The Greater Toronto Area and surrounding regions remain under a heat warning from Environment Canada on Monday, which is expected to continue until Tuesday night for most areas. 

Daytime highs are forecast to be between 33 and 36 C. Humidex values, which reflect how hot it feels, are forecast to range between 42 and 46, the government agency said.

Overnight lows are forecast to be between 22 and 25 C, which the weather agency said will provide "little relief from the heat." 

The 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. 

While Barrie, Collingwood and Hillsdale are also under a heat warning Monday, Environment Canada says it appears conditions will fall below the heat warning criteria on Tuesday. 

Daytime highs between 30 to 33 C are expected for these areas, as well as humidex values between 40 to 43, the weather agency said. 

Environment Canada has warned of "dangerous heat and humidity" in the GTA and surrounding areas starting Sunday afternoon. 

Over the weekend, some City of Toronto pools set extended hours due to the heat warning. But the city said some pools had temporary closures because of high humidex levels. 

"Given provincial requirements related to heat and humidex (45C+) protocols, some outdoor pools had intermittent closures today to ensure staff health and safety," the city said in a post on X on Sunday. 

Staff remained on site to re-open the pools as soon as possible, the post read. 

Coun. Brad Bradford called the closures "totally unacceptable" in a post on X on Sunday. 

"Pools need to be open for people to cool down on what might be the hottest day of the year," the post read. "Instead, staff were closing pools and turning people away." 

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 Monday'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.