Environment Canada has issued a heat warning for the city of Toronto and most of the GTA.
The weather agency says humidex values reaching 40 are expected in much of southern Ontario, with the hot, muggy conditions expected to kick off the last week of summer vacation.
Here are some of the areas under the warning:
The heat warning will likely be dropped on Wednesday, when a cold front is forecasted to move through the region.
The heat poses risks for young children, pregnant women, older adults, people with chronic illnesses and those exercising or working outdoors.
Even if you're not in one of those at-risk groups, you're advised to seek shade and air conditioning when possible and drink plenty of water.