"We aren’t expecting it to be as bad as [Monday's haze]," he said. About a dozen fires were burning around Sydney on Monday and the RFS was planning more fires near the Blue Mountains and in the Southern Highlands later in the week. "We are currently assessing whether we will go ahead with those burns based on the weather, how much smoke is already in the atmosphere, and whether those areas are actually suitable," the spokesman said.

Jiwon Park, a Bureau of Meteorology forecaster, said winds were likely to swing around to the north from Wednesday, reducing the chance of smoke haze from the current fires. Loading Friday was also likely to bring much-needed rain, with as much as 15 millimetres predicted for the city, or more than the total for the whole of April. This month's tally of 11.2 millimetres will most likely make it Sydney's driest April since 2006, with less than a 10th of typical rainfall, according to bureau data. Smoke detectors Fire researchers at the University of Wollongong are working to reduce the pollution impact of hazard-reduction burning while also predicting how conditions for such fires will alter with climate change.

A study published in the Medical Journal of Australia found heavy pollution from such burns over a six-day period in May 2016 led to as many as 14 premature deaths in Sydney. That event helped prompt new research led by Owen Price from the University of Wollongong's Centre for Environmental Risk Management of Bushfires to minimise smoke from controlled burns. Dr Price said fire crews could try to limit the amount of burning early in the morning or late in the day when temperature inversions were more likely to occur. Monday's haze was made worse by such an event, with cool air near the surface trapped below warmer air aloft. Wildfires typically differ from controlled ones by burning more efficiently - the drier material producing less smoke - and sending the smoke higher in the atmosphere, he said. Assumptions that southern Australia's bushfire season would continue to lengthen as temperatures rise in a warming climate don't necessarily mean the hazard-reduction programs - and hence air pollution - will become more concentrated, according to Hamish Clarke, a fellow University of Wollongong fire researcher.

His research indicates that, at least for the fire-prone forests of coastal NSW, there may be more days suitable for controlled burns in the future. "We found a complex pattern of changes, with the potential for substantial and widespread increases in the current burning seasons of autumn (March-May) and spring (August-October)," his 2019 paper in Agricultural and Forest Meteorology found. The result was "somewhat surprising", with "a fair whack" of the additional suitable burning days coming in the "shoulder seasons" either side of summer but "some in the winter as well", Dr Clarke said. As for the current pollution levels, an OEH spokesman said particulate levels "vary significantly from year to year especially during April and May, and during September to October". El Nino years typically see higher pollution and La Nina years - which are typically wetter than average - have less pollution.