Beijing on Sunday issued its highest smog alert of the year, upgrading it on Sunday from the yellow of the past two days to orange, second only to red.

The municipal weather center said humidity and a lack of wind would mean the smog will linger for another two days, before a cold front arrives on Wednesday.

On Sunday, the reading for PM2.5, airborne particles smaller than 2.5 microns in diameter, hit 274 micrograms per cubic meter in most parts of the capital.

Under the orange weather alert mechanism, industrial plants are required to reduce or shut down production; construction sites should stop the transportation of materials and waste while heavy-duty trucks are banned from the roads.

The municipal environment watchdogs have reinforced checks of discharge from coal-fired plants, outdoor barbecues and burning. Endi