Opinions of the United
1999 Decisions                                                                                                             States Court of Appeals
                                                                                                                              for the Third Circuit


2-3-1999

Heller v. Shaw Ind Inc
Precedential or Non-Precedential:

Docket 97-1735




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Filed   February 3, 1999

UNITED STATES COURT OF APPEALS
FOR THE THIRD CIRCUIT

_______________

NO. 97-1735
_______________

CAROL HELLER; THOMAS HELLER individually and as
the parents and natural guardians of EMILY HELLER and
KATHERINE HELLER, minor children,
Appellants

v.

SHAW INDUSTRIES, INC.
_______________

On Appeal from the United States District Court
For the Eastern District of Pennsylvania
(D.C. Civ. No. 97-07657)
District Judge: Honorable William H. Yohn, Jr.
_______________

Argued: October 6, 1998
Before: BECKER, Chief Judge, NYGAARD, and
NOONAN,* Circuit Judges

(Filed: February 3, 1999)

        JOSEPH C. KOHN, ESQUIRE
        JOANNE ZACK, ESQUIRE (ARGUED)
        MARTIN J. D'URSO, ESQUIRE
        DAVID G. CONCANNON, ESQUIRE
        Kohn, Swift & Graf, P.C.
        1101 Market Street, 24th Floor
        Philadelphia, PA 19107

        Attorneys for Appellants




_______________________________________________________
*Honorable John T. Noonan, Jr., Senior Judge of the United States
Court of Appeals for the Ninth Circuit, sitting by designation.



     JOHN F. KENT, ESQUIRE
     ANNE M. MANERO, ESQUIRE
     Kent & McBride
     18th & Arch Streets
     Two Logan Square, Suite 600
     Philadelphia, PA 19103

     LEE ANN JONES, ESQUIRE
     (ARGUED)
     Powell, Goldstein, Frazer & Murphy,
     LLP
     191 Peachtree Street
     Atlanta, GA 30303

     Attorneys for Appellee

_______________

OPINION OF THE COURT
_______________

BECKER, Chief Judge:

This is an appeal by plaintiff Carol Heller ("Heller"), who
sought to recover from defendant Shaw Industries ("Shaw"),
for certain respiratory illnesses allegedly caused by volatile
organic compounds emitted by Shaw carpet installed in
Heller's former home. The District Court's grant of summary
judgment against Heller and in favor of Shaw is largely a
function of its exclusion, following an extensive in limine
hearing, of key expert testimony by which Heller hoped to
establish liability. See Heller v. Shaw Indus., Inc., No.
Civ.A.95-7657, 

(E.D. Pa. Aug. 18, 1997). In
reviewing the District Court's rulings, we revisit the caselaw
interpreting Federal Rule of Evidence 702, particularly
Daubert v. Merrell Dow Pharmaceuticals, Inc., 

(1993), and In re Paoli Railroad Yard PCB Litigation, 

(3d Cir. 1994), and apply their teachings to this case.

2

After careful but deferential review, see General Elec. Co. v.
Joiner, 

, 517 (1997), we conclude that,
although the District Court erred in excluding certain aspects
of the experts' proffered testimony, it properly excluded the
central portions of their testimony, depriving Heller's claim of
its needed evidentiary support.

More specifically, the District Court was too restrictive in
requiring Heller's medical expert to rely on published studies
specifically linking Heller's illness with Shaw's product, and
in requiring Heller's medical expert to rule out all alternative
possible causes of her illness. However, it properly excluded
this expert's causation testimony because his conclusion
regarding the cause of Heller's illness was heavily based on
a flawed temporal relationship between the installation of the
Shaw carpet and the presence of Heller's illness. The District
Court also properly excluded the testimony of Heller's
environmental expert on the grounds that his environmental
testing revealed levels of dangerous compounds in the air in
Heller's home that were not significantly higher than
background levels, and his methodology for extrapolating
from these tests to estimate the (higher) levels of compounds
at an earlier time was seriously flawed. Therefore, because
the District Court did not abuse its discretion in excluding
the key elements of Heller's experts' testimony necessary to
prove causation, the grant of summary judgment will be
affirmed.

I.   Facts and Procedural History

On September 30, 1993, Heller, her husband Thomas, and
their two children moved into a nine-year old house in West
Chester, Pennsylvania. Shortly after the move, Thomas Heller
experienced allergy symptoms. In November and December
1993, an allergist advised Mr. Heller to replace the carpeting
in the home because cat hair from previous owners might
have caused his allergic reactions. On December 13 and 14,
1993, the Hellers put new carpeting--manufactured by Shaw
Industries--in certain rooms of their home, including the

3

master bedroom on the first floor and a guest room on the
second floor.

In late December 1993, Carol Heller began to experience
respiratory problems, including asthma, breathing difficulty,
wheezing, coughing, and dizziness. After seeking treatment
from her father, a physician, Heller consulted Dr. Joseph
Papano, an allergist and one of her two expert witnesses.1 Her
first visit to Dr. Papano was on February 15, 1994. Dr.
Papano took Heller's medical and family history, questioned
her about her environment (whether there were cats or dogs
in the home, etc.), and performed allergy tests, chest X-rays,
and pulmonary function tests. Based on the history, tests,
and a physical examination, Dr. Papano ruled out various
possible causes of her respiratory problems. In February
1994, the doctor recommended that Heller contact Alan Todd
of Todd Environmental Consultants (collectively "Todd") to
test both the air quality in her home and the carpet.2 When
Dr. Papano next saw Heller, on March 19, 1994, she was still
experiencing problems, but informed him that her symptoms



_______________________________________________________

1. Because, as discussed below, Dr. Papano relied heavily for
his diagnosis on the temporal relationship between the
installation of the Shaw carpet and the onset of Heller's
illness, the date on which she began to experience her symptoms
is a contested and important issue. Dr. Papano originally
testified at his deposition that her symptoms began in "January
1994," and he so stated in his expert witness report. However, at
the Daubert hearing, he testified that the symptoms began in
"mid-January 1994." Heller testified at the Daubert hearing that
her symptoms began in late December 1993 or early January 1994,
while her father testified that they began either in mid-December
or during the first two weeks of December. Given the conflicting
testimony, we find no clear error in the District Court's
reliance, in its findings following the Daubert hearing, on
Heller's testimony that her symptoms began in late December 1993.
See Heller, 

, at *17 n.29.

2. Dr. Papano's expert report represents that he referred Heller
to Todd at her March 1994 visit, while Todd's expert report
states that he was first contacted by Heller and did the initial
testing in February. The District Court apparently credited Todd
and we will not revisit this (supported) conclusion. Given that
the initial home testing was performed by Todd in February 1994,
Dr. Papano must have referred the Hellers to Todd in February.

4


improved when she was out of her house. At this time, she
brought the doctor a sample of the Shaw carpet, which he
testified had a strong odor.

The Hellers contracted with Todd to perform the testing of
the environment in the house and of the carpet. Todd initially
tested for excessive levels of dust or other possible
contaminants, finding nothing unusual. By April 7, 1994, the
Heller family had moved out of their home in an attempt to
eliminate Heller's respiratory problems. On April 14, 1994,
Todd tested the air in the closet of one of the Hellers'
bedrooms; the closet contained both some of the Shaw carpet
and some carpet from an unknown manufacturer. Todd's
initial test, conducted over approximately eight hours, found
fourteen types of volatile organic compounds ("VOC"s) present
in the air in the closet.

