Environment Canada and the provincial Ministry of the Environment issued the advisory early Saturday afternoon. It includes the GTA and stretches as far east as Kingston.
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 extreme heat alert that was issued on Friday remains in place.
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," the city says.