The Greater Toronto Area is sweltering through another day of blistering heat, but Environment Canada says the region should see some relief Tuesday night.

Daytime highs are forecast to be between 30 and 35 C, with humidex values expected to range between 40 and 45, according to the federal weather agency. 

"We need to make sure that all of us, especially the most vulnerable, have access to cool indoor spaces," said Dr. Samantha Green, a family physician at Unity Health Toronto who researches heat and health. 

Green said as outdoor temperature increases, the body needs to do everything it can to maintain an internal temperature of 37 degrees — part of homeostasis, which is the body's way of keeping internal conditions stable. 

"Our heat beats faster, we start to breathe faster and we also start to sweat. The body is focused on maintaining that homeostasis and we end up not thinking straight," she said in an interview with CBC Radio's Metro Morning on Tuesday. 

Tuesday's heat warning is in effect for the following areas: Toronto, Mississauga, Brampton, Richmond Hill, Vaughan, Markham, Newmarket, Georgina, northern York Region, Durham Region, Pickering, Oshawa, Uxbridge, Beaverton, Simcoe, Halton Hills, and Milton.

There is also a 40 per cent risk of thunderstorms in Toronto Tuesday from noon until 8 p.m., according to Environment Canada. 

Monday's extreme heat broke a temperature record at Pearson International Airport, the weather agency said. 

Temperatures reached 35.8 C at Pearson airport on Monday afternoon, said Kristina Kretchman, meteorologist with Environment Canada. The previous record of 32.8 C was set on June 23, 1921.

Environment Canada is advising people to limit exposure to sun and heat by planning activities during the coolest parts of the day, as well as wearing a wide-brimmed hat and lightweight, light-coloured, loose fitting clothing. 

People are advised to watch for symptoms of heat stroke, which is a medical emergency, in themselves and others. Symptoms can include red and hot skin, dizziness, nausea, confusion and change in consciousness. 

"While you wait for medical attention, try to cool the person by moving them to a cool place, removing extra clothing, applying cold water or ice packs around the body," Environment Canada said in Tuesday's warning. 

The weather agency also advised people to drink water often, even before feeling thirsty, and close blinds and open windows if the outside is cooler than inside. 

"If your living space is hot, move to a cool public space such as a cooling centre, community centre, library or shaded park," the heat warning says. 

People are also advised to check in regularly on those at greater risk of heat illness, including the elderly and those living alone. 

Signs of heat exhaustion may include headache, nausea, dizziness, thirst, dark urine and intense fatigue. If you experience these symptoms, Environment Canada advises stopping your activity and drinking water.