Évaluation scientifique du smog au Canada : faits saillants et messages clés : chapitre 11
Epidemiologic evidence has been published in recent years confirming earlier observations of significant harm from particulate matter (PM), especially, but not confined to, the fine fraction. Of note is the confirmation of mortality from long-term exposure to PM, and the linkage to adverse cardiac outcomes, both from acute and chronic exposures. Additionally, the finding of a robust relationship with lung cancer mortality in the most influential chronic exposure cohort study (the American Cancer Society study) has added to the understanding of specific adverse outcomes associated with fine PM. The emergence of animal toxicology studies using ambient particles has provided evidence of a broad range of mechanisms and toxicological pathways by which adverse outcomes could manifest themselves. These studies also implicate the importance of the source of PM (and hence the chemical composition of PM) in these outcomes. Overall, the database provides sufficient evidence to conclude that there are causal relationships between PM and a range of adverse effects including restricted activity days, respiratory symptoms, bronchitis (both acute and chronic), asthma exacerbation, as well a range of respiratory and especially cardiac impacts which result in increased emergency room visits, hospital admission and premature mortality.
Significant uncertainty remains as to the relative toxicity of sources and components of PM, the role of co-occurring pollutants for some endpoints, the importance of reproductive outcomes seen in an increasing number of studies, and the role of PM in eliciting effects beyond the cardio-respiratory systems. The lack of significant insights from studies with human volunteers in controlled exposure situations (i.e., clinical settings) limits understanding of some important mechanistic pathways.
The database on O3health effects confirms its role in mortality (especially acute exposure-related) and a variety of morbidity effects. The major effect is on respiratory outcomes, though there are indications of possible adverse effects beyond the lung. The most recent evidence on the health effects of ambient ozone is consistent with that reported in earlier evaluations, and provides additional details on outcomes, modifying factors, and the presence of significant susceptible groups in the population.
Evidence is very strong for an effect of O3 on a range of adverse respiratory outcomes including lung function, respiratory symptoms, inflammation, and immunological defenses. Overall, this evidence lends considerable support to the epidemiological literature, which has reported significant associations with respiratory emergency room and hospital visits (especially asthma-related), and premature mortality associated with short-term exposure to ozone.
Observations from the PM and O3 literature are sufficient to conclude that certain groups within the population can be characterized as particularly susceptible to adverse effects following exposure to these pollutants. Susceptible sub-populations include children (both healthy and asthmatic), the elderly (especially those with a pre-existing respiratory or cardiac condition), individuals who hyper-respond to respiratory irritants, and those who are more active outdoors (e.g., exercising individuals, outdoor workers). There is also an increasing body of evidence to support susceptibility related to the specific genetic makeup of the individual.
Recent evidence also confirms that exposure to PM and O3 can have negative effects on plant health which can lead to ecosystem changes. Plants take up O3 via leaf stomates which can cause direct physical damage, while PM effects are largely due to changes in soil chemistry rather than direct deposition. Vegetation response to these pollutants is dependent upon the composition of PM, plant species and development stages (in the case of O3), and can be modified when combined with other ecosystem stressors. Impacts from the exposure to PM and O3 on wildlife is an area of emerging concern in understanding ecosystem impacts; however, no research has been conducted to asses direct impacts or to identify which species are more sensitive.
Smog has wide-ranging negative impacts on our social and economic wellbeing. Although most of the focus has been on the quantification of health-related socio-economic impacts (e.g., medical treatment costs and lost worker productivity), non-health related socio-economic impacts (e.g., decline in crop and tree output from farm and forestry operations, reduced enjoyment of vistas from poor visibility, and costs associated with material breakdown) are also significant.
Recent monitoring observations and analyses show that southern Ontario and southern Quebec continue to record the highest fine PM2.5 and O3 levels in Canada, according to the Canada-wide Standards metrics. Almost all sites over the Canada-wide Standards 2010 targets are located in southwestern Ontario and in large urban centres and communities influenced by local industries in eastern Ontario and southern Quebec, although PM2.5 levels are closer to the target than is the case for O3. Southern Ontario and southern Quebec also have the highest number of days and frequency of episodes with levels greater than the Canada-wide Standards numeric targets for PM2.5 and O3(30 µg m-3 and 65 ppb, respectively). These conclusions are based on studies and data covering the period 2001-2006.
