Science can often be more complicated and nuanced than we might like. Back in 1897, the Indiana legislature attempted to establish that π was equal to 3.2.[1] Sure, that was simpler and easier to use in calculations, but also wrong. The irreducible fact is that π is an irrational number, and Indiana’s attempt to change that fact was, well, irrational. And to celebrate irrationality, consider the lawsuit’s industry’s jihad against glyphosate, including its efforts to elevate a dodgy IARC evaluation, while suppressing evidence of glyphosate’s scientific exonerations

After Bayer lost three consecutive glyphosate cases in Philadelphia last year, observers were scratching their heads over why the company had lost when the scientific evidence strongly supports the defense. The Philadelphia Court of Common Pleas, not to be confused with Common Fleas, can be a rough place for corporate defendants. The local newspapers, to the extent people still read newspapers, are insufferably slanted in their coverage of health claims.

The plaintiffs’ verdicts garnered a good deal of local media coverage in Philadelphia.[2] Defense verdicts generally receive no ink from sensationalist newspapers such as the Philadelphia Inquirer. Regardless, media accounts, both lay and legal, are generally inadequate to tell us what happened, or what went wrong in the court room. The defense losses could be attributable to partial judges or juries, or the difficulty in communicating subtle issues of scientific validity. Plaintiffs’ expert witnesses may seem more sure of themselves than defense experts, or plaintiffs’ counsel may connect better with juries primed by fear-mongering media. Without being in the courtroom, or at least studying trial transcripts, outside observers are challenged to explain fully jury verdicts that go against the scientific evidence. The one thing jury verdicts are not, however, are valid assessments of the strength of scientific evidence, inferences, and conclusions.

Although Philadelphia juries can be rough, they like to see a fight. (Remember Rocky.) It is not a place for genteel manners or delicate and subtle distinctions. Last week, Bayer broke its Philadelphia losing streak, with a win in Kline v. Monsanto Co.[3] Mr. Kline claimed that he developed Non-Hodgkin’s lymphoma (NHL) from his long-term use of Round-Up. The two-week trial, before Judge Ann Butchart, last week went to the jury, which deliberated two hours before returning a unanimous defense verdict. The jury found that the defendants, Monsanto and Nouryon Chemicals LLC, were not negligent, and that the plaintiff’s use of Roundup was not a factual cause of his lymphoma.[4]

Law360 reported that the Kline verdict was the first to follow a ruling on Valentine’s Day, February 14, 2024, which excluded any courtroom reference to the hazard evaluation of Glyphosate by the International Agency for Research on Cancer (IARC). The Law360 article indicated that the IARC found that glyphosate can cause cancer; except of course IARC has never reached such a conclusion.

The IARC working group evaluated the evidence for glyphosate and classified the substance as a category IIA carcinogen, which it labels as “probably” causing human cancer. This label sounds close to what might be useful in a courtroom, except that the IARC declares that “probably,” as used in is IIA classification does not mean what people generally, and lawyers and judges specifically, mean by the word probably.  For IARC, “probable” has no quantitative meaning.  In other words, for IARC, probable, a quantitative concept, which everyone understands to be measured on a scale from 0 to 1, or from 0% to 100%, is not quantitative. An IARC IIA classification could thus represent a posterior probability of 1% in favor of carcinogenicity (and 99% probable not a carcinogen). In other words, on whether glyphosate causes cancer in humans, IARC says maybe in its own made-up epistemic modality.

To find the idiosyncratic definition of “probable,” a diligent reader must go outside the monograph of interest to the so-called Preamble, a separate document, last revised in 2019. The first time the jury will hear of the IARC pronouncement will be in the plaintiff’s case, and if the defense wishes to inform the jury on the special, idiosyncratic meaning of IARC “probable,” they must do it on cross-examination of hostile plaintiffs’ witnesses, or wait longer until they present their own witnesses. Disclosing the IARC IIA classification hurts because the “probable” language lines up with what the trial judges will instruct the juries at the end of the case, when the jurors are told that they need not believe that the plaintiff has eliminated all doubt; they need only find that the plaintiff has shown that each element of his case is “probable,” or more likely than not, in order to prevail. Once the jury has heard “probable,” the defense will have a hard time putting the toothpaste back in the tube. Of course, this is why the lawsuit industry loves IARC evaluations, with its fallacies of semantical distortion.[5]

