Torontonians should prepare for some frigid weather ahead.

The city’s medical officer of health has issued an extreme cold weather alert as wind chill values of -15 arrive in Toronto tonight.

The alert triggers additional cold-weather related services, including more shelter spaces for the homeless and overnight street outreach.

Toronto Public Health says people who are most at risk to develop cold-related illnesses include the homeless, people who work outdoors, the elderly, infants and young children, as well as people with pre-existing heart or respiratory conditions.

Two cold weather drop-ins are open 24 hours a day, seven days a week, until February 28.

The alert will be in effect until further notice.

During an extreme cold weather alert, the city advises residents to:

• Dress in layers, making sure your outer layer is windproof, and cover exposed skin.

• Wear a hat, warm mittens or gloves, and warm boots.

• Stay dry. Your risk of hypothermia is much greater if you are wet.

• Choose wool or synthetic fabrics for your clothes instead of cotton, because cotton stops keeping you warm once it gets wet.

• Seek shelter if you normally spend long periods outside. Depending on the wind chill, exposed skin can freeze in minutes.

• Drink warm fluids, other than alcohol.

• Warm up by taking regular breaks in heated buildings when enjoying winter activities outside.

• Consider rescheduling outdoor activities, or limiting time outdoors, during colder temperatures, especially if it's windy.

• Heat your home to at least 21ºC if infants or elderly people are present.

• Call or visit vulnerable friends, neighbours and family to ensure they are not experiencing any difficulties related to the weather.