Health effects of outdoor air pollution

Case description

Mrs Smith calls you about her 11-year-old daughter, Judy, who has asthma that is difficult to control. Mrs Smith has heard on the news that the Air Quality Health Index (AQHI) is forecast to be 7 the next day, and Judy is scheduled to compete in an all-day athletics event. She is concerned that Judy’s asthma might be aggravated by the air pollution during a full day outdoors. She asks your advice as to whether Judy should attend the event.

What is your assessment of the risk to your patient and your subsequent advice to her mother?

Sources of information

In preparing this article, MEDLINE was searched using terms relevant to air pollution and adverse effects in general, as well as ozone, particulate matter, nitrogen dioxide, and traffic-related air pollution. We reviewed English-language articles published between January 2008 and December 2009, yielding 462 (117 relevant) articles and 66 (17 relevant) reviews. Articles related to tobacco smoke, indoor air, and occupational exposures were removed. Relevant articles published before 2008 were identified by searching the reference lists of studies found during the primary search.

Burden of illness from air pollution

Outdoor (ambient) air pollution has a substantial influence on the health of Canadians. Both short-term and long-term exposure to air pollution affect the respiratory and cardiovascular systems. Recent research has highlighted the extent of the effects of air pollution on the cardiovascular system, the complex mechanisms of these effects, and the fact that adverse health effects occur at low pollution levels, similar to those of the air that Canadians in many parts of the country breathe. In this article we discuss the epidemiologic and toxicologic evidence for these health effects and the burden of disease in Canada. We also discuss what family physicians can do to help their patients and introduce a new tool, the AQHI. The AQHI can be used to counsel patients in the family practice setting, especially those at high risk (patients with asthma, chronic obstructive pulmonary disease [COPD], cardiovascular disease, and diabetes, as well as seniors and children), to help them monitor their response to air pollution and reduce harmful exposure.

There is a meaningful burden of illness from air pollution, but because air pollution is “upstream” and cannot be measured in the individual patient, much like hypertension, it is usually not apparent as a causative agent in clinical practice. In 2004, Health Canada estimated that air pollution caused nearly 6000 premature deaths in 8 cities in Canada each year, which accounted for approximately 8% of deaths from all causes in the study population. Long-term exposure accounted for more than 70% of these deaths.¹ The Canadian Medical Association recently extended this analysis to the entire country and estimated that approximately 21 000 deaths from all causes (excluding accidents and violence) could be attributed to air pollution in 2008 (approximately 10% owing to short-term exposure), together with 11 000 cardiac- and respiratory-related hospital admissions and 92 000 emergency department visits. National economic damages (including lost productivity, health care costs, pain and suffering, and loss of life) have been estimated to be $8 billion.²

Health effects of air pollution

An extensive body of evidence links exposure to outdoor air pollution and health effects. Evidence is derived from epidemiologic studies (mostly time series studies and cohort studies), toxicologic studies of animals, and controlled human exposure studies (chamber studies); therefore, most studies provide level 2 evidence. The health effects considered are measured by short-term exposure to pollutants of hours, days, or weeks and long-term exposure of months or years (Table 1^3–43).

The effects of short-term exposure include exacerbation of pre-existing respiratory disease (especially asthma and COPD) and pre-existing cardiovascular disease (including ischemia, arrhythmias, and cardiac failure), with increased hospitalization and emergency department visits. Long-term exposure to air pollution is associated with increased mortality, increased incidence of lung cancer and pneumonia, and development of atherosclerosis.

Although it was previously understood that air pollution led only to exacerbation of asthma, there is now evidence from cohort studies that long-term exposure to air pollution might lead to development of new asthma and might delay development of the lungs.

The health effects of traffic-related air pollution, comprising a mixture of pollutants, is an area of recent research interest.³⁰ A study in Toronto, Ont, showed that exposure to traffic-related air pollution at time of birth was associated with increased current asthma in school-aged children.³⁹ Wood smoke, a common air pollutant in Canada from forest fires and residential wood burning for heating, is also associated with negative health effects.⁴⁴

Mechanisms of action of air pollution

Individuals vary in their response to different air pollutants. Some genetic polymorphisms contribute to increased susceptibility.^45–47 At the population level, it is believed that there is no threshold for the health effects of air pollution, so that even the relatively low levels of pollution commonly found in Canada have implications for health.

