There is also a risk of severe thunderstorms and tornadoes in some areas.
Environment Canada has issued watches of tornado for 11 regions of southern Ontario.
In addition, a heat warning remains in effect in southern Ontario, where the Humidex index is expected to approach or exceed 40 on Wednesday for the second day in a row, warns Environment Canada.
The following areas have tornado watches: Algonquin, Barrie – Orillia – Midland, Burk's Falls – Bayfield Inlet, Dufferin – Innisfil, Gray – Bruce, Huron – Perth, London – Middlesex, Parry Sound – Muskoka, Sarnia – Lambton, Waterloo – Wellington, Windsor – Essex – Chatham-Kent.
These watches will be in effect at least until Wednesday evening, can we read on the Environment Canada website.
Authorities recommend that the public go swimming or stay in an air-conditioned place to avoid heatstroke.
Very hot and humid air can also cause air quality to deteriorate and cause the Air Quality Health Index to approach the high risk category, adds Environment Canada.
The federal agency is advising the public to watch for heat-related symptoms such as swelling, rash, cramps, fainting or exhaustion.< /p>
Look for a cool place — such as shade from trees, the pool, shower or bath — or an air-conditioned place like a public building, Environment Canada recommends. Never leave people or animals in a parked vehicle.
Slightly cooler weather is expected Thursday in Greater Toronto and Niagara, in particular.
On the other hand, in the South-West, the oppressive heat episode could last until Sunday.
Environment Canada also warns that severe thunderstorms could hit the Windsor, Parry Sound and Barrie regions, in particular, on Wednesday.
Thunderstorms are expected to form over eastern Michigan or Lake Huron this afternoon and then moving eastward through the evening, Environment Canada says.
Federal agency adds: Gusts Destructive storms are the main threat, but large hail and a few tornadoes are also possible. The possibility of a powerful tornado cannot be ruled out.