Thick smoke from hazard reduction burns has blanketed much of Sydney with health authorities warning air quality across the city is "poor".
The Office of Environment and Heritage dropped the air quality level in Sydney on Tuesday following hazard reduction burns in the Blue Mountains and elsewhere.
NSW Rural Fire Service inspector Ben Shepherd says there'll likely be similar levels of smoke on Wednesday before it slowly dissipates.
Westerly winds have pushed the smoke across the city but haven't been strong enough to break up the thick haze, Bureau of Meteorology forecaster Elli Blandford says.
The hazy conditions were worst in the morning because smoke was trapped below a layer in the air overnight, she told AAP on Tuesday.
NSW Health has warned the smoky conditions could irritate the respiratory system and aggravate existing lung and heart conditions.
People with asthma, emphysema and angina are more likely to be sensitive to the effects of smoke, environmental health director Richard Broome said in a statement.
"For most people, smoke will be no more than an irritation," he said on Tuesday.
"I recommend that people with existing heart and lung conditions should avoid outdoor physical activity when there's smoke around."