Sydney has been shrouded in a blanket of smoke since this morning, caused by hazard reduction burns in preparation for the start of bushfire season.

The NSW Rural Fire Service reported early this morning that “most of the smoke should clear throughout the day and night” but warns that the smoke may settle in some areas. "If you have asthma or a lung condition, reduce outdoor activities if smoke levels are high and if shortness of breath or coughing develops," the RFS says.

Data from the NSW Office of Environment showed that the inner-west, south-west and eastern suburbs are the areas most affected by the influx of smoke.

Air quality in the city was rated “poor” at 10am with the smoke causing visibility issues for some commuters.

The permitted burning is happening in and around the northern beaches, the Hills Shire, Hawkesbury, Hornsby and Sutherland. The Blue Mountains, Central Coast, Illawarra, Southern Highlands and South Coast have also been affected.

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Warmer temperatures and low rainfall signify a high-risk summer, according to research from the Centre for Environmental Risk Management of Bushfires at the University of Wollongong.