Originally, the term "smog" referred to a mixture of smoke and fog in the air. Today, "smog" describes a noxious mixture of air pollutants: including gases and fine particles —that can often be seen as a brownish-yellow or greyish-white haze.
In Canada smog is a concern in most major urban centres but, because it travels with the wind, it can affect sparsely populated areas as well. Particularly vulnerable to smog are the elderly, those with existing heart or lung disease and small children. Even healthy adults can be adversely affected by high levels of smog.
The two key components of smog are airborne particles and ground-level ozone:
These are solid or minute droplets of liquid that are small enough to remain suspended in the air. Particles give smog its colour, which may be brown, dark gray or white, depending on the type of particles. Small particles, less than 10 micrometres, have a significant effect on human health, particularly for those who already suffer from heart or lung disease. Of particular concern are the fine particles, less than 2.5 micrometres, that can penetrate deep within the lungs. Fine particles may remain suspended in the air for days or even weeks and may be transported long distances.
Particulate matter is not limited to urban areas, it remains a problem in every region of Canada all year round.
Recent studies have shown that every major Canadian urban centre has levels of ambient particles high enough to pose a health risk.
The major environmental problem of airborne particles, besides its health impact on humans, is reduced visibility.
Ozone is a colorless and highly irritating gas that forms naturally when sunlight "cooks" the soup of air pollutants often found over urban areas on hot summer days. The precursor air pollutants, nitrogen oxides (NOx) and volatile organic compounds (VOC), react with each other to produce ground-level ozone.
Unlike the ozone that forms naturally in the stratosphere, ground-level ozone does not provide any significant protection from the sun's harmful UV rays, nor does it find its way to the upper atmosphere.
Most Canadians in urban areas live where ground-level ozone may reach unacceptable levels during the summer months. Periods of high ozone can last several days and frequently occur when a stagnant air mass traps pollutants over a region.
Recent studies have shown that every major Canadian urban centre has levels of
ground-level ozone high enough to pose a health risk.
Not only is ozone a problem for humans, it is also known to damage vegetation and cause the deterioration of some natural and synthetic materials, including paints and dyes. Ozone is also a powerful greenhouse gas, which contributes to climate change.
This fact sheet is part of a series on air issues.
Other fact sheets in this series include:
Smog, Acid Rain, Air Toxics (Fall 1999)
Cette publication est aussi disponible en français.
Minister of Public Works and Government Services Canada, 1999.
The Green LaneTM,
Environment Canada's World Wide Web Site.