What is smog?

The term "smog" was first coined more than three decades ago to describe a mixture of smoke and fog in the air. Today, "smog" refers to a noxious mixture of air pollutants that can often be seen as a haze in the air.

Smog can make breathing more difficult -- even for healthy people -- and it can make us more susceptible to cardio-respiratory diseases. Even healthy young adults breathe less efficiently on days when the air is heavily polluted, especially if exercising outdoors. Particularly vulnerable to smog are people with heart or lung disease, the elderly and small children.

The two main ingredients in smog that affect our health are ground-level ozone and fine airborne particles.

Ground-level Ozone

Ground-level ozone is a colorless and highly irritating gas that forms just above the earth's surface. It is called a "secondary" pollutant because it is produced when two primary pollutants react in sunlight and stagnant air. These two primary pollutants are nitrogen oxides (NOx) and volatile organic compounds (VOC). NOx and VOC come from natural sources as well as human activities.

NOx are nitrogen-oxygen compounds that include the gases nitric oxide and nitrogen dioxide, and are produced mostly by burning fossil fuels. VOC are carbon-containing gases and vapors such as gasoline fumes (but excluding carbon dioxide, carbon monoxide, methane, and chlorofluorocarbons).

Human activities are responsible for the increases in ground-level ozone in recent years. About 95 per cent of nitrogen oxides from human activity come from the burning of coal, gas and oil in motor vehicles, homes, industries and power plants. VOC come mainly from fuel combustion and from the evaporation of liquid fuels and solvents.

Ozone not only affects human health, it can damage vegetation and decrease the productivity of some crops. It can also injure flowers and shrubs and may contribute to forest decline in some parts of Canada. Ozone can also damage synthetic materials, cause cracks in rubber, accelerate fading of dyes, and speed deterioration of some paints and coatings. As well, it damages cotton, acetate, nylon, polyester and other textiles.

Airborne Particles

Airborne particles are microscopic and remain suspended in the air for some time. Particles can be both primary pollutants and secondary pollutants, sent directly into the atmosphere in the form of windblown dust and soil, sea salt spray, pollen and spores. Secondary particles are formed through chemical reactions involving nitrogen oxides, sulfur dioxide, VOCs and ammonia.

Particles give smog its color and affect visibility. Depending on the type of particles, the air can appear yellowish-brown, or even white. Like ozone, particles are believed to have adverse effects on vegetation, and on various synthetic and natural surfaces. (See separate factsheet on Particulate Matter).

Other Pollutants in Smog

Nitrogen Dioxide (NO₂ ) is a principal member of the family of nitrogen oxides (NOx ). It is a toxic, irritating gas that results from all combustion processes.

Sulphur dioxide (SO₂) is a colorless gas that smells like burnt matches. It can be chemically transformed into acidic pollutants such as sulfuric acid and sulfates (sulfates are a major component of fine particles). The main sources of airborne SO₂ are coal-fired power generating stations and non-ferrous ore smelters. Sulfur dioxide is also the main cause of acid rain, which can damage crops, forests and whole ecosystems.

Carbon Monoxide (CO) is a colorless, odorless and tasteless gas that comes primarily from automobile emissions.