RECOMMENDED FOR FULL-TEXT PUBLICATION
                            Pursuant to Sixth Circuit Rule 206
                                   File Name: 09a0078p.06

               UNITED STATES COURT OF APPEALS
                               FOR THE SIXTH CIRCUIT
                                 _________________


                                                 X
                        Plaintiffs-Appellants, -
 ELIZABETH A. GASS, DEBORAH DEJONGE,
                                                  -
                                                  -
                                                  -
                                                       No. 07-1733
          v.
                                                  ,
                                                   >
                                                  -
                                                  -
 MARRIOTT HOTEL SERVICES, INC., ECOLAB,

                       Defendants-Appellees. -
 INC.,
                                                  -
                                                 N
                    Appeal from the United States District Court
               for the Western District of Michigan at Grand Rapids.
                No. 05-00856—Robert Holmes Bell, District Judge.
                                  Argued: July 22, 2008
                           Decided and Filed: March 3, 2009
           Before: BOGGS, Chief Judge; MOORE and CLAY, Circuit Judges.

                                   _________________

                                        COUNSEL
ARGUED: Peter D. Bosch, STRAIN, MURPHY & VANDERWAL, Grand Rapids,
Michigan, for Appellants. Richard G. Morgan, BOWMAN & BROOKE, Minneapolis,
Minnesota, for Appellees. ON BRIEF: Peter D. Bosch, Joseph P. VanderVeen, STRAIN,
MURPHY & VANDERWAL, Grand Rapids, Michigan, for Appellants. Ronald C. Wernette
Jr., BOWMAN & BROOKE, Troy, Michigan, for Appellees.
        CLAY, J., delivered the opinion of the court, in which MOORE, J., joined. BOGGS,
C. J. (pp. 23-31), delivered a separate dissenting opinion.
                                   _________________

                                        OPINION
                                   _________________

        CLAY, Circuit Judge. Plaintiffs, Elizabeth A. Gass and Deborah DeJonge, appeal
the district court’s grant of summary judgment to Defendants, Ecolab, Inc. (“Ecolab”) and
Marriott Hotel Services, Inc. (“Marriott”). Plaintiffs sued Defendants under Michigan law,
claiming that they were poisoned by pesticides during their stay at a Marriott hotel in Maui,

                                             1
No. 07-1733          Gass, et al. v. Marriott Hotel Servs., et al.                    Page 2


Hawaii. Plaintiffs allege that employees of Ecolab, which provides extermination services
for Marriott, sprayed their belongings with an unknown pesticide and filled their hotel room
with toxic vapors, causing Plaintiffs to become ill. The district court granted summary
judgment to Defendants, holding that no reasonable jury could conclude that Defendants’
negligence caused Plaintiffs’ injuries. We disagree, and accordingly REVERSE the grant
of summary judgment and REMAND this case to the district court for trial.

                                      BACKGROUND

I.      FACTUAL HISTORY

        On September 6, 2004, Plaintiffs were guests in a Maui hotel operated by Marriott.
That day, DeJonge filed a complaint with the hotel after she discovered a dead cockroach
in her room, and a hotel employee eventually removed the roach. The next day, while
Plaintiffs were away from their room, three employees of Ecolab entered Plaintiffs’ room,
bringing with them at least one unidentified pesticide. Ecolab provides pest extermination
services to Marriott.

A.      Plaintiffs’ Exposure to Pesticides

        Although the parties offer opposing views of what happened while the Ecolab
exterminators were in Plaintiffs’ hotel room, Defendants concede that the summary judgment
standard requires this Court to credit Plaintiffs’ testimony regarding the exterminators’
actions. According to Plaintiff DeJonge, she was relaxing on the beach with Plaintiff Gass
when she decided to retrieve lunch money from the hotel room. DeJonge left the beach and
walked to the ground-level hotel room, entering through a sliding glass door. Immediately
upon entry, she discovered three men in the room. Two of the men were wearing metal tanks
on their backs and masks on their faces, and were spraying a chemical from those tanks.
According to DeJonge, there was a “thick, horrid, acrid putrid odor” in the room, and the air
was “sort of cloudy.” (J.A. 490–91.) DeJonge also states that the haze of chemicals in the
room was so thick that she could “see it, smell it, taste it, feel it.” (J.A. 494.)

        Upon noticing the three men, DeJonge immediately began screaming at the men to
stop spraying, and accused them of “ruining [her] stuff,” much of which was laid out in
suitcases on the floor. (J.A. 492–93.) In response to her demands, DeJonge testified that one
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                     Page 3


of the men “shot [her] a dirty look,” yelled something in a language she did not understand,
and resumed spraying. (J.A. 493.) At this point, DeJonge picked up the phone, called the
hotel desk, and asked for the manager to meet her outside of the hotel room. She then left
the room to wait for the manager.

        When the manager arrived, DeJonge relayed what had happened in the room, and
demanded that they be given a new room immediately because “we need to get our stuff out
of there before . . . it gets more ruined.” (J.A. 496.) DeJonge added that “I can’t stand the
smell in there. I think it’s making me sick.” (Id.) The manager acquiesced, and called a
bellhop to help DeJonge move her belongings to a new room. DeJonge then left to find Gass
and let her know what happened.

        DeJonge found Gass and explained why they needed to retrieve their belongings and
switch rooms. By the end of this conversation, which lasted about six or seven minutes,
DeJonge began to feel ill. Nevertheless, both women returned to the room, where they spent
about two-and-a-half minutes gathering their belongings. Neither DeJonge nor Gass
identified the specific chemical that Ecolab used to fumigate their room.

        Although Defendants concede that the summary judgment standard requires that this
Court accept Plaintiffs’ version of events, Defendants offer a different version of how the
exterminators acted. According to testimony by Michael Medeiros, an Ecolab exterminator,
he and one co-worker entered Plaintiffs’ room just thirty seconds before DeJonge arrived.
According to Medeiros, neither Ecolab employee had sprayed any pesticides before DeJonge
entered the room. Medeiros claims that his co-worker made “two quick squirts” of an
insecticide from an aerosol can similar to those commercially available at retail stores, and
that no other chemicals were sprayed in the room. (J.A. 481.) Medeiros further testified
that, immediately after the exterminator made these “quick squirts,” DeJonge became “very
upset,” and began yelling at Medeiros and his co-worker. (Id.) The two men left the room,
and Medeiros claims that he heard DeJonge say “my stuff is ruined” as they were leaving.
(Id.)

        Medeiros also testified regarding the kinds of chemicals Ecolab typically used in
servicing the Marriott hotel, identifying three kinds of insecticides used to target
cockroaches. The first, and least potent, of these chemicals is a pesticide called “SSI-50,”
No. 07-1733              Gass, et al. v. Marriott Hotel Servs., et al.                           Page 4


which is normally sprayed from a twelve-ounce aerosol can. Additionally, Medeiros
identified two more-potent pesticides—“Suspend SC” and “Demand CS”—that were
commonly used by Ecolab at the time of Plaintiffs’ trip to Hawaii. Suspend SC and Demand
CS are sold in concentrated form, then mixed with water in a pump sprayer or similar
container to apply the pesticide. Medeiros claims that, on September 7, 2004, the day
Plaintiffs’ room was sprayed, only SSI-50 was used, and a report he filed that day
corroborates his claim.

B.       Plaintiffs’ Illness

         A short time after their exposure to pesticides in the hotel room, Plaintiffs contacted
the hotel manager and complained of “numbness to their tongues, stomach aches, and seeing
stars.” (J.A. 370.) The manager arranged transportation to a nearby urgent care center, and
Plaintiffs received medication for their symptoms.

