NSW Health should provide more detailed paediatric advice, Ms Blakemore said. "What are we supposed to do with children?" she asked, noting that schools in the area do not have airconditioning because of the relatively mild climate in the Blue Mountain foothills. Hazardous pollution levels are present at Lindfield, Macquarie Park, Bringelly and Liverpool, with the worst levels recorded at Lindfield, which had an air quality index for visibility of 587 at 6am.

Loading Any air-quality reading over 200 is "hazardous", according to the NSW Office of Environment and Heritage (OEH). Particle levels across the east and north-west of the city have been "poor" (100 to 149) to "very poor" (150 to 199) on average over the past 24 hours. An OEH-backed site for the Blue Mountains said that readings at Springwood were nearing 1300 after earlier touching 2400 on Wednesday, Ms Blakemore said. In a statement issued on Tuesday afternoon, NSW Health warned that poor air quality could aggravate existing heart and lung conditions, and was expected to persist into Wednesday. "Smoke from hazard reduction burns can be quite unpredictable, but we're expecting poor air quality in a number of places across the Sydney basin into Wednesday morning," the statement said.

Loading Residents waking to a third day of smoke following a large number of hazard reduction burns on the weekend took to social media to express their frustration and concern. It's the second time Sydney has been blanketed in smoke haze from hazard reduction burns in the past month. The NSW Rural Fire Service said in a separate statement that smoke from fires would settle "over the next few days", noting the forecast for light winds and stable conditions. Overnight temperature inversions - which stop air from rising and place a lid on smoke - are preventing the pollution from dissipating.

South-west Sydney, the Central Coast and Newcastle were among the areas affected on Tuesday. "A number of burns have been postponed this week and the NSW RFS, along with partner agencies, are continuing to re-evaluate and reschedule burns if required," the RFS said, adding the fires were "important controlled burns which will reduce the risk to people and properties from bushfires". A high-pressure ridge has sat over NSW for days, leaving conditions unusually stable and allowing heat to build up. Tuesday's top temperature of 27.2 degrees was almost 8 degrees above the May average. It was also the second warmest day this late in the year season on record for Sydney, exceeded only by the 28 degree reading on May, 25, 1994, Blair Trewin, senior climatologist at the Bureau of Meteorology, said.

Depending on how coming days play out - and the bureau is forecasting daytime temperatures to reach 23-25 degrees until Sunday - the city could set late-season records for the sequence of days this warm so late in May. Rainfall for April and May is just 25.8 millimetres in Sydney, tracking for the second lowest on record behind 1888, Dr Trewin said. Pollution consultation The return of the smog comes as public consultation of new state and federal air quality standards opens on Thursday. Environmental groups have targeted the current standards for sulphur dioxide - much of it from coal-fired power plants - as too weak. They say existing standards are 11 times higher than the World Health Organisation recommends, and weaker than the US, the European Union and China.

They note high sulphur dioxide levels are associated with low-birth weight, respiratory disease and premature death. Small particulate pollution levels have also been rising in parts of the Hunter for the past four years. "State and federal governments in Australia have failed to set and enforce safe pollution standards that protect people living near coal-fired power stations," Kate Smolski, chief executive of the NSW Nature Conservation Council, said. Air quality index showing hazadous levels in parts of Sydney on Wednesday. Credit:NSW OEH For the current alert, Richard Broome, the director of the Environmental Health unit of NSW Health, said the smoke could irritate the respiratory system and aggravate existing lung and heart conditions. "People with conditions like asthma, emphysema and angina are more likely to be sensitive to the effects of smoke,” Dr Broome said.