Environment Canada has issued a special air quality statement for much of southwestern Ontario, including the London region.
It says Sunday’s hot and sunny conditions are expected to cause increasing ground-level ozone concentrations.
Similar conditions may persist into Monday.
It is warning of symptoms such as increased coughing, throat irritation, headaches and shortness of breath.
Children, seniors and those with cardiovascular or lung disease are especially at risk.
It is advising those who experience symptoms to consider reducing or rescheduling strenuous outdoor activities until the special air quality statement is lifted.
A heat warning remains in effect in London and Middlesex. It’s set to wrap up by Monday.
Sunday is calling for a high of 31 C and a low of 19 C. A mix of sun and cloud is expected Monday, with a high of 30 C and a low of 20 C.