Regional officials have issued an air quality advisory for eastern Metro Vancouver and the central Fraser Valley due to high concentrations of ground-level ozone.
People are being advised to avoid strenuous outdoor activities in the mid-afternoon and early evening.
The poor air quality is of particular concern to people with underlying conditions like heart or lung disease, asthma, diabetes or respiratory illnesses including COVID-19.
Pregnant women, children and people who work outdoors are also at risk, Metro Vancouver said.
Ground-level ozone is formed when pollutants produced by fuel combustion and volatile organic compounds, like those generated from solvents, react in sunlight.
It persists in highest concentrations during mid-afternoon and early evening on hot summer days.
Metro Vancouver estimated the ozone would persist with the hot, sunny weather — which is forecast to last into next week.