The Diesel Exhaust in Miners Study (DEMS)

Study Background

The National Institute for Occupational Safety and Health (NIOSH) and the National Cancer Institute (NCI) completed a 20-year large scientific study in March 2012 that included 12,315 workers from eight non-metal mining facilities located in Ohio, Missouri, New Mexico, and Wyoming. The Diesel Exhaust in Miners Study (DEMS) was done to further understand whether breathing in diesel exhaust fumes could lead to lung cancer and other health outcomes.

This study consisted of three main parts: a cohort mortality study, a nested case-control study, and an exposure assessment. The cohort mortality study, which was conducted by NIOSH, selected a group of workers and followed them to determine their vital status as of December 31, 1997. Death rates for selected diseases of interest were calculated for the overall group and for certain sub-groups of workers based on job location. Those death rates were compared to general population death rates to see if they were higher, and whether they increased with increasing levels of exposure. We also compared lung cancer mortality across different exposure groups. The nested case-control study, which was conducted by NCI, focused on the cohort study workers who had died of lung cancer. Exposure to diesel exhaust in these cases was compared to that in other workers in the cohort who did not die from lung cancer. The nested case-control study controlled for lung cancer risk factors such as smoking; such data were not available for the entire cohort study. The third main part of the study was an exposure assessment, which was conducted by both agencies. This part involved large surveys of each mining facility where measurements were taken of the exposures that workers were receiving to diesel exhaust and other possible agents that may cause lung cancer such as asbestos and radon. Historical exposure levels going back to the date when the first diesel engine was used in each mine were estimated by using past exposure information and other data such as mine ventilation and information on diesel engine use. These exposure data were used in the cohort and case-control studies.

Study Mines

Before the main investigation was started, NIOSH and NCI did a large feasibility study to select mining facilities for the study. Selection of mines depended on the following: a) whether the mines had jobs with a wide range of diesel exposure, b) whether the mines had 50 or more workers, c) whether the mines had no or low levels of other agents that may cause lung cancer, d) whether the mines had reasonably complete historical personnel records, and e) whether the mines had reasonably complete records on past diesel engine usage. This feasibility study identified 10 mining facilities in different states in the country that fit these criteria.

During the main investigation, two facilities were later found to not actually meet the above criteria and were dropped from the study. The final study facilities consisted of three potash mines in New Mexico, three trona mines in Wyoming, one salt mine in Ohio, and one low-silica limestone mine in Missouri.

Study Findings

• The death rate from lung cancer was higher than expected in the study cohort as compared to that in the general population. There were 203 deaths overall and 161 were expected.
• Few other causes of death were elevated, and none were seen to be related to level of diesel exhaust exposure.
• There was a strong relationship between level of exposure to diesel exhaust and risk of lung cancer mortality. At higher exposures to diesel exhaust, the mortality rates were about 3 to 5 times greater compared to workers who had the lowest exposures.
• The relationship between lung cancer risk and diesel exhaust exposure remained after controlling for smoking and other lung cancer risk factors.

Study Limitations

Great care was taken to ensure that the data and methods were reliable and accurate. However, all studies have limitations. For example, study findings typically depend on the valid reconstruction of past exposures. In the Diesel Exhaust in Miners Study, because individual workers did not have exposures measured over the entire course of their employment, their past exposures had to be estimated. Where possible, the exposure estimates were validated by comparing them with actual data from other sources. In addition, several alternative sets of exposure estimates were analyzed and no major differences were found in the results. Having no smoking, lifestyle, or pre- or post-employment data in the DEMS – Cohort Mortality Study is a typical shortcoming for this type of study. However, the cohort findings were similar to those from the DEMS – Case-Control Study, where information on these factors was accounted for in the analysis.

Questions and Answers

Following the announcement of the DEMS publications in the Journal of the National Cancer Institute and the Annals of Occupational Hygiene, NCI posted questions and answers about the study in their press release. See Diesel Exhaust in Miners Study: Questions & Answers .