It marks the third time this summer — and the third time in less than a month — that the city has issued an extreme heat alert.
The forecast for Friday, Saturday and Sunday calls for highs in the low 30s C, along with high UV levels.
The alert, issued by Dr. Barbara Yaffe, the city's acting medical officer of health, will be in effect in Toronto until further notice.
During an extreme heat alert the city opens cooling centres. It also suggests that anyone suffering from the heat go to a library, mall, or community centre to cool off.
"During an Extreme Heat Alert, the public is encouraged to call or visit family, friends and neighbours, especially isolated adults and seniors who are at greater risk of suffering from heat-related illness, to make sure they are cool and drinking plenty of fluids," says the news release from the city.
The public health website also provides a map of cooling centres and advice on how to beat the heat in more than 20 languages.
The smog advisory was jointly declared mid-afternoon by the Ontario government and Environment Canada.
During smog advisories, people are encouraged to take public transit or car pool, and to avoid using gas-powered motors, aerosol sprays and solvents.
The advisory says high pollution levels should ease by Saturday.