Coined by medical doctor Harold Antoine des Voeux in 1905, “smog” originally referred to the dense, black fog associated with the burning of coal. The effects of smog had also been seen well before the Industrial Revolution, in ancient Rome, where wood-burning fires blackened many of the buildings. It is also speculated that the Chinese may have burned coal as fuel starting around 1000 BC.

These days, the visible haze that hangs over cities is mostly caused by exhaust fumes from the combustion of fossil fuels in vehicles and other anthropogenic sources concentrated in urban areas. A mixture of noxious gases known as criteria air contaminants—such as ground-level ozone and sulphur dioxide— and particulate matter, smog can severely affect human health, especially amongst children, the elderly, and those with pre-existing respiratory and heart problems.

To make matters more complicated, the sunlight that normally brings warmth and life to the world also causes primary pollutants (those emitted into the atmosphere) to chemically react, producing secondary pollutants, collectively termed photochemical smog. More specifically, it is the nitrogen oxides and volatile organic compounds emitted into the atmosphere that react with sunlight to produce ozone and secondary particulate matter. While ozone in the Earth’s stratosphere filters out most of the sun’s ultraviolet radiation, ozone at the ground level in the troposphere causes severe irritation to the lungs. In other words, people are now advised to stay indoors on warm, sunny days in order to avoid a trip to the nearest hospital.

During days when air at elevated altitudes is higher in temperature than air at lower altitudes, a temperature inversion occurs. Since cold, dense air sinks, there is no vertical mixing between the two layers, resulting in that concentrated haze of pollutants that hangs over a city. This effect is especially pronounced in cities located in low lying areas where horizontal air flow is blocked by land features. While a breeze might be able to lower the concentration of pollutants, it also transports unwanted and highly reactive pollutants to rural areas, affecting agricultural lands, water sources, and ecosystems many kilometres away from the city. As a result, areas located downwind of areas emitting primary pollutants usually have the greatest ozone concentrations

Airborne particles, or aerosols, do not simply hang around waiting for an unsuspecting human being to inhale. They may be suspended in the air for hours or days, and acidic particles, such as nitrogen oxides and sulphur dioxide, may be deposited on surfaces (termed dry deposition) or washed out of the atmosphere through various forms of precipitation (termed wet deposition). This generates acid rain, which not only destroys human architecture, but also creates havoc for ecosystems. Soil, especially in eastern Canada, does not have sufficient alkalinity to neutralize the acid that is deposited. The now acidic soil, in turn, is less able to retain minerals, causing leaching and affecting organisms that rely on these nutrients. Conversely, the high content of nitrogen present in acid rain actually promotes algal growth in aquatic ecosystems, leading to eutrophication and the depletion of oxygen. This process severely affects many organisms that reside within affected lakes and streams.

Persistent organic pollutants are, as their name suggests, difficult to break down. POPs include certain pesticides (like DDT), industrial chemicals (such as PCBs), and certain chemical by-products. Once POPs enter the food chain, they are able to bioaccumulate in animal and human tissue alike. Because of their persistence, these compounds are carried well away from their sources and many become concentrated in polar regions where the cold dense air sinks, adversely affecting local people and wildlife. POPs have been linked to problems with the reproductive system, neurobehavioral disorders, and cancer.

Surprisingly, some of the substances that are referred to as “air pollutants” today are also organic vapours generated by vegetation, smoke from naturally-occurring fires, and gases from volcanic eruptions. The problem now lies in the level of concentration at which these substances are being produced through industrial activities, and secondary pollutants produced from chemical reactions between them.

About a month ago, the Southern Ontario Centre for Atmospheric Aerosol Research officially welcomed the abundant atmospheric pollutants waiting outside its doors. Comprised of U of T faculty members from the Department of Chemical Engineering and Applied Chemistry, the Department of Chemistry, and the Faculty of Medicine, the centre aims not only to decipher the sources of atmospheric aerosols, but also their chemical and physical properties, the effects on humans and the environment, and possible improvements that can be made to reduce the severity of these effects. With the help of up-to-date technology located in the Walberg, Lash Miller, and Gage buildings on the St. George campus, these researchers are set to clear the air about smog and its effects.

For more information on what you can do about improving air quality, visit:

  • socaar.utoronto.ca
  • cleanairpartnership.org
  • cleanairalliance.org
  • ec.gc.ca