Health Warning Issued As Smoke Blankets Sydney For Second Day

Thick smoke has blanketed Sydney, prompting authorities to issue a health warning for people sensitive to air issues -- especially people with heart or lung disease.

"Smoke haze will make for poor air quality today in parts of the Sydney Basin and Blue Mounts," the Bureau of Meteorology said.

It's the second day of smoky conditions for Sydney, as the Rural Fire Service takes advantage of favourable conditions to conduct hazard reduction burns in the Sydney and Blue Mountains area.

The smoke is expected to linger, NSW RFS spokesman James Morris said.

"We have postponed all burns for at least 24 hours while we get that smoke to push out and clear that air a bit," he told AAP on Wednesday.

"It's very strategic to try and reduce that smoke and for minimal impact."

The Office of Environment and Heritage has rated the air quality as "poor" due to particles in the air.

Sensitive groups, such as those with asthma, lung disease, cardiovascular disease, pregnant women, young children and older people are advised to cut back or reschedule strenuous outdoor activities.

They are also advised to stay indoors and keep windows shut.

With AAP.