Toronto Public Health issued an extreme cold weather alert for the city on Wednesday night as the city braces for a return of frigid temperatures.

Dr. Eileen de Villa, the city's medical officer of health, said in a news release on Wednesday that she issued the alert in anticipation of colder temperatures in the next 24 hours.

The alerts activate local services that help to keep vulnerable residents inside, she said.

On the day an alert is issued, the city opens a warming centre at Metro Hall by 7 p.m. and keeps it open until 12 noon on the day that the alert is terminated.

During an extreme cold weather alert, the city tells community agencies to relax service restrictions, makes transit tokens available in some drop-in centres and provides additional overnight street outreach.

The city also urges residents to help vulnerable people by calling 311 if there is a need for street outreach assistance.

Extreme cold weather alerts are issued when Environment Canada predicts temperatures will drop down to – 15 C or colder, or when the wind chill is forecast to reach –20 or colder.

"Exposure to cold weather can be harmful to your health," the city said in a news release on Wednesday.

The city said hypothermia occurs when the body's core temperature drops below 35 C and it can have severe consequences, including organ failure and death. Frostbite can also occur in cold weather when skin freezes, and in severe cases, can lead to amputation when deeper tissues freeze.

According to Environment Canada, the low temperature on Wednesday night is forecast to be –12 C, with the windchill forecast to be –15.