Environment Canada has ended a special air quality statement in London-Middlesex, Elgin, Oxford-Brant, and Huron Perth.
The weather agency says the hot and sunny conditions on Wednesday were expected to cause increased ground-level ozone concentrations.
Local residents were advised that they may experience symptoms that included increased coughing, throat irritation, headaches or shortness of breath. Children, seniors, and those with cardiovascular or lung disease, such as asthma, were the most at risk.
Sarnia-Lambton, Windsor Essex, and Chatham-Kent were under an air quality statement for two days.
More information on how to reduce health risks and personal contribution to pollution levels can be found at www.airhealth.ca.