Abbotsford residents, especially infants and the elderly with lung conditions, are advised to avoid outdoor activities during the afternoon and evening Friday due to an air quality advisory issued for the Fraser Valley.

High concentrations of ground-level ozone in the Fraser Valley and the eastern parts of the Lower Mainland are expected starting Friday, Metro Vancouver is warning.

The highest concentrations are likely to occur in the afternoon and evening and may stay elevated into Saturday.

The very young or elderly who have medical conditions such as asthma or lung or heart disease should be alert for symptoms such as chest discomfort, shortness of breath, coughing or wheezing and stay indoors in air-conditioned areas.

Julie Saxton, Metro Vancouver air quality planner, said that air quality is impacted by stretches of hot sunny weather.

Ground-level ozone is formed when nitrogen oxides, which result from the burning of fuel, and volatile organic compounds from solvents react with air in sunlight.

"Sunshine is needed to create ground-level ozone," said Saxton on Friday.

"So obviously, hot conditions mean more reaction is going on."

The advisory will remain in effect until there is a change in air quality conditions, she said.

If the cooler weather arrives as predicted this weekend, air quality should improve.

"Some cloud coming in will hopefully bring down the concentrations," Saxton said.

The Metro Vancouver air quality threshold for ozone is 82 parts per billion.

"We reached just under 80 ppb [on Thursday] afternoon," she said.

Ground-level ozone conditions are quite dynamic and affected by a range of factors, but the Fraser Valley's topography plays a role, said Saxon.

"We are surrounded mountains and when we see very calm conditions . . . it's more difficult for air contaminants to disperse.

For more information about information about real-time air quality readings and how to reduce emissions visit www.bcairquality.ca.