The market for dust masks and air purifiers is booming in Beijing because the capital has been shrouded for several days in thick fog and haze, an indicator of heavily polluted air.

There is a huge demand for dust masks, particularly those labeled "PM2.5 masks", which are among the most expensive and are said to be able to help block fine particulate matter.

"We have sold dozens of these masks today," said Wang Hu, a salesman at a 7-Eleven convenience store in Beijing's Chaoyang district, referring to the "PM 2.5 masks".

The store sold out of the masks, priced at 32 yuan ($5.15), before 2 pm on Sunday.

Other masks have also seen a spike in sales.

Liu Qiong, a staff member at Haodekuai Pharmacy, said sales of disposable masks used by hospitals have tripled in the past few days.

Around 6,920 masks were sold over a two-day period on Taobao and Tmall, two popular e-commerce websites in China.

Sales of air purifiers have also increased. Many customers wearing masks rushed into electrical appliance retailers to buy air purifiers for their families.

"My daughter has severe allergic rhinitis, and she suffers a lot because of the polluted air," Beijing Evening News quoted a man who paid 5,000 yuan for an air purifier at Dazhong Electronics as saying.

"My home is a little too big for one device, so I bought another one. I hope it can effectively improve the air quality in my home."

According to the newspaper, the majority of air purifiers that customers chose are priced between 3,000 and 5,000 yuan, with the more expensive ones being the most popular.

Online sales of air purifiers have increased 16 percent compared with last year, according to Taobao.