Environment Canada has issued a heat warning for parts of Southern Ontario, with temperatures expected to soar to 34 C.
The weather agency says hot and humid conditions are expected to begin Monday and last until a cold front sweeps through with thunderstorms Tuesday evening.
Officials say residents in Hamilton, Halton, Niagara, and surrounding areas can expect maximum daytime temperatures from 31 C to 34 C, with humidex values near 40 C.
Overnight low temperatures could fall to only 21 C to 25 C, providing little relief from the heat.
The hot and humid air can also bring deteriorating air quality and can cause the Air Quality Health Index to approach the high-risk category.
Environment Canada is reminding Ontarians to watch for the effects of heat illness. Symptoms can include swelling, rash, cramps, fainting, heat exhaustion, heatstroke, and the worsening of some health conditions. Officials say the risks are greater for young children, pregnant women, older adults, people with chronic illnesses, and people working or exercising outdoors.
