The smog Friday is a result of hot, light winds from the U.S. Midwest.
"Pollution from both the U.S. and Ontario is sitting stagnant over the city, and most of southern Ontario," said CBC meteorologist Johanna Wagstaffe.
During the smog and high pollution episode people with heart or lung disease, including asthma, may experience a worsening of their symptoms. The smog also makes it more difficult to breathe while exercising outdoors.
Friday's temperatures will sit at around 31 C with the humidex levels in the mid-30s.
The heat and humidity will remain throughout the weekend, cooling slightly on Monday, but as winds shift Saturday the air quality should improve.