A grey haze descended on the city of around 21.5 million people, with levels of PM2.5 — harmful microscopic particles that penetrate deep into the lungs — at one point above 300 micrograms per cubic metre according to the US embassy, which issues independent readings.
The World Health Organization’s recommended maximum exposure is 25.
The alert coincided with global climate change talks in Paris, where Chinese President Xi Jinping has vowed “action” on greenhouse gas emissions.
Most of China’s greenhouse gas emissions come from the burning of coal for electricity and heating, which spikes when demand peaks in winter and is the main cause of smog.
It was the first time authorities declared a “red alert” since emergency air pollution plans were introduced two years ago, although levels were far from the city’s worst.
It came a week after thick grey smog shrouded Beijing, cutting visibility severely and sending PM 2.5 levels as high as 634 micrograms per cubic metre.
Under the alert — the highest in a four-tiered, colour-coded warning system — an odd-even number plate system bans half the city’s roughly 4.4 million private cars from the streets on alternate days.
Outdoor construction sites are ordered to close and some industrial plants told to cease or reduce operations.
Some schools are also urged to close, and several Beijing residents said their children had been told to stay at home.
Authorities in the capital were heavily criticised after only issuing an orange alert for last week’s pollution.
“The red alert is a welcome sign of a different attitude from the Beijing government,” said Dong Liansai, climate and energy campaigner for environmental group Greenpeace.
“However, this, the latest of a series of airpocalypses to hit Beijing, is also a firm reminder of just how much more needs to be done to ensure safe air for all.