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 and solvents (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 gasoline 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.
For more information about ground-level ozone, view the section on Ground-Level Ozone Maps.