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 \"very unhealthy\". Beijing\'s main airport cancelled hundreds of flights due to the poor visibility on Sunday and Monday, angering passengers at the world\'s second-busiest airport. Visibility had improved by Tuesday, but 80 domestic and 10 international flights had been cancelled by midday due to light snow. Users of Sina\'s weibo -- one of China\'s hugely popular microblogging sites -- expressed frustration at the delays to their journeys, with one saying it had taken him 24 hours 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 on pollution posted to Sina on Tuesday. \"Today is another fine smog day in Beijing. I wore a mask this morning. I don\'t know how long I can live if I breathe this hazardous air all day long,\" posted another. One online retailer reported selling 100 face masks fitted with air filters to a single customer in Beijing, according to the 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 official 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 less severe assessment of air quality, leading to accusations the authorities are downplaying how serious the pollution is. \"There are always huge differences between the public data and weather broadcasts and the feelings of people,\" said one weibo user under the name \"Xuxuaimingxuan\". \"Sometimes, I suspect that what we\'re breathing isn\'t air, but politics.\"