A Heat Warning is now replacing the Special Weather Statement in Niagara.
Environment Canada says excessive heat and humidity will continue today into Tuesday.
Afternoon temperatures near 31 degrees and humidex values near 40 are expected today and Tuesday.
An overnight low near 20 degrees is forecast tonight, providing little relief from the heat.
Hot and humid air can also bring deteriorating air quality and can result in the Air Quality Health Index to approach the high risk category.
Extreme heat affects everyone, but the risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.
Drink plenty of water even before you feel thirsty and stay in a cool place.
Heat warnings are issued when very high temperature or humidity conditions are expected to pose an elevated risk of heat illnesses, such as heat stroke or heat exhaustion.
The heat wave ends on Thursday, Canada Day, when the high will be 20.