Three weeks later, on May 5, 1994, the Shaw carpet was
removed from the house. Less than a week later, on May 11,
1994, Todd again tested the air in the bedroom closet for
eight hours. During the period between the two tests, no
other changes were made in the house: no objects were added
or removed, the windows remained closed, and no persons
entered or left the by-now empty house. In the second test,
Todd found only five types of VOCs present. Four of these
were present in levels virtually indistinguishable from the
initial readings. Therefore, nine compounds completely
disappeared and one (benzene) remained present, but at
lower levels. Todd's initial conclusion, in a May 23, 1994,
letter to Heller, was that "none of the compounds identified
would be expected to typically result in asthmatic or
sensitization responses." (Later, however, in his first expert
witness report, dated January 16, 1997, he opined that the
compounds that disappeared or diminished were emitted
from the Shaw carpet, and were "the likely source of [the
Hellers'] irritation and related responses.")

The Hellers returned to the home briefly on May 11, 1994.
Although the carpet had been removed six days earlier and
the May 11 testing would reveal the presence of very few

5


VOCs, Mrs. Heller again   experienced "wheezing, shortness of
breath and an irritated   throat." The Hellers then left the
house, never to return.   In November 1994, they sold the
home for less than they   had paid for it a year earlier.

Dr. Papano's expert report stated that he performed a
differential diagnosis, which involved ruling out possible
causes of Heller's symptoms other than Shaw's carpet
(including "an infectious cause"), and, based largely on the
temporal relationship between her symptoms and the
installation of the Shaw carpet, concluded that the Shaw
carpet precipitated Heller's respiratory problems. As noted
above, Alan Todd also offered his expert opinion (in his
original expert report) that "to a reasonable degree of
scientific certainty, . . . the illness[es] suffered by the Heller
family were caused by their prolonged exposure to the VOC's
measured in their home and emitted by the carpeting
manufactured by Shaw Industries."

In December 1995, the Hellers brought a diversity action
against Shaw in the District Court for the Eastern District of
Pennsylvania, under 28 U.S.C. S 1332, alleging breach of
warranty, failure to warn, negligent and intentional
misrepresentation, defective design, and violation of state
consumer protection laws. The complaint sought
compensatory and punitive damages for both personal
injuries and property damage, as well as a medical
monitoring award. To establish defective design and failure to
warn, a plaintiff must prove that the defendant's product
caused her injuries. Causation therefore was the primary
focus of the District Court's inquiry and the primary disputed
issue in this case.

Following extensive discovery, Shaw moved for summary
judgment and, as an adjunct to that motion, moved in limine
to exclude all of Heller's expert witness testimony. The
District Court held a Daubert hearing over several days. It
then filed an unpublished opinion and order, granting
defendant's motions for exclusion of plaintiff's expert
testimony and for summary judgment. See Heller v. Shaw

6

Indus., Inc., No. Civ.A.95-7657, 

(E.D. Pa.
Aug. 18, 1997).

On appeal, we review a District Court's decision to exclude
expert testimony for abuse of discretion. See Joiner, 118 S.
Ct. at 517. The District Court's interpretation of the
requirements of Rule 702, however, is subject to plenary
review. See 

. As to the District Court's
entry of summary judgment for defendants, "we exercise
plenary review, construing all evidence and resolving all
doubts raised by affidavits, depositions, answers to
interrogatories, and admissions on file in favor of the non-
moving party." Iberia Foods Corp. v. Romeo, 

,
302 (3d Cir. 1998).

Heller does not appear to dispute that, if we determine that
the District Court properly excluded all of plaintiff's expert
testimony, summary judgment for defendant was the proper
course for the key claims of design defect and failure to warn.
This is because, without either Dr. Papano's or Alan Todd's
testimony, Heller would be left without any proof of
causation, a necessary element for each of these claims.
However, if we decide (as we do) that some of the testimony
should have been admitted, we must determine whether that
testimony is sufficient to create a material issue of fact on the
causation issue. Most of our opinion will focus on the key
underlying issue of the admissibility of Heller's expert witness
testimony, on which the causation issue hinges. While there
are other issues in the case, including breach of warranty
and misrepresentations, we will address these only briefly, for
they are easily disposed of without extended discussion.

II.   Expert Witnesses: The Legal Background

Rule 702 provides: "If scientific, technical, or other
specialized knowledge will assist the trier of fact to
understand the evidence or to determine a fact in issue, a
witness qualified as an expert . . . may testify thereto in the
form of an opinion or otherwise." Relying on the language of

7


Rule 702 and the liberal thrust of the Federal Rules of
Evidence, the Supreme Court held in Daubert that expert
testimony need be based only on a reliable and scientifically
valid methodology that fits with the facts of a case. See

.3 The Court listed four factors to
guide a district court in its preliminary assessment of these
requirements, but cautioned that these were guideposts and
not required factors in each case. The factors are: (1) whether
the methodology can and has been tested; (2) whether the
technique has been subjected to peer review and publication;
(3) the known or potential rate of error of the methodology;
and (4) whether the technique has been generally accepted in
the proper scientific community. See 

The Court
made clear that its listing of these factors should not obscure
the fact that the district court's gatekeeper role is a flexible
one, see 

& n.12, and that the factors are simply
useful signposts, not dispositive hurdles that a party must
overcome in order to have expert testimony admitted. In this
regard, a party seeking to exclude (or to admit) expert
testimony must do more than enumerate the factors from
Daubert (and the additional ones from Paoli, discussed below)
and tally the number that are or are not met by a particular
expert's testimony.

In Daubert, the Court noted that "[v]igorous cross-
examination, presentation of contrary evidence, and careful
instruction on the burden of proof are the traditional and
appropriate means of attacking shaky but admissible
evidence," and that, even if expert testimony is admitted,
summary judgment might be warranted if a party has still
failed to present sufficient evidence to get to the jury. 

see also 

n.21. Clearly, the Court
envisioned cases in which expert testimony meets the
Daubert standard yet is "shaky," and cases in which



_______________________________________________________

3. In Daubert, the Supreme Court interred the decades-old Frye
doctrine,from Frye v. United States, 

(D.C. Cir.
1923), which required that an expert's methodology be "generally
accepted" within the scientific community before the expert's
testimony could be admitted. See 

.

8

admissible expert testimony provides only a "scintilla" of
support for a claim or defense. Put differently, an expert
opinion must be based on reliable methodology and must
reliably flow from that methodology and the facts at issue--
but it need not be so persuasive as to meet a party's burden
of proof or even necessarily its burden of production.

In Paoli, filed barely a year after Daubert, we identified a
number of factors that a istrict court might use in
evaluating expert testimony in addition to the four factors
listed in Daubert. The additional factors include: (1) "the
existence and maintenance of standards controlling the
technique's operation"; (2) "the relationship of the technique
to methods which have been established to be reliable"; (3)
the expert witness's qualifications; and (4) "the non-judicial
uses to which the method has been put." 

n.8. In Paoli, we explained that even if the judge believes
"there are better grounds for some alternative conclusion,"
and that there are some flaws in the scientist's methods, if
there are "good grounds" for the expert's conclusion, it should
be admitted. 

We also emphasized in Paoli that the district court could
not exclude the testimony simply because the conclusion was
"novel" if the methodology and the application of the
methodology were reliable. See 

n.15. However, we
rejected the plaintiffs' argument in Paoli (also urged strongly
by the plaintiff here) that the district court had abused its
discretion by examining the experts' conclusions. While "[t]he
focus, of course, must be solely on principles and
methodology, not on the conclusions that they generate,"

, a district court must examine the
expert's conclusions in order to determine whether they could



_______________________________________________________

4. In addition to the "good grounds" requirement, in a diversity
case such as this, state rules on the degree of certainty
required of an expert's opinion apply. In Pennsylvania, a doctor
can give an opinion on the cause of a plaintiff's illness if he
or she can do so with a reasonable degree of medical certainty.
See 

-52.

