A young Chinese pupil wearing a face mask walks out of her primary school in heavy smog in Beijing, China, 1 December 2015. Frustration is mounting in Beijing over the municipal government's response to air pollution, which on Tuesday (1 December 2015) grew so bad that it went beyond the worst official terms to describe it. Smog blanketed the capital for a fifth straight day on Tuesday, sending visibility in some areas down to 200 metres and the city's pollution index well beyond "hazardous", the highest level on the smog scale. Concentrations of PM2.5 particles, those most dangerous to human health, were particularly severe in the city's south. But the authorities still did not issue the top pollution alert for what is the worst case of air pollution in Beijing this year. A red alert, the highest in the four-tier system, would force schools to shut down and cars to stay off the road on alternate days. It should be issued when severe air pollution persists for more than 72 hours, but the capital remains on orange alert, one step below red.