Environment Canada issued an extreme cold warning for the GTA on Tuesday, with temperatures plunging to between –20 C and –28 C in various parts of the region.
"Bitterly cold arctic air combined with light winds" is responsible for the deep freeze, the federal weather agency said.
"This is the coldest air so far this season."
In Toronto, it could feel as cold as –30 with wind chill.
Temperatures are expected to "moderate" later this morning, Environment Canada said.
"Risks are greater for young children, older adults, people with chronic illnesses, people working or exercising outdoors, and those without proper shelter," the agency said.
"Cover up. Frostbite can develop within minutes on exposed skin, especially with wind chill."