"During an extreme heat alert, 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 OK," said a release from the city.
People with breathing problems are at the greatest risk.
The temperature hit 36 C on Wednesday afternoon, and is forecast to be in the mid-30s through Friday. The humidex made it feel like 44 on Wednesday, according to Environment Canada.
The extreme heat alert also means city cooling centres are open for people who need them — and pool hours will be extended at many city-run swimming pools during the heat wave.
The Toronto Transit Commission also warns that subway trains will slow to a maximum speed of 40 km/h in open areas due to the heat. Streetcar service is unaffected.
Environment Canada says cooler temperatures should arrive by Saturday, though they will still be in the high 20s.