It's been another cold winter day in Melbourne, with temperatures struggling to make it to 10 degrees Celsius. And while it may look like the fog stuck around all day, there's more to it than meets the eye.

It's actually low cloud and haze — and it led Victoria's Environment Protection Authority (EPA) to issue a warning about poor air quality across parts of Melbourne.

Cold temperatures and still conditions over the past 24 hours led to an increase in tiny air pollution particles, especially in the inner city, northern suburbs and inner west.

People over 65, children under 15, pregnant women and those with existing heart or lung conditions are advised to avoid prolonged or heavy physical activity and limit the time spent outdoors.

A lone swimmer contemplates a dip in the fog-covered waters off St Kilda beach. ( Supplied: Clare Lax )

EPA chief environmental scientist Dr Andrea Hinwood said the lack of wind was causing pollution by sources such as motor vehicles and wood-fired heaters, to accumulate in the air.

"We've got no wind to move it anywhere," Ms Hinwood said.

"Because it's so cold and still, it's all being trapped."

The view from the Eureka Tower high over Melbourne showed a city covered in a thick layer of brown haze that stretched across the suburbs.

The city was shrouded in the foggy haze throughout the day. ( ABC News: Michael Gleeson )

Ms Hinwood said inner-city areas were particularly affected because of increased traffic, industrial activity and the number of households with wood-fired heaters.

The poor air quality is expected to continue until Friday.

"If you do have a pre-existing cardiovascular condition, please don't go for a run today," Ms Hinwood said.

Bureau of Meteorology senior forecaster Chris Godfred said the cloud and haze were trapped by an inversion layer.

Northerly winds are expected to pick up overnight and visibility should improve by Friday morning.

Dozens of flights were delayed or cancelled at Melbourne Airport this morning because of the thick fog.