Although identifying the causes of a jury verdict is more difficult than even determining carcinogenicity, Rosemary Pinto, one of plaintiff Kline’s lawyers, suggested that the exclusion of the IARC evaluation sank her case:

“We’re very disappointed in the jury verdict, which we plan to appeal, based upon adverse rulings in advance of the trial that really kept core components of the evidence out of the case. These included the fact that the EPA safety evaluation of Roundup has been vacated, who IARC (the International Agency for Research on Cancer) is and the relevance of their finding that Roundup is a probable human carcinogen [sic], and also the allowance into evidence of findings by foreign regulatory agencies disguised as foreign scientists. All of those things collectively, we believe, tilted the trial in Monsanto’s favor, and it was inconsistent with the rulings in previous Roundup trials here in Philadelphia and across the country.”[6]

Pinto was involved in the case, and so she may have some insight into why the jury ruled as it did. Still, issuing this pronouncement before interviewing the jurors seems little more than wishcasting. As philosopher Harry Frankfurt explained, “the production of bullshit is stimulated whenever a person’s obligations or opportunities to speak about some topic exceed his knowledge of the facts that are relevant to that topic.”[7] Pinto’s real aim was revealed in her statement that the IARC review was “crucial evidence that juries should be hearing.”[8]  

What is the genesis of Pinto’s complaint about the exclusion of IARC’s conclusions? The Valentine’s Day Order, issued by Judge Joshua H. Roberts, who heads up the Philadelphia County mass tort court, provided that:

AND NOW, this 14th day of February, 2024, upon consideration of Defendants’ Motion to Clarify the Court’s January 4, 2024 Order on Plaintiffs Motion in Limine No. 5 to Exclude Foreign Regulatory Registrations and/or Approvals of Glyphosate, GBHs, and/or Roundup, Plaintiffs’ Response, and after oral argument, it is ORDERED as follows:

1. The Court’s Order of January 4, 2024, is AMENDED to read as follows: [ … ] it is ORDERED that the Motion is GRANTED without prejudice to a party’s introduction of foreign scientific evidence, provided that the evidence is introduced through an expert witness who has been qualified pursuant to Pa. R. E. 702.

2. The Court specifically amends its Order of January 4, 2024, to exclude reference to IARC, and any other foreign agency and/or foreign regulatory agency.

3. The Court reiterates that no party may introduce any testimony or evidence regarding a foreign agency and/or foreign regulatory agency which may result in a mini-trial regarding the protocols, rules, and/or decision making process of the foreign agency and/or foreign regulatory agency. [fn1]

4. The trial judge shall retain full discretion to make appropriate evidentiary rulings on the issues covered by this Order based on the testimony and evidence elicited at trial, including but not limited to whether a party or witness has “opened the door.”[9]

Now what was not covered in the legal media accounts was the curious irony that the exclusion of the IARC evaluation resulted from plaintiffs’ motion, an own goal of sorts. In previous Philadelphia trials, plaintiffs’ counsel vociferously objected to the defense counsel’s and experts’ references to the determinations by foreign regulators, such as European Union Assessment Group on Glyphosate (2017, 2022), Health Canada (2017), European Food Safety Authority (2017, 2023), Australian Pesticides and Veterinary Medicines Authority (2017), German Federal Institute for Risk Assessment (2019), and others, that rejected the IARC evaluation and reported that glyphosate has not been shown to be carcinogenic.[10]

The gravamen of the plaintiffs’ objection was that such regulatory determinations were hearsay, and that they resulted from various procedures, using various criteria, which would require explanation, and would be subject to litigants’ challenges.[11] In other words, for each regulatory agency’s determination, there would be a “mini-trial,” or a “trial within a trial,” about the validity and accuracy of the foreign agency’s assessment.

In the earlier Philadelphia trials, the plaintiffs’ objections were largely sustained, which created a significant evidentiary bias in the courtrooms. Plaintiffs’ expert witnesses could freely discuss the IARC glyphosate evaluation, but the defense and its experts could not discuss the many determinations of the safety of glyphosate. Jurors were apparently left with the erroneous impression that the IARC evaluation was a consensus view of the entire world’s scientific community.