Effects on the respiratory system include pulmonary inflammation, airway obstruction, and increased susceptibility to infection and sensitivity to allergens. Cardiovascular effects associated with short-term exposure include changes in heart rate variability, blood pressure, vascular tone, and blood coagulability, while long-term exposure might accelerate the progression of atherosclerosis. Many of these effects are mediated through proinflammatory pathways and the generation of reactive oxygen species.^6,48

Sources of air pollution

Air pollutants are usually present in the air as a mixture, or “soup,” of constituent gases and particles. The main pollutants and their sources are shown in Table 2. In general, combustion of fossil fuels for transportation, home heating and cooling, and industry are the main sources of air pollution. Pollution can be regional when large geographic areas are covered by pollution (for example, the smog [mostly ozone and particulate matter] in the Quebec City–Windsor corridor, the Fraser River Basin, and southern Atlantic Canada) or local, such as pollution from traffic, local industries, or wood smoke. Motor vehicle emissions consist mostly of carbon monoxide, nitrogen oxides, volatile organic compounds, and particulate matter (PM); levels are elevated close to busy roads, but fall off rapidly within 150 m of the road. The most common pollutants, listed in Table 2, are widely monitored.

Ground-level ozone, a colourless and odourless gas, causes inflammation of the airways. Particulate matter is classified according to size. Size is important, as smaller particles with diameters of less than 2.5 μm (PM 2.5) penetrate deeper into the lungs, reaching and affecting the alveoli, while larger particles with diameters from 2.5 to 10 μm (PM 10) are filtered out at higher levels in the airways. The smallest particles, ultrafine particles with diameters of less than 0.1 μm, can cross the alveolar membrane into the bloodstream. The chemical properties of particles are complex, varying according to their sources, and also play a part in the effects on health. Some population groups are more vulnerable than others to the effects of air pollution (Table 3⁴⁹).

Air Quality Health Index

Family physicians can counsel patients to reduce exposure to air pollution so as to reduce the amount of pollutant delivered to their lungs and other organs. Short-term reduction of exposure can be achieved by modifying activities in relation to reports of air quality conditions.

The AQHI is a new health risk communication tool that indicates the level of risk from short-term exposure to air pollution. The values are derived from monitoring the 3 pollutants (ozone, PM, and nitrogen oxides) that best represented health risk in a succession of epidemiologic time series studies conducted in Canada.⁵⁰ The AQHI has been developed by Health Canada and Environment Canada, in conjunction with a number of other partners, and is currently being rolled out across the country.

The AQHI replaces existing Air Quality Indices, with which there were several problems. The Air Quality Indices were based on a threshold below which the air quality was reported as “good,” frequently ignoring the health effects on high-risk people at low pollutant concentrations; their reports were based only on the worst offending pollutant at any given time, rather than the mix of pollutants that we are exposed to; they did not have well-developed health messages; and they were inconsistent in provinces across the country.

Conversely, the AQHI indicates the current and next day’s health risk from air pollution in a coloured scale from 1 to 10; advises how to assess vulnerability to the effects of air pollution based on known risk factors, defining at-risk groups (Table 3⁴⁹); suggests that individuals self-calibrate, learning their sensitivity to each risk level of the AQHI; and delivers a protective health message to reduce exposure, advising a reduction of strenuous outdoor activities (as exercise increases respiratory minute ventilation) according to AQHI risk level and risk group (Table 4⁵¹). An example of an AQHI forecast from June 28, 2011, is provided in Figure 1.*

The AQHI scale can easily be taught to at-risk patients, for example, in an asthma action plan or in asthma education. Clinical advice of this kind is recommended in clinical practice guidelines for management of asthma, respiratory disease, and cardiac disease.^6,52,53 Although the project is still being rolled out, the AQHI is available to most patients nationally through the Weather Network and in many areas through local newspapers and weather reports. The AQHI can also be accessed via “push technology” or social media platforms, such as Twitter or widgets. Using this technology, at-risk patients no longer have to seek out the AQHI but instead can register through the Canadian Lung Association or the Asthma Society of Canada to receive daily AQHI updates. In addition to the daily AQHI, many communities also have air quality or smog advisories to advise the population when air pollution reaches a predetermined level of concern. Figures 2* and 3* are tear sheets that we have found useful for FPs to use with their patients.

