Human Health Risk Assessment for Diesel Exhaust – summary

Health Canada completed the Human Health Risk Assessment for Diesel Exhaust, a comprehensive review and analysis of the potential adverse health effects associated with diesel fuel use in Canada. The report focuses on diesel exhaust (DE) emissions from on-road and off-road vehicles (excluding rail and marine applications) and targets impacts resulting from general population exposures. The assessment includes a review of diesel fuels, engines and emissions, a review of exposure to DE, an evaluation of the health effects associated with DE exposure, as well as a quantitative analysis of the population health impacts associated with the contribution of DE to criteria air contaminant concentrations in Canada. This report does not address the health risks of diesel fuel itself, which is under review as part of the Chemicals Management Plan of the Government of Canada and will be reported elsewhere.

Internationally, the potential health effects of DE exposure have long been recognized, and great effort has resulted in substantial reductions in diesel emissions, including in Canada. A key accomplishment has been the introduction of stringent emission regulations for new diesel vehicles and engines, resulting in improved engine and emission control technologies in both the off-road and on-road diesel fleets. In addition, the quality of diesel fuel used in on-road, off-road, rail, marine and stationary engines has improved, particularly in terms of the sulphur content. Some jurisdictions have undertaken additional initiatives to mitigate in-use diesel engine emissions and human exposure to them, such as inspection and maintenance programs, retrofit and scrappage programs and idling restrictions. However, the Canadian in-use diesel fleet is still dominated by engines pre-dating the most recent emission standards.

Diesel-powered vehicles are pervasive on major roadways and in urban centres in Canada. It is reasonable to assume that most Canadians are regularly exposed to DE. Because of the variable and complex nature of DE and the fact that DE constituents are emitted by other pollution sources, it has been difficult to quantify general population exposure to DE. Several surrogates have been used to represent DE, all of which have had their limitations. The respirable fraction of elemental carbon is considered to be one of the better options used to date.

This risk assessment considered the reviews and conclusions of the California Environmental Protection Agency (1998)Footnote 1 and the United States Environmental Protection Agency (2002)Footnote 2 human health risk assessments for DE and provided detailed review of the health effects literature published since 2000. The available information supports the conclusion that DE emissions have direct effects on human health.

The newly published health studies along with supporting evidence from work published prior to 2000 provide sufficient evidence to conclude that DE is carcinogenic in humans and is specifically associated with the development of lung cancer. Although the risk estimates are generally small, the population health risks are considered to be significant given the ubiquitous presence of DE emissions in Canada. The evidence is also suggestive that DE may be implicated in the development of cancer of the bladder in humans, but further research is required to allow definitive conclusions to be drawn. A limited number of studies have investigated other cancers in association with DE exposure, but the evidence is inadequate to draw conclusions regarding causality. Overall, these conclusions are consistent with the categorization of DE as a human carcinogen (Group 1) by the International Agency for Research on Cancer.Footnote 3,Footnote 4

Regarding non-cancer health effects and the potential causal role of DE in their development, a number of conclusions are drawn from the existing literature. The evidence supports a causal relationship between acute exposure to DE at relatively high concentrations and effects on the respiratory system, including increases in airway resistance and respiratory inflammation. Under conditions of chronic exposure, DE exposure is likely to be causal in the development of respiratory effects. It was concluded that DE exposure is likely to be causal in the development of adverse cardiovascular outcomes following acute exposure and in the development of adverse immunological responses. The evidence reviewed is suggestive of a causal relationship between DE and 1) adverse cardiovascular outcomes following chronic exposure, 2) adverse reproductive and developmental effects and 3) central nervous system effects following acute exposure to DE. Currently, there is inadequate evidence to draw conclusions regarding the potential neurological impacts of chronic DE exposure.

Based on traditional risk assessment methodologies and with regard to general population exposures, a short-term exposure guidance value of 10 µg/m³ and a chronic exposure guidance value of 5 µg/m³ have been derived based on diesel exhaust particulate matter (PM) to protect against adverse effects on the respiratory system. The available evidence indicates that respiratory effects occur at lower concentrations of DE than those associated with other non-cancer adverse effects, and so these guidance values are considered protective against the non-cancer health impacts of DE exposure. However, it is recognized that there have not been adequate large scale epidemiological studies of non-cancer effects associated with either short-term or chronic DE exposure to conclusively characterize the exposure-response relationships. More research is needed to elucidate this and to evaluate the potential role of DE in the observed non-threshold population health effects of fine particulate matter (PM2.5).