Now plaintiffs’ objection has a point, even though it seems to prove too much and must ultimately fail. In a trial, each side has expert witnesses who can offer an opinion about the key causal issue, whether glyphosate can cause NHL, and whether it caused this plaintiff’s NHL. Each expert witness will have written a report that identifies the facts and data relied upon, and that explains the inferences drawn and conclusions reached. The adversary can challenge the validity of the data, inferences, and conclusions because the opposing expert witness will be subject to cross-examination.

The facts and data relied upon will, however, be “hearsay,” which will come from published studies not written by the expert witnesses at trial. There will be many aspects of the relied upon studies that will be taken on faith without the testimony of the study participants, their healthcare providers, or the scientists who collected the data, chose how to analyze the data, conducted the statistical and scientific analyses, and wrote up the methods and study findings. Permitting reliance upon any study thus allows for a “mini-trial” or a “trial within a trial,” on each study cited and relied upon by the testifying expert witnesses. This complexity involved in expert witness opinion testimony is one of the foundational reasons for Rule 702’s gatekeeping regime in federal court, and most state courts, but which is usually conspicuously absent in Pennsylvania courtrooms.

Furthermore, the plaintiffs’ objections to foreign regulatory determinations would apply to any review paper, and more important, it would apply to the IARC glyphosate monograph itself. After all, if expert witnesses are supposed to have reviewed the underlying studies themselves, and be competent to do so, and to have arrived at an opinion in some reliable way from the facts and data available, then they would have no need to advert to the IARC’s review on the general causation issue.  If an expert witness were allowed to invoke the IARC conclusion, presumably to bolster his or her own causation opinion, then the jury would need to resolve questions about:

• who was on the working group;
• how were working group members selected, or excluded;
• how the working group arrived at its conclusion;
• what did the working group rely upon, or not rely upon, and why,
• what was the group’s method for synthesizing facts and data to reach its conclusion;
• was the working group faithful to its stated methodology;
• did the working group commit any errors of statistical or scientific judgment along the way;
• what potential biases did the working group members have;
• what is the basis for the IARC’s classificatory scheme; and
• how are IARC’s key terms such as “sufficient,” “limited,” “probable,” “possible,” etc., defined and used by working groups.

Indeed, a very substantial trial could be had on the bona fides and methods of the IARC, and the glyphosate IARC working group in particular.

The curious irony behind the Valentine’s Day order is that plaintiffs’ counsel were generally winning their objections to the defense’s references to foreign regulatory determinations. But as pigs get fatter, hogs get slaughtered. Last year, plaintiffs’ counsel moved to “exclude foreign regulatory registrations and or approvals of glyphosate.”[12] To be sure, plaintiffs’ counsel were not seeking merely the exclusion of glyphosate registrations, but the scientific evaluations of regulatory agencies and their staff scientists and consulting scientists. Plaintiffs wanted trials in which juries would hear only about IARC, as though it was a scientific consensus. The many scientific regulatory considerations and rejections of the IARC evaluation would be purged from the courtroom.

On January 4, 2024, plaintiffs’ counsel obtained what they sought, an order that memorialized the tilted playing field they had largely been enjoying in Philadelphia courtrooms. Judge Roberts’ order was short and somewhat ambiguous:

“upon consideration of plaintiff’s motion in limine no. 5 to exclude foreign regulatory registrations and/or approvals of glyphosate, GBHs, and/or Roundup, any response thereto, the supplements of the parties, and oral argument, it is ORDERED that the motion is GRANTED without prejudice to a party’s introduction of foreign scientific evidence including, but not limited to, evidence from the International Agency for Research on Cancer (IARC), provided that such introduction does not refer to foreign regulatory agencies.”

