Just after 2 p.m., the weather agency said hot and humid conditions were creating high levels of air pollution.
The statement covers an area stretching from Peterborough in the north down to Windsor in the south.
“Hot and sunny conditions have resulted in increasing ground-level ozone concentrations,” the statement said.
“High-risk (air quality health index) values are expected to persist for 1 to 2 hours.”
Higher pollution levels are expected to continue through the afternoon and into the evening.
“Individuals may experience symptoms such as increased coughing, throat irritation, headaches or shortness of breath,” the statement said.
Children, seniors and individuals with lung or cardiovascular disease are at higher risk, Environment Canada said.
A heat warning also remains in effect for much of southern Ontario, with a “two-day heat event” happening Tuesday and Wednesday.