Eastern Metro Vancouver and the Fraser Valley were placed under an air quality advisory Thursday afternoon.
The Metro Vancouver Regional District issued the advisory citing high levels of ground-level ozone.
Ground-level ozone is created when pollutants from burning fuel react with organic compounds produced by solvents in sunlight.
It is most commonly seen in the mid-afternoon and early evening during summer.
It can be particularly harmful to infants, the elderly and people with underlying health conditions.
Metro Vancouver says the conditions are expected to persist until Friday.