1. The Explosion of Mold Litigation.


In recent years, concern over the potential toxic effects of airborne mold exposure has transformed traditional construction defect and product defect litigation into high-stakes personal injury litigation frequently involving scores or even hundreds of individuals. Fueled by media coverage, public concern over the protracted harm from mold has triggered a multitude of lawsuits involving schools, public buildings and private homes. At the heart of these cases is the question of whether molds are responsible for the harms being alleged. Throughout the country, courts are struggling with how to best handle evidentiary issues that arise when scientific testimony is presented in these cases. The controversies surrounding such testimony are compounded by the differing opinions that exist among the scientific community with regard to potential harmful effects from mold contamination. The purpose of this presentation is to identify and outline the issues that the courts grapple with when considering scientific opinions testimony and to suggest the sorts of foundation different courts may require before a scientific opinion may be presented in a case.

2. Scientific Testimony is Governed by Court Rules.


The law with regard to liability for harm from exposure to airborne concentrations of mold is rapidly developing and varies from state to state and according to the peculiar circumstances of each individual case. Claims of harm from exposure to molds raise questions and issues that are outside of the knowledge of the average judge or juror. As a result, scientific testimony is absolutely critical to inform the decision makers in court cases and will ultimately determine the underlying bases for the laws passed by legislatures and applied by courts faced with disputes involving airborne mold exposure.

However, as important as it is the presentation and consideration of scientific testimony in these cases is not automatic. Only on very rare occasions does a court select its own expert to inform itself or the jury. Instead, scientific testimony is typically presented in the context of our adversary system wherein each party is entitled to present their position on an issue in the best light possible, with a jury making the ultimate determination of the facts and issues in dispute. Each side of a controversy selects the witnesses, experts and scientific testimony that makes its own position the most compelling. The logical premise for this system is that each adversary’s own-self interest will result in vigorous and effective cross-examination, as well as testimony and evidence in counterpoint so that the jury will have the benefit of considering all viewpoints and possibilities before rendering its verdict. In this context, there is no guarantee that scientific testimony must be presented and considered by a jury nor is there any absolute right by a party to present such testimony. Instead, the application of both substantive law and procedural law by a court determines whether and what scientific testimony will be allowed. The former is used to ascertain whether the profferred scientific testimony is germane to the legal or factual issues that must be resolved in a case, and the latter considers whether the scientific testimony has a sufficient foundation for it to be considered in determining the legal or factual issues presented in a case.

Notably, there is no universal substantive law or procedural rules for courts in the United States. Instead, cases involving alleged harm from airborne exposure to mold, with ostensibly identical fact patterns, could have different procedural rules applied, could have different substantive law applied, and might even have differing outcomes depending upon the particular court in which each case is brought. For each case then, the individual scientific testimony allowed or refused in that particular case can have a profound effect on the outcome of the case.

1. Each State Has Differing Substantive Law.


Substantive law, that is the law that determines liability among parties to a case, may differ from state to state. Each state is allowed to develop its own substantive law through legislation and court decisions as it sees fit, bounded by the requirements of the United States Constitution and that state’s own particular state constitution. In many respects, the substantive law from state to state may be similar. For example, most states would not impose liability on a landlord who allowed a tenant to be exposed to the common flu in an apartment common area, but might impose liability for exposure to dangerous chemicals that caused harm to tenants in that same common area. One could say that the substantive law in most states with regard to this issue is that landlords are not liable for their tenants’ contracting common ailments in an apartment building, but that landlords may be liable for harm caused by exposing tenants to known dangerous chemicals.

Currently, the law with regard to airborne exposure to mold falls somewhere in between the two outcomes in the example above. Most states do not yet have court decisions defining the state’s law with regard to mold exposure. Nor, until quite recently, have legislatures stepped in to establish laws in this area. California has taken a first legislative step in establishing a legal standard with regard to the exposure to mold in its Toxic Mold Control Act of 2001, but even this legislation has not defined the liability that will attach for exposure to given amounts of mold. For most states, the courts on a case-by-case basis will determine the substantive law with regard to liability for mold exposure. The substantive law applied in each case determines whether and what scientific testimony and opinions will be considered. Scientific testimony must address either legal or factual issues that the applicable substantive law requires be answered. If the scientific testimony is not necessary to resolve an issue created by the substantive law, the testimony is deemed irrelevant and not allowed.

