A cool down is finally expected by Friday as a cold front moves in.
The warnings stretch from Sudbury and North Bay down to Windsor and east past Montreal.
It's going to be a scorcher for Canada Day.
Heat warnings are issued by Environment and Climate Change Canada to inform the public when air temperature and humidex are forecast to be above a certain criteria for two consecutive days.
The national forecaster says temperatures could reach the mid-30s with humidex values into the mid-40s, while some areas could see severe thunderstorms. However, the daytime highs are still expected near 30 and overnight lows in the high teens. Daytime temperatures will again be near 30 degrees, with humidex values in the mid 30s.
The overnight lows are expected to be in the low 20s but with the humidity sticking around, there won't be much relief from the heat.
"This will be the most significant heat event in the past few years", according to Environment Canada.
"An extreme heat event for the Canada Day long weekend", says a notice for the City of Toronto.
There are many warning signs for heat illness, including swelling, rash, cramps, fainting, heat exhaustion, heat stroke and the worsening of some health conditions.
To prevent heat stress during a heat alert, Dr. Strang suggests to stay in a shaded, cool or air-conditioned area, drink plenty of water, wear light-coloured clothing and to not leave anyone or any animal unattended in vehicles.