The courtroom “real world” outcome after Judge Roberts’ order was an obscene verdict in the McKivison case. Again, there may have been many contributing causes to the McKivison verdict, including Pennsylvania’s murky and retrograde law of expert witness opinion testimony.[13] Mr. McKivison was in remission from NHL and had sustained no economic damages, and yet, on January 26, 2024, a jury in his case returned a punitive compensatory damages award of $250 million, and an even more punitive punitive damage award of $2 billion.[14] It seems at least plausible that the imbalance between admitting the IARC evaluation while excluding foreign regulatory assessments helped create a false narrative that scientists and regulators everywhere had determined glyphosate to be unsafe.

On February 2, 2024, the defense moved for a clarification of Judge Roberts’ January 4, 2024 order that applied globally in the Philadelphia glyphosate litigation. The defendants complained that in their previous trial, after Judge Roberts’ Order of January 4, 2024, they were severely prejudiced by being prohibited from referring to the conclusions and assessments of foreign scientists who worked for regulatory agencies. The complaint seems well founded.  If a hearsay evaluation of glyphosate by an IARC working group is relevant and admissible, the conclusions of foreign scientists about glyphosate are relevant and admissible, whether or not they are employed by foreign regulatory agencies. Indeed, plaintiffs’ counsel routinely complained about Monsanto/Bayer’s “influence” over the United States Environmental Protection Agency, but the suggestion that the European Union’s regulators are in the pockets of Bayer is pretty farfetched. Indeed, the complaint about bias is peculiar coming from plaintifs’ counsel, who command an out-sized influence within the Collegium Ramazzini,[15] which in turn often dominates IARC working groups. Every agency and scientific group, including the IARC, has its “method,” its classificatory schemes, its definitions, and the like. By privileging the IARC conclusion, while excluding all the other many agencies and groups, and allowing plaintiffs’ counsel to argue that there is no real-world debate over glyphosate, Philadelphia courts play a malignant role in helping to generate the huge verdicts seen in glyphosate litigation.

The defense motion for clarification also stressed that the issue whether glyphosate causes NHL or other human cancer is not the probandum for which foreign agency and scientific group statements are relevant.  Pennsylvania has a most peculiar, idiosyncratic law of strict liability, under which such statements may not be relevant to liability questions. Plaintiffs’ counsel, in glyphosate and most tort litigations, however, routinely assert negligence as well as punitive damages claims. Allowing plaintiffs’ counsel to create a false and fraudulent narrative that Monsanto has flouted the consensus of the entire scientific and regulatory community in failing to label Roundup with cancer warnings is a travesty of the rule of law.

What seems clever by halves in the plaintiffs’ litigation approach was that its complaints about foreign regulatory assessments applied equally, if not more so, to the IARC glyphosate hazard evaluation. The glyphosate litigation is not likely as interminable as π, but it is irrational.

*      *     *      *      *     * 

Post Script.  Ten days after the verdict in Kline, and one day after the above post, the Philadelphia Inquirer released a story about the defense verdict. See Nick Vadala, “Monsanto wins first Roundup court case in recent string of Philadelphia lawsuits,” Phila. Inq. (Mar. 15, 2024).

[1] Bill 246, Indiana House of Representatives (1897); Petr Beckmann, A History of π at 174 (1971).

[2] See Robert Moran, “Philadelphia jury awards $175 million after deciding 83-year-old man got cancer from Roundup weed killer,” Phila. Inq. (Oct. 27, 2023); Nick Vadala, “Philadelphia jury awards $2.25 billion to man who claimed Roundup weed killer gave him cancer,” Phila. Inq. (Jan. 29, 2024).

[3] Phila. Ct. C.P. 2022-01641.

[4] George Woolston, “Monsanto Nabs 1st Win In Philly’s Roundup Trial Blitz,” Law360 (Mar. 5, 2024); Nicholas Malfitano, “After three initial losses, Roundup manufacturers get their first win in Philly courtroom,” Pennsylvania Record (Mar. 6, 2024).

[5][5] See David Hackett Fischer, “ Fallacies of Semantical Distortion,” chap. 10, in Historians’ Fallacies: Toward a Logic of Historical Thought (1970); see also “IARC’s Fundamental Distinction Between Hazard and Risk – Lost in the Flood” (Feb. 1, 2024); “The IARC-hy of Evidence – Incoherent & Inconsistent Classification of Carcinogencity” (Sept. 19, 2023).

