Causation is at the very heart of the toxicological world. Causation in toxicology
is defined as the action of causing or producing an effect as a result of ingestion,
inhalation, dermal absorption or other exposure routes to a toxic substance.
It is the process by which the expert toxicologist establishes or refutes whether
a potential adverse health effect is truly caused by an exposure, dose and duration.
A determination of toxicological causation must satisfy multiple investigative criteria.(a)
Establishing or refuting toxicological causation involves a highly specific and
painstaking process of investigation as well as application of generally-recognized
scientific, forensic and deductive principles. Toxicological causation analyses
require a thorough investigation and review of all available data and case facts
including generally-accepted toxicological studies, relevant deposition transcripts,
medical records, police reports and (in some cases) review or inspection of the
exposure or accident scene.
The expert toxicologist forms a causative opinion based on the relationship(s) between
scientific information and objective evidence. Assessments must be based on sound
methodologies, sometimes covering a wide range of substances. The expert must present
all evidence, methods and results in an unbiased, scientifically credible and objective
manner. It may also be incumbent upon the expert toxicologist to present his findings
clearly and succinctly to the court, jurors and (in some cases at the client's request)
the news media or other lay persons.
It is important to note that causation may or may not be established in an investigative
assessment. There is no guarantee of a conclusive outcome. However, direct
on-site investigation, exposure reconstruction supported by laboratory analyses,
dose calculations and disciplined, thorough review of available (world-wide) toxicological
studies in the peer-reviewed scientific literature will often lead to toxicologically-reasonable
conclusions demonstrating or refuting causation.
Types of Causation
Generally, the expert toxicologist must either demonstrate or refute
causation. There are two types of causation which the toxicologist may be required to
assess: (1) general causation and (2) specific causation.
- General Causation is a methodology which determines
whether or not exposure to a substance or agent is capable of producing an adverse
health effect. General causation determination can also measure the potential
for impairment or death stemming from the use of (for example) pharmaceuticals,
drugs-of-abuse or alcohol. Typically, an opinion of general causation requires supporting
evidence in the form of peer-reviewed scientific literature identifying dose-response
relationships and/or epidemiological studies describing medical conditions associated
with the exposure in question. The evidence demonstrating that the exposure (chemical,
drug, substance, consumer product, etc.) is capable of causing the health effect
must be reliable and recognized within the generally-accepted body of scientific
and/or toxicological literature.
- Specific Causation is a methodology which determines
whether or not exposure to a substance or agent did, in fact, cause the specific
adverse health effect at issue. There are significant and critical differences
in distinguishing specific causation from general causation. It is
possible that an exposure can be demonstrated to be capable of causing an adverse
health effect, but this alone does not mean that it did, in fact, cause the effect.
For example, radon exposure is capable of causing the same type of bronchogenic
lung cancer as cigarette smoking. If both exposures have occurred, each must be
appropriately quantified. This means the toxicologist must weigh the individual
doses, timeframes and exposure frequencies and calculate the degree to which each
exposure contributed to the adverse health effect at issue. Thus, individual exposure
data must be compiled, quantified and assessed in accordance with the prevailing body
of peer-reviewed toxicological literature.
Causation and Weight-of-Evidence
A causative determination is the result of following a detailed set of procedures
supported by generally-accepted methodologies. Application of a weight-of-evidence
("WOE") methodology relies in part on the list of guidelines developed
by Sir Austin Bradford Hill for inferring general causation.1
Such an approach requires review of several types of evidence relating to the effects
of exposure. These types of evidence (or "modalities") include human epidemiological
studies, case reports, reviews, studies assessing the commonalities between a group
of epidemiological studies and studies of exposures on animals or in vitro.2
Sir Austin Bradford Hill.(b)
Sir Austin Bradford Hill (1897-1991) and Sir Richard Doll (1912-2005) pioneered
the concept of case-control studies. Subsequently, Hill and Doll were the first
to apply a scientific methodology to demonstrate the causal connection between cigarette
smoking and lung cancer.3 Hill subsequently published
a groundbreaking paper in 1965 in which he formalized his approach to determining
causation.4 Today, Hill's criteria for establishing
causation (the "Hill Factors") sets the toxicological standard for defining
a causal relationship between an event and a consequence.
In a toxicological causation assessment the
"Hill Factors" are important components in compiling weight-of-evidence.