As discussed throughout this summary document, the spatial patterns of PM2.5 and O3 across the country vary regionally and seasonally. Local ambient levels of PM2.5 and O3 are influenced by a variety of factors that can lead to substantial differences in ambient concentrations from one region of the country to another and over time. These include meteorology, coastal and urban effects, the local chemical mixture, natural emissions, transboundary transport and background concentrations. High PM2.5levels occur both in the summer and winter, with the cold season receiving more attention recently due to the significant role of local emission sources leading to the build up of primary pollutants (i.e., primary PM2.5). High O3 levels occur in the spring or summer due to enhanced photochemical production, precursor emissions and favourable meteorological conditions.
Air quality observations show that between 1985 and 2006, ambient PM2.5 levels at urban sites across Canada declined by approximately 40%; though most of the decline occurred before 1996 as a result of SO2 emission controls in eastern U.S. and Canada. No long-term data are available to examine regional-scale changes in ambient PM2.5; however, since 2001, emission reductions of sulphur dioxide (SO2) and nitrogen oxides (NOX) in eastern Canada and the northeastern U.S. have resulted in declines in PM2.5constituents (namely ammonium sulphate and ammonium nitrate) and ambient SO2 and nitric acid (HNO3) precursor gases. In light of decreasing SO2 and NOX emissions in eastern North America, the influence of ammonia (NH3) on PM2.5levels is expected to increase.
Changes in O3 levels have been observed at the regional and local scale. At urban sites across Canada, median and lower percentile ambient 8-hr O3 levels increased between 1990-2006; attributed to less O3 scavenging (the removal of O3 from the atmosphere as it reacts with NO to form NO2) as a result of NOXemission reductions. Outside of these unique urban circumstances, rural sites in Ontario and Quebec experienced a decreasing trend from 1990-2006 across all percentiles in the distribution of O3concentrations. Regionally, ambient O3concentrations have been declining in Quebec and Ontario, remaining constant in Alberta, and increasing in Atlantic Canada and on the Pacific coast.
Monitoring and modelling studies show that transboundary transport of pollutants occurs all across Canada but has a greater impact on air quality over southern Ontario, Quebec and Atlantic Canada. In spite of the strong influence of emissions from the U.S., local emissions and local formation of O3 and PM2.5 in some areas of eastern Canada play a significant role in degrading air quality to levels close to or above the Canada-wide Standards.Top of Page
Baseline O3, the level measured at a given site in the absence of strong local influence is increasing in a number of areas in Canada (i.e., Georgia Basin in the Pacific coast, Atlantic coast, and continental western Canada). Changes in baseline O3 are consistent with evidence of increasing intercontinental transport of O3 into North America.
Air quality models project that with the implementation of existing North American legislation, PM2.5 and O3 levels are expected to decrease by 2015 (relative to 2002) across the country and regionally. Exceptions include large urban areas where emissions of primary PM2.5 are projected to increase and changes in the atmospheric chemical regime may lead to increases in PM2.5 formation, and parts of the Prairies where industrial emissions are increasing. The number of days exceeding the Canada-wide Standard numeric value for PM2.5is projected to increase in areas already showing exceedances, while the number of days exceeding the Canada-wide Standard numeric value for O3 are projected to decrease overall. Model simulations of additional reductions in industrial emissions, if new Canadian legislation is implemented, show widespread improvements in summertime PM2.5 and O3 levels in the Prairies but only marginal improvements in other parts of the country.
National emissions of primary PM2.5, and PM2.5 and O3 precursors have generally declined from 1985-2006, with the exception of NH3. Some of this improvement has been offset in recent years by increases in some source sectors. Emissions projections for the year 2015 show expected increases relative to 2006, namely for VOCs and NH3.
The state of knowledge of smog science in Canada has greatly improved over the last 10 years and is continually evolving. Emerging areas of research include the impact of climate change on smog and the associated ecosystem and human health effects, the role of intercontinental transport (e.g., across the Pacific from Asia) on Canadian air quality, and the use of satellite-based monitoring. Modelling studies indicate that climate change may result in increases in both PM2.5 and O3; however, further investigation is needed to understand the complex interactions between air quality and climate change and to quantify the overall impact and regional variation. The role of intercontinental transport of smog-forming pollutants on Canadian air quality is an issue needing attention, especially with changing domestic precursor emissions. The use of satellite-based measurements to study air pollution is expected to grow, particularly in areas of the country where ground-level monitoring is limited.
The information contained within the Canadian Smog Science Assessment and synthesized in this document reflects the considerable amount of research, monitoring and modelling information available across Canada. It is also clear that large gaps still remain in our understanding of smog and its impacts on Canadians and their environment. Further advancements are needed to address these gaps and to improve our ability to track changes resulting from air quality management strategies (see the sections “Knowledge Gaps” and “Recommendations for Future Research”).
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