         Upon their return to Michigan, Plaintiffs initially sought treatment from Dr. Robert
                                                                                   1
DeJonge, an osteopathic physician and Plaintiff DeJonge’s husband.                     Dr. DeJonge,
however, eventually referred Plaintiffs to Dr. Gerald Natzke, a physician specializing in
environmental medicine.2 During her appointment with Dr. Natzke on October 19,
2004, DeJonge informed Dr. Natzke that she had developed various symptoms within
fifteen minutes of her exposure to the pesticides, including “a headache, swelling of her
tongue, hands, feet and face, profuse itching, dizziness, shortness of breath and . . .
drooling.” (J.A. 846.) By the time of her appointment, DeJonge’s symptoms included
“a slurring and swollen tongue, complaints of a foul taste in her mouth, drooling and
complaints of fatigue and pain in her muscles and joints,” in addition to a need to “take
afternoon naps which wasn’t the case before the incident on September 7, 2004[,] . . .
diarrhea and [] a droopy left eye.” (Id.)




         1
             Unless otherwise indicated, “DeJonge” will refer to Ms. DeJonge, not Dr. DeJonge.
         2
            Physicians specializing in environmental medicine treat “adverse reactions experienced by an
individual on exposure to an environmental excitant,” such as a pesticide. See American Academy of
Environmental Medicine, What Is Environmental Medicine?, http://www.aaemonline.org/introduction.html
(last visited August 11, 2008).
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                    Page 5


       Gass had her first appointment with Dr. Natzke on October 22, 2004, and
complained of “achiness all over, chills, sweats, fever, blisters on her tongue, droopiness
on the right side of her face, muscle spasms, dizziness, blurred vision and memory
problems.” (J.A. 847.) Like DeJonge, Gass told Dr. Natzke that within ten to fifteen
minutes of her exposure to the pesticides, she developed “weakness and fatigue and had
a green/gray tongue which she said turned black about 1½ weeks later.” (Id.)

       According to Dr. Natzke, both women also exhibited neurological symptoms,
including “brain fog, memory loss [and] mood swings.” (J.A. 848.) He administered
a “visual contrast sensitivity test” to Gass, which Dr. Natzke said “she failed miserably”
and had “one of the worst test results I have seen.” (Id.) Gass’ poor performance on the
vision test indicated to Dr. Natzke “that she was exposed to neurotoxins.” (Id.) Dr.
Natzke added that “[a]ll pesticides contain neurotoxins.” (Id.) Dr. Natzke also noted
that both women “exhibited black tongues” at some point since he began treating them,
a symptom that he attributed to pesticides in their system. (Id.)

       Based on their symptoms, Dr. Natzke diagnosed both DeJonge and Gass with
“acute pesticide exposure.” (J.A. 846–47.) However, he could not identify the particular
pesticide to which Plaintiffs were exposed. Although a test for pyrethroids and other
chemical compounds found in SSI-50 (the least potent of the three pesticides purportedly
used at the Marriott in Maui) did not reveal “detectable levels” of such compounds in
Plaintiffs’ systems, Dr. Natzke did not rule out the possibility that Gass and DeJonge
were exposed to SSI-50 because “the concentration of chemicals from the pesticide
contamination . . . would have been diluted in their blood by the time I saw them in mid-
October.” (J.A. 847.) Dr. Natzke did not test Plaintiffs for other toxins because “there
are tens of thousands of chemicals and it is impossible to test for all such chemicals
without specifically knowing what chemical a person was exposed to.” (Id.)
No. 07-1733           Gass, et al. v. Marriott Hotel Servs., et al.                              Page 6


C.       Defendants’ Experts

         Despite the uncertainty regarding which toxin or toxins Plaintiffs were exposed
to, Defendants introduced substantial expert testimony indicating that SSI-50 could not
have caused the symptoms experienced by Plaintiffs. First, Defendants point to a report
by Marcia van Germert, Ph.D., a toxicologist with a doctorate in Pharmacology and
Biochemistry. Although Dr. van Germert did not examine the toxic effects of any
substance other than SSI-50, she concludes that “no peer reviewed study” demonstrates
that the chemicals in SSI-50 have “ever produced a toxic effect in humans, or produced
the symptoms alleged by the plaintiffs” when those chemicals are used merely for
“crack and crevice application.” (J.A. 180.)

         Similarly, the record contains a report by H. James Wedner, MD, chief of the
Division of Allergy and Immunology Medicine of the Washington University School of
Medicine. According to Dr. Wedner, “throughout the ages there have been individuals
who have developed conditions that have defied characterization.” (J.A. 196.) Dr.
Wedner suggests that Plaintiffs may be suffering from such a condition which “do[es]
not seem to have an obvious cause.” (Id.) Nevertheless, Wedner concluded that neither
Gass nor DeJonge “suffered any health problems that are related to their potential
exposure to Ssi-50 [sic] in their hotel room in Maui on September 7, 2004.” (J.A. 191.)
Dr. Wedner provides no analysis regarding the possible effect of Suspend SC, Demand
CS, or any other pesticide on Plaintiffs.3

         Finally, the record contains a report by Elissa P. Benedek, MD, a clinical
psychiatrist who holds faculty appointments at three universities. Dr. Benedek attempts
to dismiss Plaintiffs’ symptoms as merely psychosomatic, concluding that both DeJonge
and Gass have “demonstrated a tendency to react to stress in the past with physical
symptoms, and now continue[] to react to psychological stressors with physical


         3
          The record also contains a report from Dr. Gary Bennett, an Entomology professor at Purdue
University. After surveying the legal landscape governing pesticide use, Dr. Bennett ultimately concludes
that “Ecolab’s treatment was proper, including the selection of products, and the treatment method and
amount of product used.” (J.A. 129.) It is unclear from Dr. Bennett’s report, however, what the basis of
his conclusions might be.
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                   Page 7


symptoms and complaints.” (J.A. 225, 242) According to Dr. Benedek, Gass and
DeJonge’s symptoms resulted from nothing more than a psychological reaction to stress.

II.     PROCEDURAL HISTORY

        Plaintiffs filed this diversity case in the Western District of Michigan on
December 29, 2005, alleging that Defendants negligently exposed them to pesticides and
that this exposure was the cause of their subsequent illness. After discovery, Defendants
moved for summary judgment. On May 8, 2007, the district court granted Defendants’
motion for summary judgment. This appeal followed.

                                      DISCUSSION

I.      CHOICE OF LAW

        Federal courts sitting in diversity apply the choice of law provisions of the forum
state. NILAC Int’l Mktg. Group v. Ameritech Servs., Inc., 

, 358 (6th Cir.
2004). Because Plaintiffs filed this case in the Western District of Michigan, Michigan
choice of law provisions apply. 

of law provisions favor allowing Michigan residents to bring
suit in Michigan courts under Michigan law. See Olmstead v. Anderson, 

, 302–03 (Mich. 1987). Generally speaking, a tort claim filed in a Michigan court
will be governed by Michigan law “unless a ‘rational reason’ exists to displace it.”
Watkins & Son Pet Supplies v. Iams Co., 

, 611 (6th Cir. 2001) (quoting

). Moreover, the fact that a tort took place outside of
Michigan is not itself a sufficient reason to apply a different state’s law. See 

(holding that the fact that an accident occurred outside of Michigan
is of no “great or particular significance” in determining which state’s law to apply in
a tort suit). Additionally, although Michigan courts recognize that applying Michigan
law rather than the law of the state in which the tort allegedly took place might lead to
forum shopping, “[t]here is no forum-shopping concern when the forum is also the
plaintiff’s state of citizenship.” 

No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                   Page 8


         Plaintiffs are Michigan residents, and they present no arguments why the law of
Hawaii or any other state should apply. Accordingly, we see no reason to displace
Michigan’s presumption that Michigan substantive law governs tort suits brought within
its borders. See 

However, under Erie Railroad Co. v. Tompkins, 

(1938), federal law governs procedural issues, including evidentiary rulings made
pursuant to the Federal Rules of Evidence. Legg v. Chopra, 

, 289 (6th Cir.
2002).

