ARCHIVED - Chronic Diseases in Canada


Volume 29 · Supplement 2 · 2010


Air contains thousands of natural and synthetic organic and inorganic chemical compounds, most of which are present at very low levels. Many of these are discussed in the section on chemicals and are also described in the glossary. Human exposure to air pollution has been ubiquitous over time because fire, a major pollutant source has been used for cooking and heating. Most air pollutants result from the combustion of fossil fuels in motor vehicles, factories, thermal power plants and home furnaces.1a-2

Air pollutants can be classified into primary or secondary pollutants. Primary pollutants are released directly into the air from specific sources such as industry or motor vehicles. Some primary pollutants can be altered by sunlight, heat or other chemicals to form secondary pollutants.

Substances that pollute air can be solids (particles and fibres), liquids (droplets) and vapours or gases. The major pollutants are particulate matter (PM) and certain gases. Nitrogen oxides (or NOx) is a group of highly reactive gases that are formed when fuel is burned at high temperatures. Volatile organic compounds (VOCs) are compounds having a high vapor and low water pressure, and are typically industrial solvents. Nitrogen oxides and VOCs are organic compounds that convert into vapour or gas without a chemical reaction, and key precursor gases which react with other gases in the presence of sunshine to form ozone. Ozone is an example of a secondary pollutant and a major component of smog which, in turn, is an important type of air pollution.3

Sources of PM include fuel combustion from automobiles, power plants, wood burning, industrial processes, and diesel powered vehicles such as buses and trucks.4 It can also be formed in the atmosphere when gaseous air pollutants undergo certain chemical reactions. PM consists of microscopic particles that vary in size and chemical makeup. Examples are asbestos, fibreglass, silica, dusts, heavy metals (e.g., mercury and lead), pollen, spores, bacteria, fungi, cotton and other fibres. The smaller the particles, the greater their potential for damage to the human respiratory tract as they are more easily inhaled and deposited in the respiratory tract. For example, particles with an aerodynamic diameter larger than 10µm are filtered out of the nose and pharynx, whereas smaller particles can reach deeper areas within the lung. Removal of particles from the upper airways is effective and occurs within hours, but clearance from the deep lung by alveolar macrophages may take days to months.5a Particles with diameter of 2.5 µm or less (PM2.5) are the focus of numerous recent studies and warrant being singled out since they also have higher concentrations of nitrates, organic compounds and transitional metals.6 A limited body of work has recently found some associations, primarily acute effects, for coarse particulate matter (PM2.5-10). More recently, several studies have evaluated the health impacts associated with ultrafine particles which consists of particles with an average aerodynamic diamater of less than 0.1 µm. Particulate matter is a combination of both direct emissions and reactions that take place in the atmosphere, as therefore, its’ composition, unlike the common pollutant gases varies considerably by region.

Commonly studied gaseous air pollutants include ozone (O3), sulphur dioxide (SO2), oxides of nitrogen (nitrogen oxide (NO) and dioxide (NO2)) and carbon monoxide (CO). Because O3 and NO2 are less soluble than other irritant gases, they can reach the deeper areas of the lung where they cause inflammation and edema respectively.5b Pollutants with a carcinogenic potential include benzo[a]pyrene, benzene, 1,3-butadiene, formaldehyde, chloroform, chromium, other metals, particulate matter, especially PM2.5, and possibly ozone.7,8

Indoor Air Pollution

Contrary to common perception, indoor air is a greater source of air pollutants to Canadians than outdoor air. However, outdoor air pollution infiltrates buildings, so the two are not mutually exclusive. Canadians spend nearly 90% of their time indoors,9 usually in airtight, well-insulated buildings where the low rate of air exchange between the outdoor and indoor environments allows the buildup of contaminants. For example, (VOCs) are present at higher concentrations indoors than outdoors.10a The importance of indoor air pollution is heightened for infants and the elderly who, on average, spend more time indoors and are typically more susceptible to harmful effects of environmental exposures. These individuals may also be more susceptible to the effects of air pollution as they may be more likely to have pre-existing disease, and children may have less developed immune systems and growing lungs.

