Pregnant women, children and elderly residents of Melbourne and Geelong subject of official air quality warning

Vulnerable residents in Melbourne and Geelong are breathing in a cloud of wood smoke and urban pollution, leading to official air quality warnings.

Children, pregnant women, those over 65 and people with existing heart and lung conditions have been warned about poor air quality in the two Victorian cities, with a lack of wind exacerbating the incidence.

"These people should reduce prolonged or heavy physical activity and, where possible, limit the time spent outdoors," the Environment Protection Authority said in a statement today.

Smoke from wood heaters and other urban pollution has not blown away from the cities, leaving an elevated level of small particles in the air.

The "PM2.5 particles", often described as fine particles, are up to 30 times smaller than the width of a human hair.

In November, nine people died and thousands were hospitalised in Victoria due to the world's worst recorded thunderstorm asthma event.

People with asthma have been told to follow their asthma management plan.