II.      THE ADMISSIBILITY OF PLAINTIFFS’ EXPERTS’ TESTIMONY

A.       Standard of Review

         We review a district court’s ruling regarding the admissibility of expert
testimony for abuse of discretion. Aetna Cas. & Sur. Co. v. Leahey Constr. Co., 

, 544 (6th Cir. 2000). “In the context of an evidentiary ruling, abuse of
discretion exists when the reviewing court is firmly convinced that a mistake has been
made regarding admission of evidence.” 

v. Yellow Freight Sys., Inc.,

, 532 (6th Cir. 1989)).

B.       Analysis

         As a threshold matter, Plaintiffs contest the district court’s decision to exclude
statements by Dr. DeJonge and Dr. Natzke as unreliable opinion testimony. Generally,
a treating physician may provide expert testimony regarding a patient’s illness, the
appropriate diagnosis for that illness, and the cause of the illness. See Fielded v. CSX
Transp., Inc., 

, 870 (6th Cir. 2007). However, a treating physician’s
testimony remains subject to the requirement set forth in Daubert v. Merrell Dow
Pharmaceuticals, 

(1993), that an expert’s opinion testimony must “have
a reliable basis in the knowledge and experience of his discipline.” 

Under
Daubert, before allowing an expert’s testimony to be considered by the jury, a trial court
should consider: (1) whether the reasoning or methodology underlying the expert’s
testimony is scientifically valid; and (2) whether that reasoning or methodology properly
No. 07-1733            Gass, et al. v. Marriott Hotel Servs., et al.                                Page 9


could be applied to the facts at issue to aid the trier of fact.” United States v. Smithers,

, 315 (6th Cir. 2000).

         In assessing whether Dr. DeJonge and Dr. Natzke’s testimony satisfies the
requirements of Daubert, the district court concluded that Dr. DeJonge and Dr. Natzke
could testify as to Plaintiffs’ “symptoms, tests, diagnosis, and treatment.” Gass v.
Marriott Hotel Servs., Inc., 

, 1021 (W.D. Mich. 2007). The district
court determined that both doctors “are experienced physicians and are qualified to
diagnose medical conditions and treat patients.” 

However, with respect to
the doctors’ “causation opinions,” the district court concluded that “Dr. Natzke and Dr.
DeJonge have not demonstrated a scientifically reliable method to support their
conclusions as to causation in this particular matter and may not be permitted to testify
as to the cause of Plaintiffs’ symptoms.” 

The district court reasoned that
“[t]he ability to diagnose medical conditions is not remotely the same . . . as the ability
to deduce, delineate, and describe, in a scientifically reliable manner, the causes of those
medical conditions.” 

(quoting Wynacht v. Beckman Instruments, Inc., 

, 1209 (E.D. Tenn. 2000)). Because “Dr. Natzke and Dr. DeJonge have
not based their causation opinions on any testing data,” and the only blood tests which
Dr. Natzke relied on “did not reveal any detectable levels for the products the lab tested
for,” the district court found that neither physician had a scientific basis for their
“causation opinion.” 

, 1021.4

         Insofar as the district court permitted Dr. DeJonge and Dr. Natzke to offer a
diagnosis, while excluding testimony on matters outside of their professional experience,
the district court’s decision is similar to Dickenson v. Cardiac & Thoracic Surgery of

         4
           While the district court’s decision to exclude the doctors’ “causation opinion” could be seen as
inconsistent with its decision to allow the doctors to testify regarding their diagnosis of Plaintiffs’
condition as acute pesticide exposure, the district court’s conclusions likely recognize that, although both
doctors are competent to testify regarding Plaintiffs’ diagnosis (even if the diagnosis necessarily implies
exposure to pesticides), neither doctor is competent to testify regarding the specific pesticide and the time
frame of exposure. The district court emphasized that neither doctor engaged in testing that revealed the
particular pesticide Plaintiffs were exposed to, and neither doctor “referenced any scientific literature
establishing a connection between Plaintiffs’ exposure to some unknown pesticide and symptoms that they
continue to experience over two years after the exposure.” 

. Thus, the
district court ruled that while Dr. Natzke and Dr. DeJonge are competent to testify that Plaintiffs are
suffering from pesticide exposure, neither is competent to testify regarding whether Defendants caused this
exposure.
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 10


Eastern Tenn., P.C., 

(6th Cir. 2004). In Dickenson, we considered the
expert testimony of Dr. Johnson, a cardiac surgeon, regarding the cause of a patient’s
brain injuries. Dr. Johnson testified that the patient’s injuries resulted from premature
removal of her ventilation tube, 

and that the patient’s cardiac surgeon was
responsible for the decision to prematurely extubate the patient, 

In holding
that Dr. Johnson could testify that the patient suffered injuries as a result of premature
extubation, the court in Dickenson disagreed with the district court’s conclusion that,
because Dr. Johnson was neither a pulmonologist nor familiar with scholarly literature
in the field of pulmonology, he was not qualified to provide expert testimony regarding
whether the patient should have been extubated. 

       According to Dickenson, a physician need not “demonstrate a familiarity with
accepted medical literature or published standards in [an area] of specialization in order
for his testimony to be reliable in the sense contemplated by Federal Rule of Evidence
702.” 

text of Rule 702 expressly contemplates that an expert may be
qualified on the basis of experience.”       

R. Evid. 702 advisory
committee’s note); see also Kumho Tire Co., Ltd. v. Carmichael, 

, 156
(1999) (“[N]o one denies that an expert might draw a conclusion from a set of
observations based on extensive and specialized experience.”). The exclusion of a
medical doctor’s professional opinion, rooted in that doctor’s “extensive relevant
experience,” is “rarely justified in cases involving medical experts as opposed to
supposed experts in the area of product liability.” 

. Because
Dr. Johnson stated in his affidavit that he is “involved with extubation decisions on
almost a daily basis,” the court concluded that his significant experience qualified him
to testify as to whether another physician prematurely extubated a patient. 

       The court in Dickenson reached the opposite conclusion with respect to Dr.
Johnson’s testimony that the patient’s cardiac surgeon—as opposed to the patient’s
pulmonologist—was at fault for the decision to prematurely extubate the patient. 

The court reasoned that there was no support for Dr. Johnson’s opinion that the
patient’s cardiac surgeon was at fault for the premature extubation because Dr. Johnson
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 11


testified to nothing in his experience which supported his theory that the cardiac surgeon
was somehow responsible for the pulmonologist’s decision. Thus, Dickenson stands for
the proposition that a medical doctor is generally competent to testify regarding matters
within his or her own professional experience. See 

When, however, the
doctor strays from such professional knowledge, his or her testimony becomes less
reliable, and more likely to be excluded under Rule 702. See 

       In light of this reading of Dickenson, we believe that the district court did not
abuse its discretion. Both Dr. DeJonge and Dr. Natzke relied on professional experience
in diagnosing and treating Plaintiffs. Specifically, Dr. Natzke stated in an affidavit that
he relied on his experience treating “thousands of patients for environmental medicine
issues including pesticide and/or other chemical contamination.” (J.A. 846.) Under
Dickenson, both Dr. DeJonge and Dr. Natzke are competent to testify with respect to
Plaintiffs’ diagnosis to the extent that they rely on professional education or experience.

. Conversely, nothing in Dr. DeJonge’s or Dr. Natzke’s medical
expertise would provide a basis to determine the exact chemical Plaintiffs were exposed
to at the Marriott hotel. In addition, because Defendants did not disclose that Plaintiffs
possibly were exposed to Demand CS or Suspend SC until very late in the discovery
process, the doctors could not run tests to determine whether Plaintiffs actually were
exposed to such chemicals, which would have provided a basis for the doctors’ causation
opinion. Accordingly, similar to the expert witness in Dickenson who lacked a basis in
experience or personal knowledge regarding which doctor made the decision to
prematurely extubate a patient and, as a result, could not testify regarding which doctor
ordered the extubation, 

and Dr. Natzke cannot rely on their general
knowledge of pesticides to testify regarding the specific pesticide that caused Plaintiffs’
symptoms, or when Plaintffs’ exposure to that pesticide occurred.

