China's biggest online face mask sellers are running out of stock, with consumers rushing to protect themselves against smog that has shrouded swathes of northern China for an entire week.

Beijing's official reading for PM 2.5 - small airborne particles which easily penetrate the lungs and have been linked to hundreds of thousands of premature deaths - stood at 486 micrograms per cubic metre on Wednesday morning. The World Health Organization's recommended safe limit is 25.

An alternative measure by the US embassy in Beijing said PM 2.5 levels reached 557 in the city. In Xinji, in the neighbouring province of Hebei, official Chinese statistics put the figure at 761, AFP news agency reported.

The Beijing government raised its four-tiered alert system to "orange" for the first time last week, after drawing public criticism for its initial ineffective response. The warning system was unveiled in October last year.

The choking smog has seen anti-pollution product sales boom and online face mask stores were struggling to meet demand.

Of the 29 models of face masks provided by US industrial and equipment supplier 3M's flagship store on Tmall.com, a business-to-consumer shopping website, 26 were sold out or unavailable on Wednesday.

The Tmall outlet of Totobobo, which makes transparent, reusable masks in Singapore, put up a notice saying new stocks would not be available until April 1.

"I'm looking for facemasks and an air purifier as the smog is getting worse. And then I found masks were sold out and the price of air purifiers is shooting up. Is everybody panicking?" complained a user on Sina Weibo, a Chinese equivalent of Twitter.

Hazardous environment

Cities across China have been hit by intense air pollution in recent years, much of it caused by emissions from coal-burning power stations.

China's pollution problems are further blamed on rapid urbanisation, dramatic economic development, increasing car use and climatic factors. Pollution tends to worsen in winter.

A pollution index reading above 300 is deemed "hazardous", when everyone is advised to avoid outdoor activities.

"We have been trying to stay at home as much as possible for days. The pollution is really bad, and it affects our respiratory system. We just don't go out unless we absolutely have to," said 62-year-old Wang Xinrui.

The National Meteorological Centre has said the pollution is expected to continue until Thursday.