Cars run in a street in smoggy Sydney, Australia, on May 24, 2019. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)

A man jogs in smoggy Sydney, Australia, on May 24, 2019. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)

Photo taken on May 24, 2019 shows the smoggy view in Sydney, Australia. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)

Photo taken on May 24, 2019 shows the smoggy view in Sydney, Australia. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)

A tourist takes photos in smoggy Sydney, Australia, on May 24, 2019. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)

Photo taken on May 24, 2019 shows the smoggy view in Sydney, Australia. The smoke haze that choked Sydney earlier in the week has returned, again shrouding the city in a thick fog even though the New South Wales Rural Fire Service halted hazard reduction burns on Wednesday due to the poor air quality. (Xinhua/Bai Xuefei)