Thick smoke from wildfires shroud the Opera House in Sydney, Australia, Dec. 10, 2019.
Thick smoke from wildfires shroud the Opera House in Sydney, Australia, Dec. 10, 2019.

Residents of Sydney are choking on a thick cloud of smoke and haze stoked by hundreds of bushfires that have burned for weeks on the outskirts of the historic Australian city.

Officials said Sydney's air quality index Tuesday was 11 times above the limit considered to be hazardous, prompting them to urge residents with heart and lung problems to stay indoors as much as possible.

Firefighters raced from building to building to disable alarms triggered by the thick haze that settled across Sydney, while some commuter ferries were canceled as the smoke reduced visibility in the famous harbor to nearly zero.

About 100 bushfires are burning in the eastern states of New South Wales, Victoria and Queensland, fueled by high temperatures and land left dry by drought and climate change.