Diesel engine exhaust, produced by the combustion of diesel fuel in a compression ignition engine, is a complex mixture of gases and particulates. The composition of the exhaust depends on a number of factors including the type of engine (heavy or light duty), the type of fuel and oil, speed and load of operation, and emission control systems.
Gaseous compounds present in diesel exhaust can include carbon dioxide, water vapour, oxygen, sulphur and nitrogen compounds, carbon monoxide, and low molecular weight hydrocarbons and their derivatives.[2,3] Diesel particulate matter (DPM) can contain elemental carbon, organic compounds (including PAHs), metals, and other trace compounds. Almost all particulate emitted by diesel engines is respirable (PM <10 micron), with the majority having diameters less than 1.0 micron.
IARC classifies diesel exhaust as Group 1, carcinogenic to humans, based on sufficient evidence for lung cancer and limited evidence for bladder cancer in humans. Animal studies also showed increases in lung cancer rates from inhalation of whole engine exhaust and development of tumours from particulate either applied to the skin or administered internally. Studies of the gaseous phase (particles removed) were inadequate to determine carcinogenicity (gasoline engine exhaust was classified as Group 2B, possibly carcinogenic to humans).
Although epidemiological evidence supports a link between diesel exhaust and lung cancer, exposure assessment is complex due to difficulty separating diesel exposure from other ambient contaminants with similar characteristics.
Short term exposure to diesel engine exhaust can cause irritation of the eyes, throat, and bronchi, neuro-physiological symptoms such as light-headedness, nausea, and respiratory symptoms such as cough and phlegm. Diesel exhaust may initiate allergic reactions or increase immunological response to other allergens. Increases in hospital admission, higher incidence of respiratory symptoms, and decreases in lung function are associated with exposures to airborne particulate, including DPM. Increased mortality rates are also reported, particularly in the elderly and those with cardiopulmonary conditions. Children living in high traffic areas are at particular risk of respiratory symptoms.
Diesel fuels, produced by refining crude oils, are generally blended and often contain additives to meet specifications for performance. Recent regulations regarding fuel composition and emission standards in Canada (aligned with the US EPA) should result in lower diesel exhaust emissions across North America. The regulations require that the sulphur content in fuels sold for on-road use not exceed 15 ppm after August 31, 2006, and that fuels used for off-road, locomotive and marine engines meet the same level on a phased-in schedule, to be complete by 2012. Lower sulphur content in the fuel leads to lower emissions of particulate matter in the exhaust.
A series of regulations have been developed or are under review that outline allowable emissions for new models of on-and off-road vehicles and engines, covering applications where diesel fuels are used.[6,9]
Emission standards will be met through technological changes including catalytic converters and particle traps. Although the change to ultra-low sulphur fuels (ULSF) will decrease sulphur emissions, significant decreases in DPM and other diesel exhaust constituents will not be evident immediately, as older technology engines will remain in use until replacement is required.
Inhalation is the most common route of occupational exposure.
CAREX Canada estimates that approximately 781,000 Canadians are exposed to diesel exhaust in their workplace. The two largest industrial groups exposed are truck transportation and transit and ground passenger transportation. The two largest occupational groups exposed are truck drivers and heavy equipment operators.
Additional occupations that may be exposed to diesel exhaust from the use of on-road engines include bus and subway drivers, bus garage workers, trucking company workers, forklift operators, firefighters, lumberjacks, toll-booth and parking garage attendants, traffic controllers, car mechanics, taxi drivers, couriers, and other professional drivers.[1,2] Occupations with potential for exposure to non-road diesel engines include railroad workers, underground mine workers (using diesel powered equipment), forestry (loggers), and those in the marine industry.[1,2]
A consensus is yet to be reached for what exact substance(s) should be measured to accurately assess exposure to diesel exhaust. There is also no established occupational exposure limit for diesel exhaust. Therefore, exposure data is not consistently collected or catalogued appropriately enough for high quality exposure estimates in Canada.
The primary source of exposure to the general population is from outdoor and indoor air. CAREX Canada’s environmental estimates for diesel exhaust suggest that levels in both indoor and outdoor air contribute to elevated cancer risk among Canadians (data quality very low). Because diesel engine exhaust is not directly measured in Canada, our estimates are developed based on the assumption that 12% of total outdoor fine particles (PM2.5) are attributable to diesel engine emissions.
The three major sources of diesel exhaust are mobile sources (e.g. vehicles and other mobile sources such as ships, locomotives); stationary area sources (e.g. oil and gas production facilities, stationary engines, repair yards, shipyards); and stationary point sources (e.g. chemical manufacturing, electric utilities). Non-road sources can double the exposure to diesel exhaust in locations near point sources.
Data from a Health Canada assessment of respirable particulate matter estimated that over 80% of PM10 emitted from the transportation sector was attributable to diesel engines. Although regulation of engine performance and fuel formulation has decreased exhaust emissions, the increased numbers of vehicles and increased use of sports utility vehicles (SUVs) are continued issues of concern.
Diesel engine exhaust also contributes to the formation of smog, composed primarily of ground level ozone (formed by the reaction of nitrous oxides and volatile organic compounds, or VOCs, in sunlight) and particulate matter.[13,15]
Our team has performed a detailed scan of exposure control resources and assembled a compilation of key publications and resources. These are organized by type of exposure (environmental or occupational) and by specificity (general or carcinogen-specific). Please visit our Exposures Reduction Resources page to view.
We also recommend exploring the Prevention Policies Directory, a freely-accessible online tool offering information on policies related to cancer and chronic disease prevention. Providing summaries of the policies and direct access to the policy documents, the Directory allows users to search by carcinogen, risk factor, jurisdiction, geographical location, and document type. To learn more about policies specific to diesel engine exhaust on the Directory, click here. For questions about this resource, please contact Michelle Halligan, from the prevention team at the Canadian Partnership Against Cancer.