BEIJING • As winter approaches, thick hazardous smog has returned to Beijing and other parts of northern China which still rely heavily on coal-fuelled heating systems.

Severe air pollution has driven up the sales of face masks, air purifiers and bottled air harvested from a forest in Inner Mongolia, according to media reports.

Beijing and neighbouring regions have experienced three periods of "hazardous" air quality since the start of autumn, and meteorological departments have warned the situation could get worse in the coming months.

A bottled-air product on online marketplace Taobao. Thick smog has returned to Beijing and other parts of northern China. PHOTO: TAOBAO

Global Outlook Biotech, which sells bottled air collected from a mountainous area in the Inner Mongolia region, said sales of its Senlin Baobei canisters are doing much better this month.

"We sold fewer than 100 bottles in August and about 400 in September. But we sold more than 1,000 in the first 20 days of October, and have been selling more than 100 per day since then, thanks to the recent smog," said Mr Chen Zhiquan, the company's managing director.

Most purchases have been made online by people in Beijing and cities in Shandong province, he told China Daily.

Each canister is priced at 58 yuan (S$12) and comes with a breathing mask. It is designed to provide 30 minutes of "natural fresh air", he said.

Sellers of air purifiers are also seeing brisk sales. More than five million air purifiers are expected to be sold on the mainland this year, a 14.2 per cent rise from last year, according to the state-run think-tank CCID Consulting.

But in the first nationwide quality inspection of air purifiers, Chinese officials found that one in four machines failed to meet quality requirements, the South China Morning Post reported .

The General Administration of Quality Supervision, Inspection and Quarantine tested appliances from about one-fifth of all manufacturers in China. It found that 24.6 per cent of those examined could not remove pollutants efficiently.

In some cases, the users risked getting electric shocks from leaked current, according to the watchdog.

The disappointing performance of air purifiers sparked another wave of complaints about China's persistent air pollution.

"If the air could meet its quality standard, why would we need air cleaners," one Weibo user commented on a post about the inspection results.