Diesel engine exhaust, produced when diesel fuel combusts 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), 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.[3,4] Diesel particulate matter (DPM) may contain elemental carbon, organic compounds (including polycyclic aromatic hydrocarbons, or 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.
The International Agency for Research on Cancer (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 increases in tumour development when particulate was applied to the skin or administered internally. Studies of the gaseous phase (particles removed) were inadequate to determine carcinogenicity.
Although epidemiological evidence supports a link between diesel exhaust and lung cancer, exposure assessment is complex due to the difficulty in isolating diesel exposure from other ambient contaminants with similar characteristics.
Short term exposure to diesel engine exhaust can irritate the eyes, throat, and bronchi, and cause neuro-physiological symptoms such as light-headedness, nausea, and respiratory symptoms. 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 all associated with exposures to airborne particulate, including diesel particulate matter. 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.
No occupational exposure limits (OELs) for diesel engine exhaust were located in Canada. OELs exist for various components of diesel engine exhaust, but there is currently a regulatory gap for limiting exposure. Elemental carbon has emerged as the best surrogate for measuring diesel exhaust particulate. Several countries have proposed or adopted exposure limits for measuring elemental carbon, including Australia, Austria, and Finland. For more information about setting an OEL for diesel engine exhaust, please see the report from our Burden of Occupational Cancer Symposium, held in March 2015.
Diesel fuels, produced by refining crude oils, are generally blended and often contain additives to meet specifications for performance. Regulations regarding fuel composition and emission standards in Canada (aligned with the US Environmental Protection Agency) should result in lower diesel exhaust emissions across North America. The regulations, which came into effect on August 31, 2006, require that the sulphur content in fuels sold for on-road use not exceed 15 ppm; fuels used for off-road, locomotive, and marine engines were required to meet the same level by 2012. Lower sulphur content in the fuel leads to lower emissions of particulate matter in the exhaust.
A series of regulations outline allowable emissions for new models of on- and off-road vehicles and engines. These regulations cover applications where diesel fuels are used.[9,13] For example, the On-Road Vehicle and Engine Emission Regulation sets emission standards for particulate matter and nitrogen oxides for on-road heavy-duty truck engines from 2007 and onward. These new standards reduce emission limits by 90% and 95%, respectively, compared with previous standard levels.
Emission standards are met through technological changes including catalytic converters and particle traps. Although the change to ultra-low sulphur fuels will decrease sulphur emissions, significant decreases in diesel particulate matter and other diesel exhaust constituents will not be evident immediately. This is because older technology engines will remain in use until replacement is required.
Inhalation is the most important route of occupational exposure.
CAREX Canada estimates that approximately 897,000 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.[2,3] 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.[2,3]
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 in Canada. Therefore, exposure data is not collected consistently enough or catalogued appropriately enough to develop high quality exposure estimates for Canadian workplaces.
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 result in increased risks of cancer at a population level (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, 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 nearby locations.
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. Regulating engine performance and fuel formulation has decreased exhaust emissions, however the contributions of increased numbers of vehicles, and increased use of sports utility vehicles (SUVs) in particular, are ongoing concerns.
Diesel engine exhaust also contributes to smog, which is composed primarily of ground level ozone (formed when nitrous oxides and volatile organic compounds, or VOCs, react in sunlight) and particulate matter.
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. Click here to learn more about policies specific to diesel engine exhaust in the Directory. For questions about this resource, please contact a member of the Prevention Team at the Canadian Partnership Against Cancer at firstname.lastname@example.org.