Beijing on Friday issued its second red alert for pollution, after residents of North China were warned of choking smog over the weekend. The smog will stretch from Xian, across parts of central China, through Beijing, to Shenyang and Harbin in the northeast, the National Meteorological Centre said.

This red alert will last from 7 am on Saturday till midnight on Tuesday. Visibility in the worst-affected places, including Beijing, is likely to drop to less than one kilometre. As the pollution index is expected to exceed 500 in Beijing and parts of Hebei province, residents have been advised to stay indoors. The United States deems a level of more than 200 "very unhealthy".

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Beijing had declared its first red alert last week after being criticised for not issuing the alert during previous bouts of smog. A government issues a red alert when it believes air quality will reach very unhealthy levels. Once a red alert is issued in Beijing, the odd-even licence plate system comes into effect, with only half the number of vehicles being permitted on roads. Schools are encouraged to remain closed, and outdoor construction is banned.

Beijing’s neighbouring city of Tianjin plans to implement the odd-even scheme, while orange and yellow alerts, which are the second- and third-highest warnings, will be enforced in other places affected by smog.