Additionally, the expert toxicologist may apply a wide variety of experimental data
showing the biological mechanisms through which a substance can or may create adverse
health effects with respect to the metabolic pathways of the substance in animals
and humans. These and other generally-accepted methods all contribute to a scientifically-sound
causation determination.
A weight-of-evidence approach should also include review of any available information
regarding structure of the chemical at issue and the similarity of its structure
to chemicals already known to cause certain health effects in humans. However, scrupulous
care must be exercised when applying different studies to causative issues. Dr. Sheldon
Krimsky, Professor of Urban and Environmental Policy and Planning at Tufts University,
writes in his article entitled "The Weight of Scientific Evidence in Policy
and Law" that "Each type of study may provide some evidence, but each
has its limitations." He also writes, "If a chemical were known
to be one of the causal agents responsible for a human disease, then we would expect
a series of evidentiary pathways to converge on that conclusion .. But not all of
the evidence may be consistent with the result." 5
Thus, confounding factors are almost always present in a causative investigation.
The Hill factors establish a broad framework to help evaluate the various modalities
of evidence relevant to the general causation question. Hill's causative modality
factors include consideration of:
Example of "Hill Factor" Weighting of Causative Modalities.(c)
- Strength of association between the exposure and a particular health effect
- Specificity of the association
- Consistency of the association
- Dose-responsiveness of the chemical
- Biological plausibility of the causal connection
- Coherence of the association
- Temporality (time relationships)
- Relevant experimental data (i.e. animal studies)
In assessing chemical causation, as many of the Hill factors should be applied as
possible. However, it is not uncommon for there to be little or no information available
for one or more factors. Thus, application of the Hill factors to a particular causal
hypothesis (and the relative weight to assign to each) is both substance-dependent
and subject to the independent judgment of the toxicologist reviewing the
available body of data. "For example, some WOE approaches give higher weight
to mechanistic information over epidemiological data."6
Modalities beyond the Hill factors should also be considered. For example,
the National Research Council states that "the committee must rely on a combination
of evidence from different sources to reach any conclusion in accordance with its
mandate to estimate health effects...." 7
In view of Professor Hill's epidemiological background, it is not surprising that
several of his factors focus on consideration of epidemiological evidence. However,
such evidence addresses only one aspect among the several modalities of evidence
that should be considered in assessing causation. "The term WOE has come
to mean not only a determination of the statistical and explanatory power of any
individual study (or the combined power of all the studies) but the extent to which
different types of studies converge on the hypothesis." 8
Standards of Admissibility
The expert toxicologist's testimony must be responsive to answering the questions
at issue and rest on a sound, reliable scientific foundation (Rule 702, Federal
Rules of Evidence).9 In particular, toxicological
testimony cannot be considered admissible if the evidence is based upon a "novel"
scientific technique or methodology. Expert testimony must have its basis in reliable
scientific techniques that are generally accepted by the relevant scientific community.
Thus, although personal observations and professional experience may play a role
in formulating an opinion, they do not, in themselves, constitute an admissible basis
for a conclusion. The expert toxicologist must pragmatically assess the list of
contributing factors, weigh them appropriately and communicate findings in a
scientifically-sound and objective manner.
Accordingly, causation cannot be based solely on professional judgment that is difficult
to trace to a well-established scientific methodology. It cannot be vague, formulaic
or ignore the complexity and case-specificity of scientific interpretation. Although
a causative determination can have statistical support, the statistics merely support
the reliable outcome of applying a generally-accepted scientific method. Thus, a
scientifically credible causative opinion cannot be reached solely through statistics
or extrapolation of numbers. For example, so-called "evidence-based medicine"
has not achieved general acceptance in causation due to its very nature which reduces
the role of expert judgment in favor of data compiled in computer programs.
Interpretation of Toxicological Evidence
A scientifically credible weight-of-evidence approach to demonstrating or refuting
causation must do more than merely comply with the Daubert standard. In some cases
(particularly in toxic tort matters), the toxicologist's expert testimony may be
required to satisfy a "preponderance of evidence" criteria. This can be
seen in cases where a judge applies the Daubert standard not only to expert testimony
but also to each study upon which the expert relies. This interpretation of Daubert
requires each study to stand on its own merits. In his Proposal for Linking Culpability
and Causation to Ensure Corporate Accountability for Toxic Risks, McGarity
writes:10
"If the plaintiff fails to establish the relevance and scientific reliability
of a sufficient number of individual studies, the trial judge may exclude the expert's
testimony and (in the absence of other relevant and reliable expert testimony on
causation) grant the defendant's motion for summary judgment before the jury ever
enters the picture."