9


reliably follow from the facts known to the expert and the
methodology used.5

III.   Plaintiff's Expert Witnesses

In this case, Heller must demonstrate, as part of her prima
facie case, that Shaw's carpet emitted VOCs into the air; that
she inhaled these VOCs; that she has an injury; and that the
VOCs were the cause of this injury. See In re Paoli R.R. Yard
PCB Litig., 

, 860 (3d Cir. 1990). The expert
testimony of Alan Todd, who is a certified industrial
hygienist, is integral to plaintiff proving the first of these
elements, and would bolster a medical conclusion on
causation by demonstrating that the level of VOCs present in
the Heller home was significantly higher than the background
levels typically present. Without his testimony, drawn from
the tests he performed in the Heller household, Heller has
adduced no evidence that the Shaw carpet installed in the
Heller home emitted VOCs into the air (let alone emitted them
at a level sufficient to cause her illness). Further, even if
Todd's testimony is admitted and is sufficient to meet
plaintiff's burden on this first element at the summary
judgment stage, Dr. Papano's testimony is necessary to prove
that Heller became sick. His testimony also is critical for
proving that the Shaw carpet was the cause of Heller's
illness. While Todd also offered his expert testimony
regarding the fourth element, i.e., that the VOCs from the
Shaw carpet caused Heller's illness, as we will discuss below,
only Dr. Papano is qualified to testify as to this element.6

_______________________________________________________

5. The Advisory Committee on the Federal Rules of Evidence has
proposed changes to Rule 702 that would reflect the standards
from Daubert (and Paoli), requiring that an expert's testimony be
based on reliable facts, be the product of reliable principles
and methodology, and be based on a reliable application of these
principles and methods to the facts of the case. See Fed. R.
Evid. 702 (Preliminary Draft 1998).
6. We do not focus on the second element, that Heller inhaled
the VOCs. If she is able to prove, through her expert witnesses'
testimony, that the Shaw carpet installed in the home emitted
VOCs, that she became ill, and

10


A.   Dr. Papano

Shaw did not challenge Dr. Papano's qualifications, so we
mention them only briefly here. Dr. Papano is board-certified
in internal medicine and allergy-immunology. He has been a
practicing physician for more than 35 years, and currently
treats 60 to 80 patients per week. Dr. Papano has held a
number of prominent positions at Bryn Mawr Hospital in
suburban Philadelphia, and has taught fellows and residents
in allergy and internal medicine.

Dr. Papano's written expert report, from January 1997,
opines that he "can state with a reasonable degree of medical
certainty, that both Mr. and Mrs. Heller's respiratory
problems or difficulties [were] precipitated [by] the rugs
installed in their home in December 1993." Dr. Papano also
testified at the Daubert hearing that, following Mrs. Heller's
visit in May 1994, "I concluded that the carpeting in her
house was the major factor in her illness." The basis for Dr.
Papano's conclusion was a differential diagnosis drawn from
his examination of Heller, the results of a series of medical
tests, review of Heller's personal and family medical history,
and Heller's descriptions of her personal activities (smoking,
etc.) and environmental conditions ("cats, dogs, the type of
heating system, rugs, pillows, things of that sort").
Dr. Papano testified that he also relied on the temporal
relationship between Heller's exposure to the Shaw carpet
and the onset of her symptoms, as well as information from
Todd Environmental Consultants after its testing of the Heller
home in April and May 1994. Finally, Dr. Papano relied on
his more than thirty years of experience treating patients
with allergy-related medical problems and his knowledge of
environmental causes of respiratory problems gained at
professional seminars he attended.

_______________________________________________________
that the VOCs caused her illness, it certainly would be within a
jury's purview to find that Heller had inhaled the VOCs, without
further direct proof.

11


The District Court excluded all of Dr. Papano's testimony,
largely because he could point to no studies indicating at
what level the VOCs detected in the Heller home could cause
symptoms such as those experienced by Mrs. Heller, see
Heller, 

, at *15; his differential diagnosis
"failed to rule out all alternative possible causes of Carol
Heller's illness," 

and the court found that the
temporal relationship on which Dr. Papano relied was weak,
see 

We address each of these in turn.

1.   Lack of Studies

The District Court faulted Dr. Papano for citing "no
research to support his contention that the levels of VOCs
detected by Todd Environmental can and did cause the type
of illness allegedly experienced by [Mrs. Heller]." 

The court found that the lack of studies supporting Dr.
Papano's conclusion was a "defect" in his testimony. 

not believe that the court's reading of Rule 702--as
requiring research studies supporting a finding of general
causation--is correct. Assuming that Dr. Papano conducted
a thorough differential diagnosis (see infra Part III.A.2) and
had thereby ruled out other possible causes of Heller's
illness, and assuming that he had relied on a valid and
strong temporal relationship between the installation of the
carpet and Heller's problems (see infra Part III.A.3), we do not
believe that this would be an insufficiently valid methodology
for his reliably concluding that the carpet caused Heller's
problems.

A number of courts, including our own, have looked
favorably on medical testimony that relies heavily on a
temporal relationship between an illness and a causal event.
See, e.g., Zuchowicz v. United States, 

, 385 (2d
Cir. 1998); Kannankeril v. Terminix Int'l, Inc., 

,
809 (3d Cir. 1997). The temporal relationship will often be
(only) one factor, and how much weight it provides for the
overall determination of whether an expert has "good
grounds" for his or her conclusion will differ depending on

12


the strength of that relationship. For example, if there was a
minor oil spill on the Hudson River on the same day that
Heller began experiencing her symptoms in West Chester,
Pennsylvania, and she recovered around the time the oil was
cleaned up, a proper differential diagnosis and temporal
analysis by a well-qualified physician such as Dr. Papano
could not possibly lead to the conclusion that the oil spill
caused Heller's illness. See, e.g., 

(both
the methodology and the application of that methodology
must be reliable). Conversely, "if a person were doused with
chemical X and immediately thereafter developed symptom Y,
the need for published literature showing a correlation
between the two may be lessened." Cavallo v. Star Enter., 

, 774 (E.D. Va. 1995), aff'd in relevant part, 

, 1159 (4th Cir. 1996), cert. denied, 

(1998).

The present case falls between these two hypotheticals. In
this middle area, we do not believe that Daubert and Paoli
require a physician to rely on definitive published studies
before concluding that exposure to a particular object or
chemical was the most likely cause of a plaintiff's illness.
Both a differential diagnosis and a temporal analysis,
properly performed, would generally meet the requirements
of Daubert and Paoli. While again emphasizing that the
Daubert/Paoli factors are simply guideposts, we note that
differential diagnosis "consists of a testable hypothesis," has
been peer reviewed, contains standards for controlling its
operation, is generally accepted, and is used outside of the
judicial context. 

n.8.

The question we have thus posed is whether the expert's
conclusion can be considered reliable if it is based on these
scientifically valid methods, but is not based on published
studies. We acknowledge that a number of courts have
answered this question in the negative. See, e.g., Moore v.
Ashland Chem. Inc., 

, 278 (5th Cir. 1998) (en
banc) (holding that, absent a "compelling" situation such as
the Cavallo example above, a temporal relationship is to be
given little weight when there are few scientific studies

13

supporting a medical expert's specific causation diagnosis),
petition for cert. filed, 

(U.S. Dec. 17, 1998)
(No. 98-992); 

(excluding
expert testimony on causation primarily because "there is no
support for this causation theory in the scientific literature").
But see Kennedy v. Collagen Corp., 

, 1229 (9th
Cir. 1998) (finding district court abused its discretion by
excluding expert testimony that was based on reliable
methodology simply because "no epidemiological or animal
studies" linked defendant's product to plaintiff's disease).

