Heavy Diesel Exhaust Linked to Lung Cancer Deaths in Miners
Exhaust from diesel-powered equipment used in underground mines may increase workers’ risk of lung cancer.
Long-awaited results from an epidemiological study of workers in nonmetal mines suggest that exposure to exhaust from diesel-powered equipment in mines is associated with an increased risk of lung cancer. Even exposure to moderate levels of diesel exhaust was associated with some increased risk of death from the disease among underground miners in the study, researchers reported March 2 in the Journal of the National Cancer Institute. (See the first analysis, second analysis, and editorial.)
The findings are from the Diesel Exhaust in Miners Study (DEMS). Launched in 1992 by researchers from NCI and the National Institute for Occupational Safety and Health (NIOSH), the study was designed to clarify the relationship between exposure to diesel engine exhaust and the risk of death from lung cancer. DEMS includes more than 12,000 miners exposed to different levels of diesel exhaust.
Based on the data, the researchers concluded that underground mine workers exposed to the highest levels of diesel exhaust were more likely to die from lung cancer than their colleagues who worked above ground, as well as those who worked underground and were exposed to lower levels of diesel exhaust.
“The results also show that the risk of death from lung cancer grew with increasing exposure to exhaust,” said one of the lead investigators, Dr. Michael Attfield, formerly of NIOSH. This phenomenon is known as an exposure-response relationship, he explained.
Many Studies, No Consensus
Diesel engine exhaust contains fine particles that enter the body through the nose and mouth and can leave deposits in the lungs. But even after some 35 prior studies investigating lung cancer risk in relation to diesel engine exhaust, the evidence was unclear.
"The results show that the risk of death from lung cancer grew with increasing exposure to exhaust."
—Dr. Michael Attfield
A majority of the studies suggested a modest, but consistent, increased risk of lung cancer associated with this exposure. Many of these studies, however, had important limitations. In some cases, researchers inferred exposure levels based on a subject’s job title, such as truck driver, rather than quantifying subjects’ actual exposures.
To overcome these limitations, Dr. Debra Silverman of NCI’s Division of Cancer Epidemiology and Genetics (DCEG), Dr. Attfield, and their colleagues designed two studies based on DEMS. The first analysis was carried out in the full cohort of 12,315 miners from eight U.S. nonmetal mines. (The analyses did not include metal mine workers because they are often exposed to radon, which could contribute to lung cancer risk and confound the effect of diesel exhaust.)
For each participant, the researchers developed a quantitative estimate of occupational exposure to diesel exhaust based on many sources of information, including mining company records and air samples collected in seven of the eight mines. In addition, they reviewed records on inventories of diesel equipment at each mine, including the year a machine came into use, and ventilation data over time.
Smoking and Diesel Exhaust: A Puzzling Interaction
One of the unexpected findings of the case-control analysis was an apparent interaction between smoking and diesel exhaust among the most heavily exposed workers. Each of these factors was attenuated in the presence of the other factor.
"This was very interesting," said Dr. Silverman, noting that the finding is the first report of this interaction and needs to be replicated in other populations.
The researchers offered a number of possible explanations for the tapering off of smokers' risk at high levels of diesel exhaust exposure. "It may be that smokers are more likely to clear diesel exhaust particulate matter from their lungs than nonsmokers," said Dr. Silverman.
The analysis of the full cohort showed statistically significant increases in the risk of lung cancer mortality among underground workers as the level of diesel exposure increased, especially in those who worked for more than 5 years.
In a second analysis, researchers conducted a nested case-control study, in which subjects who died from lung cancer in the full cohort were compared with matched control subjects from the cohort. For the 198 workers in the study who died from this disease, as well as the matched comparison subjects, the researchers collected additional data, including information about workers’ history of smoking, employment in jobs associated with a high risk for lung cancer, and nonmalignant respiratory disease. Information on smoking and other factors often came from interviews with a worker’s next of kin.
The results of the second analysis showed that workers with heavy exposure to diesel exhaust were three times more likely to die from lung cancer than workers with the lowest exposures, after taking smoking and other lung cancer risk factors into account.
DEMS is the first study to show a statistically significant exposure-response of increasing lung cancer risk associated with increasing exposure to diesel exhaust based on quantitative levels of workers’ exposure, after adjusting for confounding factors such as cigarette smoking, the authors noted.
“Diesel exhaust appeared to have a strong effect on nonsmokers, although this is based on small numbers of nonsmoking cases,” Dr. Silverman said. Among nonsmokers in the study, exposure to high levels of diesel exhaust resulted in seven times the risk of death from lung cancer, compared with exposure to low levels. (For information on smokers and diesel exhaust, see the sidebar.)
The new studies are “an important contribution to the body of evidence about diesel engine exhaust,” wrote Dr. Lesley Rushton of Imperial College London in an accompanying editorial. The findings are also timely, because the International Agency for Research on Cancer (IARC) will meet this year to review the evidence linking diesel engine exhaust to cancer. In 1989, the agency classified the exhaust as a “probable” carcinogen.
Environmental Exposures and Urban Areas
The new findings are important not just for miners, but also for the 1.4 million American workers and millions more worldwide who are exposed to diesel exhaust in the workplace and for people who live in cities with high levels of diesel exhaust, the study authors noted.
A diesel-powered machine that inserts bolts into mine roofs to strengthen them
Their results suggest that in some urban areas high air concentrations of elemental carbon, which is considered the best index of diesel exhaust, may confer an increased risk of lung cancer. Dr. Silverman and her colleagues estimated that people who live in highly polluted cities may have about the same lifetime exposure to diesel exhaust as underground miners with low exposures in their study.
Based on these reports and other results, “stringent occupational and particularly environmental standards for diesel engine exhaust should be set and compliance ensured to have an impact on health outcomes,” Dr. Rushton wrote.
“Society should endeavor to minimize exposures to diesel exhaust, and diesel engine manufacturers are already working on this,” said Dr. Attfield. Diesel engines tend to last a long time, however, so many people will likely be exposed to exhaust from older engines for years to come.
Newer, cleaner diesel engines have been developed only recently, Dr. Silverman noted. “Our study addresses the health effects of exhaust from diesel engines that are currently in use around the world—predominantly the old technology,” she added.
—Edward R. Winstead