Science and Law
26th March 2015

Evaluating Cancer Causation in Litigation

Recent media stories have proclaimed the alleged carcinogenicity of certain chemicals found in pet collars and home insect sprays. The basis to these assertions come from the International Agency for Research on Cancer (IARC), an arm of the World Health Organization whose objective is to promote international collaboration in cancer research.

Evaluating Cancer Causation in Litigation

Recent media stories have proclaimed the alleged carcinogenicity of certain chemicals found in pet collars and home insect sprays. The basis to these assertions come from the International Agency for Research on Cancer (IARC), an arm of the World Health Organization whose objective is to promote international collaboration in cancer research. IARC brings together scientists of various disciplines “to identify the causes of cancer so that preventive measures may be adopted and the burden of disease and associated suffering reduced.”

In one news story, the author states that these chemicals “could cause cancer in humans.” A powerful statement with powerful implications. As every litigator who has tried a toxic tort or product liability case involving cancer knows, the legitimacy of this statement is buried in nuance and complexity. Let’s see if we can deconstruct the evidence and the basis for the assertion.

According to a recent IARC news release, the Agency recently classified five organophosphate pesticides. The findings were also written up in Lancet Oncology where they were put into a slightly more scientific – i.e., less reactionary – context.

So what exactly does it mean that these chemicals were classified? In order to understand this, one must understand IARC’s classification system. According to IARC’s nomenclature, compounds are classified into one of five groups as set forth in the table below.

IARC GroupDeterminationNumber of Agents in this Group
Group 1Carcinogenic to humans116
Group 2AProbably carcinogenic to humans73
Group 2BPossibly carcinogenic to humans287
Group 3Not classifiable as to its carcinogenicity to humans506
Group 4Probably not carcinogenic to humans1

Two of the compounds (tetrachlorvinphos and parathion) were classified by IARC as class 2B (“possibly carcinogenic to humans”) and the remaining three compounds (malathion, diazinon, and glyphosate) were classified by IARC as class 2A (“probably carcinogenic to humans”). One of these chemicals (glyphosate) is a primary ingredient in the widely used weed killer Roundup. So IARC’s determination that it is “probably carcinogenic to humans” is shocking to say the least.

But is IARC’s classification system meaningful?

To put the classification into some context, performing “shiftwork that involves circadian disruption” is an IARC Group 2A carcinogen. Crystalline silica, or beach sand, was given an IARC 1 classification (according to IARC beach sand is “carcinogenic to humans”). So, according to the IARC classification system, performing shiftwork and going to the beach is just as likely or more likely to cause cancer as being exposed to the ingredient in Roundup.

How much exposure? How often? Under what conditions? Which cancers?

While some of these caveats are addressed in the IARC monograph reviewing the chemical, none are addressed by the IARC classification system. Thus, the meaningfulness of boiling down complex scientific evidence into a single classification is debatable. In other words, is it really possible to take a complex set of data and boil it down to a single numerical classification system?

And then there is the issue of conflict of interest. IARC’s conflict of interest policy prohibit any scientist who has any ties to industry to participate on its working group, presumably because they have a conflict of interest. However, others with clear ties to the plaintiff bar and advocates for environmental causes are known to play a major role in the working groups and to have a heavy influence on the outcomes (Arthur Frank, Harvey Checkoway, and Lennart Hardell are three noteworthy examples).

In order to minimize bias and conflict of interest, there should be an objective protocol adhered to for each step of the process. For example, there should be a transparent and well defined systematic review process that would call for separate groups to perform each of the following aspects of the review:

  • Definitions for inclusion and exclusion
  • Data and study identification
  • Application of inclusion and exclusion criteria
  • Conduct of systematic review
  • Bias and confounding assessment
  • Conduct of meta-analysis (if deemed necessary)

Such an approach would have the maximum likelihood of providing independent and unbiased assessments.

To be fair, a number of former IARC members and other scientists have published an editorial (currently in press) defending the IARC process. And others have been critical of the process in general and as it applies to this latest determination (see here and here for some good science-based rebuttals to the premise that these latest classifications are appropriate or meaningful).

The point is that the next time you confront an IARC classification as the basis to assertions in the courtroom that a chemical or other exposure is carcinogenic, there are many unanswered questions and aspects of the process that you should be sure to address.

Ken Burns Film upcoming mini-series (based on the award-winning book by Siddhartha Mukherjee) promises to put the complexities of cancer into the proper context. The idea that it is somehow meaningful to put a single number onto the concept of cancer causation should seem incomplete in the broader context.

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