As a practical matter, the substantive law of most states will, at the very least, entertain the possibility of liability for harm from exposure to mold, thereby setting the stage for scientific testimony about airborne mold exposure and its potential ill effects. This scientific testimony from experts will be critical in the development of each state’s substantive law as it will establish an accepted belief about the potential harms caused by mold exposure and will inform the expectations of conduct and liability imposed by legislatures and courts. Precisely because it can play such a fundamental role in defining the law and individual liability, scientific testimony is subject to court rules which also vary from jurisdiction to jurisdiction.

2. Each Jurisdiction Also Has Its Own Procedural Law.


In addition to having its own substantive law, each state’s courts also have their own procedural law, essentially the rules that govern how a court case is run. With regard to scientific testimony, the most important procedural issues arise from the application of rules of evidence. Evidence rules are intended to prevent a jury from being mislead by unreliable testimony or other evidence that might otherwise be presented by a party seeking to put their position on an issue in the most favorable light possible.

Evidence rules are very similar, and often identical, from court to court among most states though different courts in different states may interpret the meaning and effect of a particular rule differently. Some of the evidentiary rules are quite familiar, such as the rule against hearsay designed to prevent unsworn statements from being accepted as true by a jury. Allowing such testimony might mislead a jury or make it unduly difficult to differentiate between the facts of a case and mere gossip.

Courts also apply evidentiary rules to proffered scientific testimony. The rules are intended to prevent a jury of laypersons, who may be unable to accurately assess the validity of scientific opinion being offered, from being mislead. In general, most states’ courts have historically been fairly generous in allowing scientific testimony in court, trusting the adverse party’s right to vigorously cross-examine the witness, and to offer rebuttal testimony, to inform the jury of any flaws in the scientific testimony. Even so, courts have long recognized that there must be some threshold showing before a witness is allowed to present expert scientific evidence to ensure that wholly misleading testimony is not allowed.

The case of Frye v. United States, 293 F. 1013, decided in 1923, established a threshold rule for assessing whether scientific testimony had sufficient foundation to be considered by a jury. Most state courts adopted the Frye standard in one form or another and many state courts continue to apply the Frye standard presently. In Frye, the scientific theory and new technology at issue concerned the systolic blood pressure deception test, or "lie detector" test. Prior to trial, the defendant in a murder case had been subjected to the test and an expert for the prosecution was prepared to testify at trial about the results. The court, recognizing the profound effect of "scientific" testimony about detecting deceit in the defendant’s statements, considered carefully the degree of reliability the new science and technology possessed. The Court ultimately decided that "the systolic blood pressure deception test has not yet gained such standing and scientific recognition… as would justify the courts admitting expert testimony deduced from the discovery, development, and experiments thus far made."

It is beyond the scope of this presentation to analyze the application of the Frye rule in each of the state court systems that still apply it. Generally, however, once a scientific expert establishes that the theory he or she bases his/her opinion on is "generally accepted" in the relevant scientific community, the opinion is deemed to be permissible or admissible, and its ultimate validity is then to be determined by a jury considering any counterpoints made during cross-examination or by other witnesses. As discussed below, the federal courts no longer apply the Frye standard to scientific testimony, and the trend in state courts is moving towards abandoning it as well.

3. Scientific Testimony in the Federal Courts.
Adding another layer of complexity to the court system in the United States are the federal courts. Federal courts exist in all fifty states, but are wholly separate from the court systems in those states. In most federal cases involving claims of harm from airborne exposure of mold, the federal court will apply the substantive law of the state in which the alleged harm occurred. Thus, identical cases brought in federal courts located in different states might be subject to differing substantive law standards and, as a consequence, might have differing outcomes.

