Invited Perspective: Diesel Exhaust and Lung Cancer—Delayed Findings Confirmed, but Consequences Continue

The publication of new analyses of lung cancer risk from the Diesel Exhaust in Miners Study (DEMS) is worth celebrating for many reasons. These analyses, known as DEMS II, extend follow-up of the original cohort by 18 y, adding 2,700 additional deaths to the cohort and 178 new lung cancer deaths to the case–control analysis.^1,2 The finding of an elevated risk of lung cancer mortality observed in the first cohort study³ was confirmed in the updated analysis. Further, the study provides suggestive evidence that diesel exposure is associated with increased risk of death from non-Hodgkin lymphoma as well as from diseases of the respiratory and cardiovascular system including ischemic heart disease and cerebrovascular disease.¹ The updated nested case–control analyses provide additional insight into the effect of the length and time of diesel exhaust exposure on lung cancer mortality risk, confirming the earlier study’s finding that risk increases with increased exposure, followed by a plateau or decline among the workers with the heaviest exposure.^2,4

First, it is worth celebrating that publication occurred without the drama, litigation, and political maneuvering that delayed, obstructed, and threatened to derail the original DEMS project altogether.⁵ This is in stark contrast to the 2012 DEMS capstone reports that were published despite overt threats of “consequences” from an industry lawyer representing U.S. mining concerns.^6,7

It is also a moment to recognize the outstanding work of excellent federal government career scientists employed by the National Cancer Institute (NCI) and the National Institute for Occupational Safety and Health (NIOSH) who have, again, provided critically important documentation of the cancer and other health risks resulting from exposure to diesel exhaust.^1,2 Their work will assist global public health authorities to address this risk more effectively in both workplaces and the general environment.

The original DEMS publications were instrumental in moving the World Health Organization’s International Agency for Research on Cancer (IARC) to revise its 1988 categorization of diesel exhaust exposure from group 2A, probably carcinogenic to humans, to group 1, a human carcinogen.⁸ This categorization has had substantial public health implications worldwide, stimulating and informing development of improved control technologies, fuel alternatives, public notification of risk, and public health regulatory efforts to reduce risk.

In the 1970s the mining industry introduced diesel equipment into underground mines.⁹ The mixture of known carcinogens, particulate matter, and irritants in diesel exhaust concerned both NIOSH and mining regulators.¹⁰ NIOSH began research to assess the human health risk and recommended to the mining industry that the use of diesel technology in underground mining might have health implications and that “further introduction of diesel equipment into underground coal mines pending completion of these studies might result in future economic disruption should their use pose an unacceptable health risk.”¹¹ Nonetheless, the adoption of diesel technology expanded greatly in both coal and other mining, construction, transportation, and eventually personal automobiles.

In the mid-1980s, the legislatively mandated Federal Mine Health Research Advisory Committee established a diesel subcommittee to evaluate the risk to miners and recommend additional scientific investigations.⁵ By 1988, NIOSH had evaluated the occupational scientific literature on the health risk of diesel exhaust exposure and recommended that whole diesel exhaust be considered a human carcinogen.¹⁰

Not long thereafter, NIOSH began exploring the feasibility of conducting a large cohort mortality study of miners and was later joined by researchers at the NCI to conduct a high-quality study designed to put to rest questions concerning diesel exhaust risk to miners. Although the government scientists conducted pilot studies to ensure that exposures could be estimated accurately and that sufficient personnel records were available, and despite two peer reviews of the original protocol, a coalition of mining companies waged a concerted campaign in courts and Congress to slow or stop the study, resulting in well over a decade of delay before the results of the original studies were published.^5,12

Industries whose operations would be adversely affected by stronger regulation protecting diesel-exposed workers also mounted a well-funded initiative to magnify and manufacture uncertainties in the original DEMS findings and the IARC reclassification. To accomplish this, corporations in the fossil fuel, mining, and engine manufacturing industries hired product defense consultants to conduct at least three literature reviews,^13–15 three attempts to dispute the DEMS exposure estimates,^16–18 and three post hoc reanalyses of the data.^19–21 All of these predictably concluded there was too much uncertainty to definitively link diesel exhaust exposure with lung cancer.¹²

Although these commissioned efforts were appropriately considered, the DEMS publications were further put under the microscope by a special scientific committee of the Health Effects Institute (HEI), an independent organization funded by the U.S. Environmental Protection Agency and the motor vehicle industry. In 2015, the HEI review found no significant flaws in the DEMS work.²²

So, the current publication of the DEMS II work by Koutros et al. and Silverman et al. is another opportunity to celebrate the focus, grit, and constancy of purpose of this team of NIOSH and NCI investigators who did not let external machinations distract them from their goal: to produce scientifically valid results, then present these results accurately and clearly. But our celebration must be tempered by the consequences of the many delays and distractions. Epidemiological work that could have been accomplished in a few years took more than a decade, delaying the adoption of control technologies, exposing thousands or perhaps millions of workers worldwide and the general public to unnecessary hazardous exposures to diesel exhaust, likely contributing to the toll of preventable cancers for years to come. We hope these studies will stimulate a new look at strengthened protections from diesel exhaust exposure.

Refers to https://doi.org/10.1289/EHP11980 and https://doi.org/10.1289/EHP12840