Beijing lifts smog red alert

Beijing's first ever red alert for smog expired Thursday, as blue skies and sunshine replaced the thick haze that covered the city for days.

The Chinese capital put its air pollution emergency plan into action earlier this week, pulling half of all private vehicles off the streets from Tuesday, ordering many factories to close and recommending that some schools allow students to remain home.

The measures were being lifted from midday Thursday, according to a social media post by Beijing's environmental protection bureau.

The red alert, the highest tier of a four-colour warning system, came as heavy smog flooded the city for the second time in as many weeks.

The unprecedented move followed scathing public criticism aimed at the city's weak response to last week's thick haze, which saw pollution sky-rocket to levels not seen in years.

Counts of PM2.5 -- harmful microscopic particles that penetrate deep into the lungs -- reached well over 600 micrograms per cubic metre last week, according to the US embassy, which issues independent readings, and were regularly above 300 in recent days.

By lunchtime on Thursday they were down to 22 as moderate winds blew from the north, below even the World Health Organization's recommended maximum exposure of 25.

In a note posted online, the city wrote that the emergency measures had "been effective in slowing down the process of smog accumulation", and expressed its "heartfelt thanks" and "sincere tribute" for residents' contributions to the effort.

The city will "fight well a hard battle to prevent and contain air pollution", it said.

But the struggle is largely out of municipal officials' hands since much of its air pollution comes from neighbouring areas, where pollution levels remained hazardous Thursday, particularly to the south in Hebei province.