Federal courts do have a universal set of procedural rules, the Federal Rules of Evidence, which govern the admissibility of scientific evidence in federal court. Even federal courts located in states that follow the Frye standard apply the federal standard, though they will likely apply the substantive law of the state in which each resides.

A further nuance is that federal courts, though all bound by the same procedural rules, do not necessarily interpret the meaning and effect of the rules in the same manner. Ideally, if two federal courts interpret a rule inconsistently, a federal Court of Appeals or the Supreme Court will resolve the conflict and issue a definitive interpretation. In practice, conflict over interpretation of evidentiary rules may persist for some time before they are resolved.

From the foregoing, it is obvious that there is no entirely uniform substantive law governing cases involving alleged harm from airborne exposure to mold, nor even any universal standard governing the presentation of scientific evidence in these types of cases. Instead, as with any emerging area of the law, the substantive law and the procedural law necessarily evolve on a case-by-case basis as courts grapple with the issues presented by the circumstances of each case. Scientific testimony promises to be at the center of the controversial procedural issues to be resolved as this law develops and also at the core of the substantive law announced by legislatures or courts to govern future disputes.

Because it is not practically possible in this presentation to analyze all of the substantive and procedural issues surrounding the admissibility of scientific testimony in all of the state and federal courts, the analysis that follows will focus upon the evidentiary rules that govern the admissibility of scientific testimony in the federal courts and their application to litigation involving claims of harm from exposure to mold.

3. The Daubert Standard.


In the case of Daubert v. Merrill Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), the U.S. Supreme Court announced the abandonment of the Frye standard in the federal courts in favor of a new rule governing scientific testimony in federal courts. In Daubert, the Supreme Court mandated that the trial judges must act as "gatekeepers" and allow a jury to hear and consider only evidence that is deemed to be both relevant and reliable. While this new rule on its face appears straightforward, its interpretation and application in the years following the U.S. Supreme Court decision has resulted in a substantial number of sometimes inconsistent court decisions. The first requirement of the Daubert standard is typically the easier to interpret and apply. To be deemed relevant, proffered scientific testimony must "logically advance a material aspect of the case." In other words, it must have some significance in making a fact or issue to be decided by a jury more or less likely to be true. By way of example, in a case involving allegedly harmful mold growth in a building in 2000, an expert opinion that harmful mold growth occurred in 2002 might be deemed inadmissible as irrelevant, unless it could be shown that the mold growth found in 2002 had some logical connection to harmful mold growth allegedly occurring in 2000.

The second element of the Daubert test focuses on the reliability of the scientific testimony being offered. The federal courts consider scientific opinion testimony to be reliable if it is grounded in methods of science and "connotes more than a subjective belief or unsupported speculation." Schudel v. General Electric Co., 120 F.3d 991 (9^th Cir. 1997), quoting Daubert. In Daubert the U.S. Supreme Court identified four non-exclusive factors to be considered in assessing whether the scientific testimony being offered is sufficiently reliable to be considered by a jury.

(1) Whether the scientific theory or technique can be or has been tested;

(2) Whether the theory or technique has been subjected to peer review and publication;

(3) Whether the particular technique has a known rate of error; and

(4) Whether the theory or technique is generally accepted in the scientific community.

While not universally accepted, the general interpretation of the Daubert standard is that it requires not only that scientific testimony have a reliable scientific basis, but also that the opinion being testified to was arrived at by the reliable application of scientific principles. In other words, a scientist, even a preeminent scientist in a particular field, is not entitled to offer testimony which is merely his or her best judgment, or unscientifically supported belief or opinion. Instead, the opinion must be grounded in reliable science and the testimony must be derived from the reliable application of scientific methodology.

The Daubert standard for the admissibility of scientific testimony has been specifically codified in Federal Rule of Evidence 702:

Rule 702. Testimony by Experts

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 by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise, if (1) the testimony is based upon sufficient facts or data, (2) the testimony is the product of reliable principles and methods, and (3) the witness has applied the principles and methods reliably to the facts of the case.

