BEIJING (AFP) - Beijing authorities cancelled hundreds of flights and shut motorways on Monday as thick smog descended on the Chinese capital, reducing visibility at one of the world's busiest airports. Air quality in Beijing reached "hazardous" levels on Monday, according to the US embassy, which conducts its own measurements, while the official Xinhua news agency said pollution was likely to reach "dangerous" levels. By the middle of Monday afternoon, Beijing's main airport - the second busiest in the world by passenger traffic - had cancelled 233 domestic and 17 international flights, according to its website. Another 400 flights were cancelled on Sunday. Television footage of the airport concourse showed thousands of stranded passengers being turned away, or waiting around in hope of booking later flights if the smog lifted. Most major motorways linking Beijing to other parts of north China were closed early Monday due to the smog, but sections of some roads began opening throughout the day as the visibility improved, CCTV reported. International organisations including the United Nations list Beijing as one of the most polluted cities in the world, mainly due to its growing energy consumption, much of it from fossil fuels. "Coal burning is the main cause of all the grey hazy days that Beijing gets," Zhou Rong, an air pollution expert with Greenpeace China, told AFP. "China has more than doubled its coal consumption in the last 10 years, so we are getting more soot in the air, as well as secondary pollutants like sulphur dioxide and nitrous oxide which also help cause the haze." Beijing's nearly five million vehicles also emit a lot of the particulates that make up the capital's air pollution, she added. Authorities in Beijing went to huge lengths to clean up the city's air ahead of the 2008 Olympics, shutting down coal-fired power stations and restricting the number of cars on the roads, but air quality in the city remains bad. Frequent smog in October and November has given fresh impetus to a growing public debate over air quality in Beijing, whose 20 million residents are increasingly worried. Their concerns are being fuelled in part by data gathered by the US embassy, which produces its own pollution readings using a different gauge to Chinese authorities and broadcasts them online and on Twitter. China currently rates air quality by measuring airborne particulates of 10 micrometres or less, adopting a standard known as PM10, while the embassy measures only levels of those that are 2.5 micrometres or smaller. Scientists say Beijing's pollution is mostly caused by these smaller particles, which are deemed more dangerous to health as they can pass through smaller airways and penetrate deeper into the lungs, and even into the blood. According to the state-run China Daily, if the US standard was adopted nationwide, only 20 percent of Chinese cities would be rated as having satisfactory air quality, against the current 80 percent.