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Draft Briefing Paper 06-1-95

IMPLICATIONS FOR HUMAN HEALTH

Ambient Air Quality

Summary of the Problem

The impact of ambient air quality on human health is significant at ambient levels encountered in Canada, including elevated rates of respiratory disorders arising from exposure to various air pollutants. Mortality of the type associated in the past with dramatic episodes of severe air pollution or of the type documented to be associated with respirable particulates is unlikely to be as great a problem in Canada today.

Canadian Recommendations1

  • In Canada, ambient air quality is primarily a provincial responsibility, responsing local studies, local monitoring, and provincial actions;
  • In the agreement of transboundary air pollution between U.S. and Canada, Canada will achieve a permanent national sulphur dioxide emissions cap of 3.2 million tonnes per year by 2000, and reduce nitrogen oxides from stationary sources by 100,000 tonnes by 2000.

Air Quality

Air quality is described in terms of not only the extent of air pollution at a given time but also relevant time intervals.2 Air quality is a judgment regarding the effects that we perceive and effects that act below the threshold of human perception but that may affect the ecosystem and so indirectly affect human life. Air pollution can be thought of as the emission into the air, or the production of secondary pollutants by chemical reaction as a consequence of emissions, of chemically active compounds at a rate that exceeds the capacity of natural processes to convert or dissipate them.3

Air pollution is the product of social activities, the result largely of economic activity and transportation planning. It is a mistake to think of air pollution control and the maintenance of air quality as only an issue in engineering and chemistry. Likewise, it is a social issue, involving education, the perception of accidents, economic costs, and political will.2 In recent years, increasing public interest in air quality and increasing scientific criticism of the basis of air quality standards or guidelines (in the case of Canada) have drawn more attention to the absolute levels of air pollution, not just levels relative to an arbitrary standard.2

Air pollution can be classified into six general types according to different chemical characteristics, distribution, and sources. These types include (1) reducing air pollution, (2) photochemical air pollution, (3) point-source air pollution, (4) indoor pollution, (5) acid deposition, and (6) greenhouse gases and ozone depletion. The latter three types of air pollution have been discussed in other individual briefing documents for CAPE. The third type of air pollution, i.e. point-source, is caused by industrial activities or accidents, which may release a relatively large quantity locally or a particular type of hazardous substances (often called "air toxics") into the air that are not regulated under usual air quality standards.

The first two types of air pollution, which generally occur in urban atmosphere, refer to air pollution resulting from criteria pollutants that are regulated under air quality guidelines. Reducing air pollution is produced by chemical reducing agents in the atmosphere, including sulfur dioxide (SO2) and particulates from stationary combustion sources.4 As this type of pollution has been largely controlled in developed countries, photochemical air pollution, i.e., "smog" (smoke plus fog), has emerged as a secondary problem.5-8

The formation of smog requires the energy in sunlight to drive complicated chemical reactions in the atmosphere. Oxidation of nitrogen and hydrocarbons and the catalytic effect of free hydrocarbons in the atmosphere can produce highly reactive chemicals: nitrogen, aldehydes, and ozone. Smog primarily results from emission from mobile sources, but is exacerbated by certain types of emission from stationary sources.9

There are two general approaches for monitoring air pollution: ambient air quality monitoring and surveillance of particular emission sources.10-11 The first involves detection of dilute concentrations of particulates and gases in a complex atmosphere, with concern about precision and documenting trends over time. The second is to detect the concentrations of air pollutants at the emission source, with concern about accuracy and extrapolation of daily or annual emissions. This emission surveillance is performed primarily for regulatory purposes in support of protecting ambient air quality. The other forms of measurement of air pollution, such as summary indices of air quality and "smog alerts", have been developed in recent years.12

Air Quality Standards and Guidelines

From 1930s to about 1980s, there were a series of air pollution disasters in Europe and the Americas.10,13-14 A possible risk factor arising from ambient air quality for respiratory disease led to a new field of investigation in human health effects of exposure to air pollutants. Most American air quality standards and many Canadian air quality guidelines in the early years were based on various epidemiological studies,14 such as the CHESS (Community Health and Environmental Surveillance Systems) epidemiological research program that was conducted in 1960s and 1970s.