       The district court correctly permitted Dr. DeJonge and Dr. Natzke to testify
regarding their diagnosis of Plaintiffs, and properly excluded their testimony regarding
where and when Plaintiffs were exposed to pesticides. Therefore, the district court did
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 12


not abuse its discretion in limiting Dr. DeJonge and Dr. Natzke’s testimony to matters
within their professional experience or personal knowledge.

III.   THE DISTRICT COURT’S GRANT OF SUMMARY JUDGMENT

A.     Standard of Review

       The district court granted summary judgment to Defendants with respect to
Plaintiffs’ negligence claims. 

. A district court’s grant of
summary judgment is reviewed de novo. Farhat v. Jopke, 

, 587 (6th Cir.
2004). The district court’s grant of summary judgment should be affirmed when “the
discovery and disclosure materials on file, and any affidavits show that there is no
genuine issue as to any material fact” as to an essential element of the non-moving
party’s case. Fed. R. Civ. P. 56(c). An issue of fact is “genuine” if a reasonable person
could return a verdict for the non-moving party. Anderson v. Liberty Lobby, Inc., 

, 248 (1986). After the moving party has satisfied its burden, the burden shifts
to the non-moving party to set forth “specific facts showing that there is a genuine issue
for trial.” Matsushita Elec. Indus. Co. v. Zenith Radio Corp., 

, 587 (1986).
When no genuine issues of material fact exist, this Court reviews de novo the district
court’s conclusions of substantive law. 

.

B.     Analysis

       Plaintiffs argue that the district court erred in granting summary judgment to
Defendants on the grounds that, absent expert testimony linking Plaintiffs’ symptoms
to a particular pesticide, no reasonable jury could have found that Defendants
negligently caused Plaintiffs’ illness. We agree.

       We first emphasize that, in deciding a motion for summary judgment, we must
draw all justifiable inferences in favor of Plaintiffs as the non-moving party, and
“[Plaintiffs’] evidence is to be believed.” Martin v. Toledo Cardiology Consultants,
Inc., 

, 410 (6th Cir. 2008) (citing Liberty 

).
According to Plaintiff DeJonge, she returned to her hotel room to find men in masks
spraying chemicals. She testified that there was a “thick, horrid, acrid putrid odor” in
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 13


the room, and that the air was “sort of cloudy.” (J.A. 490–91.) The haze of chemicals
in the room was so thick that she could “see it, smell it, taste it, feel it.” (J.A. 494.)
Within fifteen minutes of her exposure to the “cloud,” DeJonge claims that she became
ill. Similarly, Plaintiff Gass testified that she began to experience symptoms shortly
after her exposure to the cloud of pesticides in the hotel room. We conclude that, should
a jury credit Plaintiffs’ testimony and the other evidence supporting Plaintiffs’ claim, a
reasonable jury could find that Defendants are liable for causing Plaintiffs’ injuries.

       Plaintiffs claim that Defendants’ negligence caused their illnesses. Accordingly,
as in any case alleging simple negligence under Michigan law, Plaintiffs must
demonstrate: “(1) that defendant owed them a duty of care, (2) that defendant breached
that duty, (3) that plaintiffs were injured, and (4) that defendant’s breach caused
plaintiffs’ injuries.” Henry v. Dow Chem. Co., 

, 688 (Mich. 2005).
Defendants argue on appeal, and the district court held, that Plaintiffs have failed to
demonstrate both that Defendants breached a duty of care and that such a breach caused
Plaintiffs’ injuries. We consider each of these arguments below.

       1.      Duty of Care

       Defendants characterize Plaintiffs’ negligence claim as an allegation that
Defendants violated the professional standard of care governing exterminators, and
therefore conclude that Plaintiffs’ claim must fail because Plaintiffs failed to present
expert testimony to establish the standard of care applicable to exterminators. The
dissent agrees and, although it acknowledges that the case is “not a ‘professional care’
case,” Dissenting Op. at 26, it argues that, “[i]n Michigan, expert testimony in
professional negligence (and toxic tort) cases like this one is required to avoid summary
judgment . . . ,” 

law does not require expert testimony under the
circumstances presented in this case. The cases the dissent cites, such as Lince v.
Monson, 

(Mich. 1961), address medical malpractice claims against, for
example, a surgeon performing a complex medical procedure, not an exterminator
spraying pesticides.
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 14


        The dissent also asserts that “these cases require expert testimony in complex,
professional, or scientific-based negligence cases.” Dissenting Op. at 26. While the
dissent’s statement is literally true—medical malpractice cases do involve “complex,
professional, and scientific-based” questions—the dissent’s use of such cases to require
Plaintiffs to produce expert testimony under the circumstances is entirely misleading.
In the medical malpractice context, as demonstrated by the cases the dissent relies on,
the applicable standard of care generally is “beyond the ken of laymen.” Thomas v.
McPherson Cmty. Health Ctr., 

, 705 (Mich. Ct. App. 1986). As a result,
“in an action for malpractice against a hospital, expert testimony is required to establish
the applicable standard of conduct, the breach of that standard, and causation.” 

Michigan Supreme Court has recognized, proving negligence and causation in
medical malpractice cases is different than in ordinary negligence cases. In a case
involving “conduct, like that of a surgeon, resting upon judgment, opinion, or theory, the
ordinary rules for determining negligence do not prevail.” 

(internal quotation marks omitted) (citation omitted). Pervasively spraying pesticides
within an enclosed room inhabited by humans, however, is not “conduct . . . resting upon
judgment, opinion, or theory,” and is not a “matter[] of special knowledge strictly
involving professional skill.” 

courts require plaintiffs to produce
an expert in medical malpractice cases to explain the applicable standard of care,
Michigan law does not require Plaintiffs to present expert testimony regarding the
standard of care applicable to spraying chemicals in the confined quarters of an occupied
room.

        Accordingly, Plaintiffs’ claim in the present case that Defendants negligently
sprayed pesticides is more akin to an ordinary negligence case than a claim involving
professional negligence. The dissent’s attempt to classify Plaintiffs’ claim as one
involving professional negligence is not supported by Michigan case law. Thus, even
in the absence of expert testimony regarding “how the room should have been sprayed
or how the risk of chemical poisoning should have been reduced,” Dissenting Op. at 25,
Michigan law does not prevent Plaintiffs from avoiding summary judgment in the
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 15


absence of expert testimony where the factfinder is able to weigh and evaluate the
evidence based on his or her ordinary experience.

       Through their testimony, Plaintiffs allege that men in masks entered their hotel
room and, despite the presence of Plaintiffs’ suitcases and similar indications that the
room was in use, sprayed such a thick concentration of pesticides that a “thick, horrid,
acrid[,] putrid” cloud of toxic chemicals filled the room. (J.A. 490.) Expert testimony
is not necessary to allow a reasonable jury to conclude that such actions are negligent,
inasmuch as an ordinary person understands that it is unacceptable to enter a place where
another is residing and fill that place with airborne poison, without providing for
evacuation of the inhabitants, appropriate ventilation, or taking other precautions.

