Dr. David McKeown says the alert will be in effect until further notice.
Environment Canada has forecast a high of 30 C for Toronto both Thursday and Friday.
During a heat alert, people are encouraged to keep cool and drink plenty of fluids — and to check on others at risk of suffering from heat-related illness, especially children and isolated or vulnerable adults.
Health officials also say people should avoid the sun, keep outdoor activities limited to the cooler times of day and never leave seniors, children or pets unattended in a car.
Those without air conditioning at home are urged to find public places where they can go to beat the heat, such as shopping malls, libraries or community centres.
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