Anyone who rides a bicycle or walks in the city, whether commuting or for exercise, knows the questions: are the health benefits outweighed by sucking the fumes of the autos sharing the street? Are urban dwellers and people living close to major transportation thoroughfares at risk?
But a different question lurks behind the suppression of a decades-old study on diesel exhaust and cancer: Are workplaces taking adequate measures to protect employees exposed to diesel exhaust on their jobs?
Diesel-Cancer Link Long Suspected
Concern about lung cancers caused by diesel exhaust is old (not news). The International Agency for Research on Cancer (IARC) classifies diesel engine exhausts as "probably carcinogenic to humans," the middle classification between known human carcinogens and possible human carcinogens. The listing of "diesel engine exhaust" as known to the State of California to cause cancer (pdf) since 1 October 1990.
As early as 1988, government agencies began considering regulation of worker exposure to diesel particulates, especially in the mining industry where operation of diesel machinery in the confined spaces underground results in some of the highest exposures. The existing data was challenged.
So in 1992, the National Cancer Institute began developing a study (the Diesel Exhaust in Miners Study) intended to finally review a large enough population with enough quantitative exposure data (that means knowing how much diesel exhaust people were really breathing) to make what scientists call an exposure-response relationship. This means that it can be proven that if you breathe more diesel exhaust, you are more likely to get lung cancer. It is one gold standard of proof that the substance studied really is causing or contributing to the disease. The exposure-response relationship also allows regulating agencies to establish safe and hazardous exposure levels.
Industry Suppresses the Study DataThe Diesel Coalition of the Methane Awareness Resource Group (MARG), an organization representing the mining industry, successfully derailed publication of the study data and results for a decade and a half. In a case study published in 2006, Weight of the Evidence or Wait for the Evidence?, Celeste Monforton details MARG tactics.
MARG used every trick in the lobbying and lawyering book -- from slowing the study by lobbying the Senate to question its design to challenging the release of the data in court. Simultaneously, MARG worked to delay and diminish federal regulations to improve worker protection.
MARG's main delaying tactic in the courts relied on the argument that the mining industry should be given an opportunity to review the study data in advance of publication, a tactic that allows the spin-control machine a head start in the race to peer-review the scientific data.
Although the courts ordered release of the documents related to the NCI studies as early as June 5, 2001, it was not until a 2011 court order finally resolved the ongoing litigation that the studies have finally come to light.
What the Diesel Exhaust in Miners Study FoundThe Diesel Exhaust in Miners Study reviewed the cases of 12,315 workers at eight non-metal mining facilities,the largest population ever studied. Researchers looked at the available data from two angles: a Cohort Mortality Study with Emphasis on Lung Cancer, and a Nested Case-Control Study of Lung Cancer and Diesel Exhaust.
A cohort study looks at a large population with similar characteristics, in this case 12,315 mine workers, and compares the rates of death or disease to a control population representative of the average rates of death or disease. The Diesel Exhaust in Miners Cohort study confirmed the link between working in mines and getting lung cancer, strengthening the suspicion that diesel exhaust could be causing cancer.
The case-control study looks deeper at the population in the cohort that did suffer cancer, and compares their exposures to members of the cohort that did not have cancer. By adjusting for other factors that might cause cancer (like smoking) and by looking at the quantitative exposure (the weight of diesel particulates each study member breathed), the case-control study proved that the more diesel particulate a miner breathed, the greater their risk of cancer.
Industry will attempt to poke the study full of holes. Already comments questioning the methods used to establish the quantitative diesel exhaust exposure have been lodged. Critics point out that modern diesel engines run far cleaner than those which caused the exposures long ago (the study found around a 15-year lag between exposure and presentation of disease), and that miners are exposed at much higher levels than the general public. Questions were raised by the study itself, due to findings that workers with the highest smoking and exposure rates had a slight drop in risk, although this may be explained by mechanisms the body uses to defend itself when we try to poison ourselves so severely.
But when the dust settles, regulators will be reviewing the pace of implementation of diesel emissions controls and the residual exposures to both workers and the general population. The costs of lowering miners' exposures are estimated at $128,000 per mine, a seemingly low price to pay to prevent cancer -- and certainly a societal benefit compared to the costs of treating cancer alone, never mind the pain and suffering.
Conclusions for the General PopulationWe cannot say it better than the authors themselves:
Our findings are important not only for miners but also for the 1.4 million American workers and the 3 million European workers exposed to diesel exhaust (29), and for urban populations worldwide.
The authors list some of the locations which report high levels of elemental carbon (an indicator of exhaust particulate exposure) in the air:
- Los Angeles (4.0 μg/m3),
- the Bronx (a borough in New York City) (6.6 μg/m3)
- Nine urban sites in China (8.3 μg/m3)
- Mexico City (5.8 μg/m3), and
- Estarreja, Portugal (11.8 μg/m3).
The authors conclude:
Environmental exposure to average elemental carbon levels in the 2-6 μg/m3 range over a lifetime as would be experienced in highly polluted cities approximates cumulative exposures experienced by underground miners with low exposures in our study. Because such workers had at least a 50% increased lung cancer risk, our results suggest that the high air concentrations of elemental carbon reported in some urban areas may confer increased risk of lung cancer.