China's biggest online face-mask sellers were running out of stock Wednesday as consumers rushed to protect themselves from smog that has shrouded large swathes of northern China for a 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 501 micrograms per cubic metre on Wednesday afternoon. The World Health Organisation's recommended safe limit is 25.

The situation was improving, however, with the reading for a location in the centre of the capital recorded at 100 by Wednesday evening.

An alternative measure by the US embassy in Beijing said PM 2.5 levels were at 542 in the city in the afternoon but had fallen to 180 in the evening.

The capital was on its sixth day of an "orange" smog alert -- the second-highest on the scale -- with the air tasting gritty and visibility down to a few hundred metres.

The choking smog has seen sales of anti-pollution products 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 platform, 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.

Another seller, Vogmask, had only children's models left on its Tmall store.

"I'm looking for face-masks 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 with the online handle Simao's Early Riser Mum on Sina Weibo, a Chinese equivalent of Twitter.

Concerns at the impact on children's health were particularly high, with most schools in Beijing keeping students indoors all day long according to Chinese media reports.

One school in the city defied education authorities by keeping its junior high department closed on Wednesday.

"Due to the city's air quality forecast... the junior high section will continue to use the online study and question and answer model on February 26," the Affiliated High School of Peking University said in a statement.

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

It has become a major source of popular discontent with the authorities.

President Xi Jinping paid a rare visit to one of Beijing's smog-hit streets Tuesday -- without a face-mask -- drawing praise from Internet users for his latest apparent attempt to portray himself as close to ordinary citizens. One widely shared online headline read: "Breathing the same air, sharing the same fate".

The pollution, which tends to worsen in winter, is blamed on the use of coal for energy, dramatic economic development, increasing car use and climatic factors.

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

In China a pollution index reading above 300 is deemed "hazardous" and everyone is advised to avoid outdoor activities.