Diesel Exhaust in Miners Study: Questions & Answers
How was the diesel study designed?
The Diesel Exhaust in Miners Study (DEMS) was designed to evaluate the risk of death associated with diesel exhaust exposure, particularly as it may relate to lung cancer. The study was carried out by the National Cancer Institute (NCI) and the National Institute for Occupational Safety and Health (NIOSH). It includes two complementary components: a retrospective cohort mortality study of 12,315 workers at eight non-metal mining facilities; and a nested case-control study of lung cancer deaths in the cohort to investigate risk of lung cancer after taking into account smoking and other lung cancer risk factors. Elevated mortality from other causes among diesel exhaust-exposed miners was examined as well. The researchers undertook an extensive effort to characterize current and historical exposures to diesel exhaust and to develop estimates of personal exposures for each worker in the cohort.
What was the main result of the study?
The researchers observed increased risk for lung cancer death with increasing levels of exposure to diesel exhaust. Risks among heavily exposed workers were about three times greater than risk among workers with low exposure to diesel exhaust.
They also reported that non-smokers with the high levels of diesel exposure were seven times more likely to die from lung cancer than non-smokers in the lowest exposure category.
The study shows risk for lung cancer death dropped off for heavy smokers in the highest diesel exhaust exposure category. Why?
Among low and moderate smokers, the risk of lung cancer increased with increasing diesel exposure. In contrast, among miners who were heavy smokers (two or more packs per day) the risk of lung cancer death for those with the highest levels of exposure to diesel exhaust was attenuated. Their risk remained elevated but tapered off with increasing levels of exposure. Despite the leveling off of risk in the heavily diesel-exposed smokers, the researchers stress that there is no safe level of smoking.
The researchers offered a number of possible explanations for the tapering off of risk at high levels of diesel exhaust exposure. First, it is possible that smokers are more likely to clear diesel exhaust particulate matter from their lungs than non-smokers. This phenomenon has been reported among coal miners who are heavy smokers. Second, carcinogens in diesel exhaust and cigarette smoke may operate in the same metabolic pathway in the body. If so, they may compete with each other, resulting in a saturation of the pathway, thus diminishing the effects of either component.
In this study, what is “lagging” exposure? Why is lagging important?
Lagging is an analytic technique that removes from consideration exposures in the years just preceding death which are likely to be too recent to be responsible for cancer. It is generally thought to take approximately 15 to 20 or more years to develop lung cancer. The strongest associations in this study were observed when exposures lagged for 15 years.
In the case-control study, information on smoking and other confounders was based on interview data that were not available in the cohort study. These interviews were mainly conducted with next of kin because the subject was deceased. How did the researchers address possible recall bias from next of kin of the cases?
In order to evaluate possible recall bias, the researchers carried out a comparison of data on possible confounders (for example, smoking history) gathered directly from living study participants and from next of kin for deceased control subjects. This comparison suggested that data from next of kin and data from directly interviewed subjects on smoking and other confounders were fairly comparable. Still, the possibility of some recall bias cannot be completely ruled out.
How were the mines selected?
The study mines were selected from all U.S. underground non-metal mining facilities with at least 50 employees. Non-metal mines were used to avoid confounding from radon in particular. The mines that were eligible for the study had high ambient levels of diesel exhaust but also low levels of other potential carcinogens associated with lung cancer risk, such as radon, silica, and asbestos. The facilities were located in Missouri (1 limestone mine), New Mexico (3 potash mines), Ohio (1 salt mine), and Wyoming (3 trona mines; trona is an ore used in making soda ash).
What is unique about this study?
According to the investigators, this is the first study based on estimates of quantitative historical exposure to diesel exhaust to yield a statistically significant, positive increase in lung cancer risk with increasing diesel exposure after taking smoking and other potential lung cancer risk factors into account.
Also, the alternate effect of heavy diesel exhaust among heavy smokers has not been previously observed (see question 3) and will need to be replicated.
How was diesel exhaust measured in this study?
Diesel exhaust is made up of many components, which makes it difficult to measure. In this study, diesel exhaust levels were quantified by measurements and estimates of one of its components, respirable elemental carbon, which is considered the best index of diesel exhaust in underground mining. Quantitative estimates of exposure were determined by piecing together information from many different sources for each of eight study mines over time. The methods for this effort were published previously in four papers in the Annals of Occupational Hygiene (Volume 54, Issue 7, October 2010) and can be found at http://annhyg.oxfordjournals.org/content/54/7.toc.
Were exposures different at the surface of a facility compared to underground?
All of the mines in the study had operations both at the surface and underground. The levels of diesel exhaust sustained by workers underground far exceeded exposures for workers at the surface.
Where should future research focus?
Research is needed to understand the mechanisms of action in diesel exhaust-induced lung cancer, and to confirm the observed interaction between diesel exhaust exposure and heavy smoking, where the effect of one was attenuated in the presence of high levels of the other. This relationship has not been previously observed (though studies of coal miners and residents in rural China, whose indoor cooking stoves run on smoky coal, have shown a similar diminution of the smoking effect). Future research will be needed to replicate this finding before drawing any conclusions about the interaction between diesel exhaust and smoking.
