Toronto's medical officer of health issued an extreme cold weather alert for the city on Wednesday with the temperature forecast to plunge overnight.

Dr. Eileen de Villa said such alerts are issued when temperatures are forecast to reach –15 C or colder, or when the wind chill is forecast to reach–20 or colder. Environment Canada said the low on Wednesday night is forecast to be –18 C with a wind chill of near –25.

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

"Hypothermia occurs when the body's core temperature drops below 35 degrees Celsius and 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."

People most at risk of cold-related illnesses are those who are homeless, who work outside, who have heart conditions or respiratory problems, the elderly, infants and young children.

Extreme cold weather alerts allow the city to activate local services aimed at getting and keeping vulnerable residents inside.

At Metro Hall in downtown Toronto, for example, a warming centre opens at 7 p.m. the day an alert is called, and it remains open until noon on the day an alert is terminated.

As well, community agencies are told to ease any service restrictions, transit tokens are available at some drop-ins and there is additional overnight street outreach.