A 660,000 square kilometer smog cloud has smothered much of northern China, engulfing the national capital as well as 70 other cities in neighbouring Hebei, Henan and Shandong provinces.

Chinese media have said the expanse of air pollution is more than 40 times the size of Beijing itself, which has issued its second-ever red alert over air pollution levels.

Air Quality Index readings for north China on Monday morning. Photo: HKFP.

The red alert, the highest tier in a four-colour warning system, will see factories closed and half of the city’s private cars taken off the road. It is only the second time a red alert has been issued since the warning system was introduced in 2013.

China’s first pollution red alert was issued in Beijing earlier in the month, on December 6.

Harmful PM2.5 particles measured more than 260 micrograms per cubic meter early on Saturday—close to ten and a half times the World Health Organization’s recommended maximum exposure of 25 over 24 hours.