       Plaintiffs also have introduced evidence indicating that Defendants were aware
of potential injuries that result from contact with at least two of the pesticides commonly
used in exterminating cockroaches. By federal regulation, manufacturers of hazardous
chemicals must produce a “material safety data sheet” (“MSDS”), and employers using
such chemicals must keep a copy of the MSDS for each chemical that they use. 29
C.F.R. § 1910.1200(g)(1). Each MSDS must detail “[t]he health hazards of the
hazardous chemical, including signs and symptoms of exposure, and any medical
conditions which are generally recognized as being aggravated by exposure to the
chemical.” § 1910.1200(g)(2)(iv). According to the MSDS for Suspend SC, one of the
pesticides used by Defendants, Suspend SC is “[h]armful if inhaled,” and the MSDS
warns that a person who does inhale the product should be “remove[d] to fresh air” and
given “medical attention.” (J.A. 527.) The MSDS for Demand CS offers even more
serious warnings, instructing persons who may come in contact with the pesticide to
wear protective clothing to avoid “[u]nprotected contact” with the chemical, and to
either ensure that areas where Demand CS is being applied are well-ventilated or to have
persons in the area wear respirators. (J.A. 535.) Indeed, mere skin contact with Demand
CS is dangerous, and the MSDS for this pesticide warns that if the pesticide gets on a
person’s skin or clothing, that person should “[t]ake off contaminated clothing,” [r]inse
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 16


skin immediately with plenty of water for 15-20 minutes,” and “[c]all [the
manufacturer], a poison control center or doctor for treatment advice.” (J.A. 534.)

       A reasonable person would understand that he or she could seriously injure
another person by filling an occupied hotel room with a cloud of toxic or hazardous
chemicals. Based on this fact, and the evidenced introduced by Plaintiffs indicating that
Defendants were aware that at least some of the chemicals they routinely use could cause
serious illness, a jury reasonably could find that Defendants were negligent in inundating
an occupied hotel room with pesticide spray in the absence of any warnings to the
occupants. Accordingly, the district court erred in granting summary judgment to
Defendants on the ground that Plaintiffs failed to introduce expert testimony establishing
that Defendants breached a duty of care.

       2.      Causation

       Defendants offer two arguments supporting their belief that no reasonable jury
could find that the chemicals Plaintiffs allegedly were exposed to caused their illness.
First, Defendants argue that, because Plaintiffs do not know exactly which chemical they
were exposed to, a reasonable jury could not conclude that they were exposed to a
chemical, dispensed by these Defendants, which could have caused their particular
symptoms. Additionally, Defendants claim that expert testimony is required to establish
causation. Neither of these claims has merit.

       Defendants use three different chemicals to exterminate cockroaches at the
Marriott in Maui: SSI-50, Demand CS, and Suspend SC, and Defendants admit that
exterminators were in Plaintiffs’ hotel room in response to Plaintiffs’ earlier complaint
of a dead cockroach in the room. Nevertheless, Defendants argue—without citing any
cases—that, because Plaintiffs do not know exactly which chemical they were exposed
to, “as a matter of law they cannot competently establish that such a mystery substance
caused their claimed ailments.” (Def.’s Br. 39.)

       Michigan law, however, requires only that a plaintiff claiming negligence prove
his or her case by a preponderance of the evidence, and does not require that a plaintiff
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                   Page 17


alleging exposure to a harmful substance prove with certainty that he or she was exposed
to a particular chemical. See Liberty Mut. Ins. Co. v. Bay City, Water Dept., 

, 200 (Mich. 1962). Therefore, Plaintiffs may survive summary judgment if a
reasonable jury could find that it is more likely than not that Defendants caused Plaintiffs
to be exposed to a sufficient quantity of a hazardous substance capable of causing their
injuries.

        Although Defendants introduced substantial expert testimony indicating that
exposure to SSI-50 could not have caused Plaintiffs’ illness, they declined to offer any
evidence regarding the toxic properties of Demand CS and Suspend SC. While the
record contains little evidence regarding the toxic effects of Suspend SC, based on the
MSDS for Demand CS, a reasonable jury could conclude that Demand CS is capable of
producing many of the symptoms experienced by Plaintiffs, especially because
Defendants failed to introduce any evidence rebutting the MSDS’s description of
Demand CS as a highly dangerous toxin capable of causing a myriad of symptoms.

        According to Dr. Natzke, Plaintiff DeJonge experienced a wide range of
symptoms, including “a headache, swelling of her tongue, hands, feet and face, profuse
itching, dizziness, shortness of breath . . . a slurring and swollen tongue, complaints of
a foul taste in her mouth, drooling and complaints of fatigue and pain in her muscles and
joints,” in addition to a need to “take afternoon naps which wasn’t the case before the
incident on September 7, 2004 . . . diarrhea and [] a droopy left eye.” (J.A. 846.)
Plaintiff Gass experienced “achiness all over, chills, sweats, fever, blisters on her tongue,
droopiness on the right side of her face, muscle spasms, dizziness, blurred vision and
memory problems . . . weakness and fatigue and [] green/gray tongue which she said
turned black about 1½ weeks later.” (J.A. 847.)

        The MSDS for Demand CS warns that it can cause many of these symptoms,
including “central nervous system depression,” “irritation to eyes, skin and respiratory
tract,” “headaches, dizziness, anesthesia, drowsiness . . . and other central nervous
system effects.” (J.A. 536.) Moreover, according to the MSDS, the chemicals in
Demand CS target the liver, nervous system, kidney, blood, respiratory tract, skin and
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 18


eye. Although the MSDS does not account for all of Plaintiffs’ symptoms, such as their
discolored tongues, Dr. Natzke stated in his affidavit that this symptom could be
attributed to pesticide poisoning.

       Accordingly, viewing the evidence in the light most favorable to Plaintiffs as the
non-moving parties, a reasonable jury could find that Plaintiffs’ symptoms were caused
by their exposure to the pesticides sprayed by Defendants. Plaintiffs have produced
ample evidence to demonstrate that at least one of the chemicals Defendants routinely
used to exterminate cockroaches, Demand CS, is capable of causing their symptoms.
Plaintiffs further have testified that they were exposed to a visible and pungent cloud of
pesticides after Defendants sprayed pesticides in their room while Plaintiffs occupied the
room. It is also significant that Plaintiffs began experiencing symptoms within fifteen
minutes of their alleged exposure to pesticides in their hotel room. In addition,
Defendants have offered no evidence to refute the MSDS’s representation of Demand
CS as a chemical which could have caused Plaintiffs’ symptoms.

       Despite this evidence, Defendants argue that this Court’s decision in Kalamazoo
River Study Group v. Rockwell International Corp., 

(6th Cir. 1999),
requires Plaintiffs to introduce an “essential element” of “admissible expert testimony”
in order to prove causation. That case, however, cannot be read so broadly. Kalamazoo
River was an environmental contamination case, involving 38 miles of shoreline which
was polluted by the chemical polychorinated biphenyl (“PCB”). 

The
defendant in Kalamazoo River owned an automotive parts manufacturing plant, located
3200 feet from an entry into the tainted waterways, which leaked PCB into the
surrounding soil in 1989. 

In 1993, the defendant, acting with the approval
of state environmental authorities, undertook to repair any environmental damage caused
by the 1989 leak. As a result of these efforts, the defendant excavated approximately
800 cubic yards of soil from the area surrounding the leak, and conducted soil studies
revealing that PCB had traveled no farther then 1400 feet away from the site leak—1800
feet short of the nearby waterway. 

1069. Nevertheless, in 1995, the
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                 Page 19


defendant was named in the Kalamazoo River lawsuit, which alleged that the 1989 leak
had contributed to the PCB contamination along the 38 miles of shoreline. 

       In holding that the defendant could not be held liable for the PCB contamination
along the shoreline, the court noted that the plaintiff presented no reliable expert
testimony which refuted evidence showing that PCB from the 1989 leak never reached
the nearby waterway. 

Accordingly, the court held that, “[t]he analytical
gap between the evidence presented [by the plaintiff] and the inferences to be drawn
. . . is too wide. Under such circumstances, a jury should not be asked to speculate on the
issue of causation.” 

(quoting Turpin v. Merrell Dow Pharms., Inc., 

, 1360–61 (6th Cir. 1992)).