Rule 702 is the foundational rule that every expert witness offering scientific testimony in a federal court must satisfy before his or her testimony will be allowed to be presented or considered in determining the outcome of a particular case. In making the determination of whether scientific testimony meets the requirements of Daubert and Rule 702, federal courts attempt to strike a balance between, on the one hand, not requiring proof that the scientific evidence offered is irrefutable or certainly correct, but on the other, recognizing that proffered scientific testimony has to potential to be powerfully persuasive and that, if it is not reliable, it may mislead rather than enlighten a jury. See, e.g., Cavallo v. Star Enter., 100 F.3d 1150 (4^th Cir. 1996) and United States v. Dorsey, 45 F.3d 809 (4^th Cir. 1995).

While the typical case involving claimed harm from airborne exposure to mold calls for scientific testimony on a number of issues, for illustrative purposes, the focus here will be on scientific testimony offered to establish that airborne exposure to mold caused the toxic effects complained of by individuals. In determining whether such testimony is scientifically reliable and admissible for consideration by a jury, a federal court will usually consider two aspects of causation termed general causation and specific causation.

1. General Causation and Specific Causation.


Federal courts typically consider two separate but related components of causation to determine the reliability of scientific testimony concerning claims harm from exposure to toxic substances. The first component, general causation, addresses whether a reliable scientific basis exists for concluding that the substance at issue is capable of causing the kind of harm alleged. The second component, specific causation, focuses on the existence of a reliable scientific basis for concluding that the substance at issue actually caused the harm alleged. Both issues are focal points of controversy in cases involving claims of toxic effects from airborne exposures to mold compounds.

First, in most cases alleging toxic harm from airborne mold exposures, there is insufficient data to establish an amount, concentration or even a duration of an alleged airborne exposure to mold, so no dose or duration can be accurately identified. Second, given the paucity of epidemiological studies or other scientific studies establishing a dose/response relationship between an exposure airborne concentrations of mold and toxic effects, there is no means of reliably ascertaining whether the uncertain amount, concentration or duration is capable of causing the alleged toxic effect in humans, particularly in the circumstances (i.e. non-industrial) most often presented. A conclusion to be drawn is that there is no scientifically credible basis in such cases to establish the element of general causation, that is, that molds of the type and in the amounts found in a particular case were sufficient to cause toxic effects in humans as a result of airborne exposures. Without a scientific basis for establishing the element of general causation, there can be no credible basis for establishing specific causation, that airborne exposure to mold did in fact, cause the toxic effects alleged in a particular case.

On the other hand, experts retained by plaintiffs often rely upon anecdotal reports of toxic effects, the frequency of complaints of similar symptoms associated with the presence of like types and amounts of mold, indirect or analogous studies, and their own experience to contend that a credible basis exists for believing that molds of the type and amount at issue in a particular case are capable of causing the sorts of toxic effects complained of. To bolster the argument that these bases should be sufficient for a general causation showing, attorneys for plaintiffs frequently argue that the burden of proof in a court case is much less than the standard for credible science; that to carry a point in court requires only that the point be the slightest bit more likely to be true than false, that is, only 50.1% certainty is required. These attorneys also point out that no epidemiological studies are or may be available in time for use in a case and that such studies have not been required in many other types of cases where general causation has been deemed to be established.

To establish specific causation, plaintiffs alleging toxic effects from airborne exposure to mold often rely upon medical experts who opine on a cause and effect relationship after performing a differential diagnosis, a technique in which a medical provider first "rules in" all of the potential causes for symptoms and them systematically "rules out" different causes until left with the most likely cause or causes for the symptoms.

