With a thick smog wrapping the Chinese capital since Friday, the city’s pollution monitoring center warned the city’s 20 million residents to stay indoors.

Data posted on Sunday by the monitoring center showed particulate matter measuring less than 2.5 micrometers in diameter (PM2.5) had reached more than 600 micrograms per square metre at some monitoring stations in Beijing, and was as high as 900 on Saturday evening.

The recommended daily level for PM2.5 is 20, according to the World Health Organisation. Such pollution has been identified as a major cause of asthma and respiratory diseases.

“This is really the worst on record not only from the official data but also from the monitoring data from the U.S. embassy — some areas in (neighboring) Hebei province are even worst than Beijing,” said Zhou Rong, climate and energy campaigner at Greenpeace.