Canadian Environmental Health Atlas

Understanding Our Environment is Key to Promoting Health and Preventing Disease

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Urban Air Pollution

Urban Air Pollution

Toxins in Traffic Emissions
In Canadian communities, as in those Dockery and his colleague monitored for their Six Cities Study, air pollution is primarily from roadway traffic: cars, trucks, buses, and other motorized vehicles. Traffic-related emissions expose residents of cities to a range of toxins, including nitrogen dioxide (NO2), sulphur dioxide (SO2), and particulate matter or PM—tiny solid particles or liquid droplets. Size is often used to describe particulate matter: particles 20 to 50 microns in diameter (total suspended particles or TSP), particles less than 10 microns in diameter (PM10), and particles less than 2.5 microns in diameter (PM2.5). The smallest particles, which can easily penetrate deep in airways, are the most harmful.

Particulate Matter and Mortality
In a large national cohort study of Canadian adults, Crouse and his colleagues (2012) found that long-term exposure to fine particulate matter was associated with increased risk of death from ischemic heart disease and other nonaccidental causes.1 The study relied on census data for mortality causes from 1991 to 2001 and pollution estimates derived from satellite remote sensing observations and Environment Canada monitoring stations.

The research results showed that ground-based readings for PM2.5 levels from stations in 11 of Canada’s largest cities were highly correlated with satellite images of particle concentrations for other parts of Canada. This novel use of technology allowed the researchers to map particulate pollution in detail and study a larger population. In the end, they showed that there were health impacts with concentrations of particulate matter as low as a few micrograms per cubic metre—an important finding that leads us to ask, Is there a safe level of exposure to particulate matter?

Air Pollution in Urban Environments
As the study by Crouse demonstrates, research into air pollution over the last decade has evolved. Time series and cohort studies have provided important insights about the effects of pollutants in cities and will continue to do so. Meanwhile, we are learning that the health effects of ambient air quality vary over short distances and, crucially, this likely leads us to underestimate exposure and health impacts with large-area models. This is especially true in the dense and complex landscape of big cities, where air quality can vary from one street to the next. Today many study designs are being used to uncover different health impacts in Canada’s three largest cities—Toronto, Montreal, and Vancouver.     

A study of Toronto air quality by Buckeridge and his colleagues (2002) focused on particulate matter in 334 neighbourhoods of about 500 people (Figure 1)2. The researchers used fine particles (PM2.5) as a measure of urban air pollution. They looked at neighbourhoods with different levels of exposure to PM2.5 concentrations and compared hospital admission rates for respiratory conditions and genitourinary conditions. As expected, they found that higher exposure to fine particles did not raise admission rates for genitourinary conditions but did raise admission rates for asthma, bronchitis, chronic obstructive pulmonary disease, pneumonia, and upper respiratory tract infection by about 24%.

In another study of Toronto air quality, Buzzelli and Jerrett (Figure 2) relied on land use regression (LUR) to find variations in traffic-related nitrogen dioxide (NO2) estimates.3 Land use regression is a way for researchers to estimate the distribution of airborne pollutants using air monitors that are distributed across a community. The investigators found the highest NO2 concentrations in the downtown core (Inset 1) were near major roadways, such as Toronto’s 401 highway (Inset 2). The widest expressway corridor in North America, the 401 carries over 400,000 vehicles per day. The NO2 concentrations in some areas of Toronto exceed the US Environmental Protection Agency (EPA) National Ambient Air Quality Standards for 24-hour values. Anyone living near these heavy traffic areas is persistently exposed to high concentrations of PM2.5 and NO2—a cause for concern, since NO2 is known to irritate the lungs and worsen asthma symptoms.

In Montreal, Smargiassi and her colleagues (2006) showed that exposure to traffic emissions increased respiratory-related hospital admissions.4 Their study differed from the Buckeridge Toronto study in four important ways: (1) they used morning peak traffic intensity as a proxy for auto emissions; (2) they used a case-control approach to study hospital admission and discharge data, with cases defined as subjects with respiratory diagnoses and controls defined as subjects with non-respiratory diagnoses; (3) they focused on persons 60 years and older; and, finally, (4) they took account of subjects’ socioeconomic status (SES) as indicated by home prices. They found that higher traffic intensity was associated with an 18% higher hospital admission rate and concluded that respiratory-related hospital admissions could be attributed to air pollution exposures (as represented by the traffic intensity proxy) independent of the subjects’ socioeconomic status.

In a Vancouver study, Gan and his colleagues (2010) wanted to find out if proximity to roadways and the air pollution generated by traffic on them is a risk factor for coronary heart disease.5 They did this by mapping the residential location of 450,000 adults aged 45 to 84 years who lived in Vancouver and did not have heart disease, and then followed these subjects for several years. By comparing subjects living away from traffic with subjects living near traffic, the researchers found those living close to a highway or major road were about 29% more likely to die from coronary heart disease (Figure 3). 

Importance of Population Characteristics
Recent studies of urban air pollution illustrate the range of approaches currently in use: some studies are population-based while others focus on at-risk groups; some use pollution monitoring data while others model exposures; some use a case-control design while others follow a cohort. Whatever the approach, researchers recognize that population characteristics—including socioeconomic status, gender, age, and disease status—are often important considerations in the impact of air pollution on health. For instance, Brook and colleagues (2008) found gender differences in response to NO2 exposures among respiratory health clinic patients with diabetes mellitus in Toronto and Hamilton.6

When Clark and colleagues (2010) studied Vancouver subjects exposed in utero and as infants to local pollutants, especially those generated by traffic, they found that these exposures led to the development of asthma in children up to 4 years of age.7 When Smargiassi and colleagues (2009) used a time-series approach to study children in Montreal living close to a source of industrial sulfur emissions, they found emission spikes were associated with increased emergency room visits for asthma.8 Finally, a study by Ebelt and colleagues (2005) showed that even though patients with cardiopulmonary disease were exposed to a smaller proportion of outdoor particles than indoor particles (such as those generated by cooking and smoking), adverse health effects were associated primarily with outdoor particles.9

Lessons from Research in Canada’s “Big Three”
Studies undertaken in Canada’s “big three”—Toronto, Montreal, and Vancouver—show that the health effects of ambient air quality vary over short distances and that we have likely been underestimating exposure and health impacts with large-area models. This research also provides lessons for all communities in Canada. In each of these cities, exposure to air pollution from motor vehicles is a risk factor for heart disease, asthma, and lung cancer. There are exceptions to these findings, of course, owing to measurement problems and the diversity of groups observed for different periods and using different approaches. Nevertheless, the impact of exposure to pollutants seen in Toronto, Montreal, and Vancouver is not unique to these urban centres and shows that all communities should be concerned about air quality.