The effects of outdoor air pollution on chronic illnesses.

Chronic diseases, especially cancer, cardiovascular disease, and respiratory diseases, are the leading causes of morbidity and mortality worldwide. In Canada alone, chronic diseases account for 87% of total disability and 89% of all deaths (1). It is estimated that half of the Canadian population live currently with a chronic illness and more than 90% of them have these three diseases (1). Chronic diseases develop typically over long periods of time and have multiple risk factors. The major causes of cardiovascular disease include high cholesterol, high blood pressure, and obesity (2–6). For lung cancer and respiratory disease, tobacco smoking and exposure to toxic chemicals are important risk factors (7–15). More recently, ambient air pollution has been implicated in increasing the incidence and mortality from lung cancer and from cardio-pulmonary diseases (16–19). Present-day urban air pollution comprises hundreds of substances, including sulphur dioxide, ozone, nitrogen oxide, nitrogen dioxide, carbon monoxide, carbon dioxide, particulate matter, rubber dust, polycyclic aromatic hydrocarbons, and many different volatile organic compounds. Particles are a heterogeneous mixture of solid and liquid droplets with wide distributions of size and mass. Coarse particles, greater than 2.5 μm in median aerodynamic diameter, derive from a variety of sources including windblown dust and grinding operations; fine particles are primarily from the combustion of fossil fuels (20). Common constituents of particulates include elemental and organic carbon, sulphates, nitrates, pollen, microbial contaminants, and metals (20). Fine particles can react with sulphur dioxide and oxides of nitrogen in the atmosphere to form strong acids, such as sulphuric acid, nitric acid, hydrochloric acid, and acid aerosols (20). In addition, urban air also contains benzene and 1,3-butadiene that are considered carcinogenic (21). The health impact of outdoor air pollution became apparent during the smog episodes in London, England, (Figure 1) in the 1950s and in some other places. In London, the lethal fog, which occurred because a temperature inversion trapped heavy combustion-related emissions of particles and SO2 (traffic and coal-fired heating), resulted in approximately 3,000 more deaths than normal during the first three weeks of the smog event (Figure 2) (22). During five days of a smog episode in Donora, Pennsylvania, a small town of 14,000 residents, 20 people died and over 7,000 were hospitalized (23). These episodes demonstrated conclusively that the confluence of adverse weather conditions and extremely high levels of pollution from ambient particles and sulphur dioxide can cause immediate and dramatic increases in mortality (24;25). Figure 1: The London smog episode of 1952. A photograph during the day. Figure 2: The association between total suspended particles, sulphur dioxide, and nonaccidental deaths during 15 days in the London smog episode of 1952. The right hand vertical axis shows the number of deaths per day and the left hand axis represents concentrations ... In the subsequent decades, especially in economically developed countries, changes in fuels (e.g., low sulphur fuels), improved combustion technology, and regulations (e.g., the clean air acts in the UK, USA, and Canada) have led to significant reductions in the levels of ambient air pollution (Figure 3). Unfortunately, the situation in some other less developed countries is not encouraging. For example, Delhi, India, is subjected frequently to high levels of total suspended particulates, with an annual mean concentration well exceeding 600 μg/m3 (26). In many other parts of the world, such as Mexico City and Beijing (Figure 3), similarly high levels of total suspended particles are observed frequently (27;28). In contrast, the maximum annual level of total suspended particles in Windsor, Ontario, one of the most polluted cities in Canada, is usually below 120 μg/m3 (29). Figure 3: Photographs of air pollution in Montreal and Beijing. Panel A: a “clean” day in Montreal, August 27, 2002, mean concentration of fine particles 3 μg/m3. Panel B: a “dirty” day in Montreal, August 14, 2002, mean ...