Pollution levels 50 times higher than that deemed safe by the World Health Organisation were recorded in northern China at the weekend.
According to the Associated Press news agency, the levels found in the city of Shenyang could be the highest pollution levels ever recorded in China since the country started monitoring and releasing data in 2013.
The Shenyang Environmental Protection Agency released data displaying levels of the airborne matter PM2.5 at more than 1,000 micrograms per cubic metre.
WHO's recommended level is 25 micrograms per cubic metre on a 24-hour average.
China's Xinhua News agency reported on Twitter readings as high as 1,400 micrograms per cubic metre in some areas.
The BBC reports that activists have labelled the recordings the “worst ever”.
The high level is considered extremely hazardous and residents were advised to stay indoors after members of the public took to social media complaining of sore throats.
At this time of year coal production is particularly high in the northern region as it is needed to heat up homes during the winter months. Industrial plants and the amount of cars also contribute to the hazardous levels.
In August it was revealed that air pollution kills 4,000 people per day in China.
