BEIJING (AP) — Air quality reached extremely hazardous levels in the northeastern Chinese city of Shenyang on Sunday, as northern China began to burn coal to heat homes for winter.

Real-time data released by the Shenyang Environmental Protection Agency showed the density of the poisonous tiny airborne matters known as PM2.5 was more than 1,000 micrograms per cubic meter. Nearby cities also reported very high levels of the harmful particulates.

The World Health Organization considers the safe level of PM 2.5 to be 25 micrograms per cubic meter on a 24-hour average basis.

Local authorities say they have warned residents to stay indoors and demanded power plants to cut output to reduce pollution.

Photos posted on social media showed gray skies, and members of the public complained of sore throats.

Northern China typically burns coal to heat homes in the winter, a practice believed to have fouled the air. Emissions from industrial plants and the increasing use of cars also are major causes of air pollution in China.