Sydney's air quality worse than Beijing

A blanket of smoke smothering Sydney has seen the city's air quality fall to levels worse than is being experienced in parts of Beijing.

It's prompted NSW health authorities to issue warnings that people with respiratory conditions, and the young and elderly, stay indoors.

The hovering smoke is from hazard reduction burns carried out around Sydney at the weekend, with NSW Fire & Rescue saying the air is not expected to completely clear until Wednesday.

According to the state's official air quality index, Sydney's north west and south west recorded "hazardous" levels of air quality. The city as a whole was given a "very poor" rating as of Tuesday afternoon.

On a wider scale, the World Air Quality Index rated Sydney's air quality as 164, while inner Beijing is listed as an "unhealthy" 152.

Air pollution in Beijing is generally better during the northern hemisphere summer months.

By mid-afternoon, paramedics had treated more than 50 people with asthma or breathing difficulties across Sydney.

Almost a third of the cases related to young children, NSW Ambulance said.

"Paediatric cases don't normally make up one third of our workload on a daily basis," Inspector Giles Buchannan said.

"Our advice at this stage is if you can stay inside and out of the haze covering Sydney you are best to do so."

Paramedics have been called out to more than 80 cases across the state.

The elderly, the very young and those with respiratory conditions are being urged to take precautions as the air quality impact is three or more times the national standard, NSW Health said on Tuesday.

"But also those without respiratory conditions should take precautions, (they) should stay inside, and avoiding vigorous exercise is really important on a day like today," Dr Ben Scalley told reporters.

"There are lots of large particles in the air but it is the ones you can't see that have the worst impact."

NSW Rural Fire Service spokesman Ben Shepherd said more smaller burns are scheduled for later this week but these fires are not expected to impact the city's air quality.

"They will be weather-dependent looking at the rain ahead but they are smaller burns at around six hectares in size," he told AAP.

Australian Associated Press