matter
determines the claims of others; and the kind of evidence
presented on the issue(s) certified and potentially presented
on the remaining issues, including the risk subsequent triers




                               36
of fact will need to reexamine evidence and findings from
resolution of the common issue(s). See Principles of the Law
of Aggregate Litigation §§ 2.02-05 (2010); Hohider, 574 F.3d
at 201. This non-exclusive list of factors should guide courts
as they apply Fed. R. Civ. P. 23(c)(4) “to „treat common
things in common and to distinguish the distinguishable.‟”
Chiang, 385 F.3d at 256 (quoting Jenkins, 400 F.2d at 35).

       When certifying an issue class the court should clearly
enumerate the issue(s) to be tried as a class as required by
Fed. R. Civ. P. 23(c)(1)(B). See Wachtel v. Guardian Life
Ins. Co. of Am., 453 F.3d 179, 184-85 (3d Cir. 2006). It
should also explain how class resolution of the issue(s) will
fairly and efficiently advance the resolution of class
members‟ claims, including resolution of remaining issues.
See Principles of the Law of Aggregate Litigation §§ 2.02(e)
(2010).

        The trial court here did not abuse its discretion by
declining to certify a liability-only issue class when it found
liability inseverable from other issues that would be left for
follow-up proceedings. Nor did the court err in finding no
marked division between damages and liability.

        Plaintiffs have neither defined the scope of the
liability-only trial nor proposed what common proof would be
presented.22 The claims and issues here are complex and

22
  Plaintiffs appear to rely on the same purported common
proof used for the medical monitoring class. But the common
evidence presented for the medical monitoring class shows
present levels of contamination to be very low, undercutting
the claims of the class seeking damages for present
contamination of their property.




                              37
common issues do not easily separate from individual issues.
A trial on whether the defendants discharged vinlydine
chloride into the lagoon that seeped in the shallow aquifer and
whether the vinyl chloride evaporated from the air from the
shallow aquifer is unlikely to substantially aid resolution of
the substantial issues on liability and causation.

       Certification of a liability-only issue class may unfairly
impact defendants and absent class members. Plaintiffs‟ bald
assertion that class members claims share “the same nucleus
of operative facts” is a mere “assurance to the court that it
intends or plans to meet the requirements.” Hydrogen
Peroxide, 552 F.3d at 318 (citing Newton, 259 F.3d at 191).
Plaintiffs appear to rely on the same “common” evidence
used for the medical monitoring class, but fail to explain how
their estimates of exposure to residents over substantial
periods of time corresponds to the level of contamination
currently present at each home. It may prejudice absent class
members whose properties may be shown to have suffered
greater contamination.23

       Given the inability to separate common issues from
issues where individual characteristics may be determinative,
23
  Cf. Boughton, 65 F.3d at 827 n.1 (“[W]here, as here, there
are multiple types of claims, more than one form of relief
sought and the parties disagree about the number of models
necessary to deal with the various ways in which properties
may have become contaminated it may not be so simple as to
err on the side of certification just to keep the option open
because there may be mutually exclusive ways of defining
subclasses and any attempt to certify subclasses before it is
clear what the common issues are carries with it the potential
for making the case less manageable.”)




                               38
the District Court did not abuse its discretion in refusing to
certify a liability-only property damage class.

                              III.

        For the foregoing reasons the District Court did not
abuse its discretion in denying the plaintiffs‟ motion for class
certification under Fed. R. Civ. P. 23(b)(2) and (b)(3). We
will affirm its judgment.




                              39
