[BEIJING] Half of Beijing's private cars were ordered off the streets on Tuesday and many construction sites and schools were closed under the Chinese capital's first-ever red alert for pollution.
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 Organisation'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 - outdoor construction sites are ordered to close, and some schools are urged to do so, with several Beijing residents saying their children had been told to stay at home.
An odd-even number plate system also means that half the city's private cars are not allowed onto the streets on alternate days. According to official statistics, almost 4.4 million private cars were registered in the Chinese capital at the end of 2014.
Some industrial plants are told to cease or reduce operations, as well.
Authorities in the capital were heavily criticised after only issuing an orange alert for last week's pollution.
An editorial in the government-published newspaper the China Daily on Tuesday said the decision to issue the alert showed that "authorities have listened to residents' concerns."
"Of course," it added, "we don't expect the frequent issuance of a red alert, and we hope that we will be able to forget about it in the near future", when the government can "keep the air clean for good".