Several municipalities in eastern Metro Vancouver remain under an air quality advisory due to high amounts of smog.

In a statement Thursday, the regional district said high concentrations of ground-level ozone — commonly known as smog — are expected to persist until at least Friday in Coquitlam, Port Coquitlam, Surrey, Langley, Maple Ridge, Pitt Meadows and the Fraser Valley

The advisory has been expanded to include higher levels of fine particulate matter in the eastern Fraser Valley caused by wildfire smoke.

In other parts of Metro Vancouver, skies are hazy due to wildfires, but air quality remains within guidelines.

Ground-level ozone, the district said, forms when nitrogen oxides (from pollutants emitted when fuels are burned) and volatile organic compounds (emitted from solvents like paint, cleaning products and liquid gas) react in sunlight.

"The highest levels of ground-level ozone are generally observed between mid-afternoon and early evening on summer days," the district said in a written statement.

The Fraser Valley Regional District issued a similar advisory Wednesday.

An Air Quality Advisory has been issued for the Fraser Valley because of high concentrations of ground-level ozone that are expected to persist until a change in weather. Avoid strenuous activities during mid-afternoon to early evening when ozone levels are highest. —@FVRD1

To reduce the amount of ozone, the district is encouraging residents to drive less and not refuel cars at the hottest times of the day.

Metro Vancouver advised residents of affected cities to avoid strenuous outdoor activities from mid-afternoon to early evening. Seniors, infants and people with medical conditions like heart disease, lung disease or asthma are most likely to suffer ill effects from the ozone.

Anyone who is experiencing chest discomfort, shortness of breath, coughing or wheezing may want to seek medical advice.