Beijing has issued its first ever “red alert” over the city’s smog, with the Chinese capital going into shutdown in an attempt to protect people from the deadly air.

It is the first time ever that the highest possible state of caution has been declared in the city, which has more than 21 million residents.

The warning means schools will have to close down and construction and other industry in the capital will be limited.

Smog levels are actually lowe than they have been in recent weeks. But authorities expect an extreme amount of pollution over the coming days, and have imposed the order to ensure safety during that time.

The order will last from Tuesday morning local time until midday on Thursday, when the weather is expected to change and blow away the smog.

The city had already been in a state of orange alert, which meant some construction and industry was being curbed. The city had also issued a ruling that said cars with odd and even number plates would be stopped from driving on alternate days.

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In some parts of Beijing, people can only see around 200m. The air is also packed with poisonous particles that mean that people could become ill simply from being outside.

Air pollution monitors showed areas of Beijing had more than 256 micrograms per cubic metre of the poisonous particles. The World Health Organisation (WHO) says anything over 25 micrograms is considered unsafe.

The poisonous smog in Beijing is caused by the burning of coal for industry and heating, and huge amounts of dust from the city’s many construction sites. The problem is being made yet worse by high humidity and low wind.

Last week, Greenpeace called on Beijing to issue a red alert, after four days of what it called “Airpocalypse”.

Choking smog in China

“The city is blanketed in a thick, choking smog that has covered an area of North China the size of Spain and Beijing’s most famous landmarks have been completely obscured by the yellow haze,” wrote Zhang Kai on the group’s blog.

Greenpeace praised the Orange Alert that had been declared for putting restrictions on construction and industry, but said that it was “clearly not enough”.