Tobacco companies did it.  Asbestos-peddlers did it.  Chromium users did it.  The list goes on and on.  When polluters and manufacturers of dangerous products feel threatened by scientific evidence that their pet compound is carcinogenic to humans, they will do everything money can by to avoid the “cancer-causing” label.

The latest example comes from diesel-engine manufacturers.   Their efforts come just in time for a meeting of the International Agency for Research on Cancer (IARC) scheduled for June 5-12, 2012.  IARC, an agency within the World Health Organization, is convening an expert panel to consider the body of worldwide scientific evidence on the carcinogenic risk to humans of diesel and gasoline engine exhausts.  The last time an IARC expert panel conducted such a review on these hazards was 1988.   At the time and based on evidence from experimental (laboratory animal) studies and limited evidence from human studies the group classified diesel-engine exhaust as “probably carcinogenic to humans.”  In the 20+ years since that IARC panel met, the body of evidence has grown.  Most recently, the U.S. National Cancer Institute and CDC’s National Institute for Occupational Safety and Health (NIOSH) published two long-anticipated mortality analyses of more than 12,000 underground mine workers.  Both the cohort mortality analysis and the case-control study reported an increased risk of death from lung cancer with exposure to diesel exhaust.  Those studies, combined with dozens of other epidemiological analyses (e.g., here, here, here, here, here, here, here,) provide ample scientific evidence that diesel exhaust is ripe for the cancer-causing label.

Diesel equipment manufacturers, including Navistar, Inc. and Cummins, Inc., are following the same-old play book used by tobacco, asbestos and others industries to manufacture uncertainty about a serious health risk.  This time the topic is the carcinogenicity of diesel exhaust.  They’ve hired consultants to reanalysis data assembled by the original researchers, and have published “new” papers to raise doubt about those authors’ work.  Several of these industry-funded papers are hot off-the-presses just in time for the IARC meeting.

I couldn’t help but notice that one of them—a 30-page article for the journal Regulatory Toxicology and Pharmacology cruised through the journal’s peer-review process with lightning speed.  The paper “Evaluation of carcinogenic hazards of diesel engine exhaust needs to consider revolutionary changes in diesel technology,” by Roger O. McClellan, Thomas W. Hesterberg (of Navistar) and John C. Wall (of Cummins, Inc.) was received by the journal on February 23, 2012 and published on-line on April 27, 2012.  All three authors are now listed a “observers” for the IARC meeting, and I suspect they shared copies of this paper with members of the expert panel.

The trouble with this paper and many of the others funded by entities that have an economic stake in the cancer-causing label, is it is not original research.  Instead, it is a verbose critique of other people’s work.  It sets out to expose each study’s limitations—-as if a perfect study could ever be conceived or conducted—-and cast doubt on the body of epidemiological research that shows an association between exposure to diesel exhaust and lung cancer.  The contribution of the McClellan, et. al. paper is the assertion that the emissions from today’s diesel engines are significantly different in composition than emissions from older diesel engines.  They use the acronyms TDE (traditional diesel exhaust) and NTDE (new technology diesel exhaust) to distinguish exposures pre- and post-2006.

I agree that diesel-engine technology has changed substantially over the last two decades, due largely in part to U.S. EPA regulations and global standards, which required reductions in particulate matter emissions.  The trouble is, the vast majority of diesel engines in use today  is not this “clean” diesel.  The most heavily-exposed individuals, like industrial workers, are exposed currently to exhaust from older (some very old) diesel engines.  Those older engines are the source of the diesel exposure and the health risk that the IARC expert panel is addressing this week.

Another industry-funded paper that fits this manufacturing-uncertainty category includes one published in Critical Reviews in Toxicology, a journal edited by the above-mentioned Roger O. McCellan.  The paper entitled “Lung cancer and diesel exhaust: a critical review of the occupational epidemiological literature, was written by John F. Gamble.  Dr. Gamble worked at CDC’s National Institute for Occupational Safety & Health (NIOSH) from 1977-1990, and is an independent consultant providing expert servics to CONCAWE, a European trade association of oil companies involved in refining and distribution.  This 2010 paper published is a 50-page “evaluation” of other researchers’ work.  Dr. Gamble concludes:

“…the weight of evidence suggests most (70%) studies are indeterminate because limitations are too great. The remaining 30% of studies allow a positive or negative conclusion, but the lack of consistency in results from these four studies does not outweigh the inconclusive pattern from the bulk of the data.”

Dr. Gamble adds that in order to reach a definitive conclusion about the link between traditional diesel exhuast (TDE) and lung cancer, “more studies with longer follow-up and quantitative exposure-response analyses.”  I’ve not seen any evidence, however, of the oil companies and diesel engine manufacturers opening up their pockets to fund independent epidemiological studies of diesel-exposed workers.  They seem to only have an interest in funding long-winded critiques and re-analysis of other researchers’ data.

A third paper of this ilk was also funded by the European business group CONCAWE as well as Mining Awareness Resource Group (MARG).  MARG is an interest group I wrote about in the 2006 article “Weight of the evidence or wait for the evidence,” published in the American Journal of Public Health.   For nearly two decades, mining-industry interests operating as MARG tried all the tricks in the book to obstruct a NIOSH/National Cancer Institute study on cancer risk for diesel-exposed underground miners.  The MARG- and CONCAWE-funded researchers published a 50-page follow-up in 2012 to Dr. Gamble’s 2010 “evaluation.”  It appeared in the same journal, Critical Reviews in Toxicology, with the title “Lung cancer and diesel exhaust: an updated critical review of the occupational epidemiology literature, and with two additional authors, Mark Nicolich, Paolo Boffetta.  The authors wrote frankly about their purpose for the article:

“The publication of recent meta-analyses, cohort studies, and case-control studies relating to the possible association of occupational exposures to diesel exhaust and an increased incidence of lung cancer has raised the question whether the available epidemiological evidence is different from what the International Agency for Research on Cancer (IARC) determined it to be in 1989 –’limited.'”

So for anyone reading the article, it was clear from the start that the authors were engaged in a doubt-sowing exercise.  It was no surprise therefore to read their critique which used all the phrases familiar to first-year epidemiology students: selection bias, exposure misclassification, exposure assessment uncertain, confounding, inappropriate surrogate for exposure, etc., etc., and more.  The industry-funded paper predictably concluded:

“…As detailed in our critical review, neither the results of the population and pooled analyses nor the [NCI/NIOSH] Diesel Exhaust in Miners Study (DEMS) results (which include a cohort and case-control study) are sufficient to change the conclusion that the available epidemiological data base is inadequate to support a definitive causal association between occupational exposures to diesel engine exhaust and increased risks for lung cancer.” (emphasis added)

“In sum, the recent publication of new epidemiology studies has not altered the state of the epidemiological data base to the point where the epidemiological data can be deemed sufficient to support a definitive causal association between occupational exposures to diesel engine exhaust and an increased risk of lung cancer.  To the contrary, the evidence remains ‘limited’ and inconclusive.”

The WHO’s IARC scientific panel is expected to complete its meeting in Lyon, France on June 12.  It is expected to issue its expert opinion on whether diesel exhaust is “carcinogenic to humans” (Group 1) or “probably carcinogenic to humans” (Group 2A).  I’ll be surprised if they give more weight to the industry-funded critiques and re-analyses than the most recent epidemiological research.  This original research, along with studies from decades past, consistently shows an association between workers’ exposure to diesel exhaust and increased risk of lung cancer.