NSW Health issued a bushfire smoke alert for a third day on Friday, advising those with asthma or other lung conditions to limit outdoor activity.

AirVisual, a website ranking pollution among major cities, placed Sydney as high as 5th for poor air quality on Thursday. By Friday morning, Sydney was still in the top two dozen cities globally for poor air quality, two places ahead of Beijing.

Tourists wearing face masks walk in smog at Tiananmen Square in December 2016 and, right, Frank Heatherton of Balmain takes a dip in Sydney's harbour on Thursday. Credit:AAP/Getty

It came as an expert said the NSW government should consider issuing temporary curbs on polluting industries during periods of poor air quality such as during the current bout of smoke haze over Sydney.

Gabriel Da Silva, an atmospheric chemist lecturing at the University of Melbourne, said that while much of the pollution shrouding the city was coming from fires to the north, the mixing with pollutants from coal-fired power plants, transport and factories was amplifying the impact.