An air quality advisory has been issued for eastern parts of Metro Vancouver and the Fraser Valley due to high concentrations of ground-level ozone.
The regional district says the ozone could persist until Monday or longer if hot weather sticks around.
Ground-level ozone is created when pollutants from fossil fuel emissions mix with organic compounds in sunlight.
It is generally a problem in the mid-afternoon and early evening on hot days.
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People with underlying health conditions, seniors and children are particularly vulnerable to ground level ozone.
People are advised to avoid strenuous activities later in the day, when concentrations are hightest.