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. 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, fine particulate matter and nitrogen dioxide are all pollutants that are included in the calculation of the Air Quality Health Index (AQHI). 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. All combustion in air produces nitrogen oxides, of which nitrogen dioxide is a major product. The formation and transport of both ozone and fine particulate matter is strongly dependent on meteorological conditions, as is the build-up of nitrogen dioxide.

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, and can occur throughout the entire year. The biggest contributor to nitrogen oxides (NOX) emissions, in Ontario is the transportation sector, so nitrogen dioxide is often highest in cities with heavier traffic. Stagnation periods when meteorological conditions are not conducive for the dispersion of pollutants often lead to elevated levels of fine particulate matter and nitrogen dioxide.

Map showing wind-blown Ozone coming from Mid-Western U.S.A. into Southern Ontario

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.