This reminds us that there are both judicial and regulatory approaches to interpreting
toxicological evidence. In regulation, the strands of evidence are not assumed to
stand by themselves but are seen as pieces of a larger puzzle. Scientific interpretation
of the puzzle forms the basis of the peer-review process in which each piece is
assessed on its merits by professionals with good education, experience and reputation.
The expert toxicologist must also be mindful of the fact that good communication
is paramount and judges and juries are not toxicologists. Toxicological assessments
can be extremely complex and findings must be presented such that the court can
understand them. A judicial interpretation may apply different weights to different
pieces of evidence based on comprehension of the evidence as presented.
Thus, whether submitting a report or offering testimony in deposition, being "right"
and being concise are equally important. Conscientious application of brevity and
meticulous attention to detail can go a long way toward avoiding misunderstandings
in forensic matters.
To that end, it is helpful to remember that the "weighing instrument"
for "weighing evidence" is always human cognition. Forensic toxicological
testimony can be complex and the more simplistically it can be presented, the greater
the likelihood that it will be understood by a judge and jury. By sticking strictly
to the science and presenting each piece of information in its most understandable
form, the expert toxicologist assists the court by assembling the puzzle pieces
into a comprehensive picture that either demonstrates or refutes causation.
Concordantly, causation is based on weight-of-evidence which must always be based
on good science. Ideally, WOE turns subjective perceptions in a forensic matter
into a purely rational, objective experience. Since interpretation is involved,
criticism must not only be expected, it must be anticipated. The expert toxicologist
must be fully prepared to defend both his opinions and the data upon which his opinions
are based — with concise professionalism, integrity and objective evidence.
Summary
In forensic causation matters, the expert toxicologist proceeds in the direction in which the evidence
leads. The goal of any toxicological analysis of causation is to establish the true
cause to reasonable toxicological certainty. Since toxicological expert
testimony has the potential to significantly impact the outcome of a case, accurate
compilation of evidence, thorough collation of Hill factors and conscientious application
of weight-of-evidence, are all central to an accurate toxicological causation assessment.
Notes and References
- Hill, A.B., "The Environment and Disease: Association or Causation?"
1965, Proceedings of the Royal Society of Medicine, Vol. 58(5), pages 295-300.
- National Research Council, "Environmental Epidemiology: Public Health and
Hazardous Wastes," 1991, Washington, D.C., The National Academies Press, Vol.
1.
- Doll, R. and Hill, AB, "Smoking and carcinoma of the lung. Preliminary report."
1950, British Medical Journal, pages 739-748., http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2557577/pdf/10063665.pdf
- Hill, A.B., "The Environment and Disease: Association or Causation?"
1965, Proceedings of the Royal Society of Medicine, Vol. 58(5), pages 295-300.
- Krimsky, Sheldon, Ph.D., "The Weight of Scientific Evidence in Policy and
Law," 1995, American Journal of Public Health, No. S1, http://www.tufts.edu/~skrimsky/PDF/AJPH_WOE.PDF
- Krimsky, Sheldon, Ph.D., "The Weight of Scientific Evidence in Policy and
Law," 1995, American Journal of Public Health, No. S1, http://www.tufts.edu/~skrimsky/PDF/AJPH_WOE.PDF
- National Research Council, "Environmental Epidemiology: Public Health and
Hazardous Wastes," 1991, Washington, D.C., The National Academies Press, Vol.
1.
- Krimsky, Sheldon, Ph.D., "The Weight of Scientific Evidence in Policy and
Law," 1995, American Journal of Public Health, No. S1, http://www.tufts.edu/~skrimsky/PDF/AJPH_WOE.PDF
- Federal Rules of Evidence, "Rule 702. Testimony by Expert Witnesses,"
http://www.law.cornell.edu/rules/fre/rule_702
- T.O. McGarity "Proposal for Linking Culpability and Causation to Ensure Corporate
Accountability for Toxic Risks," William and Mary Environmental Law and Policy
Review, Fall 2001.
Images
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Sir Austin Bradford Hill Archive, Cardiff University Library Sciences Division, London School of Hygiene and Tropical Medicine
- TCAS demonstrative (redacted), graphical image © Copyright 2017
TCAS, LLC.