Smoke from wildfires in northwest Ontario prompted Environment Canada to issue an air quality advisory for a large stretch of the province on Monday.
An area spanning from Toronto, north to Algonquin and east to Kingston could be affected if the smoke makes its way to ground level, the federal weather agency said.
It says smoke is currently "expected or occurring," and high levels of air pollution are possible as a result.
People in those regions could experience symptoms such as coughing, throat irritation, headaches or shortness of breath.
"If you or those in your care are exposed to wildfire smoke, consider taking extra precautions to reduce your exposure," Environment Canada said.
"Wildfire smoke is a constantly-changing mixture of particles and gases which includes many chemicals that can be harmful to your health."
The City of Toronto is expected to see a high of 31 C by midday, with a chance of rain showers in the afternoon and early evening.