Where does smog come from?

Smog in Ontario is usually the result of the migration of pollutants from the United States combining with unfavourable weather patterns. There are, however, local sources of pollution that can contribute to the severity of smog events. Ontarians can take personal actions -- such as driving better-tuned cars or driving less, car-pooling, avoiding the use of gas-powered lawn mowers, or lowering the use of air conditioning during the summertime -- to lessen the impact of smog during a smog alert. All of these "spare the air" actions will help, especially for vulnerable persons who suffer from breathing difficulties or other health concerns.

Ground-level ozone and fine particulate matter are key components of smog. Ground-level ozone is formed when nitrogen oxides and volatile organic compounds react in the presence of sunlight. Ground-level ozone is different from the ozone layer high above the earth that protects us from the sun's harmful UV rays. Fine particulate matter is also formed from chemical reactions in the atmosphere as well as through direct emissions. The formation and transport of both ozone and fine particulate matter is strongly dependent on meteorological conditions.

In Ontario, elevated concentrations of ozone are generally observed on hot, sunny days from May to September, between noon and early evening. On such days, fine particulate matter levels are often elevated but unlike ozone they can remain high throughout the day and night. Elevated levels of fine particulate matter can occur throught the entire year, especially during stagnation periods when meteorological conditions are not conducive for the dispersion of pollutants.

Significant amounts of ozone and fine particulate matter are carried into Ontario from the U.S. During periods of widespread elevated smog, it is estimated that more than 50 per cent of Ontario's ground-level ozone and fine particulate matter come from the U.S. The map above shows the area from which southern Ontario air originates during days of widespread elevated smog. Historically, most episodes of elevated smog in Ontario come from emission sources in the U.S. Midwest.