The Supreme Court has held that it was not an abuse of a
district court's discretion to exclude expert testimony when
there was "too great an analytical gap between the data [of
scientific studies] and the opinion proffered," Joiner, 118 S.
Ct. at 519, but we do not read the Supreme Court as
requiring a medical expert to always rely on published
studies indicating the exposure necessary to cause a
particular illness. Rather, given the tenuous link in Joiner
between plaintiff's exposure to PCBs and the onset of his
cancer a number of years later, the lack of studies linking
PCBs to cancer in humans left only "the ipse dixit of the
expert" to support his conclusion. 

Court
held that it was not an abuse of discretion for the district
court to exclude the expert's testimony. See 

liberal thrust of the Federal Rules of Evidence,
the flexible nature of the Daubert inquiry, and the proper
roles of the judge and the jury in evaluating the ultimate
credibility of an expert's opinion, we do not believe that a
medical expert must always cite published studies on general
causation in order to reliably conclude that a particular
object caused a particular illness. Cf. McCullock v. H.B. Fuller
Co., 

, 1043 (2d Cir. 1995) (affirming admission
of treating doctor's testimony despite the fact that he "could
not point to a single piece of medical literature that says glue
fumes cause throat polyps"). To so hold would doom from the
outset all cases in which the state of research on the specific
ailment or on the alleged causal agent was in its early stages,
and would effectively resurrect a Frye-like bright-line

14


standard, not by requiring that a methodology be "generally
accepted," but by excluding expert testimony not backed by
published (and presumably peer-reviewed) studies. We have
held that the reliability analysis applies to all aspects of an
expert's testimony: the methodology, the facts underlying the
expert's opinion, the link between the facts and the
conclusion, et alia. See 

. However,
not only must each stage of the expert's testimony be reliable,
but each stage must be evaluated practically and flexibly
without bright-line exclusionary (or inclusionary) rules.

In the actual practice of medicine, physicians do not wait
for conclusive, or even published and peer-reviewed, studies
to make diagnoses to a reasonable degree of medical
certainty. Such studies of course help them to make various
diagnoses or to rule out prior diagnoses that the studies call
into question. However, experience with hundreds of patients,
discussions with peers, attendance at conferences and
seminars, detailed review of a patient's family, personal, and
medical histories, and thorough physical examinations are
the tools of the trade, and should suffice for the making of a
differential diagnosis even in those cases in which peer-
reviewed studies do not exist to confirm the diagnosis of the
physician. The Federal Rules of Evidence recognize as much.
See, e.g., Fed. R. Evid. 703 advisory committee's note ("[A]
physician in his own practice bases his diagnosis on
information from numerous sources and of considerable
variety . . . . The physician makes life-and-death decisions in
reliance upon them. His validation, expertly performed and
subject to cross-examination, ought to suffice for judicial
purposes.").

We repeat that all of these reliable methods for making a
diagnosis cannot sanitize an otherwise untrustworthy
conclusion. See 

-46; see also 

("A court may conclude that there is simply too
great an analytical gap between the data and the opinion
proffered."). In this case, however, there is certainly evidence
in the record--from Shaw's own records and from reliable
studies--that carpets emit VOCs and that VOCs can cause

15


certain health problems. This might be sufficient to give Dr.
Papano "good grounds" for making his conclusion, even
though the District Court (or a jury) may not agree with that
conclusion. Therefore, to the extent that the District Court
excluded Dr. Papano's testimony on the basis that it was not
grounded in scientific studies, it erred. However, it was not
necessarily error to exclude Dr. Papano's causation
conclusion as unreliable if he relied on no scientific studies
and the remaining foundation for his conclusion was shaky.

2.   Dr. Papano's Differential Diagnosis

The District Court also found it important that Dr. Papano
"failed to rule out all alternative possible causes of Carol
Heller's illness." Heller, 

, at *16 (emphasis
added). Applying plenary review, we hold that this is a more
stringent standard for a medical expert's differential diagnosis
than is required under Rule 702. A medical expert's
causation conclusion should not be excluded because he or
she has failed to rule out every possible alternative cause of
a plaintiff's illness. As Professor Capra, Reporter to the
Advisory Committee on the Federal Rules of Evidence, has
put it:
      [T]o require the experts to rule out categorically all other
      possible causes for an injury would mean that few
      experts would ever be able to testify . . . .

       . . . Obvious alternative causes need to be ruled out.
      All possible causes, however, cannot be and need not be
      eliminated before an expert's testimony will be admitted.

Daniel J. Capra, The Daubert Puzzle, 

. Rev. 699, 728
(1998).

Differential diagnosis, as we noted in Paoli, is "the basic
method of internal medicine." 

. Dr.
Papano engaged in this basic method in a reliable manner,
ordering standard laboratory tests, physically examining the

16


plaintiff, taking medical histories, and considering alternative
causes of the plaintiff's illness. See 

758. That he
used this technique to "testify to a novel conclusion" is not
sufficient grounds for excluding his testimony. 

n.27. Dr. Papano was not required to rule out all alternative
possible causes of Heller's illness. Rather, only "where a
defendant points to a plausible alternative cause and the
doctor offers no explanation for why he or she has concluded
that was not the sole cause, that doctor's methodology is
unreliable." 

Dr. Papano at the Daubert hearing,
Shaw offered a number of plausible alternative causes,
including dust from other carpets, benzene and 2-
butoxyethanol from other sources, and paint and new
hardwood floors in the house. Dr. Papano did not offer
detailed explanations for why he concluded that these were
not the causes of plaintiff's illness, but his responses,
grounded in the alleged temporal relationship, the results of
Todd's testing showing a reduction in VOCs when the carpet
was removed, and Heller's medical history and physical
examination, certainly are more than "no explanation." See,
e.g., App. at A602 (Dr. Papano's discussion of his
consideration of other possible causes). Had the District
Court applied the proper standard for evaluating a differential
diagnosis, we might conclude that it had not abused its
discretion in finding that Dr. Papano's responses were
inadequate, but it did err in requiring him to "rule out all
alternative possible causes."

As we concluded in Paoli, a physician need not conduct
every possible test to rule out all possible causes of a
patient's illness, "so long as he or she employed sufficient
diagnostic techniques to have good grounds for his or her
conclusion." 

. More recently, we held
that a district court erred in excluding expert medical
testimony because a defendant's suggested alternative causes
(once adequately addressed by plaintiff's expert) affect the
weight that the jury should give the expert's testimony and
not the admissibility of that testimony. See Kannankeril, 

. In Kannankeril, we held that even absent hard
evidence of the level of exposure to the chemical in question,
a medical expert could offer an opinion that the chemical
caused plaintiff's illness. See 

The medical expert
there relied primarily on the temporal relationship and the
nature of the plaintiff's complaints, as in the present case.
While the potential harm of the chemical in that case was
clearer than in this case, there was also some information
indicating that there may not have been a harmful level of the
chemical in Kannankeril's home. Nonetheless, we emphasized
that the district court should take care not to "mistake
credibility questions for admissibility questions." 

medical expert's "opinion on causation has a factual basis
and supporting scientific theory" that is reliable, it should be
admitted. 

Relationship

Neither Heller nor Dr. Papano disputes the absence of
definitive studies establishing the level at which the VOCs
detected in the Heller home could cause respiratory illnesses
such as those Heller experienced. Nor do they dispute that
studies linking Shaw carpeting to such illnesses do not exist.
Rather, they rely heavily on the temporal relationship
between the installation of the carpeting and the onset of
Heller's illness, as well as the fact that she appeared to
improve in health when she was away from her home. As we
noted in Part 

, we do not believe that the lack of
studies linking an alleged defective product to a plaintiff's
illness is fatal to a plaintiff's case on causation. However, as
noted, some reliable basis for a causation conclusion must
exist--and here, that basis was largely the alleged temporal
relationship between the installation (and removal) of the
Shaw carpet and the presence of Heller's illness.

