A Metro Vancouver statement says high concentrations of ground-level ozone are expected to persist throughout the day.
Residents are encouraged to avoid strenuous outdoor activities during mid-afternoon and early evening, when ozone levels are highest.
Young children, the elderly and people with underlying medical conditions such as heart disease or asthma are especially at risk.
Anyone experiencing chest discomfort, shortness of breath, coughing or wheezing is advised to stay indoors and contact a doctor.
The advisory is expected to continue until there is a change in the hot and sunny weather.