In 1980s and 1990s, a series of studies conducted in Canada and many other countries suggested a plausible association of increasing mortality of respiratory diseases, particularly asthma, with increasing levels of air pollution.15-49 These recent developments influence the management of air quality and policy initiatives on the part of Canadian federal and provincial governments. The federal government targeted air quality in the Green Plan for a Health Environment and developed its NOx/VOCs strategy in 1990 to control emissions of oxides of nitrogen (NOx) and Volatile organic compounds (VOCs) and by doing so to control atmospheric levels of ozone.50-51

There is interplay between the federal and provincial responsibilities for air pollution control in Canada. Local and regional air quality is compared to National Air Quality Objectives or their provincial counterparts that define desirable, acceptable and only tolerable levels of exposure. Among the provinces, Alberta has demonstrated particular concern over the issues, including ambient air quality, acid deposition and hazardous air pollutants.52

There has been a great deal of interest in the concept that air quality objectives can be set based on the risk of adverse health effects, so-called "risk-based standards".2, 53 The standards can be designed to attain targets for the reduction of human diseases, principally asthma. Therefore, air quality can be managed optimally beyond a "safe" level. Another approach for establishing standards is to use indication of ecosystem damage to human exposure, that is, indicator-driven standards.2, 54

However, the alternative to a risk-based strategy of air quality regulation is an open-ended policy of continuos improvement based on the best available control technology feasible to reduce emission at the source and continuous review of standards in the light of new information.

Human Health Effects

Human health effects are well known to occur as a result of exposure to air pollutants.55-67 Air pollutants include primary pollutants (sulfur dioxide, particulate matter with a diameter 2 10 µ (PM10), lead, and carbon monoxide) and secondary pollutants (ozone, and oxides of nitrogen). Exposure to certain types of air pollutants has been found to increase mortality rates, respiratory disorders (acute and chronic illness and reactive airway diseases), and even some cancers.

Increased Mortality Rates Several studies have now provided persuasive evidences that air pollution is directly linked to mortality.24,30,33,35,43,68-70 Particulates (sulfur dioxide with particulates rather than its individual) play an important role in mortality rates.24,33,43,55,71 The mechanism of PM10 in causing excess mortality from respiratory and other causes is not clear at all. Neither show a threshold for mortality. A primary meta-analysis study indicated that airborne particle concentration of 100 µg/m3 increase in total suspended particles concentration was a significant risk factor for elevated mortality.43

Respiratory Disorders The primary and secondary pollutants can produce various types of respiratory disorders, including respiratory symptoms, acute or chronic respiratory diseases, reactive airway diseases, and impaired pulmonary functions.

Respiratory symptoms (such as cough, nose and throat irritation, and mild shortness of breath) and lung function change are the most common effects, particularly in children.72-87 These symptoms are often associate with mucosal irritation (acute or chronic bronchitis, nasal tickle, or conjunctivitis) at high level of air pollution. Asthmatic and patients with chronic obstructive pulmonary disease often experience worsening of their symptoms during air pollution episodes.

There is convincing evidence that exposure to some air pollutants (ozone and sulphates) exacerbates asthma attacks and increases the frequency of episodes in asthmatic children.28,79,88-89 The evidence for adults is weaker for these pollutants but is well established for nitrogen dioxide.88-94 However, it has not been show that exposure to these air pollutants causes asthma.57

Respiratory tract infections are probable the most sensitive effect of adverse air quality to express itself in clinical disease. Chronic obstructive lung disease has been shown to be exacerbated during heavy pollution episodes.95-96

The mechanism of these respiratory disorders is exceedingly complex, depending on types of air pollution, species of pollutants and individual susceptibility to their effects at low concentrations. Some of air pollutants may enhance airway reactivity and compromise host defenses.97-98

Cancer Cancer has always been a major theoretical concern because there are known carcinogens (such as benzene, benz[a]pyrene and 1,3-butadiene) in ambient air pollution. There is little evidence to suggest that community air pollution is a significant cause of cancer except in unusual cases of point-source emission, such as a smelter with emission of arsenic.

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