Canadian Smog Science Assessment Highlights and Key Messages: chapter 1

Introduction to Smog

Air pollution has received significant attention over the past several decades from both the scientific and the environmental risk management communities in Canada and internationally. This is especially true of smog’s two major components: particulate matter (PM) and ground-level ozone (O3). The primary reason for this attention has been the scientific literature indicating the extensive impacts exerted by these pollutants on human health. Although they can also impact the environment, these impacts are less critical at the levels of smog currently experienced in Canada. In the last decade alone, at least 4500 new primary scientific publications on the health effects of PM and O3 have been published, with 1200 of these in the 2002-2006 period covered by this review. 

The World Health Organization (WHO) periodically undertakes an analysis titled Global Health Risks. In the most recent 2009 analysis3, the WHO lists the most important quantifiable human health risk factors. While these risk factors vary considerably between developing and developed countries, urban air pollution ranks 13th in the top 20. In developed countries, urban air pollution is eighth among the top 10 risk factors, and is the most important population health risk factor that cannot be significantly reduced by changes to personal behaviour, arguably making it the most important risk factor for which the primary control mechanism is regulatory in nature.

In Canada, efforts have been made over the last decade to quantify the health burden of smog, facilitated by the evolving health effects literature. While a number of different endpoints have been associated with air pollution, mortality has received the most attention due to the availability of data for analysis and the severity of the endpoint. A recent 2008 analysis by the Canadian Medical Association4 estimated that some 21 000 deaths per year were associated with air pollution in Canada, along with tens of thousands of hospital visits, hundreds of thousands of asthma and respiratory symptom days and millions of minor illnesses and restricted-activity days.

There is evidence that all Canadians are at some risk from the effects of ongoing exposure to air pollution, especially PM. However, much of the evidence indicates a heightened level of sensitivity for those with cardiovascular or respiratory disease. This is important due to the prevalence of such diseases in the Canadian population where cardiovascular causes account for 30% of mortality and respiratory disease for 10%. Asthma, which is exacerbated by both PM and O3, has been diagnosed in at least 8% of the Canadian population over the age of 12. Diabetes, a common and increasing disease currently affecting one in ten Canadians, is also adversely affected by smog.

Not only does smog affect a broad swath of the population, but adverse health effects occur across the full range of ambient levels experienced in Canada, even very low levels. For all practical purposes, both PM and O3 need to be treated as having no safe level (i.e., on a population basis, there is no lower threshold at which there are no effects). This is likely due to the ubiquitous exposure to smog and the common nature of the illnesses and conditions affected by air pollution. One of the ramifications of this is that, while smog days are the most noticeable, the cumulative effects of smog on what are normally regarded as “good” days are significant and, in total, exceed the total effects on “smog” days. For example, a 2001 analysis for the City of Toronto5 found that over 90% of the health impacts of air pollution in that city occurred on days classified as “good” under the province’s Air Quality Index. 

While human health effects are regarded as the primary impact of concern, smog also exerts considerable adverse effects on a range of environmental factors, most notably in relation to acid deposition (or acid rain) and the effects of O3 on vegetation. Visibility is also degraded by air pollution (primarily PM) and this has an aesthetic impact, especially in areas where natural vistas are culturally and economically important.

Because of its impacts, smog pollutants have been the subject of significant attention from policy makers and risk managers for several decades. Measures to address acid rain in eastern Canada and the eastern U.S. have delivered major reductions in sulphur dioxide and nitrogen oxides (two of the precursors of PM and O3). This has also led to some improvement in levels of PM2.5 and, to a lesser extent, levels of O3. Canada-wide Standards for PM2.5 and O3 were established in June 2000 in order to reduce the risks to human health and the environment associated with these pollutants.

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Smog Facts

Smog is a general term used to describe poor air quality that is often associated with reduced visibility. Although smog is a mixture of pollutants, the Canadian Smog Science Assessment focuses on its two main components; particulate matter and ground-level ozone. Particulate matter (PM) is a complex mixture of small airborne liquid and solid particles that are characterized by their size. PM can be emitted directly (primary PM) or formed in the atmosphere (secondary PM) by reactions involving nitrogen oxides (NOX), sulphur dioxide (SO2), volatile organic compounds (VOCs), and ammonia (NH3). Ground-level ozone (O3) is a gas that forms in the atmosphere through reactions between NOX and VOCs in the presence of sunlight. 

Primary PM, and the gaseous precursors to O3 and secondary PM originate from both natural (e.g., forest fires, windblown soil, sea salt spray, volcanic dust) and anthropogenic sources (e.g., fossil fuel burning, various industrial processes, agricultural activity, road dust), both within and outside Canada.

The PM of most concern is that which can penetrate into the human lungs and is classified by size as coarse PM (between 10µm and 2.5µm in diameter), fine PM (less than 2.5µm) and ultrafine PM (less than 0.1µm). Since smaller particles penetrate deeper into the lungs, the Canadian Smog Science Assessment focuses on PM2.5 but, where appropriate, discusses results oriented to other sizes as well.

The Canada-wide Standards For PM2.5 and O3

The Canada-wide Standards for PM2.5 and O3 are numeric targets published in June 2000 by the Canadian Council of Ministers of the Environment (CCME) in order to minimize the exposure risk to human health and the environment. These targets were based on the scientific understanding of the issue at the time and represented a balance between the desire to achieve the best health and environmental protection possible in the near-term and the feasibility and costs of reducing the emissions that contribute to PM and ozone levels in ambient air. The numeric target for PM2.5 is 30 µg m-3 (24-hour average) to be achieved by 2010 based on the 98th percentile annual value averaged over three consecutive years. The numeric target for O3 is 65 parts per billion (8-hour average) to be achieved by 2010 based on the fourth highest annual value averaged over three consecutive years.

3. World Health Organization, 2009. Global health risks: mortality and burden of disease attributable to selected major risks. WHO Press, Geneva.

4. Canadian Medical Association, 2008. No breathing room: National illness costs of air pollution. CMA, Ottawa.

5. Toronto Public Health, 2001. Toronto Air Quality Index: Health Links Analysis. Toronto, Ontario.

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