Beijing issued its second-ever red alert for smog and put its emergency response plan into action once again as severe pollution hit China's capital on Saturday.

The notice from the capital's environmental bureau ordered factories to close and pulled half of all private cars off the streets, among other measures, as bad air flooded into the city for the third "airpocalypse" of the month.

The red alert, the highest tier of a four-colour warning system, will last from Saturday to Tuesday, according to a statement on the Beijing Municipal Environmental Protection Bureau's website.

It is the second time the highest level alert has been issued since the city established a pollution warning system in 2013.

Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- were above 260 micrograms per cubic metre earlier Saturday, according to the US embassy, which issues independent readings.

The World Health Organisation's recommended maximum exposure is 25 over a 24-hour period.

By mid-afternoon the reading lowered to 140, but the pungent smelling smog still reduced visibility in the capital even though the streets were not as clogged with vehicles as usual.

"Environmental protection is an important objective the Chinese government has been firmly working on," foreign ministry spokesman Hong Lei said at a regular scheduled briefing in Beijing.

But, he added: "We are still facing some challenges in environmental protection".

The national environmental bureau said that other cities in the region would raise their alert level to red, adding it had urged regional governments to "implement heavy air pollution emergency response measures".

- Excercising in smog -

The soupy haze did not turn off several elderly people who headed to Beijing's parks and public squares to do their morning exercises.

But residents on social media were seemingly more bothered by the latest bout of heavy haze and said the government was not doing enough to tackle the problem.

"Whether or not I drive my petrol car has little to do with the main causes of smog," one user on China's Twitter equivalent Weibo said.

"If alternating traffic is effective in controlling smog, why not set it up permanently?" said another commenter.

Beijing issued its first-ever red alert on December 7, declaring emergency pollution measures following scathing public criticism of the city's weak response to choking smog that settled on the city earlier in the month.

PM2.5 rates regularly exceeded 300 micrograms per cubic metre during the last red alert.

Beijing lifted the alert on December 10, after winds from the north dispersed the bad air, leaving skies clear and blue.

During an earlier airpocalypse at the beginning of the month, PM2.5 levels climbed to as high as 600 micrograms per cubic metre but the government did not issue a red alert, prompting strong public criticism.

Most of the country's greenhouse gas emissions come from the burning of coal for electricity and heating -- particularly when demand peaks in winter -- which is also the key cause of smog.

The recurrent pollution has driven Beijing residents to hospitals in growing numbers, according to a report on Internet giant Tencent's news portal.

During the last month's periods of severe pollution, it said, trips to medical facilities using hailing app Didi Kuaidi -- backed by Tencent -- went from 3.4 percent of all journeys to 4.1 percent, an increase of more than a fifth.

The report also cited online retailer JD.com as saying pollution mask sales soared by as much as 400 percent in response to the bad air.

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JD.COM

Tencent

Weibo