Air quality reached extremely hazardous levels in the northeastern Chinese city of Shenyang as northern China began to burn coal to heat homes for winter. Real-time data released by the Shenyang Environmental Protection Agency on Sunday showed the density of the poisonous tiny airborne matters known as PM2.5 was more than 1,000 micrograms per cubic metre. Nearby cities also reported very high levels of the harmful particulates. The World Health Organisation considers the safe level of PM 2.5 to be 25 micrograms per cubic meter on a 24-hour average basis. READ MORE: Air pollution killing 4,000 people a day in China, says US report The readings from Shenyang are possibly the highest pollution levels ever recorded in China since the country began to monitor air quality and release real-time data in 2013. The local authorities said the density of PM 2.5, considered extremely harmful to human health, peaked on Sunday afternoon at more than 1,200 micrograms per cubic metre. The state-run Xinhua News Agency reported that the reading had reached as high as 1,400 micrograms per cubic meter at some monitoring sites. The local authorities issued the highest alert, warning residents to stay indoors and demanding that factories cut output to reduce pollution. Photographs posted on social media showed grey skies and members of the public complained of sore throats. READ MORE: Air quality rule changes put heat on local authorities 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.