Although there is no direct experimental evidence to support the effectiveness of air quality index or advisory programs, there is indirect evidence that supports the expectation of short-term health benefits. A recent epidemiologic study in Ontario has shown that each unit increase of daily AQHI values is associated with a substantial increase in emergency and outpatient department visits for asthma, up to 2 days later.⁵⁴ Wen and colleagues showed that awareness of air pollution media alerts and health professional advice might change behaviour,⁵⁵ while Stieb et al have shown that following smog advisory advice reduces exposure to outdoor air pollution (although potentially increasing exposure to indoor source pollutants).⁵⁶ McCreanor et al demonstrated that exercising away from traffic mitigated short-term lung inflammation and reductions in lung function in patients with asthma.¹⁹ A pilot study to examine the applicability and effectiveness of the AQHI in patients with asthma is under way, and further studies are recommended. Air quality index and advisory programs that advise reducing physical activity might conflict with broader recommendations promoting physical activity⁵⁷; the benefits and risks must be appropriately balanced on a case-by-case basis.

Other interventions to protect patients from air pollution

As the greatest burden of disease related to air pollution is associated with long-term exposure, interventions that separate people from sources of pollutants over the long term are likely to be associated with the greatest health benefit. As an example, California’s school siting bill (SB 352) mandates that no new schools be built within 500 feet of a highway and prohibits school buses from idling near schools.^58,59 Similar recommendations have been made with respect to siting of day-care centres,^60,61 hospitals, and long-term care facilities.⁶² The same rationale might also be used to support advising people to avoid areas of higher air pollution, such as those with heavy traffic and increased industrial emissions.⁵⁶

Ultimately, the most effective intervention is primary prevention—the reduction in emissions of air pollutants. Active commuting (eg, biking, walking) reduces air pollution (and greenhouse gas emissions) and supports a more active lifestyle, which also has important health benefits.

Disease control is also important. Better asthma control, especially with anti-inflammatory medications, is a protective measure against the effects of air pollution.⁶³ Acute exacerbations of chronic cardiac or respiratory conditions triggered by air pollution should be managed clinically in the same manner as exacerbations triggered by other factors.

Case resolution

The forecast for the next day, when Judy’s athletics event is planned, calls for an AQHI of 7, indicating a high health risk from air pollution. Patients with ashthma are considered an at-risk group. Judy would be outdoors and exposed to air pollution for a long time, and her exposure would be further increased when running, as cardiovascular activity amplifies minute ventilation. The health message at an AQHI of 7 advises at-risk groups to “reduce or reschedule strenuous outdoor activities” (Table 4⁵¹). However, the AQHI also encourages patients to “self-calibrate”—to determine their sensitivity to the effects of air pollution on their health. If Judy has self-calibrated, she would have a better idea as to what effect an AQHI of 7 would have on her asthma. Also, better asthma control, especially with anti-inflammatory medication, is protective. Depending on what she knows about her response to air pollution from her self-calibration results, Judy might be advised to not participate and to stay indoors, or if she does participate, to optimize her controller medication and have her rescue medication with her.

Someone with COPD, cardiac failure, angina, or an arrhythmia, or a healthy person going for a run, would be advised to check AQHI levels and health messages daily and plan accordingly. With a high-risk AQHI of 7, the at-risk patient should be advised to stay indoors and the runner, although healthy and not at risk from any pre-existing disease, would be advised to reduce exposure by running at a lower intensity or a shorter distance and not running near heavy traffic. Some at-risk and healthy patients will find at self-calibration that they are sensitive to moderate-risk AQHI levels of 4, 5, or 6.

Conclusion

The AQHI in general encourages exercise in accordance with other public health guidelines except in situations of higher risk from air pollution, which occur infrequently. In those situations, people are advised to reduce strenuous outdoor activity, to move indoors, or to reschedule. They are also advised to avoid cumulative exposures by not exercising near traffic and to be aware of the risks of exercising in excessive heat.

The AQHI is a public health and clinical tool that can easily be taught to vulnerable patients, such as patients with asthma, COPD, and cardiovascular disease, by family physicians and other health professionals. Family physicians can learn more about the health effects of air pollution and about the AQHI through accredited online courses and educational brochures.^64,65