[6] Malfitano, note 2 (quoting Pinto); see also Law360, note 2 (quoting Pinto).

[7] Harry Frankfurt, On Bullshit at 63 (2005); see “The Philosophy of Bad Expert Witness Opinion Testimony” (Oct. 2, 2010).

[8] See Malifanto, note 2 (quoting Pinto).

[9] In re Roundup Prods. Litig., Phila. Cty. Ct. C.P., May Term 2022-0550, Control No. 24020394 (Feb. 14, 2024) (Roberts, J.). In a footnote, the court explained that “an expert may testify that foreign scientists have concluded that Roundup and· glyphosate can be used safely and they do not cause cancer. In the example provided, there is no specific reference to an agency or regulatory body, and the jury is free to make a credibility determination based on the totality of the expert’s testimony. It is, however, impossible for this Court, in a pre-trial posture, to anticipate every iteration of a question asked or answer provided; it remains within the discretion of the trial judge to determine whether a question or answer is appropriate based on the context and the trial circumstances.”

[10] See National Ass’n of Wheat Growers v. Bonta, 85 F.4th 1263, 1270 (9th Cir. 2023) (“A significant number of . . . organizations disagree with IARC’s conclusion that glyphosate is a probable carcinogen”; … “[g]lobal studies from the European Union, Canada, Australia, New Zealand, Japan, and South Korea have all concluded that glyphosate is unlikely to be carcinogenic to humans.”).

[11] See, e.g., In re Seroquel, 601 F. Supp. 2d 1313, 1318 (M.D. Fla. 2009) (noting that references to foreign regulatory actions or decisions “without providing context concerning the regulatory schemes and decision-making processes involved would strip the jury of any framework within which to evaluate the meaning of that evidence”)

[12] McKivison v. Monsanto Co., Phila. Cty. Ct. C.P., No. 2022- 00337, Plaintiff’s Motion in Limine No. 5 to Exclude Foreign Regulatory Registration and/or Approvals of Glyphosate, GHBs and/or Roundup.

[13] See Sherman Joyce, “New Rule 702 Helps Judges Keep Bad Science Out Of Court,” Law360 (Feb. 13, 2024) (noting Pennsylvania’s outlier status on evidence law that enables dodgy opinion testimony).

[14] P.J. D’Annunzio, “Monsanto Fights $2.25B Verdict After Philly Roundup Trial,” Law360 (Feb. 8, 2024).

[15] “Collegium Ramazzini & Its Fellows – The Lobby” (Nov. 19, 2023).

Recently, two lawyers wrote an article in a legal trade magazine about excluding epidemiologic evidence in civil litigation.[1] The article was wildly wide of the mark, with several conceptual and practical errors.[2] For starters, the authors discussed Rule 702 as excluding epidemiologic studies and evidence, when the rule addresses the admissibility of expert witness opinion testimony. The most egregious recommendation of the authors, however, was their recommendation that counsel urge the classifications of chemicals with respect to carcinogenicity, by the International Agency for Research on Cancer (IARC), and by regulatory agencies, as probative for or against causation.

The project of evaluating the evidence for, or against, carcinogenicity of the myriad natural and synthetic agents to which humans are exposed is certainly important. Certainly, IARC has taken the project seriously. There have, however, been problems with IARC’s classifications of specific chemicals, pharmaceuticals, or exposure circumstances, but a basic problem with the classifications begins with the classes themselves. Classification requires defined classes. I don’t mean to be anti-semantic, but IARC’s definitions and its hierarchy of carcinogenicity are not entirely coherent.

The agency was established in 1965, and by the early 1970s, found itself in the business of preparing “monographs on the evaluation of carcinogenic risk of chemicals to man.” Originally, the IARC set out to classify the carcinogenicity of chemicals, but over the years, its scope increased to include complex mixtures, physical agents such as different forms of radiation, and biological organisms. To date, there have been 134 IARC monographs, addressing 1,045 “agents” (either substances or exposure circumstances).

