Environment Canada has issued a heat warning for Innisfil, New Tecumseth, Angus and Orangeville, Ont.
On Monday, high temperatures are forecast to be between 30 and 33 C, while humidex levels are expected near 40.
Environment Canada says a cold front will pass through southern Ontario from west to east on Monday evening and overnight, bringing a slightly cooler and drier air mass for Tuesday.
The Simcoe Muskoka District Health Unit advises people to protect themselves during a heat wave by drinking lots of fluids, staying cool indoors and keeping physical activity to a minimum.
The health unit also says people who must be outside should stay in the shade and wear sunscreen, a hat and sunglasses.
“Infants and young children, the frail elderly and people with chronic lung conditions or taking certain prescription drugs are more vulnerable to harm from overheating,” the health unit adds.
“Check in on these people to make sure they’re staying safe.”
Environment Canada says hot and humid air can bring deteriorating air quality and result in the air quality health index reaching the “high risk” category.