Heavy smoke is blanketing Sydney as hazard reduction burns across New South Wales.

Sydney is currently in the top 20 cities for the worst air quality around the world alongside Bangladesh, India and China.

Air quality and pollution city ranking (IQAir)

Conditions are particularly bad in Sydney’s south-west, including the suburbs of Bargo, Bringelly, Camden, Campbelltown West, Liverpool and Oakdale.

Critical hazard reduction burns are currently taking place in the Blue Mountains area (near Woodford) and in the Wattamolla area.

NSW Health is urging those with asthma or other health conditions to remain indoors, if possible.