From its beginnings, the IARC has conducted its classifications through working groups that meet to review and evaluate evidence, and classify the cancer hazards of “agents” under discussion. The breakdown of IARC’s classifications among four groups currently is:

Group 1 – Carcinogenic to humans (127 agents)

Group 2A – Probably carcinogenic to humans (95 agents)

Group 2B – Possibly carcinogenic to humans (323 agents)

Group 3 – Not classifiable as to its carcinogenicity to humans   (500 agents)

Previously, the IARC classification included a Group 4 for agents that are probably not carcinogenic for human beings. After decades of review, the IARC placed only a single agent in Group 4, caprolactam, apparently because the agency found everything else in the world to be presumptively a cause of cancer. The IARC could not find sufficiently strong evidence even for water, air, or basic foods to declare that they do not cause cancer in humans. Ultimately, the IARC abandoned Group 4, in favor of a presumption of universal carcinogencity.

The IARC describes its carcinogen classification procedures, requirements, and rationales in a document known as “The Preamble.” Any discussion of IARC classifications, whether in scientific publications or in legal briefs, without reference to this document should be suspect. The Preamble seeks to define many of the words in the classificatory scheme, some in ways that are not intuitive. This document has been amended over time, and the most recent iteration can be found online at the IARC website.[3]

IARC claims to build its classifications upon “consensus” evaluations, based in turn upon considerations of

(a) the strength of evidence of carcinogenicity in humans,

(b) the evidence of carcinogenicity in experimental (non-human) animals, and

(c) the mechanistic evidence of carcinogenicity.

IARC further claims that its evaluations turn on the use of “transparent criteria and descriptive terms.”[4] This last claim is, for some terms, is falsifiable.

The working groups are described as engaged in consensus evaluations, although past evaluations have been reached on simple majority vote of the working group. The working groups are charged with considering the three lines of evidence, described above, for any given agent, and reaching a synthesis in the form of the IARC classificatory scheme. The chart, from the Preamble, below roughly describes how working groups may “mix and match” lines of evidence, of varying degrees of robustness and validity (vel non) to reach a classification.

Agents placed in Category I are thus “carcinogenic to humans.” Interestingly, IARC does not refer to Category I carcinogens as “known” carcinogens, although many commentators are prone to do so. The implication of calling Category I agents “known carcinogens” is to distinguish Category IIA, IIB, and III as agents “not known to cause cancer.” The adjective that IARC uses, rather than “known,” is “sufficient” evidence in humans, but IARC also allows for reaching Category I with “limited,” or even “inadequate” human evidence if the other lines of evidence, in experimental animals or mechanistic evidence in humans, are sufficient.

In describing “sufficient” evidence, the IARC’s Preamble does not refer to epidemiologic evidence as potentially “conclusive” or “definitive”; rather its use of “sufficient” implies, perhaps non-transparently, that its labels of “limited” or “inadequate” evidence in humans refer to insufficient evidence. IARC gives an unscientific, inflated weight and understanding to “limited evidence of carcinogenicity,” by telling us that

“[a] causal interpretation of the positive association observed in the body of evidence on exposure to the agent and cancer is credible, but chance, bias, or confounding could not be ruled out with reasonable confidence.”[5]

Remarkably, for IARC, credible interpretations of causality can be based upon evidentiary displays that are confounded or biased.  In other words, non-credible associations may support IARC’s conclusions of causality. Causal interpretations of epidemiologic evidence are “credible” according to IARC, even though Sir Austin’s predicate of a valid association is absent.[6]

The IARC studiously avoids, however, noting that any classification is based upon “insufficient” evidence, even though that evidence may be less than sufficient, as in “limited,” or “inadequate.” A close look at Table 4 reveals that some Category I classifications, and all Category IIA, IIB, and III classifications are based upon insufficient evidence of carcinogenicity in humans.

Non-Probable Probabilities

The classification immediately below Category or Group I is Group 2A, for agents “probably carcinogenic to humans.” The IARC’s use of “probably” is problematic. Group I carcinogens require only “sufficient” evidence of human carcinogenicity, and there is no suggestion that any aspect of a Group I evaluation requires apodictic, conclusive, or even “definitive” evidence. Accordingly, the determination of Group I carcinogens will be based upon evidence that is essentially probabilistic. Group 2A is also defined as having only “limited evidence of carcinogenicity in humans”; in other words, insufficient evidence of carcinogenicity in humans, or epidemiologic studies with uncontrolled confounding and biases.