       Contrary to Defendants’ assertions, the principle governing Kalamazoo River is
not applicable to Plaintiffs’ claims. Unlike in Kalamazoo River, where the defendant
presented scientific evidence affirmatively demonstrating that it was not responsible for
any PCB contamination along the shoreline, Defendants have presented nothing more
than   statements    by    their   own    agent—an       exterminator   who   works    for
Defendants—claiming that neither Demand CS nor a similarly toxic chemical was used
in Plaintiffs’ hotel room. In other words, while the Kalamazoo River defendant proved
an absence of causation by introducing objectively verifiable scientific evidence,
Defendants have not done so. Though it is certainly reasonable, as this Court held in
Kalamazoo 

–73, to require a party to refute scientific evidence
with scientific evidence, Plaintiffs are not required to produce expert testimony on
causation where Defendants have failed to offer scientific evidence regarding the effects
of Demand CS or Suspend SC.

       The complexity of the factual issue presented in Kalamazoo River also justified
requiring the plaintiff to support its case with expert testimony. Kalamazoo River
concerned whether a 1989 chemical spill, allegedly cleaned up in 1993, traveled through
3200 feet of soil to a nearby waterway, and then spread out along 38 miles of shoreline.

Such an inquiry is beyond the capacity of twelve lay people, absent
some assistance from an expert.
No. 07-1733             Gass, et al. v. Marriott Hotel Servs., et al.                           Page 20


         In contrast, Plaintiffs allege that they entered a room Defendants filled with a
cloud of toxic chemicals, and became ill within fifteen minutes of their exposure to the
toxins. Moreover, the record shows that exterminators acting on behalf of Defendants
entered the room to exterminate cockroaches, and that at least one pesticide that
Defendants use to control cockroaches—Demand CS—is capable of producing many of
the symptoms from which Plaintiffs suffer. It does not take an expert to conclude that,
under these circumstances, Defendants more likely than not are responsible for
Plaintiffs’ injuries.

         If anything, the decision in Kalamazoo River bolsters Plaintiffs’ case. Like the
defendant in Kalamazoo River, who introduced scientific evidence showing that it was
not responsible for PCB contamination along the shoreline, Plaintiffs have introduced
scientific evidence—the MSDS—which shows that Demand CS is capable of causing
their symptoms. 

. Defendants, like the plaintiff in Kalamazoo
River, have introduced no evidence regarding the toxic effects of Demand CS. See 

Defendants cannot excuse their failure to introduce expert testimony by
accusing their adversaries of the same failure where other scientific evidence exists that
tends to prove Plaintiffs’ case.

         Should a jury credit Plaintiffs’ testimony, it reasonably could conclude that the
alleged cloud of unidentified toxic chemicals sprayed in Plaintiffs’ hotel room caused
Plaintiffs’ injuries. The dissent, however, asserts that Plaintiffs were required to produce
expert testimony linking their exposure with their symptoms. According to the dissent,
“ordinary understanding of everyday medical problems does not include the proposition
that black tongue is ordinarily caused by spraying of pesticides.” Dissenting Op. at 30-
31. The dissent appears to believe that a jury in Plaintiffs’ case would have to use its
own knowledge to link pesticide exposure to Plaintiffs’ illness and its symptoms.5


         5
           The dissent asserts that Plaintiffs were required to produce expert testimony to establish
causation in this case. To support its argument, the dissent cites to an unpublished decision from the
Michigan Court of Appeals. See Dissenting Op. at 6-7 (citing Trice v. Oakland Dev. Ltd. P’ship, No.
278392, 2008 Mich. App. LEXIS 2484, at *32 (Mich. Ct. App. Dec. 16, 2008)). While similar to the facts
involved in this case, Trice, as the dissent acknowledges, relies primarily on the reasoning of the very
district court decision which is before this panel on appeal. Further, under Michigan law, an unpublished
opinion from the state court of appeals lacks precedential value and is not binding on state courts. Mich.
No. 07-1733            Gass, et al. v. Marriott Hotel Servs., et al.               Page 21


However, Dr. Natzke stated in his affidavit that, based on his experience in treating
patients who have been exposed to pesticides, a discolored tongue is attributable to
pesticide poisoning. Thus, contrary to the dissent’s assertions, our conclusion that
Plaintiffs were not required to produce expert testimony with respect to causation is not
a conclusion that the causes of a black tongue are within the “ordinary understanding”
of the jury.

         We conclude that when a plaintiff claims that a defendant was negligent in filling
a hotel room with a cloud of a poisonous substance, and there is evidentiary support for
such claims, expert testimony is not required to show negligence, and the district court
erred in holding otherwise.

IV.      PLAINTIFFS’ MOTION TO EXTEND DISCOVERY

         Finally, we note that, at oral argument, Plaintiffs claimed that they would have
presented more evidence indicating that they were injured by Demand CS or Suspend
SC, but they were frustrated in their efforts to conduct discovery because Defendants did
not provide Plaintiffs with documents showing that these two pesticides were used at the
Maui Marriott until after the close of discovery. Although Plaintiffs filed a motion in
the district court seeking to extend the length of discovery in light of Defendants’ alleged
failure to reveal damaging documents in a timely manner, the district court denied this
motion, and Plaintiffs do not appeal the district court’s ruling. Accordingly, we decline
to review the district court’s decision not to permit additional discovery. See United
States v. Corrado, 

, 611 n.12 (6th Cir. 2002) (“Arguments not developed
in briefs on appeal are deemed waived by this court . . . .”). We leave it to the district
court on remand to determine whether to reopen discovery to expand the record prior to
trial.




App. R. 7.215(C)(1).
No. 07-1733       Gass, et al. v. Marriott Hotel Servs., et al.             Page 22


                                   CONCLUSION

         Accordingly, for the reasons set forth above, we REVERSE the grant of
summary judgment to Defendants and REMAND this case to the district court for a jury
trial.
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 23


                                  ________________

                                      DISSENT
                                  ________________

       BOGGS, Chief Judge, dissenting. This case basically boils down to the relative
import of two Latin phrases: “post hoc ergo propter hoc” and “res ipsa loquitur.” The
former is a well known logical fallacy (recognized as such since Aristotle’s Rhetoric).
It is the fallacy of saying that because effect A happened at some point after alleged
cause B, the alleged cause was the actual cause. Such logic has never been enough to
survive summary judgment. See, e.g., Abbott v. Federal Forge, 

, 875 (6th
Cir. 1990) (“[P]ost hoc, ergo propter hoc is not a rule of legal causation.”).

       The latter phrase applies to a narrow class of cases in which the connection
between an untoward effect and some type of fault is so clear (and the likelihood of an
alternative explanation so low) that no other evidence is required to uphold a jury
verdict. The original, and classic, exposition of this principle is Byrne v. Boadle, 159
Eng. Rep. 299 (Exch. 1863), a nineteenth century English case where a pedestrian on the
streets of Liverpool was struck by a barrel of flour that came flying out of the second-
story window of a commercial storeroom. As I will explain below, it seems clear to me
that the venerable British case does not describe the case before us. A barrel of flour is
extremely unlikely to come flying out onto a city street without some fault by those
charged with the care of similar barrels, and it is also extremely unlikely that the flying
barrel of flour came from any place other than the adjacent flour warehouse.

       In our case, the plaintiffs’ symptoms, which worsened at a later time and after
medical care, and which are known to have a wide variety of possible causes, are much
less obviously connected to an unspecified dose of a potentially poisonous pesticide.
Instead, finding fault, without more, in the latter circumstances represents classic post
hoc reasoning. Something potentially causative happened at one time; something
untoward happened at a later time. Therefore, plaintiffs allege, the latter must have been
caused by the former.       The applicable law of Michigan does not permit that
unsubstantiated connection to be made, and that is why expert opinion is required. I
No. 07-1733            Gass, et al. v. Marriott Hotel Servs., et al.               Page 24


therefore respectfully dissent from the majority’s invocation of res ipsa in a case where
it is not warranted.

