Air quality has reached extremely hazardous levels in the northeastern Chinese city of Shenyang, as northern China begins 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 on Sunday. 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.

Photos posted on social media show gray skies, and members of the public have been complaining of sore throats.