PUBLIC HEALTH IMPACT
Can findings from this study be generalized to other occupationally exposed groups (e.g., truck drivers, longshoremen)?
Although the investigators studied miners with very high exposure levels, workers in other settings are also exposed to diesel exhaust. These study results should be broadly applicable to other workers with similar levels of exposure to diesel exhaust.
How do exposures in the study relate to average exposures in the general population (e.g., commuters)?
The workers in this study experienced a wide range of exposure levels. In urban areas with heavy pollution from diesel exhaust, environmental exposures for the general population may be in the range of 2 to 6 micrograms (µg) of respirable elemental carbon per cubic meter (m3). If accumulated over a lifetime (e.g. 60 years), this exposure approximates the cumulative exposures experienced by underground miners with the lowest exposures in this study (60 years x 2-6 µg/m3=120-360 µg/m3). Workers with this exposure level experienced about a 50 percent increased lung cancer risk.
However, based on this study, researchers cannot estimate with certainty the risks from diesel exposure for very low levels of pollution in the general environment. The study will provide valuable data for risk assessors to extrapolate down from high to low levels of diesel exhaust exposure.
What governmental organizations set standards or recommend limits for exposure?
For the workplace
For workplace settings, there are a number of safety standards in place to limit exposure to diesel exhaust. The Mine Safety and Health Administration (MSHA) has rules to limit diesel particulate matter exposure for underground miners in both coal and metal/nonmetal mines. For other workers, the Occupational Safety and Health Administration (OSHA) is the responsible agency.
For the general public
For the general public, the Environmental Protection Agency (EPA) has put in place policies to reduce diesel pollution and help ensure cleaner air.
How will these findings impact U.S. regulations for diesel exhaust levels for underground mining?
It will be up to the regulators to determine whether the current acceptable level should be lowered. For the mining industry, the responsible agency is MSHA. MSHA’s diesel particulate final rules for non-metal miners are available online. As of May 20, 2008, a miner’s personal exposure to diesel particulate matter (DPM) in an underground non-metal mine must not exceed an average eight-hour equivalent full shift airborne concentration of 160 micrograms of total carbon per cubic meter of air (160μg/m3).
For all other occupationally-exposed workers, the responsible agency is OSHA.
How common is lung cancer?
It is estimated that there will be 226,160 new cases of lung cancer in the United States and 160,340 deaths from lung cancer (non-small cell and small cell combined) in 2012. For more information on lung cancer, visit the NCI Lung Cancer page.
Worldwide, there were an estimated 1.6 million new cases of lung cancer in 2008 and nearly 1.4 million deaths, although these estimates have some uncertainty due to variability of reporting across various continents. Figures are from IARC’s Globocan.
What is diesel exhaust?
For information on diesel emissions, please visit the National Clean Diesel Campaign (NCDC) website at the Environmental Protection Agency (EPA), or the Diesel Exhaust page at the Department of Labor’s Occupational Safety and Health Administration website.
EPA’s diesel particulate matter web page has helpful information describing diesel exhaust, how individuals may be exposed, and how to limit exposure. See the full list of contaminants on the OSHA web page on diesel exhaust.
How does diesel fuel differ from other types of petroleum products?
Refineries process crude oil into different petroleum products. For more information, visit the U.S. Energy Information Administration: http://www.eia.gov/energyexplained/index.cfm?page=oil_refining#tab2; http://www.eia.gov/energyexplained/index.cfm?page=oil_refining#tab3.
What health problems are associated with exposure to diesel exhaust?
The EPA’s Diesel Exhaust page has information on health risks from diesel exhaust.
How have regulatory agencies classified diesel exhaust?
EPA has issued a Health Assessment Document for Diesel Engine Exhaust (Final 2002), which states “the assessment concludes that long-term (i.e., chronic) inhalation exposure is likely to pose a lung cancer hazard to humans, as well as damage the lung in other ways depending on exposure”. The NIH-National Toxicology Program’s 2011 Report on Carcinogens classifies diesel exhaust as ‘reasonably anticipated to be a human carcinogen’.
Silverman DT, Samaniac CM, Lubin JH, et al. The diesel exhaust in miners study: a nested case-control study of lung cancer and diesel exhaust. J Natl Cancer Inst. March 2, 2012. doi:10.1093/jnci/djs034.
Attfield MD, Schlieff PL, Lubin JH, et al. The diesel exhaust in miners study: a cohort mortality study with emphasis on lung cancer. J Natl Cancer Inst. March 2, 2012. doi:10.1093/jnci/djs035.
To read the March 2012 press release on the Diesel Miners Study, please go to http://www.cancer.gov/newscenter/pressreleases/2012/DieselMinersPressRelease.
For more information about NCI’s Division of Cancer Epidemiology and Genetics, please visit: http://dceg.cancer.gov.
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NIOSH is the federal agency that conducts research and makes recommendations for preventing work-related injuries and illnesses. It was established under the Occupational Safety and Health Act of 1970. For further information, visit http://www.cdc.gov/niosh.