Importing IARC 2A classifications into legal or regulatory arenas will allow judgments or regulations based upon “limited evidence” in humans, which as we have seen, can be based upon inconsistent observational studies, and studies that fail to measure and adjust for known and potential confounding risk factors and systematic biases. The 2A classification thus requires little substantively or semantically, and many 2A classifications leave juries and judges to determine whether a chemical or medication caused a human being’s cancer, when the basic predicates for Sir Austin Bradford Hill’s factors for causal judgment have not been met.[7]

An IARC evaluation of Group 2A, or “probably carcinogenic to humans,” would seem to satisfy the legal system’s requirement that an exposure to the agent of interest more likely than not causes the harm in question. Appearances and word usage in different contexts, however, can be deceiving. Probability is a continuous quantitative scale from zero to one. In Bayesian analyses, zero and one are unavailable because if either were our starting point, no amount of evidence could ever change our judgment of the probability of causation. (Cromwell’s Rule). The IARC informs us that its use of “probably” is purely idiosyncratic; the probability that a Group 2A agent causes cancer has “no quantitative” meaning. All the IARC intends is that a Group 2A classification “signifies a greater strength of evidence than possibly carcinogenic.”[8] Group 2A classifications are thus consistent with having posterior probabilities less than 0.5 (or 50 percent). A working group could judge the probability of a substance or a process to be carcinogenic to humans to be greater than zero, but no more than say ten percent, and still vote for a 2A classification, in keeping with the IARC Preamble. This low probability threshold for a 2A classification converts the judgment of “probably carcinogenic” into little more than precautionary prescriptions, rendered when the most probable assessment is either ignorance or lack of causality. There is thus a practical certainty, close to 100%, that a 2A classification will confuse judges and juries, as well as the scientific community.

In addition to being based upon limited, that is insufficient, evidence of human carcinogenicity, Group 2A evaluations of “probable human carcinogenicity” connote “sufficient evidence” in experimental animals. An agent can be classified 2A even when the sufficient evidence of carcinogenicity occurs in only one of several non-human animal species, with the other animal species failing to show carcinogenicity. IARC 2A classifications can thus raise the thorny question in court whether a claimant is more like a rat or a mouse.

Courts should, because of the incoherent and diluted criteria for “probably carcinogenic,” exclude expert witness opinions based upon IARC 2A classifications as scientifically insufficient.[9] Given the distortion of ordinary language in its use of defined terms such as “sufficient,” “limited,” and “probable,” any evidentiary value to IARC 2A classifications, and expert witness opinion based thereon, is “substantially outweighed by a danger of … unfair prejudice, confusing the issues, [and] misleading the jury….”[10]

Everything is Possible

Category 2B denotes “possibly carcinogenic.” This year, the IARC announced that a working group had concluded that aspartame, an artificial sugar substitute, was “possibly carcinogenic.”[11] Such an evaluation, however, tells us nothing. If there are no studies at all of an agent, the agent could be said to be possibly carcinogenic. If there are inconsistent studies, even if the better designed studies are exculpatory, scientists could still say that the agent of interest was possibly carcinogenic. The 2B classification does not tell us anything because everything is possible until there is sufficient evidence to inculpate or exculpate it from causing cancer in humans.

It’s a Hazard, Not a Risk

IARC’s classification does not include an assessment of exposure levels. Consequently, there is no consideration of dose or exposure level at which an agent becomes carcinogenic. IARC’s evaluations are limited to whether the agent is or is not carcinogenic. The IARC explicitly concedes that exposure to a carcinogenic agent may carry little risk, but it cannot bring itself to say no risk, or even benefit at low exposures.

