Toronto has about seven to nine smog advisory days a year. The summer smog capital of Canada is Windsor, and it averages more than 30 smog advisory days a year. During a warmer than average summer, the number of days with poor air quality can more than double. The Ontario location with the highest measured ozone levels is not a city. The effects of the Great Lakes force concentrations of pollution along shorelines, making Long Point on Lake Erie the record holder.
Air pollution comes from several different sources in Ontario.
Transportation accounts for a significant portion. In 1988, there were 2.1 million vehicles registered in the Greater Toronto Area. Industrial sources include combustion processes such as fossil fuel-fired power generation, and industrial processes for iron and steel, chemical processing, petroleum refining, pulp and paper, and base metal smelting. Residential wood stoves are also a source of pollution in Ontario. Transboundary air pollutants from the United States account for up to 50 per cent of the smog affecting Ontario.