Environment Canada has issued heat warnings for most of Ontario, including in Niagara.
The agency says to watch for the effects of heat illness, like swelling, cramps, fainting and heat exhaustion, as well as symptoms like dizziness, nausea, vomiting and extreme thirst.
In Niagara, it will feel like its 44 degrees later today, and the low overnight is 24.
Similar conditions are expected tomorrow as well.
Slightly cooler temperatures are expected on Thursday.