The District Court relied on three major weaknesses in the
temporal relationship to find Heller's burden to prove
causation unmet. We review the factual findings of the
District Court for clear error and can find none, as the

18

background facts to its critique of Dr. Papano's temporal
conclusion are undisputed: (1) the Shaw carpeting was
installed in the Heller home in mid-December 1993; (2) Carol
Heller first experienced respiratory problems no earlier than
late December 1993; (3) Mr. and Mrs. Heller experienced
renewed symptoms upon returning to the home in May 1994,
almost a week after the carpet had been removed; and (4)
although Dr. Papano originally relied on the same temporal
relationship to conclude that the carpeting was the cause of
Thomas Heller's illness, Mr. Heller actually experienced his
symptoms prior to the installation of the Shaw carpet.

In reaching its legal conclusion regarding the temporal
relationship, the court first noted that Heller did not
experience symptoms until at least two weeks after the Shaw
carpeting was installed.7 Dr. Papano himself testified that a
reaction to VOCs in the home would typically occur within 24
hours of exposure to the VOCs. See App. at A638. While
Heller contends that this can be explained by her use of an
upstairs bedroom (rather than the downstairs master
bedroom) after the Shaw carpet was first installed, she
admitted that both of these rooms contained the Shaw carpet.

Not only did Heller's symptoms not appear until at least
one or two weeks after the Shaw carpeting was installed, but
they remained after the carpet was removed in May 1994.
The District Court properly faulted Dr. Papano's testimony for
not accounting for this fact as well. Plaintiff attempts to
explain this weakness in the temporal relationship by
reference to the "sink" effect, by which VOCs sink into objects
other than the ones from which they are emitted and then are
re-emitted at later times. See Appellants' Br. at 21 n.6. This
explanation is dubious, however, as the air was actually


_______________________________________________________

7. Plaintiff relies on the testimony of her other expert, Alan
Todd, to establish the level of VOCs at the time the carpet was
installed in mid-December. While we find serious problems with
his calculations, see infra Part III.B.2, we note that the level
of VOCs in mid-December would appear immaterial, as Heller did
not experience any adverse reactions until at least one or two
weeks later, when, according to plaintiff's own experts, the
level of VOCs would be substantially lower.

19


measured on the day Heller returned to the home and
experienced renewed symptoms--and there were virtually no
VOCs present at that time. Further, this "sink" effect theory
was disavowed by Heller's own environmental expert. See
App. at A882-883. Most importantly, it is not evident that Dr.
Papano relied on any "sink" effect or any other explanation
for why Heller continued to suffer ill effects from the Shaw
carpeting after it had been removed from the house. Finally,
the District Court noted that Dr. Papano's temporal analysis
failed to explain why Thomas Heller (who Dr. Papano also
concluded suffered ill effects from the Shaw carpet) exhibited
allergy symptoms prior to the installation of the Shaw carpet.

These weaknesses, according to the District Court,
"disprove the existence of a temporal relationship." Heller,

, at *17. While we review such a conclusion
for abuse of discretion, as noted above, our review of the
court's legal analysis--i.e., whether it properly followed Rule
702 as prescribed in Daubert and Paoli--is plenary. The
court's analysis of the temporal relationship included the
criticism that "plaintiffs proffer no statistical evidence to show
the existence of a statistically significant correlation" between
the Hellers' symptoms and their exposure to the Shaw carpet.

physician's diagnosis, based in part on a
strong temporal relationship between symptoms and
exposure, need not necessarily be supported by "a
statistically significant correlation." What is required is that
the physician have "good grounds" for his or her diagnosis.
See 

(noting that, even if the judge
believes "there are better grounds for some alternative
conclusion" and that there are some flaws in the expert's
methods, if there are "good grounds" for the expert's
conclusion, it should be admitted). Further, when the
temporal relationship is strong and is part of a standard
differential diagnosis, it would fulfill many of the
Daubert/Paoli factors. See 

n.8.

Here, however, we have no problem concluding that the
temporal relationship between the exposure to the Shaw
carpeting and the onset of Heller's illness was questionable

20


at best and exculpatory at worst. While the district court may
not reject an expert's conclusion simply because the court
finds it wanting, it is surely within the court's province to
ensure that the conclusion, particularly a medical expert's
ultimate conclusion on causation, "fits" with the data alleged
to support it. See 

("[T]he expert's view that a
particular conclusion 'fits' a particular case must itself
constitute scientific knowledge . . . ."). Had the Hellers
experienced a prompt reaction at the time the Shaw carpeting
was installed in mid-December 1993, and had they suffered
no reaction upon return to their home after the Shaw carpet
was removed in May 1994, this would be the type of temporal
relationship that might reliably support a conclusion that the
carpet was the cause of plaintiff's illness. However, that is not
the case here.

4.   Dr. Papano's Testimony: Summary and Conclusion
We have explained that the District Court erred to the
extent that it required Dr. Papano's testimony to be backed
by scientific studies linking the type and level of VOCs
detected in the Heller home to Heller's illness, and to the
extent that it required Dr. Papano to rule out all other
possible causes of Heller's illness before concluding that the
Shaw carpet was the cause. The District Court could,
however, properly consider the fact (rather than requiring it
as a prerequisite to admissibility) that Dr. Papano relied on
few, if any, studies linking exposure to the VOCs allegedly
emitted by the Shaw carpet to the illnesses suffered by Heller.
It could also properly consider Dr. Papano's (weak) responses
to Shaw's proffered alternative theories on the cause of
Heller's illness in evaluating whether he truly had "good
grounds" to arrive at the causation conclusion he reached.

Dr. Papano relied extremely heavily on the temporal
relationship between the installation of the carpet and
Heller's illness, and the District Court did not err in
concluding that this relationship was unreliable. Without
either scientific studies pointing to VOCs of the type and

21


amount detected as the culprit or a reliable temporal
relationship, Dr. Papano was left with no valid means for
concluding that the Shaw carpet was the cause of Heller's
illness. Dr. Papano's conclusion had to "fit" with the data and
the methodology that precedes it. See 

.
Even if the data (e.g., the medical history, the laboratory
studies, evidence of VOCs in the Heller home) and the
methodology (i.e., the differential diagnosis) were reliable, the
District Court did not err in finding that the conclusion Dr.
Papano reached did not reliably flow from this data and
methodology. Under these circumstances, the District Court
did not abuse its discretion in ultimately deciding to exclude
Dr. Papano's testimony regarding the cause of Heller's
illness.8

B.   Alan Todd

Plaintiff's second expert witness, Alan Todd, of Todd
Environmental Consultants, opined in his initial expert report
that "the illness[es] suffered by the Heller family were caused


_______________________________________________________

8. We add that the District Court should not necessarily have
excluded all of Dr. Papano's testimony. In many cases, a treating
physician whose methods and data are reliable, but whose
causation conclusion is excluded as unreliable, may still have
other reliable testimony to offer. In such a case, the medical
expert should be permitted to "testify about his examination of
[the plaintiff], the tests he conducted, and the diagnosis he
reached," 

, as these are all based on
reliable methods. See also 

("There
is no question that Dr. Bellanti is qualified to testify
regarding the nature of Ms. Cavallo's illnesses . . . . Rather,
the focus of the dispute is whether his opinion regarding the
cause of these illnesses is scientifically valid and therefore
admissible under Daubert." (first emphasis added)). Thus, even if
it was proper to exclude Dr. Papano's expert testimony regarding
the cause of Heller's illness, as we conclude it was, testimony
as to his examination and treatment of her illness was almost
certainly relevant and reliable. It would be relevant to at least
one of the elements in most of Heller's claims, i.e., whether or
not she suffered an injury (as well as the extent of her
injuries, a relevant factor in any damages analysis). Of course,
without Dr. Papano's causation testimony, summary judgment may
still have been warranted--and we conclude that it was--because
without it there was insufficient evidence of causation to get to
the jury.

