Millions of Chinese went online Tuesday to vent their anger over the thick smog that has blanketed Beijing in recent days, raising health fears and causing hundreds of flights to be cancelled.

Sales of face masks were reported to have surged as residents of China's heavily polluted capital sought to protect themselves from the air, which US embassy figures ranked as "very unhealthy".

Beijing's main airport cancelled hundreds of flight due to the poor visibility on Sunday and Monday, angering passengers at the world's second-busiest airport.

Visibility had improved by Tuesday, but 89 domestic and 11 international flights had been cancelled or delayed by late afternoon.

Users of Sina's weibo -- one of China's most popular microblogs -- expressed frustration at travel delays, with one saying it had taken him 24 hours, instead of four, to travel to Beijing from the southern city of Shenzhen.

"I'm exhausted. All of this was because of the thick smog," wrote Hu Yueyue in one of 4.4 million comments about pollution posted to Sina on Tuesday.

Competitor Tencent's weibo users' posts on pollution generated a whopping 68 million comments, boosting the topic to the fourth most-discussed of the day.

Some linked the toxic air to other hot issues of the day, such as a manhunt for the bomber of a bank in central China, or a yoghurt drink made by Coca Cola that was deliberately poisoned, killing one.

"The bank explosion in Wuhan, poisoned milk in Changchun, serious air pollution in Beijing... Can we live or not? Or is it that I pay too much attention to the dark side of society?" web user Gaiyong wrote.

"The reality has crushed my confidence. No wonder rich people all go to foreign countries to avoid disasters."

One online retailer reported selling 100 face masks fitted with air filters to a single customer in Beijing, according to the official Global Times daily.

Taobao.com, China's biggest online retailer, sold 30,000 masks on Sunday, when the US embassy in Beijing rated the air as "hazardous", the state-run Xinhua news agency said.

The US embassy conducts its own air quality measurements, measuring the finest particles, which experts say make up much of Beijing's pollution.

The official government figures are based on measurements of larger particles and often give a better assessment of air quality, leading to accusations the authorities are downplaying how serious the pollution is.

On Tuesday, the US embassy pollution index called the air "very unhealthy" while Xinhua's measurement said it was "slightly polluted."

Weibo user "T_maoyangshenghuo" reacted angrily at comments from the spokesperson of the Beijing Environmental Protection Bureau saying the smog in Beijing caused "slight pollution" over the last two days.

"Beijing citizens are speechless. Where is the serious pollution? In the brain of the spokesperson?" the message said.

Another web user said there were "always huge differences between the public data and weather broadcasts and the feelings of people."

"Sometimes, I suspect that what we're breathing isn't air, but politics."