Soaring summer temperatures continued to heat up much of southern Ontario, as Toronto’s medical officer of health upgraded the city’s weather warning to an "extreme heat alert."

Dr. David McKeown issued the alert on Tuesday, urging citizens to drink plenty of water and check on loved ones, especially isolated adults and seniors, to make sure they are safe.

Environment Canada predicts the mercury will remain in the 30 C range until Saturday, when the temperature is expected to drop to 25 C with a 30 per cent chance of showers.

Tuesday’s high was 32 C with a humidex value of 40 C.

Health Canada recommends people stay in air-conditioned places, drink lots of water and limit physical activity.

Tanya Gulliver, an extreme heat expert and a PhD student at York University with the faculty of environmental studies, said people should stay alert to signs of heat exhaustion and heat stroke.

She said telltale signs include sweating and suddenly not sweating. "That doesn’t always mean that you’re cooling down," Gulliver told CTV News Channel on Tuesday. "Sometimes that can mean that your body is dehydrating so much that it’s time to really rehydrate again."

Gulliver added that some people can also develop heat rashes, an indication that their sweat ducts have started to close up.

She said individuals working outside, homeless people, children under the age of one and seniors are most at risk of developing complications from the extreme heat.

The city has opened cooling centres and extended hours for some pools to help residents cope with the sweltering weather.

The following pools will be open until 11:45 p.m. Tuesday:

  • Alex Duff Memorial Outdoor Pool, 779 Crawford Ave.
  • Alexandra Park Outdoor Pool, 275 Bathurst St.
  • Cummer Park Indoor Pool, 6000 Leslie St.
  • Giovanni Caboto Outdoor Pool, 1369 St. Clair Ave. W.
  • McGregor Park Outdoor Pool, 2231 Lawrence Ave. E.
  • Monarch Park Outdoor Pool, 115 Felstead Ave.
  • Smithfield Outdoor Pool, 175 Mount Olive Dr.
  • Sunnyside-Gus Ryder Outdoor Pool, 1755 Lake Shore Blvd. W.

The following cooling centres are open for those in need of some relief from the heat:

  • Metro Hall, 55 John St. (24 hours)
  • East York Civic Centre, 850 Coxwell Ave. (11 a.m. to 7 p.m.)
  • North York Civic Centre, 5100 Yonge St. (11 a.m. to 7 p.m.)
  • Driftwood Community Centre, 4401 Jane St. (11 a.m. to 7 p.m.)
  • Etobicoke Civic Centre, 399 The West Mall (11 a.m. to 7 p.m.)
  • McGregor Community Centre, 2231 Lawrence Ave. E (11 a.m. to 7 p.m.)
  • Centennial Park Community Centre, 1967 Ellesmere Rd. (11 a.m. to 7 p.m.)

Toronto Hydro is also encouraging businesses to close their front doors to conserve electricity. According to the company, leaving one door open all summer can add an additional $500 to the hydro bill.

With a report from CTV Toronto’s Scott Lightfoot