An air quality advisory was issued Sunday evening for eastern parts of Metro Vancouver and the Fraser Valley.
Metro Vancouver officials said the advisory was issued because of high concentrations of ground-level ozone that are expected to persist for a few days.
Residents in affected areas are urged to avoid strenuous outdoor activities in mid-afternoon, when ozone levels are highest.
Exposure is particularly a concern for infants, the elderly and those with underlying medical conditions, such as lung or heart disease and asthma.
Anyone experiencing symptoms such as chest discomfort, shortness of breath, cough or wheeze should follow their health care provider's advice and stay indoors in air conditioned spaces.
The advisory is expected to continue until there is a change in weather pattern.