22


by their prolonged exposure to the VOC's measured in their
home and emitted by the carpeting manufactured by Shaw
Industries." Todd based his conclusion on his testing of the
air in a closet of the Heller home in which the Shaw carpet
had been installed, and on his extrapolation from the results
of these closet tests. He thereby estimated the level of VOCs
emitted by the Shaw carpet at the time it was installed,
approximately four months prior to his testing. We note
preliminarily that we are doubtful that a non-medical expert
such as Todd is qualified to testify as to the cause of
someone's illness.9 We need not address that issue here,
however, because we conclude that the District Court did not
abuse its discretion in excluding Todd's extrapolations as
being unreliable so that any arguable basis for Todd's
causation conclusion was missing, making it appropriate for
the District Court to exclude Todd's causation testimony.

Todd is a certified industrial hygienist, who consults on
environmental problems in occupational and residential
settings. His qualifications were not challenged by defendant,
though as we discuss below, his methodologies were
thoroughly attacked. Because Heller must show that her
exposure to VOCs was at a greater level than "the normal
'background' level," 

, and that this
exposure came from defendant's carpet, see 

Todd's
testimony was necessary for her to survive summary
judgment. Todd testified that, at the time the Shaw carpet
was installed (December 13-14, 1993), the level of benzene in
the air in the Heller home was approximately 1712 parts per
billion ("ppb"), and that this benzene came from the Shaw
carpet. He also estimated that, at that time, the level of other
VOCs was approximately 11,469 ppb, and that these VOCs

_______________________________________________________

9. While Todd was knowledgeable about studies on VOCs and
illness, and on recommended maximum occupational VOC levels, he
is not a physician and did not examine the Hellers nor discuss
with them their symptoms or their medical histories. Thus,
whatever his qualifications for testifying about the source and
level of VOCs in the Hellers' house or his expertise regarding
dangerous levels of VOCs, his qualification to offer an opinion
on the ultimate cause of the Hellers' illnesses is another
matter.

23


came from the Shaw carpet. If his methodologies were
reliable, and his application of these methodologies to the
facts of the case was reliable, his conclusion that there were
VOCs, emitted from Shaw's carpet, at levels higher than
background levels could support a proper medical diagnosis
that the Shaw carpet caused the plaintiff's illness.

1.   Subtraction Methodology10

Todd's method for determining the source and level of
VOCs in the Heller house was to take air samples in a
bedroom closet before and after the Shaw carpet had been
removed from the house (and the closet). Comparing the two
measurements, he determined the amount of VOCs emitted
by the Shaw carpet, the only item present for the first test
and absent for the second one. If the methodology for
collecting air samples and for measuring the VOCs present in
the air was valid and reliable, and the difference in the level
of VOCs was significant, this part of Todd's testimony would
be probative of whether or not the Shaw carpeting emitted
VOCs, and should have been admitted.

While the District Court faulted Todd's subtraction
methodology on a number of counts, we uphold its decision
to exclude this testimony largely because the conclusions
Todd reached could not reliably flow from the data and
methodology he used. We first consider the District Court's
criticisms of Todd's subtraction methodology (not all of which
we find warranted) before turning to our own critique of his
testimony.

The District Court found that the studies cited by Todd for
the proposition that carpet such as Shaw's could cause
Heller's illness did not support this conclusion. See Heller,

, at *9. We address this aspect of the matter
_______________________________________________________

10. Although not so labeled by Todd, the parties and the
District Court utilized the terms "subtraction methodology" and
"back-extrapolation methodology" to describe Todd's two major
methodologies. For the sake of consistency, we will do likewise.

24


in the margin, for, given our ultimate conclusion, we need not
decide whether the District Court's analysis of the studies
was an appropriate factor in determining whether Todd's
subtraction methodology itself was valid and reliable.11

In addition, although Todd testified that he used an
accepted methodology for collecting the air samples and
described this method in detail, Shaw and the District Court
criticized him for not using some other test, specifically a
closed chamber laboratory test. While the latter is an
accepted test for measuring compounds in new carpet, it is
neither the only nor necessarily the best test for measuring
VOCs from carpet already installed in a home. Further, the
record provides ample evidence that laboratory tests and on-
site tests produce similar results, see, e.g., App. at A88, and
defendant's own expert conducted on-site tests in a number
of carpet-emission studies, see 

SA1013,
SA1030. That expert also admitted using in a prior study
essentially the same "subtraction method" used by Todd. See


_______________________________________________________

11. In our view, the record can be read to support Todd's claim
that carpeting such as Shaw's can emit some of the VOCs detected
in the Heller home and may cause illnesses similar to those
suffered by Heller after prolonged exposure. In any event, we do
not believe that unequivocal studies are required before a
qualified expert may opine that a product emits a certain
compound or causes a certain irritation, if the basis for the
opinion is otherwise reliable and scientifically valid.

Further, this aspect of Todd's testimony was based on first-hand,
field testing of the object in question--the Shaw carpet. If Todd
was qualified to conduct such tests, and if his means of
collecting air samples was scientifically valid and the initial
conclusion he drew--that the Shaw carpet was the source of a
certain level of VOCs--was reliably drawn from the field testing,
this testimony would be both reliable and relevant. We note that
this is not a case in which a party sought to avoid the
application of Daubert by labeling such testing "non-scientific."
See, e.g., Carmichael v. Samyang Tire, Inc., 

, 1435-
36 (11th Cir. 1997) (finding that testimony based on "skill- or
experience-based observation," rather than on "application of
scientific principles or theories," was not subject to Daubert),
cert. granted sub nom. Kumho Tire Co. v. Carmichael, 

(1998). Here, plaintiff argued that Todd's testimony met the
requirements of Daubert in that it was reliable and valid.

25

Properly performed, such on-site tests
would appear to meet most of the factors suggested in
Daubert and Paoli.

Finally, the District Court placed great weight on Todd's
alleged failure to "insure that other variables did not [affect]
the air sampling tests." Heller, 

, at *12.
However, Todd testified that the contents of the closet (and
the house) remained constant and that the environmental
conditions in the house were essentially static (i.e., no
persons came or went, the windows were not opened, the rate
of air flow was not changed, etc.). While it is true that the
concentration of VOCs is affected by more than the emission
from a source such as carpeting, a substantial decline in the
amount of VOCs would constitute strong (and reliable)
evidence that at least some of these VOCs were coming from
the Shaw carpet--the only item that was removed before the
lower readings were taken.

Our decision does not turn, however, on the validity of
Todd's air sampling methodology, i.e., on whether the testing
was unreliable because he did not conduct different tests or
did not control for other possible sources of VOCs. This is
because the District Court was correct to question the
reliability of Todd's conclusions. The level of VOCs detected
by Todd's closet tests, even if they could all be attributed to
the Shaw carpet, were substantially lower than any amounts
ever known or believed to cause illnesses in humans; in fact,



_______________________________________________________

12. The court also faulted Todd's testimony because he "did not
conduct further tests to ascertain whether changes in the levels
of VOCs were attributable to the removal of the carpet or whether
the changes were attributable to the natural fluctuation in VOC
levels within the home." Heller, 

, at *12. It is
not clear what "further tests" the District Court would have
required. Todd did not take snapshot tests at single moments,
which would have been subject to natural fluctuations and random
error; such tests likely would not be reliable enough to meet
the Daubert standard. Rather, Todd took air sample readings over
eight hours on one day, with the carpet present, and then took
readings for eight hours on a second day, without the carpet
present. Such prolonged readings may be sufficient to account for
"natural fluctuations in VOC levels."