Some courts have been persuaded by the argument that 50.1% certainly constitutes sufficient foundation to allow scientific testimony regarding causation. However, the argument actually misapplies the principles of Daubert and Federal Evidence Rule 702. Arguing that a scientific theory need only be 50.1% likely to be to true to be presented as true to a jury confuses the respective functions of scientists and jurors and abrogates the Court’s duty as "gatekeeper" to prevent unscientific, unreliable testimony from misleading a jury. The jury, of course, is not capable of independently evaluating the scientific reliability of testimony being presented to it as true. Replacing the standard of certainty required for a scientific theory to gain credible acceptance in the scientific community with the burden of proof applied by a jury in a civil case creates the preposterous situation in which a jury may decide the outcome of a case based upon a "scientific" theory that is 49.9% likely to be incorrect. Certainly the intent of Daubert and Federal Rule of Evidence 702 was to decrease, not increase, the likelihood that juries would be mislead by theories that do not have a reliable foundation in science.

Properly applying the Daubert standard and Federal Rule of Evidence 702 to a scientific testimony about toxic effects from an airborne exposure to mold should require first, a general causation showing that there is a credible and reliable scientific basis, with a scientific degree of certainty, that molds of the type and in the amount encountered in a particular instance can have the sorts of toxic effects complained of in that particular case. Then a showing might be made, by using a differential diagnosis carried out using proper scientific methodology, that the harm an individual suffered was more likely than not caused by the airborne exposure to mold.

After the presentation of all of the evidence a jury could then decide, on the standard "more probable than not basis" whether the claimed harm likely came from an airborne exposure to mold or from some other cause altogether. It remains to be seen how individual courts will in practice apply the Daubert standard and Federal Rule of Evidence 702. An illustrative case will give one example of how a court might go about the analysis.

2. An Illustrative Example.


While not a case involving claimed harm from exposure to molds, Westberry v. Gislaved Gummi AB, 178 F.3d 257 (4^th Cir. 1991) provides a good illustration of how a federal court might undertake a Daubert analysis to determine the reliability of proffered scientific testimony.

In that case, the plaintiff, Westberry, worked in a window manufacturing plant. As part of his job he installed window gaskets. The window gaskets were supplied by a separate company, the defendant, and came covered in talc. Westberry developed severe sinus problems after, according to him, inhaling enormous quantities of talc in connection with his job. In the lawsuit, Westberry called experts to provide scientific testimony that: 1) as a matter of general causation the inhalation of talc in large quantities could cause the sinus symptoms complained of; and 2) a differential diagnosis ruled out the other likely causes for Westberry’s sinus symptoms making it more likely than not that the symptoms were caused by the inhalation of talc. As a matter of substantive law, the court determined that the defendant supplier could be liable for causing harm from the talc on its product. A core issue of fact to be determined by the jury was whether the plaintiff really had inhaled the substantial amounts of talc as he had claimed. For purposes of the Daubert analysis, that is whether there is a credible basis for the testimony, the court accepted the fact of substantial talc inhalation as a given.

The defense contended that as a matter of general causation there was not a scientific basis to conclude that the inhalation of talc could have caused the sinus problems complained of because there were no epidemiological studies or other scientific studies that established a dose response relationship. The court found the defense’s argument unpersuasive because in that case it was undisputed that talc in sufficient quantities could cause the sinus problems complained of and even the scientists retained by the defense acknowledged that if Westberry had inhaled talc in the amounts he claimed, the harm he claimed was a possible effect. In essence the defense had argued that because no studies existed, general causation could never be scientifically established. The court rejected the argument that such a stringent showing was required in circumstances where it was undisputed that sufficient quantities the talc could in fact have caused such harm.

The defense next contended with regard to the issue specific causation, that the differential diagnosis performed by the plaintiff’s medical expert did not meet the requirements of scientific methodology or certainty and therefore did not comply with the Daubert requirements. The court in Westberry also rejected these arguments finding that where it undisputed that talc could cause the type of harm complained of, where there was evidence of substantial enough exposure to cause harm, and where a medical expert had considered and ruled out other "likely causes," that the proffered testimony sufficiently met the requirements of Daubert. The court further reasoned that the flaws or uncertainties in the "more likely than not" specific causation assessment made by plaintiff’s medical expert could be sufficiently countered through effective cross-examination inquiry and rebuttal testimony. In essence, the court determined that using the technique of differential diagnosis to establish the element of specific causation did not require the same degree of scientific certainty in its application or conclusions that another form of expert scientific testimony might require.

