Metro Vancouver has lifted an air quality advisory issued Wednesday for the lower Fraser Valley.

Offshore winds and cooler temperatures have helped disperse smog that formed, mainly affecting the Abbotsford to Hope area.

The ground-level ozone had built up due to the recent hot sunny weather, causing air quality to deteriorate Tuesday night.

Air quality worsened to fair to poor levels in some areas Wednesday, posing what's considered a moderate risk to respiratory health in the eastern Fraser Valley.

Metro Vancouver urged residents to reduce emissions by minimizing their use of cars or other polluting activities.

A moderate health risk is still forecast throughout the Lower Mainland Thursday, declining to low on Friday.

Air quality advisories have become a rare occurrence.

There were none last year and in 2006 the only air quality trouble was due to smoke from a fire in Burns Bog.

"We haven't seen very many advisories in the last couple of years because the cars are getting cleaner," said Metro Vancouver air quality planner Laurie Frymel-Bates.

Ground-level ozone is formed by a reaction between nitrogen oxide and volatile organic compounds (VOCs) in combination with sunlight and heat. Nitrogen oxide is a pollutant emitted by cars and trucks. VOCs are mainly released by plants, but also come from gasoline and other solvents.

Air Quality Health Index for Surrey/Langley

For more air quality information, including current readings by community, see

www.airhealthbc.ca.