Beijing: China's biggest online facemask sellers were running out of stock Wednesday as consumers rushed to protect themselves against smog that has shrouded large swaths 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.
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 anti-pollution product sales boom and online facemask stores were struggling to meet demand.
Of the 29 models of facemasks 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 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 with the online handle Simao's Early Riser Mum on Sina Weibo, a Chinese equivalent of Twitter.
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 blamed on rapid urbanisation, dramatic economic development, increasing car use and climatic factors. Pollution tends to worsen in winter.
The National Meteorological Centre has said the pollution is expected to continue until Thursday.
In China a pollution index reading above 300 is deemed "hazardous", when everyone is advised to avoid outdoor activities.