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Diesel engine exhaust arises from the combustion of diesel fuel in compression ignition engines. The sources of occupational exposure include motor vehicles, locomotives and diesel powered heavy equipment such as tractors and forklifts. Workers may either be exposed because they operate equipment that has a diesel engine or because their work takes them into situations where diesel engines are operating. Diesel exhaust also makes up an important part of the particulate air pollution found in cities and towns.
Emissions from diesel engines are a complex mixture of gasses, liquids and solids. IARC has identified that the carcinogenic components of diesel exhaust are probably associated with the particulate emissions and as a consequence it is common to use elemental carbon (EC), which makes up a significant fraction of these emissions, as a marker of exposure.
We have estimated that there are 3.6 million workers in the EU potentially exposed to diesel engine exhaust above background levels. The average occupational exposure level in Europe is probably around 13 μg/m3, although the levels are quite variable (geometric standard deviation of 2.7). There are very few countries that have an occupational exposure limit (OEL) for EC from diesel exhaust. In Austria, where there is a limit, it is set at 100 μg/m3; fewer than 5% of exposures in Europe exceed this figure. To have any impact on cancer risk the OEL would need to be much lower than this, perhaps less than 20 μg/m3.
Over about the last 20 years EU legislation has required engine manufacturers to reduce emissions from vehicles and, for example, the permitted particulate emissions from trucks and lorries will decreased by a factor of more than 50 times between 1992 and 2013. This emissions reduction has been driven by changes to the fuel and engine design. Despite the increase in the proportion of diesel vehicles in the EU the exposure to diesel engine exhaust has probably been decreasing over the last 20 years by about 7% per annum. However, IARC say "existing fuels and vehicles without these modifications will take many years to be replaced, particularly in less developed countries, where regulatory measures are currently also less stringent. It is notable that many parts of the developing world lack regulatory standards, and data on the occurrence and impact of diesel exhaust are limited."
The IARC Working Group also concluded that gasoline exhaust was possibly carcinogenic to humans (Group 2B), a finding unchanged from the previous evaluation in 1989.