                                                 I

        This case involves assessing the connection between exposure to some chemical
substance and a series of physical symptoms that are generally attributable to a wide
variety of causes. The district court held that the experts that the plaintiffs relied on to
connect the alleged exposure (and the defendant’s behavior relating to it) to the
plaintiffs’ symptoms could not testify to any causal relationship. The majority affirms
that decision but nevertheless reverses summary judgment, holding that ordinary
experience suffices to connect any chemical used (regardless of composition or dose) to
the symptoms. In my view, this resolution is not supported by common sense or by the
Michigan law that governs the case. Both counsel that a lay juror cannot be expected
to understand the complex medical and scientific facts that necessarily underlie any such
an attribution of fault and, accordingly, require expert explanation prior to allowing a
jury verdict. The majority’s assessment that in this case (and, one supposes, unlike in
most toxic tort cases) there is sufficient evidence for the jury to charge plaintiffs’
illnesses to the defendants because the illness began to develop reasonably soon after the
exposure – which is only post hoc ergo propter hoc – reinforces rather than refutes this
preference for expertise.

        To fully understand the weight that the majority asks the post hoc fallacy to bear,
it is useful to begin with a point of agreement between the majority and this opinion.
The plaintiffs hoped that their treating physicians could testify not just to the physical
symptoms with which they were diagnosed but also to the likely cause of such
symptoms. That testimony would have concluded that the plaintiffs’ illnesses were
explained best by chemical poisoning and that the exposure to whatever pesticides
defendants had used was the best explanation of how plaintiffs came in contact with the
chemicals that poisoned them. As rehearsed and affirmed in the majority opinion, the
district court held that these doctors lacked sufficient expertise to make the causal
connection alleged by plaintiffs.
No. 07-1733           Gass, et al. v. Marriott Hotel Servs., et al.                 Page 25


           Plaintiffs also proffered the testimony of a second set of experts to demonstrate
duty and breach. These liability experts (whose exclusion was not challenged on appeal)
would have testified to the dangers of pesticides and an appropriate standard of care for
their use. The district court excluded them because they prepared no report and did not
plan to give an opinion about the chemicals that defendants actually used or the
precautions defendants actually undertook.

           Together, this missing expert testimony means that the summary judgment record
contains no admissible evidence that directly shows a breach of duty or that shows
causation between the alleged breach and the illnesses. To be sure, the plaintiffs’
evidence recounts the spraying of the pesticide and details the potential toxicity at some
unspecified dose of the chemicals that probably were sprayed by defendants. But
establishing these propositions does not establish how the room should have been
sprayed or how the risk of chemical poisoning should have been reduced. That is, the
evidence does not tend to prove that the defendants’ activities, in light of the relevant
standard of care, were negligent or that the chemicals that were sprayed caused the
illness.

           Of course, the absence of evidence is not the same as evidence of absence. The
majority holds that in the place of the absent evidence there is sufficient circumstantial
evidence of breach and causation. Specifically, the historical fact that some illness came
quickly on the heels of exposure speaks for itself and makes up for the otherwise missing
causal links.

                                                II

           Thus presented, the question is whether the plaintiffs needed expert testimony
in this case to prove how much chemical exposure is too much chemical exposure or to
prove whether the amount of exposure actually caused the alleged harmful consequence.
In my view, the majority pays too little attention to this issue, rushing from the fact of
exposure and odd symptoms to the legal conclusion of fault. It is of course correct that
under Michigan law some complex cases involve breach or causation questions within
the ken of the jury notwithstanding the professional or scientific nature of the litigation.
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                   Page 26


See Thomas v. McPherson Health Center, 

, 631 (Mich. Ct. App. 1986).
But the majority assumes, without citation or authority, that this case is such a case.

        In Michigan, expert testimony in professional negligence (and toxic tort) cases
like this one is required to avoid summary judgment “unless the lack of professional care
is so manifest that it would be within the common knowledge and experience of the
ordinary layman that the conduct was careless . . . .” Lince v. Monson, 

,
848 (Mich. 1961). The Michigan courts do not provide a test for what is common
knowledge, but do require more than “a bad result,” Jones v. Porretta, 

,
874 (Mich. 1981), and have frequently held that negligence cannot be inferred based on
ordinary knowledge simply from an unexpected injury. See, e.g., Woodard v. Custer,

, 525 (Mich. 2005) (“[W]hether a leg may be fractured in the absence
of negligence when placing an arterial line . . . in a newborn’s leg is not within the
common understanding of the jury . . . .”). This approach comports with the general
view that injuries in professional negligence cases, especially those involving complex
chemicals and human health, “are usually not immediately obvious and the connection
between exposure and injury is not a matter of common sense or everyday experience.”
In re Meridia, 

, 798 (N.D. Ohio 2004).

        As I understand it, these cases require expert testimony in complex, professional,
or scientific-based negligence cases in order to limit the dangers associated with
indulging the post hoc impulse: it is too easy to charge an uncommon harm to the
presence of a mysterious substance. Properly credentialed expert testimony operates
as a bulwark against such fallacious attribution of guilt. As in the Daubert context, our
concern in applying these cases should be to “assure that the powerful engine of tort
liability . . . points towards the right substances and does not destroy the wrong ones.”
General Electric v. Joiner, 

, 148-49 (1997) (Breyer, J. concurring).

        While our case is not a “professional care” case, in a very recent, albeit
unpublished, decision, the Michigan Court of Appeals applied this wisdom to a case
similar to ours. The court, citing favorably to the district court opinion in this case, held
that without expert testimony directly connecting the level of pesticide exposure to the
No. 07-1733         Gass, et al. v. Marriott Hotel Servs., et al.                   Page 27


plaintiff’s mysterious illness, a plaintiff alleging pesticide poisoning could not get to the
jury. See Trice v. Oakland Development Ltd. Partnership, 2008 Mich. App. LEXIS
2484 at *30 (Mich. Ct. App. Dec. 16, 2008) (citing Gass v. Marriott, 

, 1023 (W.D. Mich. 2007)). Specifically, “the dose of chemicals to which plaintiff
had been exposed had not been determined,” 

and so “without evidence that
plaintiff had been exposed to any chemicals at a level that would be harmful, plaintiff
could not establish specific causation.” 

        It is against this background that I disagree with the majority’s conclusion that
the plaintiffs’ proof, without expert testimony, survives summary judgment.

                                             III

        As a general matter, the weakness in the majority’s reasoning is demonstrated
by reference to the emotive language used to characterize the facts. The words assume
negligence and, accordingly, make it easy to agree that a lay person could come to an
informed conclusion about the case. For instance, the cloud of pesticide was “toxic or
hazardous,” Op. at 16, and it delivered a “high dose,” Op. at 20, of chemical exposure
because of defendants’ “unacceptable behavior,” Op. at 15. But one cannot know that
these conclusions (that should be based on scientific facts of how a chemical impacts the
human body and legal standards) are appropriate without an expert explaining what
amount constitutes a high dose or how much exposure makes a chemical toxic to the
human body. Cf. 

. It is surely common experience that
pesticides are poison, but that does not resolve the question at issue in this litigation: it
may be that being exposed to a room “sort of cloudy” with Demand CS will cause no
lasting effects if the exposure is five minutes but not ten; ten but not twenty.

        The majority avoids the difficulty of scientific judgments by simply defining the
exposure as a high dose and the defendants’ behavior as unacceptable. Of course a jury
can decide for a plaintiff if it is shown that because of a defendant’s action a poisonously
high dose of a pesticide was administered to the plaintiff. But a closer examination of
the summary judgment record reveals that the evidence the plaintiffs have adduced does
not establish anything close to that description of defendants’ behavior and the
No. 07-1733          Gass, et al. v. Marriott Hotel Servs., et al.                 Page 28


majority’s holding is premised on a mistaken belief that a lay jury is competent to set the
standard of care for the administration of pesticides and to determine the cause of a
mystery illness.

