As northern China entered its high-smog season, one city’s visibility was less than half a football field on Monday and the manager of jazz singer Patti Austin said she had cancelled a Beijing concert because of asthma likely linked to pollution.

Winter typically brings the worst air pollution to the region because of a combination of weather conditions and an increase in the burning of coal for homes and municipal heating systems, which usually start on a specific date. For the northern city of Harbin, the city’s heating systems kicked in Sunday, and on Monday visibility there was less than 50 meters, according to state media.

Primary and middle schools and some highways were closed, said authorities in the city, which is in China’s northernmost province bordering Russia. At least 40 flights flying to destinations in southern China and Beijing among others had been cancelled or postponed at Harbin’s Taiping International Airport on Monday morning.

Some of the city’s buses also stopped running, the official Xinhua News Agency said.

The density of fine particulate matter used as an indicator of air quality was more than 24 times higher in Harbin than the level considered safe by the World Health Organization. The PM2.5 density was above 617 micrograms per cubic meter, according to official readings. A safe level recommended by WHO is 25 micrograms per cubic meter.

Austin’s management team said the 63-year-old singer had been treated in hospital on Friday morning for the asthma attack in combination with respiratory infection. She returned to her hotel later Friday to rest, but she was unable to physically perform at her concert scheduled for Beijing on Friday evening. Her Saturday night concert in Shanghai went ahead.

On the morning ahead of her concert Friday, Beijing’s air was visibly polluted, with the city’s environmental monitoring centre warning children, the elderly and those with respiratory illnesses to reduce outdoor activity.

China’s major cities have some of the world’s worst smog. The government was long indifferent to the environment as it pursued economic development, but has begun launching some anti-pollution initiatives after mounting public frustration.

Last month, China’s Cabinet released an action plan that aims to make a small reduction in the country’s heavy reliance on coal to below 65 percent of total energy usage by 2017. According to Chinese government statistics, coal consumption accounted for 68.4 percent of total energy use in 2011.