Environmental tobacco smoke (ETS) has been a major source of indoor air pollution and adversely affects the health of both smokers and non-smokers.11a Cigarette smoke contains more than four thousand chemical compounds (including heavy metals such as lead and cadmium, pesticides and fertilizers) which are absorbed by tobacco plants from the soil.12 Nicotine and roughly half of the other chemical compounds in tobacco smoke are naturally present in the green tobacco leaves themselves. The remaining compounds are produced by chemical reactions when tobacco is cured and burned. At least 40 of the compounds present in tobacco smoke are known to cause or promote cancer.10b Persons exposed to ETS inhale tar, carbon monoxide, nicotine, polycyclic aromatic hydrocarbons (PAHs) and other harmful compounds.13

External air, biological contaminants including fungi and dusts, and combustion products are also important components of indoor air pollution.14 Radon, which has been shown to increase the risk of lung cancer, is a naturally occurring radioactive gas which is discussed in the radiation section of this monograph. Radon has been identified as the second leading cause of lung cancer, after smoking.15

Outdoor Air Pollution

As mentioned earlier, in most areas, the largest single source of outdoor air pollution, often visible as smog, is motor vehicle exhaust.10c Other major sources include industrial processes and the burning of fossil fuels to generate electricity. Smog is a mixture of ground-level ozone, particulate matter, acid aerosols, oxides of sulphur and associated sulphates, oxides of nitrogen, VOCs and carbon monoxide. NO2, an irritating dark brown gas, gives smog its characteristic yellowish-brown colour and, as a surrogate of traffic-related air pollution, it has been associated with a modestly increased lung cancer risk.16-18

There are two types of smog—photochemical and sulphurous. Photochemical smog is pollution produced by the action of sunlight on vehicle exhaust. Ozone levels, an index for this type of pollution, are highest during the summer months, with daily peaks between 12 noon and 6 p.m. Ground-level ozone concentrations sometimes exceed the current air quality standards in some areas of Canada—such as the Windsor-Quebec City corridor, the Lower Fraser Valley in British Columbia and the Southern Atlantic region.10d

Sulphurous smog, with the main ingredient SO2, arises from the combustion of sulphur-containing fossil fuels such as coal and oil. Major outdoor sources include power plants, smelters and oil refineries (>80% combined). Sulphurous smog episodes are more common in winter, possibly from the higher demand for heat and to atmospheric inversions associated with fog formation and higher levels of primary pollutants such as SO2 and soot.5c

The burning of fossil fuels is also the major source of both acid rain and greenhouse gases. SO2, released high into the atmosphere from stacks, interacts with water, sunlight and chemical ions to form a variety of acidic particles (sulphates), which are important components of both PM and acid rain.19 Carbon dioxide, along with methane and chlorofluorocarbons (other important by-products of the burning of fossil fuels), reflect radiant infrared energy back to earth that normally would escape through the atmosphere back into space.5d

Levels of different contaminants in outdoor air are influenced by factors such as population density, the degree of industrialization, local pollution emission standards, season, climate and weather patterns.10e Air pollutants are carried by winds and can travel to areas such as the Arctic that are thousands of miles from urban and industrial centres.

The following chapters deal with ETS (the major contributor to indoor PM2.5)11b and with outdoor air pollution. The chapter on outdoor air pollution highlights some of the difficulties in characterizing exposure, as well as an examination of the relationship between outdoor air pollution and cancer.