While Westberry is an instructive illustration of how a court might apply the Daubert standard it must be remembered that it is only one such case and that the court’s analysis and conclusions might not be followed by other federal or state courts considering similar issues. Also, it must be borne in mind that the circumstances of Westberry differ significantly from many cases involving alleged airborne exposures to toxic mold. Frequently missing from airborne mold exposure cases is evidence: (1) that substantial amounts of mold were actually inhaled; and (2) that the amounts of mold compounds allegedly inhaled; are capable of causing the type of toxic effects alleged (an undisputed point in Westberry).

3. The Differential Diagnosis Debate.
The application of a differential diagnosis as a valid scientific methodology for determining the cause of harm in toxic exposure cases is currently a central point of debate in the federal courts and has profound implications for cases involving alleged exposure to, and claimed harm from, exposure to airborne molds.

On the one hand, some courts considering the admissibility of scientific opinions based upon a differential diagnosis reason that differential diagnoses are a commonly accepted and relied upon method used by physicians to diagnose ailments and, therefore, should be considered sufficiently credible to be considered by a jury. These courts also often point out that it is simply not practically possible to have peer-reviewed, epidemiological studies to establish or refute theories of harm and that, in any event, the law does not require such stringent proof as a basis for the admissibility of theories of scientific causation.

However, a differential diagnosis may be more effective as a diagnostic tool than it is as a forensic tool. To be forensically valid, the technique must accurately "rule out" other potential likely causes, which in many cases may not be possible with any degree of scientific certainty. In addition, and perhaps more importantly, the technique must accurately "rule in" possible causes to be considered. Regrettably, there is no inherent mechanism that prevents an inaccurate "ruling in" of wholly unreliable causes based upon anecdotal information.

This later concern is particularly present in many cases involving alleged toxic effects from airborne exposure to mold. A medical provider employing a differential diagnosis might "rule in" tension, a virus, poor lighting, a migraine, airborne exposure to toxic mold or even a phase of the moon as possible causes for a headache. There may not be any scientific basis to "rule out" any of these possible causes, nor may there be any reliable scientific basis to "rule in" airborne exposure to toxic mold as a cause, or to "rule in" a phase of the moon as a cause. As a result, testimony about the likely cause of a harm from which a differential diagnosis may in fact amount to nothing more than an expert’s "subjective belief or unsupported speculation" which, as noted by the Supreme Court in Daubert, is not an acceptable foundation basis for the testimony.

Of course, in the case of alleged toxic harm from exposure to airborne mold compounds, the very premise of a differential diagnosis may be the focus of the dispute, that is, is there any scientific basis to potentially "rule in" exposure to airborne concentrations of mold as a potential cause of the complained of harm? Such a fundamental concern appears especially appropriate in the absence of scientifically reliable evidence establishing toxic effects from the airborne concentration of molds in the amounts involved in typical cases. Many courts are persuaded that the lack of a threshold basis to "rule in" an alleged toxic substance as a possible cause renders causation opinions based on differential diagnoses unreliable.

The controversy over whether a differential diagnosis, or a "more likely than not" analysis, is sufficient foundation for scientific testimony in federal courts has created a notable, almost absurd, situation wherein different federal courts, applying the same standards to the same types of cases involving the same lactation-suppressing drug, Parlodil, have reached directly contrary results regarding the admissibility of scientific testimony. In eight separate federal court cases, judges have determined that scientific testimony based upon experts’ "more likely than not" evaluations about harm caused by the drug is sufficiently reliable to be considered by a jury. In seven other federal court cases, however, the same sort of scientific testimony and opinions have been ruled inadmissible on the grounds that it is not sufficiently reliable under Daubert or Federal Rule of Evidence 702.

Similar splits in determinations of the admissibility of scientific testimony are likely in cases involving alleged exposure to, and harm from, airborne concentrations of mold compounds until Courts of Appeal or the Supreme Court once again establish a definitive interpretation and methodology regarding the use of differential diagnoses in proving the elements of causation in toxic tort cases.

4. Conclusion

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