Pollution of all forms is one of the greatest concerns of Chinese citizens and undermines the legitimacy of the Communist Party. Officials are struggling with how much information on pollution to disclose to citizens. Since 2012, public pressure has resulted in greater official transparency on air quality.

The coming red alert is expected to last longer than the previous one. It is scheduled to start at 7 a.m. on Saturday and end at midnight on Tuesday, according to a phone text message sent to Beijing residents on Friday morning. City officials are supposed to inform the residents 24 hours before emergency measures start. Under the crisis plan first announced in 2013 and revised this year, Beijing is supposed to issue a red alert every time officials predict a spell of smog in which the air quality index exceeds 200 — deemed “very unhealthy” by United States standards — for 72 hours or more.

Weather forecasts show almost zero wind this weekend, which explains the prediction of smog. If the smog does come as predicted, then this would be the third prolonged spell of hazardous air in northern China in the last month, and it would be the fourth time in that period in which the air quality moves into the truly toxic range. (There was a brief surge of smog last weekend, after the second red alert had ended.)