Environment Canada has placed the Greater Toronto Area and surrounding cities under a heat warning, saying dangerously hot and humid conditions will begin Sunday afternoon and continue into next week.

Daytime highs are expected between 31 and 36 C. Humidex values, which reflect how hot it feels, may range between 40 and 45, Environment Canada said.

Overnight lows are forecast between 21 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, Milton, Barrie, Collingwood and Hillsdale.

WATCH | Environment Canada senior climatologist discusses the heat wave heading for the GTA:

Extreme heat is expected to continue until Tuesday night. It may continue Wednesday for southwestern Ontario and the GTA, depending on the movement of an expected cold front.

“I am urging all residents to stay cool, drink lots of water and look after each other,” Toronto Mayor Olivia Chow said Saturday at an unrelated city news conference.

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 advised.

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.

When going outside, people are advised to limit exposure to sun and heat by planning activities during the coolest parts of the day and wearing lightweight, light-coloured, loose fitting clothing.

The City of Toronto’s 58 outdoor pools will have extended hours this weekend, staying open from 10 a.m. to 8 p.m., the city said in a news release Saturday.

While the heat warning is in effect, the following pools will be open until 11: 45 p.m. daily:

• Alex Duff Pool, 779 Crawford St.

• Giovanni Caboto Pool, 1367 St. Clair Ave. W.

• Sunnyside Park/Gus Ryder Pool, 1755 Lake Shore Blvd. W.

• McGregor Park, 2231 Lawrence Ave. E.

• North Toronto Outdoor Pool, 200 Eglinton Ave W.

• Smithfield Park, 175 Mount Olive Dr.

• Halbert Pool, 24 Rockwood Dr. (open until 9 p.m. only)

The city’s splash pads will operate daily from 9 a.m. to 9 p.m. and 15 wading pools will be open this weekend from 12 p.m. to 5 p.m.

There are more than 500 cool spaces across the city — including libraries, community centres and pools — where people can find relief during the extreme heat. A full list of cool spaces is available on the city’s website.

The city said it will also conduct enhanced outreach efforts to people experiencing homelessness, including through referrals to shelters and distributing water bottles. It said it will deliver 14,000 water bottles to outreach staff on Saturday.

Additional staff will focus exclusively on distributing water bottles to people experiencing homelessness, including those in encampments.

On Sunday, three of the city’s HTO To Go water trailers, which provide free tap water, will be located at Nathan Phillips Square, Trinity Bellwoods Park and Sankofa Square, Chow said.