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NEW YORK (Reuters Health) - Miners, railway workers and others with years of on-the-job exposure to diesel exhaust may have a heightened risk of developing lung cancer, a new research analysis suggests.
The study, which combined the results from 11 previous studies in Europe and Canada, found that workers with the greatest lifetime exposure to diesel exhaust had a 31 percent higher risk of lung cancer than people with no such occupational exposure.
The findings, reported in the American Journal of Respiratory and Critical Care Medicine, do not themselves prove that diesel exhaust caused the excess lung cancer cases. And the researchers describe the association as "small."
However, the results add to evidence that occupational exposure to diesel exhaust is a hazard to lung health.
Diesel exhaust contains very fine particles that can be inhaled deeply into the lungs, and it's believed that chronic exposure can lead to or worsen lung diseases like asthma.
Diesel exhaust is also considered a probable human carcinogen by health authorities including the U.S. Environmental Protection Agency (EPA) and the International Agency for Research on Cancer (IARC), a part of the World Health Organization.
While a number of studies have pointed to an association between on-the-job exposure to diesel exhaust and lung cancer, most have been undermined by limitations -- such as a failure to account for smoking or other factors that could explain the link.
For the new study, researchers led by Dr. Kurt Straif, of the Lyon, France-based IARC, combined the results of the 11 studies in order to have a large sample of workers with and without diesel exhaust exposure.
All of the included reports were case-control studies, in which researchers compared a group of lung cancer patients with a group of healthy individuals, asking them about past work exposures to diesel exhaust as well as additional potential carcinogens, such as asbestos and other mineral particles, and lifetime smoking habits.
Straif's team considered certain workers -- including miners, railway and road-vehicle loaders, and diesel-engine and farm-equipment mechanics -- as having heavy exposure to diesel fumes. Certain other workers, such as truck drivers, railway engineers and farmers, were deemed to have relatively lower diesel-exhaust exposure.
The researchers divided the study participants into five groups. One group consisted of men and women with a job history that would have exposed them to little to no diesel exhaust; the rest were separated into four groups based on their cumulative diesel-exhaust exposure, determined by each worker's particular job and the number of years spent in that occupation.
Of the 13,300 lung cancer patients in the analysis, 12 percent had a job history that put them in the top 25 percent for diesel-exhaust exposure. The corresponding figure was 9 percent among the nearly 16,300 individuals in the control group.
When Straif's team considered factors such as participants' age, sex and reported smoking habits, they found that those in the group with the highest cumulative exposure to diesel exhaust were 31 percent more likely to develop lung cancer than those with no such on-the-job exposure.
In subjects whose jobs involved higher intensity exposure, such as mining, the excess lung cancer risk began to become apparent with as few as 10 years on the job, according to the researchers. In jobs with less intense exposure, such as truck driving, the risk was seen only after 30 years or more.
Overall, Straif told Reuters Health in an e-mail, the magnitude of the lung-cancer risk associated with diesel exhaust was on par with the risks linked to habitual exposure to second-hand cigarette smoke and indoor radon.
The study has a number of key limitations, one being a lack of data on the subjects' actual exposure to diesel exhaust. Moreover, case-control studies can offer only limited evidence of an association between two variables (in this case, diesel-exhaust exposure and lung cancer risk). And they cannot establish the extent to which diesel exhaust might affect any one worker's absolute risk of developing lung cancer.
Studies that follow a population of initially healthy people over time offer stronger evidence of whether a particular exposure is related to a disease risk.
Straif noted that results from a long-term U.S. government study of diesel-exhaust exposure and lung cancer among miners are expected to be released soon.
He said that study "will be very influential for the overall assessment of the carcinogenicity of diesel motor exhaust."
Another open question is how relevant the current findings are to workers with on-the-job exposure to diesel exhaust only in recent years. Some people in the 11 studies had work exposures dating as far back as the 1920s.
It is difficult to say how exposures among today's workers would compare with those of workers in these studies, Straif said.
Diesel engines have become "cleaner" in the past 20 years, with innovations such as "ultra-low" sulfur fuel. And in jobs such as mining, measures like modern ventilation systems and use of respiratory protective equipment have been adopted.
However, Straif and his colleagues note, workers may still be exposed to a high number of diesel-exhaust particles and the potential consequences for their cancer risk are not yet clear.
SOURCE: link.reuters.com/ref64q American Journal of Respiratory and Critical Care Medicine, online October 29, 2010.