AFP/ Beijing

Beijing issued its second-ever red alert for smog Friday ahead of severe pollution forecast to hit China's capital, weeks after putting its emergency response plan into action for the first time.

The notice from the capital's environmental bureau orders factories to close and pulls half of all private cars off the streets, among other measures, as bad air floods into the city for the third time this 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 will be the second time the highest level alert has been issued since the city established a pollution warning system in 2013.

Beijing issued its first 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.

Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- regularly exceeded 300 micrograms per cubic metre during last week's red alert, according to the US embassy, which issues independent readings.

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

"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. "We are still facing some challenges in environmental protection."

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

Other cities in the region will also raise their alert level to red, the national environmental bureau said in a separate statement, adding it had urged regional governments to "implement heavy air pollution emergency response measures".

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 smog.

China's meteorological bureau has said it expects at least one and possibly two more bouts of heavy pollution in December, with the first expected as soon as Saturday.