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

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.

The temperature is set to fall to –3 C Thursday afternoon but will feel like –12 with a wind chill, Environment Canada says.

By the evening the temperature is expected to dip even further.

"We'll be looking at wind chill values almost reaching –20, so certainly do bundle up if you're out and about," Gerald Cheng, a meteorologist with Environment Canada told CBC Toronto.

"As you head out to work [or] to school Friday morning, again the wind chill will almost be down to –20, so certainly it's going to be a cold night."

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

"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.

According to Environment Canada the cold should subside Saturday, and rain or snow is expected with a high of 2 C.