Air quality in Beijing was the "worst on record", according to environmentalists, as the city's pollution monitoring centre warned residents to stay indoors with pollution 30-45 times above recommended safety levels.
The Chinese capital, home to around 20 million people, has been wrapped in thick smog since Friday, reducing visibility and disrupting traffic.
Data posted by the monitoring centre (www.bjmemc.com.cn) showed particulate matter measuring less than 2.5 micrometres 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 US embassy -- some areas in (neighbouring) Hebei province are even worse than Beijing," said Zhou Rong, climate and energy campaigner at Greenpeace.
Hourly pollution data
The Beijing Municipal Environmental Monitoring Center said heavy pollution had been trapped by an area of low pressure, making it harder to disperse, and the conditions were likely to last another two days.
Pollution has been identified as one of the biggest challenges facing China's leaders, with outgoing president Hu Jintao saying during his address to the Communist Party Congress last November that the country needed to "reverse the trend of ecological deterioration and build a beautiful China".
China said at the end of last year that it would begin releasing hourly pollution data for its biggest cities.
Beijing has already committed to a timetable to improve air quality in the city, and has relocated most of its heavy industry, but surrounding regions have not made the same commitments, said Zhou.
"For Beijing, cleaning up will take a whole generation but other regions don't even have any targets to cut coal burning. I bet the pollution here is mainly from those surrounding regions."
(Reuters Health, January 2013)