"Overnight low temperatures are expected to be in the low twenties, except high teens in some rural areas, providing little or no relief from the heat".
A cooldown isn't expected until Friday, when a cold front finally passes through. Humidex values this week were expected to reach as high as 43 C in southern Ontario, and Environment Canada issued a heat warning for the densely populated region.
"People waking up Tuesday morning might feel a bit of relief from the extreme heat of the weekend, but it is only a short cooldown", she said.
The week-long heat wave started Friday and is forecast to last until the end of the week.
The heat wave has hit the Atlantic provinces, too - a heat warning is in effect for Halifax as Nova Scotia has been experiencing temperatures of around 30 degrees, or up to 39 degrees with the humidex.
Dr. Robert Strang, chief medical officer of health, says the former criteria included higher temperatures like when the humidex was 40 degrees Celsius of higher.
Extreme heat affects everyone, Environment Canada said.
They planned to visit thousands more as the heat was expected to continue until Friday. The risks are greater for young children, pregnant women, older adults, people with chronic illnesses and people working or exercising outdoors.
The weather agency says a "very warm and humid air mass" has settled over the Maritimes and above normal temperatures and humid conditions will stick around into Thursday.
The City of Toronto continues to have seven cooling stations open across the city as well as public, air-conditioned spaces like libraries and community centres available so people can catch a break from the never ending heat wave.