The Ministry of the Environment has issued a smog advisory for the Greater Toronto Area and much of the southern part of the province.
The area under the advisory includes an area that stretches from Windsor to as far east as Midland and the Niagara region.
The advisory says stagnant weather in Ontario will allow a local build-up of pollution, while light southerly winds will blow in pollution from the U.S.
This is the fourth smog advisory of the year.
During smog, the ministry encourages people to avoid strenuous outdoor activity, car idling, or the use of gasoline-powered tools.