Beijing has issued a second pollution red alert, little more than a week after the first ever such warning
The Chinese capital will see hazardous smog from Saturday until Tuesday, the official meteorological service said.
Nationwide, a vast area from Xian in central China to Harbin in the north-east would also be badly hit, the National Meteorological Centre said.
Beijing authorities have advised residents to avoid outdoor activity and for schools to stop classes.
The red alert - the highest of a four-level alert system instituted two years years ago - also triggers restrictions on vehicle use, factories and construction work.
The World Health Organization considers PM2.5 readings of 25 micrograms per cubic metre as the maximum safe level.
- Particulate matter, or PM, 2.5 is a type of pollution involving fine particles less than 2.5 microns (0.0025mm) in diameter
- A second type, PM 10, is of coarser particles with a diameter of up to 10 microns
- Some occur naturally - e.g. from dust storms and forest fires, others from human industrial processes
- They often consist of fragments that are small enough to reach the lungs or, in the smallest cases, to cross into the bloodstream as well
- A build-up of PM2.5 in the lungs has been associated with causing respiratory illnesses and lung damage
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