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BLUR: Even as Beijing's Forbidden City gets shrouded in smog, a man still takes his dog for a walk, albeit with precautions.

WHAT: Smog in large parts of China

HOW LONG: Ongoing (Started a week ago)

HOW BAD IS IT: Beijing's official reading for PM 2.5 - small airborne particles which easily penetrate the lungs - stood at 516 micrograms per cubic metre at 4pm on Wednesday.

WORLD HEALTH ORGANISATION'S RECOMMENDED SAFE LIMIT: 25

ANY ALERTS? Beijing is on its sixth day of "orange" smog alert, the second highest

VISIBILITY: a few hundred metres. air tastes gritty

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

Of the 29 models of face masks provided by US industrial and equipment supplier 3M's flagship store on business-to-consumer shopping platform Tmall.com, 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 that new stocks will not be available until April 1.

Another seller, Vogmask, has only children's models left on its Tmall store, AFP reported.

"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?" worried a user with the online handle Simao's Early Riser Mum on Sina Weibo, a Chinese equivalent of Twitter.

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

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

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" and everyone is advised to avoid outdoor activities.

As of 7pm on Wednesday, the official reading was 180.

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