Sydney ranked in the top five most polluted cities in the world for air quality on Wednesday, as planned hazard reduction burns blanketed parts of the city in thick smoke.

The burns, which saw the city experience worse air quality than New Delhi and Beijing, according to IQAir air quality rankings, started at the weekend in national parks across Greater Sydney with the aim of reducing bushfire fuel, like dry leaves and branches.

Hazard reduction burning can lessen the potential spread and intensity of future bushfires and are common practice in the country. It comes as Australia heads into a fire season that could be the riskiest since the so-called Black Summer blazes of 2019 to 2020 that burned 24 million hectares of land — an area roughly equivalent to half of California.

Read More: Wildfire’s Toxic Legacy Has Children Gasping for Air Years Later

The New South Wales Rural Fire Service said that many of the larger burns across Sydney had been completed, with some smaller fires still blazing. Smoke was expected to remain in Sydney in the coming days and clear over the weekend, it said.