                                               A

          In reversing the district court’s breach holding, the majority asserts that a
“factfinder is able to weigh and evaluate the evidence based on his or her ordinary
experience,” Op. at 15, because the plaintiffs testimony established that the defendants’
action resulted in a “thick, horrid, acrid, putrid” cloud of pesticides in the room. The
“because” in the previous sentence, the effect of which is to make it unnecessary for the
plaintiffs’ to introduce expert testimony establishing a breach of duty, is not supported
by law.

          As to the danger of the chemicals (and presumably the duty of care), the evidence
cited by the majority is the following: (1) the substance sprayed left the hotel room “sort
of cloudy” and (2) the MSDS report for two of the substances possibly used
demonstrates that exposure may result in certain symptoms. As to the defendants’
behavior, the majority cites the following evidence: (1) men in masks entered the hotel
room and sprayed pesticides despite the presence of suitcases and other indicia of
occupancy and (2) defendants were aware that “some of the chemicals they routinely use
could cause serious illness . . . .” Op. at 16.

          This is underwhelming proof of defendants’ alleged breach of a duty of care.
The evidence leaves open more questions than it answers. We do not know how harmful
the chemicals are or under what circumstances those harms obtain. For instance, how
long do the symptoms persist? How much exposure triggers what symptoms? What
measures (besides ventilation) can prevent harm? How many parts per million make a
room “cloudy”? How much chemical concentration before a “sort of cloudy” room
becomes dangerous? Similarly, the evidence is silent about defendants’ behavior in
relation to a standard of care. For instance, which chemical was sprayed? Is a hotel
room “well ventilated”? Does the chemical effect dissipate? How fast? What is its
No. 07-1733        Gass, et al. v. Marriott Hotel Servs., et al.                  Page 29


effect on articles in a room? How long does it last? Must a room be vacant to be
sprayed?

       As I understand Michigan’s tort law, the gaps in the evidence suggested by these
questions are too wide to be bridged by jury inference. To be sure, the difference
between “common knowledge” and a fact that must be explained by expert testimony
has not been precisely defined. But wherever the line may be, the questions posed in the
previous paragraph about health effects and proper precautions to mitigate them appear
to me well beyond the ordinary ken of a juror. See 

(upholding a directed verdict for the defendant where “[p]laintiffs provided expert
testimony that [their proffered theories of liability] would constitute a breach of the
appropriate standard of care” but “they did not produce evidence in the form of expert
opinion that the health center had in fact breached the standard of care.”).

       The majority makes no attempt to argue to the contrary based on the plaintiffs’
evidence. They assert only that “[e]xpert testimony is not necessary to establish that
such egregious behavior does not conform to the standard of care” because “an ordinary
person understands that it is unacceptable to enter a place where another is residing and
fill that place with airborne poison, without providing for evacuation of the inhabitants,
appropriate ventilation, or taking other precautions.” Op. at 15. This assertion fails for
two reasons.

       First, there is no evidence (lay or expert) to support the majority’s premises about
what the defendants did. There is no evidence as to which pesticide was used (that is,
how poisonous the “airborne poison” actually was – on the plaintiffs’ allegations, there
are differences between Demand CS and Suspend SC); that it would linger long enough
to seriously harm someone; that complete evacuation of all property in the room was
necessary; that a modern hotel room is not appropriately ventilated; or that “other
precautions” were necessary. The assertion thus fails on its own terms.

       Second, it asserts without argument that the standard of care is common
knowledge. This is unsupportable. Some pesticides can be used in a home by a private
individual without supervision or extra preparation and some require the tenting and total
No. 07-1733            Gass, et al. v. Marriott Hotel Servs., et al.                            Page 30


evacuation of the home. Simply reading the MSDS reports (whose warnings as to
Suspend SC and Demand CS mirror the warnings on the can of over-the-counter Ant &
Roach Killer in my chambers1) as the majority does cannot explain the difference. The
majority may be correct that if the jury knew all that was required of the defendants
under the appropriate level of care, it could compare the plaintiffs’ version of events to
that duty and make a breach determination. But that is not the case we have before us.
In our case an expert is required to explain the potential hazards associated with certain
pesticides and the best practices for avoiding those hazards.

                                                   B

         The majority’s causation analysis is even less persuasive. It boils down to an
assertion that there is evidence of causation because the defendants sprayed a pesticide,
scientific evidence shows that one of the pesticides they may have sprayed causes certain
physical symptoms, and the plaintiffs did in fact experience those symptoms. The
missing premises from this argument – that defendants did spray the substance known
to be dangerous, that the spray was in sufficient amount to cause harm, that plaintiffs’
minutes-long exposure was sufficient to cause harm, that other causes can be excluded
with confidence, etc. – are almost too many to list.

         The majority holds that those premises can be supplied by inference. The
opinion, however, makes no attempt to explain why the causal link between the
defendants’ actions and the plaintiffs’ illnesses are within common experience. There
is good reason to think that it is not. For instance, ordinary understanding of everyday
medical problems does not include the proposition that black tongue is ordinarily caused
by spraying of pesticides.2 To be sure, an ordinary lay person probably begins with an


         1
           For example, compare the warning the majority cites for the proposition “mere skin contact with
Demand CS is dangerous” at page 15 of the opinion with the over-the-counter warning regarding contact
with skin or clothing: “Take off contaminated clothing. Rinse skin immediately . . . for 15-20 minutes.
Call a poison control center or doctor for treatment advice.”
         2
          Indeed, publically available information suggests many other, more common, causes: (1)
changes in the normal bacteria or yeast content of the mouth following antibiotic treatment; (2) poor oral
hygiene; (3) medications containing bismuth, such as Pepto-Bismol; (4) regular use of mouthwash
containing oxidizing agents; and (5) drinking excessive amounts of coffee or tea. See Alan Carr,
What Causes a Black Hairy Tongue? Mayo Clinic: Ask a Dental Specialist, available at
No. 07-1733          Gass, et al. v. Marriott Hotel Servs., et al.                      Page 31


assumption that black tongue is evidence of something gone wrong, but the question here
is what that something is and whether it is chargeable to the defendants’ actions. See

(rejecting an argument similar to plaintiffs’ because the
“injury was susceptible to a number of explanations, all of which required medical
knowledge to discern.”).

        The majority’s reliance, at pages 20-21, on Dr. Natzke’s statement that pesticide
exposure could cause black tongue to show that the jury would possess the knowledge
necessary to make a reliable attribution of fault is not persuasive. First, Dr. Natzke does
not say that the plaintiffs’ injuries are the result of pesticides or even that these pesticides
could cause black tongue. Second, not only is there a lack of knowledge about how the
condition comes about, reliance on Natzke’s statement would permit lay people to make
a determination about the cause of an unfamiliar medical condition based only on a post
hoc temporal connection and an abstract statement of a risk of harm.

                                              IV

        We need to look no further than this case for an illustration of the concerns
underlying my belief that these standard of care and causation issues require expert
explanation. The flaws in the majority’s reasoning – eliding the difficult scientific
questions; conflating colloquial usage of terms like “toxic” and “high dose” with
scientific conclusions about the health effects of the plaintiffs’ exposure; and attributing
causation on the basis of order of events – are the reasons lay people (jurors and judges
alike) are advised to take expert guidance in drawing scientific conclusions. I believe
our courts should require that guidance. I respectfully dissent.




http://www.mayoclinic.com/health/black-hairy-tongue/HQ00325. Similarly, an OSHA document included
by plaintiffs in the summary judgment record that describes their diagnosis of “Multiple Chemical
Sensitivities” admits that “[t]here is insufficient scientific evidence to confirm a relationship
between . . . possible causes and symptoms.” J.A. 564.