Environment Canada has issued a heat wave warning, expected to last until Thursday morning.
The warning encompasses most of Ontario and parts of Quebec.
Temperatures in London are expected to fluctuate between the high 20s and low 30s C, with a humidex in the high 30s to low 40s.
Temperatures will drop in the evenings and overnight to the low 20s and high teens.
Air quality is also expected to decline as the temperature rises.
Temperatures are predicted to drop back down to seasonal averages on Thursday with a high of 26 C.
Trudy Kidd, an Environment Canada warning preparedness meteorologist, warns that the extreme heat poses greater risks to older adults, young children and those with chronic illnesses.
“(Older adults) are often faced with compounding factors that can put them at increased risk during extreme heat events. They can have things like chronic illnesses, medications that interfere with cooling mechanisms,” she explains.
“Individuals with chronic illnesses, things like breathing difficulties, heart problems and various psychiatric illnesses can put people at high risk of heat-related health effects.”
She adds that pregnant women and those working physical jobs or exercising outdoors are also at high risk of heat-related illness.
The weather agency says it issues heat warnings when high temperatures pose an “elevated risk” of illnesses such as heat stroke or heat exhaustion.
Warning signs of heat illness include swelling, rash, cramps, fainting and exhaustion.
During a heat wave, Environment Canada recommends monitoring alerts, paying attention to how you and others are feeling, drinking water before you feel thirsty and staying cool inside.
“There’s various ways to stay cool, things like an air conditioner, visiting a cool space like a library, community centre or shopping mall, wearing loose-fitting, light-coloured clothes, cold showers, a dip in a cold pool and avoiding strenuous outdoor activities,” Kidd says.
She also reminds people to not leave children or pets alone in a parked vehicle, as it can get extremely hot in the interior of a car.
Weather alerts can be monitored at weather.gc.ca.