26


they appear to be extremely close to the background amounts
(i.e., the levels naturally occurring in the air) for each of the
VOCs.13 A number of studies cited by the parties and
contained in the record have concluded that any ill effects
from these particular VOCs (and related ones) only occur at
much higher levels than those found in the Heller home. See,
e.g., App. at SA0835-0840 (finding basic irritation at 50-750
ppb of VOCs; headaches at 750-6250 ppb; and additional
neurotoxic effects at levels above 6250 ppb); 

(reporting pulmonary irritation at 275 ppb; slight sensory
irritation at 600 ppb); 

(finding that persons
exposed to 25,000 ppb of benzene for eight hours
demonstrated no acute effect). Another major study found
that the background levels of benzene averaged 5 ppb overall
and about 3 ppb indoors, see 

actually above
the levels (2.2 ppb) detected in the Heller home in April, when
the Shaw carpet was still in the house.14


_______________________________________________________

13. We note that a number of the VOCs allegedly attributable to
the Shaw carpet were detected at levels well below the maximum
amounts recommended by various federal agencies and professional
groups. For example, only 2.22 ppb of benzene was detected in
April 1994; the lowest suggested limit for exposure to benzene is
500 ppb. Other VOCs detected in the closet, with the maximum
amount attributable to the Shaw carpet and the lowest recommended
limit, include:

VOC                         Amount Found   Recommended
                            (in ppb)       Limit (in ppb)

2-Butoxyethanol              5.60           20,000
Carbon Tetrachloride         0.13            2,000
Cumene                       0.11           50,000
Methyl Chloroform            0.09          200,000

See Richard J. Lewis, Sr., Hazardous Chemicals Desk Reference
113, 180, 235, 318, 745 (4th ed. 1997). We note that these limits
are for long-term occupational exposure and assume exposure at
these levels for 40 hours per week, indefinitely.

14. Unfortunately, comparing these studies and industry
documents to Todd's findings can be difficult, as the latter are
reported in terms of ppb (parts per billion), while many of the
studies and industry documents measure air concentration in mg/m3
(milligrams per cubic meter) or ug/m3 (micrograms per cubic
meter). The conversion from one to the other is

27


Todd attempted to address this shortcoming in his
testimony by dividing the suggested occupational limits by
420, based on two assumptions whose validity is problematic.
First, because the limits are based on a 40-hour work week
and there are potentially 168 hours per week in which a
person could occupy her home (if she never left), Todd
reduced the limits by 4.2. Then, because the occupational
limits are based on an average healthy adult, Todd testified
that it is standard practice to reduce these limits further by
a factor of 100, to account for the fact that homes include
children, older adults, unhealthy persons, etc. As a result, he
opined that the limit for exposure to benzene in the home is
actually approximately 1.2 ppb (i.e., 500 ppb / 420), slightly
lower than the amount attributable to the Shaw carpet.

This methodology is suspect. In one study in the record,
the American Society of Heating, Refrigeration, and Air
Conditioning Engineers ("ASHRAE") is cited as recommending
a maximum level of contaminants arrived at by dividing the
OSHA limit by 10, see App. at SA0860-0861, for essentially
the same reason that Todd gave for dividing the permissible
limits by 420. The OSHA limit for benzene is 1000 ppb; the
ASHRAE limit, then, would be 100 ppb, still well above the
amount detected in the Heller home in April (2.22 ppb). The
OSHA limit for 2-butoxyethanol is 25,000 ppb, making the
ASHRAE limit 2500 ppb, significantly higher than the
amount detected in the Hellers' house (5.60 ppb). At all
events, even if the methodology is valid, the levels measured
in April (the earliest period at which Todd actually took air
concentration measurements) do not even approach the
(modified) recommended maximums for any of the other
VOCs.

Thus, while the closet tests conducted by Todd were not
necessarily unreliable, because the level of VOCs detected

_______________________________________________________

different for every compound. For benzene, it is about 3 ug/m3 for every
1 ppb; for other compounds, it ranges from about 3 ug/m3 for every 1 ppb
to 7 ug/m3 for every 1 ppb. For the sake of analyzing Todd's
testimony, we have made rough conversions, using 4 ug/m3 for
every 1 ppb. No difference in outcome would result from using
another conversion equation.

28


and (arguably) attributable to the Shaw carpet were so low
and so close to background levels, the District Court did not
abuse its discretion in excluding Todd's testimony that the
Shaw carpet was emitting VOCs sufficient to cause Heller's
illness--a conclusion that was unreliable if based on the
closet tests alone. Todd, however, attempted to reinforce his
closet tests--and to provide a stronger foundation for his
opinion about the dangerous level of VOCs in the house--by
introducing his back-extrapolation methodology in a
supplemental expert witness report (issued in March 1997,
following his original January 1997 report). We turn to this
aspect of his testimony.

2.   Back-Extrapolation Methodology

If Todd's sampling of the air in the closet: (1) was reliable
and adequately controlled for factors other than the Shaw
carpet, and (2) demonstrated that the Shaw carpet was
emitting VOCs at potentially harmful levels, his testimony
would be sufficiently reliable to meet the Daubert standard
and hence would be admissible. While we believe that the
first proposition may be true, as noted above, the closet tests
themselves fail to demonstrate the second. If it was possible
to use the results of the closet tests to estimate, in a
scientifically valid way, the level of VOCs emitted by the
carpet at some earlier time, and if these estimated levels were
potentially harmful, again, Todd's testimony would be reliable
and relevant. We express no opinion as to whether
extrapolation back in time, using known levels of compounds
and a scientifically valid mathematical formula for the
extrapolation, would meet the standards of Rule 702 and
Daubert. Cf. Ruiz-Troche v. Pepsi Cola of P.R. Bottling Co., 

, 85 (1st Cir. 1998) (approving of a back-extrapolation
and half-life methodology that "has been subjected to, and
survived, the rigors of testing, publication, and peer review,
and . . . appears to have won significant (if not universal)
acceptance within the scientific community"). Here, however,
it is clear that Todd's formula for his extrapolation was based

29


on speculation and estimation that was subject to gross
error, and that the District Court did not abuse its discretion
in excluding Todd's testimony based on the back-
extrapolation.15 Indeed, we doubt that Todd's back-
extrapolation methodology would meet even one of the eight
suggested factors from Daubert and Paoli.

Heller argues that the back-extrapolation method is "a
standard reversibility of chemical processes equation,"
Appellants' Br. at 27, but provides no support for the
reliability of Todd's equation or for the suggested relationship
between chemical half-lives and the level of VOCs in the air.
Todd's back-extrapolation method relies on at least three
questionable assumptions: (1) the concentration of VOCs in
the air declines exponentially by half-lives (i.e., the level of
VOCs are cut in half every X days or weeks or years); (2) the
half-life of VOCs in the air can be estimated based on
information on the decay curve of VOC emissions from
carpets; and (3) the concentration of VOCs in the air is not
affected by anything other than its natural half-life decline.

None of these assumptions, however, appears supported by
reliable scientific methods or the reliable application of any
valid theory. In fact, Todd admitted as much in his own
testimony and expert report. See, e.g., App. at A350-351,
A761-762; 

(Todd's Expert Report: "The precise
magnitude of difference quantitatively in off-gassing
emissions at the carpet between April 1994 and December
1993 is not readily evident from the published literature and
or studies conducted by the rug manufacturers or their trade
associations."). In fact, numerous published studies and
industry documents consistently demonstrate that the actual
rate of decline of emissions from carpet is nothing like a half-

_______________________________________________________

15. Plaintiff relies heavily on a Louisiana state case to
support Todd's back-extrapolation theory. See Appellants' Br. at
38-40. The judgment in that case, however, has been reversed and
a new trial ordered, specifically because the trial court failed
to hold a Daubert hearing and to find whether the Daubert
criteria were met, as required by Louisiana law. See Caubarreaux
v. E.I. duPont de Nemours, 

, 71-72 (La. Ct. App.
1998).

