Beijing authorities have decided to close schools and to stop outdoor construction after the Chinese capital issued its first “red alert” over smog levels.
The red alert is the highest possible, and has not been used in Beijing before, the state-run Xinhua news agency says.
Chinese authorities expect more than three consecutive days of severe smog.
Cars with odd and even number plates will be banned from driving on alternate days.
The alert comes as China, the world’s largest polluter, takes part in talks on carbon emissions in Paris.
Current pollution levels in Beijing are actually lower than last week’s, but the red alert has been placed because of levels expected over the coming days.
The order will last from 07:00 local time on December 8 until 12:00 on December 10, when a cold front is expected to arrive and clear the smog.
China’s CCTV news channel reported at the weekend that some parts of Beijing had visibility of only 660ft.
Coal-powered industries and heating systems, as well as vehicle emissions and dust from construction sites, all contribute to the smog which has been exacerbated by humidity and a lack of wind.
At 18:00 local time on December 7, the air pollution monitor operated by the US Embassy in Beijing reported that the intensity of the poisonous, tiny particles of PM 2.5 was 10 times above the recommended limit.
The level in Beijing reached more than 256 micrograms per cubic meter in some of the worst-affected areas. The World Health Organization considers 25 micrograms per cubic meter to be a safe level.
Activists said the level hit 1,400 micrograms per cubic meter in the north-east city of Shenyang last month, saying it was the worst seen in China.
Last week, activists from Greenpeace had urged the Chinese government to declare a red alert. Another Chinese city, Nanjing, issued a red alert in December 2013.
On November 30, Beijing issued an orange alert – the second-highest of the four-tier system adopted in 2013.
Correspondents say Chinese officials had been unwilling to commit to hard targets on reducing carbon emissions, but have now realized that the dependence on fossil fuels has to stop.
Chinese President Xi Jinping vowed to take action on the emissions at the current global climate change talks in Paris.
Beijing residents have been warned to don air masks and offices and homes to put electric air purifiers on overdrive after air pollution readings soared.
Air pollution readings on Thursday registered more than 20 times the recommended exposure levels by the World Health Organization (WHO).
Beijing Municipal Environmental Protection Bureau readings for PM 2.5 – air particulate smaller than 2.5 microns blamed for a range of severe respiratory ailments – registered over 500 micrograms per cubic meter. The WHO recommends no more than 25 micrograms per cubic meter.
Officials in Beijing issued a severe air warning and urged residents to wear protective masks while outdoors, and said the elderly and schoolchildren should stay indoors until conditions improved.
Beijing residents have been warned to don air masks and offices and homes to put electric air purifiers on overdrive after air pollution readings soared
The winter months in the north of the country tend to be periods of extended bad air pollution. Biting cold forces the region’s coal-burning power plants to meet heating demands, while increased car usage and relentless construction chokes the skies with dangerous particulate.
Beijing’s topography – with hills surrounding much of this city of 20 million people – can also keep the capital immersed in dirty air unless strong winds blow it out.
Certain at-risk residents who find themselves exposed to the bad air over extended periods of time complain of watery eyes, difficulty breathing or other respiratory issues. Long-term exposure can lead to asthma, heart disease and cancer. Prolonged exposure to PM2.5 has also been tied to knocking years off people’s life expectancy.
Despite ideal conditions for poor air quality, the air warning issued Friday was just the first of 2014.
China’s cities are frequently blanketed by pollution caused by coal-burning power plants, factories, and millions of vehicles on the roads.
China’s smog has forced schools, highways and airport to shut in northern city of Harbin, Heilongjiang province.
The density of airborne particles was several times above World Health Organization (WHO) recommended limits.
Visibility was reduced to below 160ft in parts of the city, reports said.
Local media have linked the pollution to the city switching on its public heating system for winter.
China’s smog has forced schools, highways and airport to shut in northern city of Harbin
PM2.5 levels, used to measure the amount of particulate matter smaller than 2.5 micrometres in the air, were above 500 micrograms per cubic metre on Monday morning, state-run news agency Xinhua reported.
WHO guidelines say average daily concentrations of PM2.5 should be no more than 25 microgrammes per cubic metre.
All of Heilongjiang province’s highways, and the Taiping International Airport in Harbin were forced to close, Xinhua reported.
A red alert for thick smog had been issued in the provinces of Heilongjiang, Jilin and Liaoning, the news agency added.
Earlier this year, air pollution in China’s capital, Beijing, also soared past hazardous levels.