In general, it has been shown that sensitive subpopulations, such as the elderly, children and asthmatics, can be at greater risk of adverse respiratory effects due to DE exposure. Exposure of the elderly and asthmatics to traffic-related DE has been shown to increase respiratory inflammation. Also, pulmonary function decrements have been demonstrated in asthmatics exposed to traffic-related DE. Furthermore, traffic-related DE exposure in children has been implicated in potential asthma development later in life. The guidance values for short-term and chronic DE exposure presented above account for the enhanced sensitivity of subgroups in the population.

Overall, it is concluded that DE is associated with significant population health impacts in Canada and efforts should continue to further reduce emissions of and human exposures to DE.

As part of this assessment, efforts were also made to quantify the population health impacts associated with the contribution of DE to criteria air contaminant concentrations in Canada. The analysis of population health impacts was conducted in a stepwise manner with the use of computer simulation tools to 1) estimate emissions from the Canadian diesel fleet, 2) estimate the impact of those emissions on ambient concentrations of criteria air contaminants across the country and 3) estimate population health impacts resulting from the incremental contribution of DE to air pollution levels. This was undertaken for calendar year 2015, and results were assessed on a national, provincial/territorial and regional basis. This analysis is complementary to the traditional risk assessment approach presented above.

The air quality scenarios modelled with A Unified Regional Air Quality Modelling System (AURAMS) and the Air Quality Benefits Assessment Tool (AQBAT) were selected in order to provide an indication of the potential air quality and health impacts associated with diesel fuel use in on-road and off-road applications in Canada. On-road and off-road diesel applications are responsible for substantial levels of pollutant emissions. Compared with other mobile sources, diesel vehicles and engines contribute significantly to nitrogen dioxide (NO2) and PM2.5 emissions, whereas gasoline mobile sources contribute the majority of carbon monoxide (CO) and volatile organic compound (VOC) emissions. Diesel source emissions are notably important in large urban areas, such as Greater Vancouver, Toronto and Montréal, where a large fraction of the Canadian population resides. Diesel emissions are also important along major trucking routes and roadways connecting major cities (e.g. Windsor-Québec corridor), as well as in agricultural and mining areas (e.g. Alberta). The characteristics of the mobile fleet and the dominating economic sectors in a particular region determine the influence of diesel emissions. The concentration of diesel emissions in specific geographic areas leads to distinct air quality impacts across Canada.

Diesel emissions are estimated to contribute significantly to ambient concentrations of NO2, PM2.5 and ground level ozone (O3). The air quality modelling results show that on-road diesel emissions contribute significantly to air pollutant concentrations in urban and economically active areas and along major transportation routes. Off-road diesel emissions, which are more widely distributed than on-road diesel emissions, affect air quality in both rural and urban areas. The combination of on-road and off-road emissions leads to greater air quality impacts in the largest Canadian urban centres, notably Greater Vancouver, Edmonton, Calgary, Winnipeg, Toronto and Montréal. Off-road diesel emissions also have a relatively large impact in less developed areas characterized by few other sources of pollutant emissions (e.g. remote mining communities).

Based on the current health impact analysis, on-road and off-road diesel emissions result in significant and substantial population health impacts and societal costs in Canada via the contribution of DE to ambient concentrations of criteria air contaminants. The modelling undertaken estimates that on-road diesel emissions are associated with 320 premature mortalities for 2015 (valued at $2.3 billion), with 65% and 35% of the estimated mortalities attributable to ambient PM2.5 and NO2, respectively. On-road and off-road diesel emissions are associated with 710 premature mortalities (valued at $5.1 billion), with 65%, 32% and 3% of the estimated mortalities being attributable to ambient PM2.5, NO2 and O3, respectively. Diesel emissions are also associated with significant numbers of acute respiratory symptom days, restricted activity days, asthma symptom days, hospital admissions, emergency room visits, child acute bronchitis episodes and adult chronic bronchitis cases across Canada. Results from the AQBAT simulations for the current assessment suggest that on-road and off-road emissions each contribute approximately equally to population health impacts. The results also indicate that both on-road and off-road diesel applications have significant health impacts in major Canadian urban centres. Diesel emissions have higher health impacts in the most populated provinces, such as British Columbia, Alberta, Ontario and Quebec, and in the most populated census divisions, which correspond to the Greater Vancouver, Calgary, Winnipeg, Toronto and Montréal areas. The greatest air quality impacts are also observed in those areas. Overall, it is concluded that efforts should continue to further reduce emissions of DE in Canada, particularly in areas with large populations.

To obtain an electronic copy of the Human Health Risk Assessment for Diesel Exhaust, please contact hc.air.sc@canada.ca. The report is also available for download.

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