30


life progression. See, e.g., 

A145, A242, A248-250,
A268-270, SA0860, SA0956, SA1020. Rather, emissions
decline rapidly in the first hours and days after installation,
reaching a level of about 10% of the original emission rate in
only one week and as low as .05% of the initial emission rate
in only one month. The differences between the emission
rates indicated in these studies and those estimated by
Todd's back-extrapolation theory are fairly substantial. (We
express them graphically in the margin.16) For example,
under Todd's theory, about ten days after installation of new
carpet, VOCs would remain at levels approximately 50% of
their initial level, while the studies in the record indicate that
the levels would actually be less than 10% of their initial
level. Within three weeks of installation, under Todd's theory,


_______________________________________________________

16
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VOCs would be at 25% of their initial level, while the studies
show that carpet, at this point, is emitting only about 1% of
the initial amount of VOCs emitted at the time of installation.
In short, the VOC levels estimated by Todd greatly exceed
those which more likely existed, and follow a very different
curve. Therefore, the District Court properly exercised its
discretion to exclude this part of Todd's testimony.

Heller contends that even if Todd's "calculations were
imprecise, it is undisputed that the level of VOC emissions in
December 1993 were significantly higher than the VOC levels
measured in April 1994." Appellants' Br. at 10 n.2. The
problem with this argument, however, is that even if the
levels were higher in December 1993, the calculations of
plaintiff's expert were "imprecise" because his methodology
was unreliable, and therefore Heller has presented no reliable
evidence to demonstrate what the actual (or even reliably
estimated) level of VOCs was in December 1993. Without a
reliable method to determine how much higher the levels
were in December 1993, only the actually measured levels in
April 1994 are admissible evidence--and, as noted above,
these levels were far too low to prove that the Shaw carpet
was emitting harmful levels of VOCs.

There are other flaws in Todd's back-extrapolation
testimony that also support the District Court's decision to
exclude this testimony. First, Todd conflates emission rates
and air concentration rates in his analysis. All of the record
data on which Todd claims he relied to estimate his back-
extrapolation formula involve the declining emission rate:
VOCs are emitted at a certain rate at the time of carpet
installation; by 24 hours later, they are emitted at
approximately half this rate; by a week later, they are emitted
at approximately 10% the original rate, etc. The air
concentration of VOCs (which is what Todd measured in the
closet in April and May 1994, and used as the starting point
for his back-extrapolation) is a function of not only the rate
at which VOCs are emitted from the carpet to the air, but
also such factors as the rate at which VOCs dissipate in the
air, the size of the room or house in which the VOCs are

32


emitted, the molecular weight of the particular VOC, the rate
of air flow, the moisture, light, and air temperature in the
room, and other factors. See, e.g., John C. Little et al.,
Modeling Emissions of Volatile Organic Compounds from New
Carpets, 28 Atmospheric Env't 227 (1994) (describing
development of a model indicating that air concentration of
a VOC is a function of time, distance from carpet, diffusion
rate of the VOC, carpet thickness, air flow rate, carpet and
room area, and air volume). Therefore, to measure the air
concentration at one point in time (as Todd did) and to
attempt to estimate the air concentration four months earlier,
one would need to know (or have a good estimate of) each of
these factors, i.e., emission rates, room size, air flow,
dissipation rate of each compound, etc.--none of which Todd
considered in his back-extrapolation formula.

Second, there is at least one study in the record that
appears to indicate that while emission rates of VOCs decline
rapidly (see supra note 16), air concentration levels remain
fairly constant after an initial slight increase, making Todd's
conflation of these two factors even more problematic. See
Alfred T. Hodgson et al., Emissions of Volatile Organic
Compounds from New Carpets Measured in a Large-Scale
Environmental Chamber, J. Air & Waste Mgmt. Ass'n, Mar.
1993, at 316, 323 (describing study indicating that emission
rates of styrene and 4-phenylcyclohexene decline rapidly,
while air concentration rates fluctuate within a relatively
narrow range). Therefore, it is entirely plausible that the level
of VOCs in the air was not much higher in December 1993
than the very low level measured in April 1994, the only time
such VOCs were actually measured.

Finally, if one were to credit Todd's back-extrapolation
theory, it would actually invalidate his closet studies, thereby
eliminating the only basis for his opinion that the Shaw
carpet was the source of the VOCs in the Heller home. This
is because the decline in benzene and 2-butoxyethanol, two
of the key VOCs on which plaintiff rests her case, could be
explained almost entirely by the back-extrapolation theory,
eliminating the possibility that it was the removal of the

33


carpet that caused the levels of these VOCs to decline. Under
Todd's back-extrapolation theory, the benzene would be
expected to decline from 2.22 ppb on April 14, 1994, to
approximately 0.40 ppb on May 11, 1994, even without the
removal of the Shaw carpet; it actually declined to only 0.55
ppb. The 2-butoxyethanol would have been expected to
decline from 5.6 ppb to approximately 1.0 ppb; it actually
declined to 0.0 ppb, making the maximum amount
attributable to the removed carpet only 1.0 ppb--
substantially lower than the recommended limit of 20,000
ppb for this compound. The District Court noted this
inconsistency, see Heller, 

, at *12, as did
Todd himself implicitly on cross-examination at the Daubert
hearing, see App. at A543, A886.
3.    Todd's Testimony: Summary and Conclusion

Although we believe that the District Court may have been
overly critical of Todd's closet tests and that those aspects of
its unreliability finding may have been inconsistent with the
exercise of sound discretion, given the patent unreliability of
Todd's back-extrapolation theory and the fact that the closet
tests did not indicate levels of VOCs anywhere near the levels
found to cause illnesses in humans, we hold that the District
Court did not abuse its discretion in excluding all of Todd's
testimony. Cf. 

n.19 (noting that all of an
expert's testimony could be excluded as irrelevant if it no
longer assists plaintiff's case after certain parts are excluded
as unreliable).

IV.   Summary Judgment

Without Dr. Papano's testimony on specific causation or
Todd's testimony on the allegedly higher levels of VOCs in
December 1993 (both of which we hold the District Court was
correct to exclude), the remaining expert testimony and other
evidence in the record are insufficient to create a material
issue on causation. We note that the District Court granted

34

summary judgment at least in part because "defendant's
carpeting is not the obvious cause of plaintiffs' illnesses."
Heller, 

, at *18. This appears to place a more
stringent burden on plaintiff than is warranted at summary
judgment, but the District Court also relied on the total lack
of causation evidence absent the expert testimony, which is
a proper ground for summary judgment.

Certain of plaintiff's claims do not rely on the causal
connection between Heller's illness and the Shaw carpet to
survive. However, without Todd's testimony, plaintiff has
failed to offer admissible proof that the Shaw carpet was
defective. The only claim that does not require proof of either
the causal connection or defectiveness is plaintiff's
misrepresentation claim. We are satisfied, however, that the
District Court properly granted summary judgment on this
claim as well. See 

("[T]here is no evidence of record
to support plaintiffs' assertion that they were injured by
reliance on [Shaw's] alleged misrepresentation.").

The order of the District Court granting summary judgment
to Shaw will be affirmed.

A True Copy:
Teste:

      Clerk of the United States Court of Appeals
      for the Third Circuit
35