  1. ^ Godish T. Air Quality. 4th ed. Boca Raton, Florida: CRC Press LLC; 2004.
  2. ^ Canada-United States Air Quality Committee. The Canada – United States Air Quality Agreement: 2004 Progress Report. Ottawa: Environment Canada; 2004. Cat. No. En40-388/2004E.
  3. ^ Health Canada and Environment Canada. National ambient air quality objectives for ground level ozone. Science assessment document. 1999. ISBN 0-662-29011-9. Catalogue No: En42-17/7-2-1999E. Available at: air/naaqo-onqaa/ground_level_ozone_tropospherique/ summary-sommaire/index-eng.php
  4. ^ Health Canada – National ambient air quality objectives for particulate matter – Part 1 Science Assessment Document. Minister, Public Works and Government Services, 1999. ISBN 0-662-63486-1 Cat H46-2/98-220. Available at: ewh-semt/pubs/air/naaqo-onqaa/ particulate_matter_matieres_particulaires/ summary-sommaire/index-eng.php
  5. a,b,c,d Brooks S, Gochfeld M, Herzstein J, et al. Environmental medicine. St. Louis, Missouri: Mosby Year Book Inc.; 1995.
  6. ^  Pritchard RJ, Ghio AJ, Lehmann JR, Winsett DW, Tepper JS, Park P. Oxidant generation and lung injury after particulate air pollutant exposure increase with the concentrations of associated metals. Inhal Toxicol 1996;8:457-77.
  7. ^  Cohen AJ, Pope CA. Lung cancer and air pollution. Environ Health Perspect, 1995;103 (Suppl 8):219–24.
  8. ^  Shy CM. Air pollution. In: Schottenfeld D, Fraumeni JF, editors. Cancer Epidemiology and Prevention. 2nd ed. Philadelphia: W.B. Saunders Company; 1996. 407–417.
  9. ^ Leech JA, Wilby K, McMullen E, Laporte K. The Canadian Human Activity Pattern Survey: report of methods and population surveyed. Chronic Dis Can 1996;17(3-4): 118-23. Available at: http://www.phac-aspc. publicat/cdic-mcbc/17-3/d_e.html
  10. a,b,c,d,e Health Canada. Health and environment: partners for life. Ottawa: Minister of Public Works and Government Services Canada; 1997. Cat.: H49-112/1997E.
  11. a,b Health Canada. Exposure Guidelines for Residential Indoor Air Quality. Environmental Health Directorate, Health Protection Branch. Ottawa, April 1987. Cat.H46-2/90-156E.
  12. ^ U.S. Department of Health and Human Services (2005). Report on Carcinogens. 11th Edition. Research Triangle Park, NC: U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program. Available at:
  13. ^ National Cancer Institute. Smoking and Tobacco Control Monograph 10: Health Effects of Exposure to Environmental Tobacco Smoke. Bethesda, MD;1999. Available at: tcrb/monographs/10/index.html.
  14. ^ Editorial Board Respiratory Disease in Canada. Health Canada. Ottawa, Canada, 2001. Catalogue no. H35-593/2001E.
  15. ^ U.S. Department of Health and Human Services. Office of the Surgeon General. Available at: pressreleases/sg01132005.html
  16. ^ Nyberg F, Gustavsson P, Jarup L et al. Urban air pollution and lung cancer in Stockholm. Epidemiology 2000;11(5):487-495.
  17. ^ Vineis P, Hoek G, Krzyzanowski M, Vigna-Taglianti F, Veglia F, Airoldi L, Overvad K, Raaschou-Nielsen O, Clavel-Chapelon F, Linseisen J, Boeing H, Trichopoulou A, Palli D, Krogh V, Tumino R, Panico S, Bueno-De-Mesquita HB, Peeters PH, Lund E E, Agudo A, Martinez C, Dorronsoro M, Barricarte A, Cirera L, Quiros JR, Berglund G, Manjer J, Forsberg B, Day NE, Key TJ, Kaaks R, Saracci R, Riboli E. Lung cancers attributable to environmental tobacco smoke and air pollution in non-smokers in different European countries: a prospective study. Environ Health 2007 Feb 15;6:7.
  18. ^ Filleul L, Rondeau V, Vandentorren S et al. Twenty five year mortality and air pollution: results from the French PAARC survey. Occup Environ Med 2005 Jul;62(7):453-60.
  19. ^ Derwent RG and Malcolm AL. Photochemical Generation of Secondary Particles in the United Kingdom. In: Brown LM, Collings N, Harrison RM, Maynard AD, Maynard RL, editors. Ultrafine Particles in the Atmosphere. London: Imperial College Press; 2003. 103–22.
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