As noted, the IARC classification scheme refers to the strength of the evidence that an agent is carcinogenic, and not to the quantitative risk of cancer from exposure at a given level. The Preamble explains the distinction as fundamental:

“A cancer hazard is an agent that is capable of causing cancer, whereas a cancer risk is an estimate of the probability that cancer will occur given some level of exposure to a cancer hazard. The Monographs assess the strength of evidence that an agent is a cancer hazard. The distinction between hazard and risk is fundamental. The Monographs identify cancer hazards even when risks appear to be low in some exposure scenarios. This is because the exposure may be widespread at low levels, and because exposure levels in many populations are not known or documented.”[12]

This attempted explanation reveals important aspects of IARC’s project. First, there is an unproven assumption that there will be cancer hazards regardless of the exposure levels. The IARC contemplates that there may circumstances of low levels of risk from low levels of exposure, but it elides the important issue of thresholds. Second, IARC’s distinction between hazard and risk is obscured by its own classifications.  For instance, when IARC evaluated crystalline silica and classified it in Group I, it did so for only “occupational exposures.”[13] And yet, when IARC evaluated the hazard of coal exposure, it placed coal dust in Group 3, even though coal dust contains crystalline silica.[14] Similarly, in 2018, the IARC classified coffee as a Group 3,[15] even though every drop of coffee contains acrylamide, which is, according to IARC, a Group 2A “probable human carcinogen.”[16]

[1] Christian W. Castile & and Stephen J. McConnell, “Excluding Epidemiological Evidence Under FRE 702,” For The Defense 18 (June 2023) [Castile].

[2] “Excluding Epidemiologic Evidence Under Federal Rule of Evidence 702” (Aug. 26, 2023).

[3] IARC Monographs on the Identification of Carcinogenic Hazards to Humans – Preamble (2019).

[4] Jonathan M. Samet , Weihsueh A. Chiu , Vincent Cogliano, Jennifer Jinot, David Kriebel, Ruth M. Lunn, Frederick A. Beland, Lisa Bero, Patience Browne, Lin Fritschi, Jun Kanno , Dirk W. Lachenmeier, Qing Lan, Gerard Lasfargues, Frank Le Curieux, Susan Peters, Pamela Shubat, Hideko Sone, Mary C. White , Jon Williamson, Marianna Yakubovskaya , Jack Siemiatycki, Paul A. White, Kathryn Z. Guyton, Mary K. Schubauer-Berigan, Amy L. Hall, Yann Grosse, Veronique Bouvard, Lamia Benbrahim-Tallaa, Fatiha El Ghissassi, Beatrice Lauby-Secretan, Bruce Armstrong, Rodolfo Saracci, Jiri Zavadil , Kurt Straif, and Christopher P. Wild, “The IARC Monographs: Updated Procedures for Modern and Transparent Evidence Synthesis in Cancer Hazard Identification,” 112 J. Nat’l Cancer Inst. djz169 (2020).

[5] Preamble at 31.

[6] See Austin Bradford Hill, “The Environment and Disease: Association or Causation?” 58 Proc. Royal Soc’y Med. 295 (1965) (noting that only when “[o]ur observations reveal an association between two variables, perfectly clear-cut and beyond what we would care to attribute to the play of chance,” do we move on to consider the nine articulated factors for determining whether an association is causal.

[7] Id.

[8] IARC Monographs on the Identification of Carcinogenic Hazards to Humans – Preamble 31 (2019) (“The terms probably carcinogenic and possibly carcinogenic have no quantitative significance and are used as descriptors of different strengths of evidence of carcinogenicity in humans.”).

[9] See “Is the IARC lost in the weeds” (Nov. 30, 2019); “Good Night Styrene” (Apr. 18, 2019).

[10] Fed. R. Evid. 403.

[11] Elio Riboli, et al., “Carcinogenicity of aspartame, methyleugenol, and isoeugenol,” 24 The Lancet Oncology P848-850 (2023);

IARC, “Aspartame hazard and risk assessment results released” (2023).

[12] Preamble at 2.

[13] IARC Monograph 68, at 41 (1997) (“For these reasons, the Working Group therefore concluded that overall the epidemiological findings support increased lung cancer risks from inhaled crystalline silica (quartz and cristobalite) resulting from occupational exposure.”).

[14] IARC Monograph 68, at 337 (1997).

[15] IARC Monograph No. 116, Drinking Coffee, Mate, and Very Hot Beverages (2018).

[16] IARC Monograph no. 60, Some Industrial Chemicals (1994).