Following a week-long meeting of international experts, the France-based International Agency for Research on Cancer (IARC) has classified diesel engine exhaust as “carcinogenic to humans,” based on “sufficient evidence that exposure is associated with an increased risk for lung cancer.”  This puts it in the same category as other noxious substances including arsenic and asbestos.

IARC, which is a part of the World Health Organization (WHO), first classified diesel exhaust as “probably carcinogenic to humans” back in 1988. And since 1998, an Advisory Group had been recommending the substance as a high priority for re-evaluation.

There has been mounting concern about the cancer-causing potential of diesel exhaust, particularly based on findings in epidemiological studies of workers exposed in various settings. This was re-emphasized by the publication in March 2012 of the results of a large US National Cancer Institute/National Institute for Occupational Safety and Health study of occupational exposure to such emissions in underground miners, which showed an increased risk of death from lung cancer in exposed workers.

The scientific evidence was reviewed thoroughly by a Working Group of IARC, which found “sufficient evidence” that diesel exhaust is a cause of lung cancer. It also noted a “positive association (limited evidence)” with an increased risk of bladder cancer.

The Working Group concluded that gasoline exhaust was “possibly carcinogenic” to humans, a finding unchanged from the previous evaluation in 1989.

According to IARC, large populations are exposed to diesel exhaust in everyday life, whether through their occupation or through the ambient air. People are exposed not only to motor vehicle exhausts but also to exhausts from other diesel engines, including from other modes of transport (such as diesel trains and ships) and from power generators.

In a media release, IARC said, “Given the Working Group’s rigorous, independent assessment of the science, governments and other decision-makers have a valuable evidence-base on which to consider environmental standards for diesel exhaust emissions and to continue to work with the engine and fuel manufacturers towards those goals.”

They also noted that increasing environmental concerns over the past two decades have resulted in regulatory action in North America, Europe and elsewhere with successively tighter emission standards for both diesel and gasoline engines.  For diesel engines, this required changes in the fuel such as marked decreases in sulfur content, changes in engine design to burn diesel fuel more efficiently and reductions in emissions through exhaust control technology. However, while the amount of particulates and chemicals are reduced with these changes, it is not yet clear how the changes may translate into altered health effects. More research into this question is needed.

In addition, existing fuels and vehicles without these modifications will take many years to be replaced, says IARC, particularly in less developed countries, where regulatory measures are currently also less stringent. Unfortunately, many parts of the developing world lack regulatory standards, and data on the occurrence and impact of diesel exhaust are limited.

Dr Kurt Straif, Head of the IARC Monographs Program, indicated that “The main studies that led to this conclusion were in highly exposed workers. However, we have learned from other carcinogens, such as radon, that initial studies showing a risk in heavily exposed occupational groups were followed by positive findings for the general population. Therefore actions to reduce exposures should encompass workers and the general population.”

A summary of the evaluation will appear in The Lancet Oncology as an online publication ahead of print on June 15, 2012.